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Fault detection and isoloation in low-voltage dc -bus microgrid systems

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Fault detection and isoloation in low-voltage dc -bus microgrid systems
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Candelaria, Jared M
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FAULT DETECTION AND ISOLOATION IN LOW-VOLTAGE DC-BUS
MICROGRID SYSTEMS
by
Jared M. Candelaria
B.S.,Metropolitan State College of Denver, 2007
A thesis submitted to the
University of Colorado Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
Electrical Engineering
2012


This thesis for the Master of Science
degree by
Jared M. Candelaria
has been approved
by
Jae-Do Park,Ph.D
Yiming Deng,Ph.D
Hamid Fardi,Ph.D
Date


Candelaria, Jared M. (M.S., Electrical Engineering)
Fault Detection and Isoloation in Low-Voltage DC-Bus Microgrid Systems
Thesis directed by Assistant Professor Jae-Do Park,Ph.D
ABSTRACT
Unlike traditional AC distribution systems, protection has been challenging
for DC systems. Multi-terminal DC power systems do not have the years of
practical experience and standards that AC power systems have. Also, the cur-
rent power electronic devices can not survive or sustain high magnitude faults.
Conveters will shut down to protect themselves under faulted conditions. This
makes locating faults in DC system difficult, and causes the DC bus to de-
energize. A fault protection algoritm and method for a low-voltage DC-bus
microgrid system is presented in this thesis in order to revolve the above issues.
The main goal of the protection method is to detect and isolate faults in the
DC system without degenergizing the entire DC bus. In order to achieve this a
ring bus was utilized for the main DC bus. The bus was then segmented into
individual zones with solid state bi-directioanl switches used to isolate the zone
in the event of a fault. Each zone is monitored and controlled by an individual
Intellegent Electrical Device. A grounding resistor was added in order to limit
the amount of ground current. The concepts have been verified in OrCAD/Psice
and MATLAB.


This abstract accurately represents the content of the candidates thesis. I
recommend its publication.
Jae-Do Park,Ph.D


DEDICATION
This thesis is dedicated to my two young daughters, Madilynn and Kennedy. I
hope that this can serve as an example the value of importance of an education.


ACKNOWLEDGMENT
I would like to first thank my wife for allowing me the time to attended my
graduate classes and conduct research while she took care of our children. I
would like to thank my grandparents for supporting me in pursuing this degree
and helping watch my children so that I could do so. I would like to acknowledge
Dr. Keith Malmedal for sparking my interest in power systems and hirig me for
my first job in power systems. Without that first job I would not be where I am
today. Lastly, I would like to thank Dr. Jaedo Park for his encouragemnet and
support during my time at the University of Colorado Denver. Without him
this thesis would not have been possible.


CONTENTS
Figures ................................................................ ix
Tables.................................................................. xi
Chapter
1. Introduction.......................................................... 1
2. Microgrid Systems .................................................... 5
2.1 AC vs. DC .......................................................... 5
2.2 HVDC vs. LVDC....................................................... 5
2.3 LCC vs. VSC......................................................... 8
3. Fault Protection of Low-Voltage DC-bus Microgrid Systems ......... 10
3.1 Possible Faults.................................................... 10
3.1.1 Line to Ground................................................... 10
3.1.2 Line to Line..................................................... 12
3.1.3 Over current..................................................... 12
3.2 Current Protection Methods......................................... 13
3.2.1 DC Protection with AC Devices.................................... 13
3.2.1.1 AC Circuit Breakers............................................. 14
3.2.1.2 Fuses ......................................................... 15
3.2.2 DC Protection with DC Devices................................... 17
3.2.2.1 IGBT Circuit Breakers.......................................... 17
3.2.2.2 Converter Embedded Devices..................................... 20
vii


3.2.3 Multi-Terminal System Protection ............................... 23
3.2.3.1 AC Protection.................................................. 23
3.2.3.2 DC Protection.................................................. 26
3.2.4 Controllers .................................................... 29
3.2.5 Current Limiting Techniques .................................... 31
4. Proposed Protection Method and Algorithm............................ 33
4.1 Operation Modes................................................... 33
4.1.1 Fault Detection and Isolation................................... 34
4.1.2 Breaker Failure Detection....................................... 40
4.1.2.1 Status and Current............................................. 40
4.1.2.2 Current........................................................ 41
4.1.3 Reclose and Restore............................................. 42
4.2 Solid State Circuit Breakers................................... 42
4.2.1 Grounding....................................................... 43
5. Simulation Results.................................................. 46
6. Conclusion and Future Works......................................... 63
Appendix
A. MATLAB Code......................................................... 65
References............................................................ 134
viii


FIGURES
Figure
1.1 Adams Hydroelectric 3 Phase AC Generating Plant......................... 2
2.1 Conceptual diagram of a DC-bus microgrid system......................... 6
2.2 VSC Operation (a) Normal, (b) Positive Line-to-Ground Fault. . 9
3.1 a) Line-to-Ground Fault b) Line-to-Line Fault.......................... 12
3.2 Back to Back Differential Protection................................... 16
3.3 Two-Terminal Differential Protection................................... 16
3.4 IGBT-CB Fault Blocking Capabilities.................................... 19
3.5 ETO Based VSC-HVDC Converter........................................... 22
3.6 VSC-HVDC Converter Submodule........................................... 22
3.7 Hand-Shaking Method:(a) Current Flow During a Fault, (b) Fault, Isolation. 24
3.8 Hand Shaking Method Re-Closing......................................... 24
3.9 IGBT-CB Protection..................................................... 28
3.10 Overvoltage Chopper Circuit........................................... 30
4.1 Microgrid Protection Zones............................................. 35
4.2 Algorithm Flow Chart................................................... 36
4.3 Uni-Directional Zone: Normal and Faulted Current Flow.................. 39
4.4 Bi-Directional Zone: Normal and Faulted Current Flow................... 39
4.5 Link Zone: Normal and Faulted Current Flow............................. 39
4.6 Zone Controller Connections............................................ 41
IX


4.7 a)Bi-direct.ional IGBT circuit breaker. b)Bi-directional IGCT circuit
breaker............................................................... 43
5.1 OrCAD/PSpice Simulation Model.......................................... 47
5.2 Analytical Simulation Circuit for Line-to-Ground Fault in Zone 1 Without.
Resistance Grounding.................................................. 49
5.3 Analytical Simulation Circuit for Line-to-Ground Fault with Resistance
Grounding............................................................. 49
5.4 Voltage at AC and DC Loads during a Line-to-Ground Fault with Resis-
tance Grounding....................................................... 50
5.5 Current at AC and DC Loads During a Line-to-Ground Fault with Re-
sistance Grounding.................................................... 51
5.6 Zone 1 Circuit Breaker Status During a Line-to-Ground Fault ......... 52
5.7 Zone 1 IED Trip Signals During a line-to-Ground Fault................ 53
5.8 Zone 4 Current with Breaker 5 Failing to Open........................ 54
5.9 Zone 3 with Breaker 5 Failing to Open ............................... 55
5.10 Zone 4 Breaker Status with Breaker 5 Failing to Open................. 55
5.11 Zone 3 Breaker Trip Signals After Breaker 5 Fails to Open............ 56
5.12 DC Voltage at Loads During a Zone 4 Breaker Fail..................... 57
5.13 Current at Loads During a Zone 4 Breaker Fail......................... 58
5.14 Zone 1 Current During a Reclose and Lockout Cycle.................... 59
5.15 Zone 1 Reclose Cycle Trip Signals.................................... 60
5.16 Zone 1 Reclose Cycle Close Signals .................................. 61
5.17 Zone 1 Breaker Status During a Reclose and Lockout Cycle............. 62
x


TABLES
Table
5.1 Simulation Parameters........................................... 48
XI


1. Introduction
At the end of the \Qth century the war to electrify the world was waged. On
one side sat Thomas Edison with his DC system and on the other was George
Westinghouse and Nikola Tesla with their AC system. In the beginning Edison
was winning with the opening of the Pearl Street Station in New York, 1882 [39].
Unfortunately for Edison the technology to increase a DC voltage to transmission
level was not yet available. AC systems had the ability to reach such voltages
with the technology of the time. In 1893 Westinghouse and Tesla successfully
supplied 11MW of electricity to the Chicago Worlds Fair. Then in 1895 the
Adams Hydroelectric Generating Plant (Fig.1.1) was opened at Niagara Falls,
serving local utilities in Niagara Falls. By 1910 transmission lines were serving
loads in Southern Ontartio[40]. This marked the birthplace of AC power and
set in motion its dominance in the world.
Even though AC clearly won the war, research and development into DC
did not stop. In the 1920s mercury arc valves became available for use in power
transmission. In the 1950s high power transistors like the thyristor were intro-
duced allowing higher voltage and power ratings [11, 5]. These inventions led
to the use of High Voltage Direct Current (HVDC) Transmission, to transmit
power over great distance. HVDC has also been used as an asynchronous ties
between separate power grids. These systems using thyristor valves are com-
monly referred to as Classical or Line Commutated Converter (LCC) HDVC
have been in place since the 1970s, and have been a vital part of the modern
1


Figure 1.1: Adams Hydroelectric 3 Phase AC Generating Plant.
2


grid.
Recently many distributed power systems have been researched and devel-
oped, especially to meet the need for high penetration of renewable energy re-
sources such as wind turbines and photovoltaic systems. The distributed power
systems have advantages such as the capacity relief of transmission and distri-
bution, better operational and economical generation efficiency, improved relia-
bility, eco-friendliness and power quality [19, 14, 9]. The current energy policy
of many governments in the world is to competitively increase the requirement
of the penetration of renewable energy sources and distributed generation. For
instance, California is trying to increase the usage of renewable generation up
to 33% by 2020 [54] and the State of Colorado has set specific requirements for
distributed generation from eligible renewable energy resources [55].
The microgrid system is a small-scale distributed power system consists
of distributed energy sources and loads, and it can be readily integrated with
the renewable energy sources [33, 42, 29, 41, 46, 25]. Due to the distributed
nature of microgrid approach, the connection to the central dispatch can be
removed or minimized and in turn the power quality to sensitive loads can
be enhanced. Most microgrid systems have their connected distributed energy
sources interfaced through power electronics converters. Generally they have
two operation modes: standalone (islanded) and grid-connected operation.
Microgrid systems can be divided into AC-bus and DC-bus systems, based
on the bus that the component systems such as energy sources, loads and stor-
ages are connected to. AC-bus based microgrids are advantageous because the
existing AC power grid technologies can be readily applicable. Currently, all of
3


the commercially installed microgrids are AC [7]. DC microgrids have several
advantageous over AC which will be coved in this thesis. One of the main reasons
that DC microgrids are not as prevalent as AC microgrids is that the system
protection is in its infancy. The protection of DC systems is not standardized
and often requires a complete shutdown of the DC bus[15, 53, 52],
This thesis presents a fault protection method and algorithm for a low-
voltage DC-bus microgrid. The primary goal of the proposed method is to
detect the fault in a bus segment between devices and isolated the faulted section
so that the system keeps operating without disabling the entire system. This
thesis will provide a comparison of AC and DC systems, present the types of DC
systems available, and give an overview of the current DC protection methods.
Finally, the protection method and algorithm will be provided with simulation
results.
4


2. Microgrid Systems
2.1 AC vs. DC
Microgrid systems can be divided into AC-bus and DC-bus systems, based
on the bus that the component systems such as energy sources, loads and stor-
ages are connected to. AC-bus based microgrids are advantageous because the
existing AC power grid technologies can be readily applicable. However, AC grid
issues including synchronization, reactive power control, and bus stability are
inherited as well. DC-bus based systems can become a feasible solution because
microgrids are small, localized system that the transmission loss is negligible,
unlike the traditional power systems that have a long line of transmission and
distribution. Moreover, it does not need to consider the AC system issues and
system cost and size can be reduced compared to the typical AC-DC-AC conver-
sion configuration because DC power is generally used in the power electronics
devices as a medium. A conceptual diagram of DC-bus microgrid is shown in
Fig. 2.1.
While the advantageous of DC microgrids are great, protection of DC dis-
tribution systems has posed many challenges such as autonomously locating a
fault within a microgrid, effectively breaking a DC arc, DC protection devices,
and certainly the lack of standards, guidelines and experience [47, 15].
2.2 HVDC vs. LVDC
When it comes to power transmission systems HVDC is the most prominent.
HVDC lines have been a part of the United States grid for over 40 years. HVDC
5


AC Grid

AC/DC
DC Bus
/ \
DC/AC
Flywheel
M/G
DC load
DC/DC
V
Figure 2.1: Conceptual diagram of a DC-bus microgrid system.
6


lines help transmit bulk power overlong distances and interconnecting the three
grids in the in the United States. In traditional power systems there are three
main components: generation, transmission and loads. Generally the generation
is a large power plant (coal, nuclear, etc.) and is located outside large cities,
while most of the loads he within these large cities. The transmission component
delivers energy from the generation sites to the loads. This is how the power
systems around the world currently work. Thus the reason for the dominance
of HVDC over LVDC.
Attitudes and views on how the power system should operate have begin
to shift and decentralization of the grid is beginning to gain attention. This
would mean that traditional loads would install local generation often called
independent power producers (IPP). The local generation can be used to supply
the local loads, thus decreasing the transmission losses. When the IPP produces
more electricity than the load consumes it can sell the power back to the utility,
and when production falls short the grid is there to make up the difference. It
is in this application that LVDC has started to gain attention. Although, the
electric grid is primarily AC many of the loads in our homes and businesses are
DC. This means that each of these devices requires a small AC-to-DC converter,
which increases losses. If local IPPs produce DC power that the loads are
able to use directly it would decrease these conversion losses. This is especially
advantageous for data centers where almost all of the equipment is DC. Recently,
the European Telecommunications Standards Institute (ETSI) began drafting a
standard on 380VDC wiring for building-wide power distribution [43].
7


2.3 LCC vs. VSC
Currently line commutated converters (LCC) or classical thyristor-based DC
systems hold the market in bulk power transmission. Recently voltage source
converter based DC systems are becoming more of a competitor of classical
thyristor-based DC systems [21]. Not only is VSC a competitor for transmis-
sion but it can also be used in multi-terminal systems, which have become an
attractive option for renewable energy applications or for distribution in large
cities. As the converter power rating increases it may one day replace thyristor-
based converters. VSCs are attractive because, unlike classical converters, no
reactive power support is needed to operate the system. In fact VSCs can pro-
duce reactive power, and control active and reactive power independently [3].
This controllability allows VSC converters to operate in systems with little or
no AC support, something that classical converts cannot achieve without ex-
pensive support [13, 17, 48, 22], VSCs are also advantageous in multi-terminal
systems. Multi-terminal systems consist of three or more converters to create
a DC network. Applications of multi-terminal systems include distribution into
large cities, microgrids, and even ship power systems [52, 20, 53, 6]. VSCs are
better suited for multi-terminal systems as the power flow can be changed by
changing the direction of the current. Classical DC converters require the DC
voltage polarity to be changed, which can be difficult [35, 44, 13].
VSC systems are, by design, vulnerable to faults on the DC systems. Clas-
sical HVDC systems are naturally able to withstand short circuit currents
8


Figure 2.2: VSC Operation (a) Normal, (b) Positive Line-to-Ground Fault.
due the DC inductors limiting the current during fault conditions [20, 56].
When a fault occurs on the DC side of a VSC system the IGBTs lose con-
trol and the freewheeling diodes act as a bridge rectifier and feed the fault
[52, 34, 23, 53, 57, 6, 56, 18], as shown in Fig. 2.2. The types of faults possible
on a HVDC system are as follows.
A challenge associated with the protection of VSC systems is that the fault
current must be detected and extinguished very quickly as the converters fault
withstand rating is generally only twice the converter full load rating [6]. Fault
detection is also important, especially on multi-terminal systems, in order to
isolate the fault and restore the system to working order.
9


3. Fault Protection of Low-Voltage DC-bus Microgrid Systems
3.1 Possible Faults
For DC system two types of faults exist, line-to-line and line-to-ground,as
can be seen in Fig. 3.1. A line-to-line fault occurs when a path between the
positive and negative line is created, shorting the two together. A line-to-ground
fault occurs when a path between either the positive or negative pole and ground
is created. A line-to-ground fault is the most common type of fault [10].
VSCs may experience internal switch faults that can cause line to line short
fault. This is a terminal fault for device that cant be cleared and in most cases it
requires replacement of the device. Hence, DC fuse would be proper protection
measure for
3.1.1 Line to Ground
A line-to-ground fault (ground fault) occurs when the positive or negative
line is shorted to ground. In overhead lines faults may occur when lightning
strikes the line. This may cause the line to break, fall to the ground and create
fault. In this situation the fault is always permanent and the line must be
isolated for repair. Ground faults may also occur by objects falling onto the
line, such as trees, providing a path to ground. In some cases when an object
causes the ground fault it may fall away from the line and the system can be
restored. If the fault persists the line would have to be taken out of service until
the fault path can be cleared.
Underground cable is almost completely immune to line-to-line faults, as
insulation, conduit and the earth separate the cables. However, they can still
10


occur. The insulation of the cable can fail due to improper installation, excessive
volt age/current, exposure to the environment (water, soil, etc) or cable aging
[56]. When this occurs, the broken insulation will allow a path for current to
flow to ground. As the fault persists the integrity of the insulation is reduced
causing the fault to worsen. A ground fault may also occur when a person
inadvertently cuts through one of the lines. This generally happens during
construction projects. In either case the fault will always be permanent and will
require a complete shutdown of the line as well as a costly repair.
When a line-to-ground fault occurs, the faulted pole rapidly discharges ca-
pacitor to ground. This causes an imbalance of the DC link voltage between the
positive and negative poles. As the voltage of the faulted line begins to fall, high
currents flow from the capacitor as well as the AC grid. These high currents
may damage the capacitors and the converter [57].
11


Figure 3.1: a) Line-to-Ground Fault b) Line-to-Line Fault
3.1.2 Line to Line
As stated before, a line-to-line fault on a cable-connected system is less
likely to occur on the cable. In an overhead system, line-to-line faults can
be caused by an object falling across the positive and negative line, they may
also occur in the event of the failure of a switching device causing the lines to
short. A switching fault, which is independent of how the converter stations are
connected together, causes the positive bus to short to the negative bus inside
the converter. A line-to-line fault may be either temporary or permanent.
3.1.3 Overcurrent
While overcurrent protection is important during line-to-line and line-to-
ground faults, it must also operate when the system is being overloaded. Over-
load conditions may occur in two-terminal systems when the load increases past
12


the rating of the converter or as a result of a fault on another part of the system.
For example, if three VSCs are feeding a common load and one VSC is dropped
due to a permanent fault, the remaining two must supply the load. This will
result in elevated currents that may overload the converters. In this situation
the overcurrent protection would need to operate. Another option to avoid a
wide spread blackout would be to shed non-critical loads.
3.2 Current Protection Methods
Although, a standard has not been agreed upon on how to best protect
DC systems, many methods have been proposed. Most of these methods are
proposed for high voltage direct current (HVDC). This is because more HVDC
systems have made it to consumer production, and therefore has been a popular
research topic. However, the concepts and ideas are the same for LVDC systems.
The below sections will overview these proposed methods.
3.2.1 DC Protection with AC Devices
Traditionally protection of DC systems has been done with conventional AC
devices such as circuit breakers and fuses. The advantages of using AC devices
include:
Less expensive than DC counterparts
Shorter lead time
Mature science
More familiar devices
The two options when choosing an AC device is either a circuit breaker or
a fuse.
13


3.2.1.1 AC Circuit Breakers
Placing AC circuit breakers on the AC side of the VSC is the most eco-
nomical way to protect the DC system. They are commonly available and can
be replaced in a shorter amount of time. However, AC circuit breakers result
in the longest interruption time as a result of their mechanical restrictions [52],
Currently, the best interrupting time for an AC circuit breaker is two cycles [2].
When using an AC circuit breaker, the voltage of the DC capacitors will be
monitored as well as the current in each DC line at each converter. These val-
ues will be fed back to a standard relay, which will monitor over/undervoltage,
as well as overcurrent. When a DC fault occurs, the capacitors will discharge
rapidly causing the voltage to decrease. The current on the faulted line will
increase over the rated value. Once the relay senses one or more of these con-
ditions, it will trip the breaker. In an attempt to restore the system, the relay
will enter a re-closing cycle in which the relay will close back in and sense the
voltage and current of the DC system. If the fault is cleared the system will
return to normal, but if a permanent fault is detected the relay will lock out the
breaker. The relay identifies a permanent fault by the re-closing sequence. A
typical industry standard for re-closing on AC systems is that two attempts will
be made; this can be applied to the VSC systems as well. After two attempts
without success, the relay determines that the fault is permanent and will not
allow the breaker to close.
In back-to-back or two-terminal transmission systems, differential protection
may be used to protect each converter, as shown in Fig. 3.2. The differential
relay (Note: 87 is the ANSI standard number for a differential relay) will measure
14


the current entering the converter as well as the current leaving the converter.
If the current entering does not match the current leaving the differential relay,
it will trip the AC breaker [4, 30]. In back-to-back systems one relay could
also monitor the AC current at the sending VSC as well as the receiving VSC.
If the current at one end does not match the current at the other end, the
relay would know a fault has occurred and the VSCs would be tripped offline.
In a two terminal transmission system, two relays would be required and the
current readings would have to be sent to the other relay via communication
as can be seen in Fig. 3.3. This type of differential protection is common in
AC systemsflO] as could be applied to VSC protection. Also, compensation for
losses would have to be taken into account.
While AC circuit breakers are inexpensive, easy to use, and widely used,
they also shut down the entire converter. This is problematic in the case of
ground faults where the faulted line could be isolated and the system could
run mono-polar using ground as a return path[57]. AC circuit breakers are also
inconvenient in multi-terminal systems, which will be discussed later.
3.2.1.2 Fuses
Fuses on the AC side are generally not a good solution for protection of
the VSC [38]. This is because a fuse is a thermal device that is only allowed
one operation. Fuses do not have the ability to distinguish whether a fault is
temporary or permanent. To a fuse every fault is permanent and therefore the
system would not be able to be restored until the fuse was physically replaced.
The only place that a fuse may be an acceptable alternative is for a non-critical
load, or in areas where space is limited, such as a ship. Fuses are used for
15


Figure 3.2: Back to Back Differential Protection.
Figure 3.3: Two-Terminal Differential Protection
16


protection [38], but are mainly for AC protection and other DC devices protect
the DC line. The DC devices coordinate with the fuse such that they will trip
before the fuse. The fuse will only operate in the event that the DC protection
fails. Overall, fuses should be used only when necessary and all other options
are not possible.
3.2.2 DC Protection with DC Devices
While AC devices are an economical way to protect the DC system, DC
devices are a better option whenever possible. DC protective devices can act
faster than their AC counterparts, as well as sectionalize lines. This allows the
operation of unfaulted lines to continue. The methods of protection using DC
devices are shown below.
3.2.2.1 IGBT Circuit Breakers
An IGBT circuit breaker (IGBT-CB) utilizes the blocking capability of the
solid-state device. Like the other IGBTs in the converters, the IGBT-CBs are
configured with an anti-parallel diode. The only drawback to the IGBT-CB is
that it is a uni-directional device. This is illustrated in Fig. 3.4. When a fault
occurs on the DC line, the IGBT is able to block the fault current (represented
by the dashed line in Fig. 3.4). If the fault occurs on the converter side, the
anti-parallel diodes conduct and allow current to flow (represented by the solid
line in Fig. 3.4). In this scenario the IGBT-CB must rely on the blocking of the
IGBTs in the converter [52],
For two-terminal systems, IGBT-CBs can be placed at each converter sta-
tion, one on the positive line and one on the negative line, as can be seen in Fig.
3.4. Fast acting DC switches are used in conjunction with the IGBT-CB, which
17


is used to isolate the line once the fault current has been cleared. It should be
noted that the switch cannot break current and may only be opened once the
fault has been extinguished. As with AC, the DC current of each line and the
DC voltage of each capacitor will be sensed. Once the control system senses a
fault on the line, an appropriate IGBT-CB will receive a gate signal to block
the current. Once the fault current has been extinguished the fast acting DC
switches will open, isolating the line. To determine if the fault is temporary
or permanent, the DC switches and the IGBT-CB will close. If the fault has
cleared the system will return to normal operation. If the fault is still present,
the line will be isolated again and a permanent fault will be determined.
The advantage of using an IGBT-CB is that the entire converter is not
shutdown in the case of a ground fault. This allows the faulted line to be
isolated and have the system continue to run mono-polar. The IGBT-CB also
opens faster than its AC counterpart. The disadvantage to the IGBT-CB is that
it cannot protect against DC rail faults in the rectifier [38].
18


4
Figure 3.4: IGBT-CB Fault Blocking Capabilities.
19


3.2.2.2 Converter Embedded Devices
Converter embedded devices are active, protective components that are in-
stalled inside a VSC to detect and isolate DC faults. This method eliminates
the use of additional devices, reducing the footprint of the converter station and
also possibly cutting cost. However, a redesign of the converter is required. In
[6], the converter uses two Emitter Turn Off (ETO) devices in an anti-parallel
configuration to achieve both switching and protection. The ETO has a higher
voltage and current rating than IGBT. Fig. 3.5 illustrates the converter config-
uration. In normal operation the ETOs X act as the switching devices, while
ETOs Y act as the anti-parallel diode, and are constantly fired on. In the event
of a fault the ETOs X are blocked while ETOs Y continue to feed the fault.
Once the fault has been identified as permanent the Y ETOs will be gated off.
Another protection method has replaced the typical IGBT and anti-parallel
diode is a combination switching device with the submodule shown in Fig. 3.6
[18, 22], The converter submodule provides two different levels of protection.
The first level protects the converter from being shutdown during a switching
device failure. In the event of a switching device failure, the submodule will
close switch Ki, shorting out the defective submodule. This allows the con-
verter to continue to operate by using redundant modules for un-altered system
performance. The second level of protection reacts under fault conditions. As
stated earlier, the switching device blocks and the anti-parallel diode conducts to
feed the fault under fault conditions. The freewheeling diodes in VSCs are not
20


able to withstand large surge currents, and may be damaged before the fault is
cleared. The solution in [18] is to bypass the IGBT and the anti-parallel diode
with a press-pack thyristor K2. The proposed press-pack thyristor is able to
withstand high surge currents, protecting the anti-parallel diode until the fault
can be cleared.
This protection method allows the converter additional control and increases
the current rating of the switching devices. It also cuts down on the number of
components required for protection because the protection is embedded in the
converter. The disadvantage is that the entire converter must be shut down in
the event of a permanent fault. This works well in two-terminal systems but
may cause problems in multi-terminal systems.
21


Figure 3.5: ETO Based VSC-HVDC Converter.
r
Protective
Switches
Â¥
! ( £ '
IK.\ aK2;

lSM
UflVf

Co
U,
c
Figure 3.6: VSC-HVDC Converter Submodule.
22


3.2.3 Multi-Terminal System Protection
VSC systems are very appealing in multi-terminal systems, as power flow
can be changed not by voltage polarity but the direction of the current. The
possible applications for multi-terminal VSC (MT-VSC) systems are used in
renewable energy applications and in distribution of power in mega cities. The
protection strategies for MT-VSC utilize both AC and DC protection.
3.2.3.1 AC Protection
As stated previously, DC protection can be achieved by using AC circuit
breakers on the AC systems. This strategy can be applied to MT-VSC as well.
A hand shaking method is proposed in [53]. This method, in addition to using
AC circuit breakers, implements fast acting DC switches. The switches are only
used to isolate lines and cannot break load or fault current. Each VSC will
receive current measurements from their respective DC switches. When a fault
occurs all of the AC circuit breakers associated with the MT-VSC system will
trip. Next, each VSC must determine which one of its respective switches to
open. This is done by measuring the magnitude and direction of the current
through each switch. The switch that will be selected is the one with the largest
positive fault current. The hand shaking method defines positive as out of the
node and negative into the node. Fig. 3.7 illustrates the example system given
in [53],
23


AC3
(a)
AC3
(b)
Figure 3.7: Hand-Shaking Method:(a) Current Flow During a Fault, (b) Fault
Isolation.
AC3
Figure 3.8: Hand Shaking Method Re-Closing.
24


When a fault occurs on Line 1, VSC1 receives current measurements from
SW11 and SW31. VSCf senses that the current through SW11 is positive and
the current through SW31 is negative. Through the hand shaking method VSC1
opens SW11. VSC2 receives current measurements from SW12 and SW22. Once
again the current through SW12 is positive and the current through SW22 is
negative and switch SW12 is selected. VSC3 receives current measurement
from SW33 and SW23. The current direction for both switches is measured as
positive. The switch with the highest magnitude of current is selected. At this
point Line 1, the faulted line, is isolated, and Line 3 is open at one end. At this
point the system must enter a re-closing mode. First, all of the AC breakers
will close back in, re-energizing the VSCs. Next, the fast DC switches of the
non-faulted DC lines must be closed. The VSCs only re-close switches when
the voltage of its respective line is near the voltage of the VSC terminals. Fig.
3.8 shows the re-closing method presented in [53], where it can be seen that
only SW33 will be able to re-close. During the fault VSC1 chose to open SW11,
leaving SW31 closed. Once the AC breakers re-close, and the VSC1 is back on
line, Line 3 will recharge. VSC3 will sense the Line 3 voltage and allow SW33 to
close. Both SW11 and SW12 will remain open as Line 1 was discharged during
the fault and both VSC1 and VSC2 will sense no voltage on Line 1, therefore
not allowing the switches to close.
25


3.2.3.2 DC Protection
DC Protection utilizes IGBT-CBs and fast acting DC switches. The IGBT-
CBs can be placed at the terminals of each VSC or at the end of each line, as
show in Fig. 3.9.
The voltage of the capacitors will be monitored as well as the current through
each line. When the current exceeds the maximum setpoint and the voltage
begins to rapidly discharge the respective IGBT will begin to block and the fast
acting DC switch will open once the fault is extinguished. This type of protection
is very advantageous in MT-VSC systems as you can isolate individual lines
without interrupting the entire network. This is especially true in Fig. 3.9 (b)
where each line has its own IGBT-CB. While this is a more effective method of
protection, it is the most expensive option with further challenges. Unlike Fig.
3.9 (a) case, IGBT-CB cannot begin to block when a fault is detected on the
positive line because two or more lines split from the positive or negative node.
Since all lines that are connected to a particular node will feed fault on any
other line connected to the same node, the faulted line must be detected. Three
different methods to achieve this are presented in [52]; they are large current
change, rise time, and oscillation pattern.
The large current change method determines which lines are faulted by
comparing the current magnitude of all lines feeding the fault. The line with
the largest current change in a given time will be chosen as the faulted line. The
rise time method measures the rise time of the first wave front of the current.
26


When a fault is detected, each VSC will measure the rise time of the current
in their respective lines. The line with the fastest rise time will be identified as
the faulted line. The oscillation pattern method looks for wide pulses without
a change in polarity. This identifies the faulted line.
While isolating the fault is important, limiting the amount of fault current
is as well. The DC link capacitors contribute high fault currents in a very short
amount of time. Typically, capacitor protection is done with snubber circuits.
However, the snubber only limits the discharge rate of the capacitor; it does
not interrupt the discharge current [6]. The idea of placing a circuit breaker
in series with the capacitor is introduced in [38]. The type of circuit breaker
chosen is a Capacitor DC Circuit Breaker (CDCCB). The advantage of using
a CDCCB is speed: it is a very fast acting device, operating in approximately
10 seconds. This fast operation protects the capacitor from extreme stress and
destruction. The voltage will hold because the capacitor does not discharge
under fault conditions. This creates a shorter charging time when the VSC is
put back on line.
DC protection devices not only protect against overcurrent, but they can
also protect against overvoltage. If a converter is lost on an MT-VSC system
the voltage on the system will drop, but once the converter is back on line the
voltage can overshoot. One method presented to mitigate this problem is the
implementation of a chopper circuit [34, 12, 8]. In Fig. 3.10 the addition of an
IGBT with a series resistor can be seen.
27


/
Figure 3.9: IGBT-CB Protection.
28


3.2.4 Controllers
The previous sections over viewed the devices that interrupt fault condi-
tions. This section will cover the active controllers that will attempt to change
the operation of the VSCs under fault conditions in order to keep the system
running. Traditionally, a single controller will operate during steady state and
fault conditions. This can be seen in [34], where the proposed controller supplies
the normal and protective gate trigger pulses. Another option is to implement
a parallel controller. The parallel controller is proposed to mitigate overcur-
rents and overvoltages [31, 32]. Both a current and a voltage controller can be
provided in parallel configuration. Within each of the respective controllers, a
steady state and a fault controller can be connected in parallel using PI con-
trollers to regulate the current and voltage. Each is running during normal
operation but depending on the condition of the system one will take control of
the firing pulses.
Overload problems may be solved by implementing some techniques found
in motor control [12]. A two-terminal VSC-HVDC system can be looked at as a
double-sided converter feeding a motor. When power levels begin to exceed the
contingency rating of the system the VSC-HVDC system can enter regenerative
braking mode, returning the power back to the AC grid. This is an alternative
to using a chopper circuit as the energy is not dissipated, rather it is redirected.
As mentioned, the loss of a converter can lead to overvoltages in the system
and can adversely affect MT-VSC systems. To combat this problem an advanced
29


Figure 3.10: Overvoltage Chopper Circuit.
DC voltage controller (ADCVC) is proposed in [36]. This ADCVC operates
in two stages; lower and higher hierarchical. The lower hierarchical control
operates during normal system operation. This lower hierarchical controller
is responsible for maintaining active power, reactive power and DC voltage.
The higher hierarchical controller monitors the system and only reacts during
transient disturbances, i.e., converter loss in MT-VSC. The higher hierarchical
controller recognizes transient disturbances by changes in the local voltage and
current, as the loss of a converter will redirect power flow in the DC network.
Upon recognition of a disturbance, the ADCVC will take control and alter the
performance of its respective converter or shut down the converter in order to
protect it from harm in some cases. The idea of lower and higher controllers is
similar to coordination of protective devices on AC systems.
30


3.2.5 Current Limiting Techniques
There are several techniques that attempt to limit the current under a
faulted condition. Some of the fault current Uniting techniques are:
Superconductor
Positive Temperature Coefficient
Saturaed Inductor
Power Electronics
The supercondcutor technique uses a superconductor as a part of the trans-
mission or distribution line. Under normal conditions the superconductor resis-
tance will be nearly zero. When a faulted condition is detected the supercon-
ductor will begin to increase its resistance in order to limit the current. This
technique require a special cooling system s well as separate protection for the
supercondcutor [37, 24],
The Positive Temperature Coefficient is similar to the superconductor
method but uses a variable resistor that is temperature controlled. Under normal
condition the current is low and therefore the resistance is low. Under faulted
conditions the increased current will cause an increase in temperature thus in-
creasing the resistance of the resistor. While this technique is less complex than
the supercondcutor it has low capacity for current and voltage. Therefore under
high fault conditions the resister can fail, opening the circuit[37].
31


The saturated inductor places an inductor in the line and uses a separate
exciting circuit to drive the inductor into saturation, keeping the resistance low.
Under faulted conditions the inductor will be pulled out of saturation, increasing
resistance and decreasing current. This method requires the use of a large iron
core, increasing size and cost. Also, the inductor on limits the rate of rise and
not the maximum fault level[37, 50]. Also,the increased inductance in the line
adversely affects the response to the fast load dynamics.
The use of power electronics suggests limiting fault current by switching of
the bus circuit breakers [37, 28]. However,it is not easy to switch large current
and required inductance could make the dynamic response of normal operation
sluggish. Also, if a fault is permanent then limiting the fault current in order to
ride through the fault is not necessary.
32


4. Proposed Protection Method and Algorithm
4.1 Operation Modes
The novel protection method and algorithm for the DC bus microgrid system
is proposed in this research. Unlike may other methods [53, 52], the proposed
scheme does not require a complete shutdown of the grid. Rather, only the
affected section of the microgrid is isolated and de-energized. This is achieved
through use of a ring bus configuration for the main DC bus, creating several
zones of protection within the ring bus, and installing a grounding resistor to
limit the fault current. This is done for both the positive and negative bus. The
proposed protection scheme can be split into three sections:
Fault Detection and Isolation
Breaker Failure Detection
Reclose and Restore
The ring bus will be split into zones and each zone is monitored by an
Intelligent Electrical Device (IED). The IED will continually monitor the current
through its assigned breakers. Once a fault is detected the IED will open the
zone breakers. The IED will then ensure that all of the breakers have opened and
that the faulted zone is de-energized. If the zone has not been de-energized the
zone IED will send signals to adjacent IED until the fault is extinguished. If the
zone was successfully de-energized the IED will attempt to restore the faulted
zone by reclosing the breakers. If a fault is then detected the zone breakers are
33


again tripped and the zone is isolated. A flow chart of the protection logic can
be seen in Fig. 4.2.
4.1.1 Fault Detection and Isolation
The microgrid under study consists 12 zones of protection. Each zone is
classified by the type of energy device it has been assigned to. The zones can
be split into 4 categories: uni-directional, bi-directional, load, and link which
is shown in Fig.4.1. Each zone consists of 2-3 breakers and a section of cable.
A local IED is assigned to each zone. The IED will monitor and control each
of the breakers within its assigned zone. Each IED is programmed with the
specific set of rules that define a normal zone operation. This is dependent on
the source that the IED is monitoring. It should be noted that due to the ring
bus configuration, current has several paths to flow. Therefore several normal
operating conditions must be accounted for, and they must all fail before a fault
is declared. Once fault has been detected all breakers in the affected zone are
tripped, regardless of the pole the fault is on. This is done to keep the microgrid
and converters in balance. In the examples specific rules that are needed to
define a fault are provided. One rule for each zone type is given.
Once the faulted segment is isolated, the remainder of the sources and loads
can continue to operate on the ring bus. Even with multiple faulted segments,
the system can operate partially if the segments from the main source to some
loads are intact.lt has been assumed that the segment controllers can detect it
and open/close Solid State Circuit Breakers in 500p.sec. For example, turn-off
time for an IGCT is approximately 11 nsec.
34


AC Grid
Figure 4.1: Microgrid Protection Zones
35


Figure 4.2: Algorithm Flow Chart
36


Uni-Directional Source Zones (Zone 5 Example)
Ib" = Ib8 + Ib9 (4.1)
Ibs = Ibt + Ib<9 (4.2)
Ib9 = Ib7 + Ibs (4.3)
Bi-Directional Source Zones (Zone 3 Example)
Discharge Mode
Ib3 = Iba + Ibt, (4.4)
Iba = Ib3 + Iba (4.5)
IB5 = Ib3 + Iba (4.6)
Charge Mode
Ib3 = Ib5 ~ Iba (4.7)
Iba = Ib3 + Iba (4.8)
Ibt = Ib3 ~ Iba (4.9)
37


Load Zones (Zone 7 Example)
Ibio = Ib\2 ~ -Ten (4-10)
Ibw = Ibio Ten (4.11)
Ibi2 = Ibio Ibii (4-12)
Link Zones (Zone 8 Example)
Ebi2 = 7bi3 (4-13)
Ibis = Ib\2 (4-14)
38


a) Normal Condition 1
b) Normal Condition 2
c) Normal Condition 3
d) Fault Condition
Figure 4.3: Uni-Directional Zone: Normal and Faulted Current Flow
a) Normal Condition 1 b) Normal Condition 2 b) Normal Condition 3 d) Fault Condition
Figure 4.4: Bi-Directional Zone: Normal and Faulted Current Flow
a) Normal Condition 1 b) Normal Condition 2 c) Faulted Condition
Figure 4.5: Link Zone: Normal and Faulted Current Flow
39


4.1.2 Breaker Failure Detection
Each zone IED continually monitors the status (open/closed) on their re-
spective zone breakers. Under normal conditions this can be used for information
purposes to operators of the microgrid. Once a fault inside a zone is detected
and trip signals are sent, the IED waits 1 second then enters breaker monitoring
mode. The breaker monitoring mode operates under two different conditions:
Status and Current
Current
4.1.2.1 Status and Current
In the Status/Current conditions the zone IED flags that a trip to all break-
ers has been sent. The IED then checks to see if all of the breakers are showing
a closed status. If a breaker status is closed and the IED expect it to be open
a breaker fail condition is suspected. The IED must also confirm that current
continues to flow is the faulted zone. If both the closed status and current in the
zone are detected the faulted zone IED will then send a signal to the appropriate
zone controller to trip its zone breakers. For example, if a fault is detected in
Zone 4, but Breaker 5 fails, then a signal from the Zone 4 IED would be sent to
the Zone 3 IED. The Zone 3 IED would trip its associated breakers (Fig.4.1). At
this point both Zones 3 and 4 have been de-energized and locked out. Locking
out the zone means that the controllers will not try to automatically reclose
and restore the zones. Restoring the zones after a lockout condition requires a
40


Figure 4.6: Zone Controller Connections
manual restore, after the fault had been removed from the system. This condi-
tion requires two pieces of information, but extinguishes the fault with the least
amount of impact to the DC bus.
4.1.2.2 Current
If all of the breakers in the faulted zone are providing an open status, but
the IED continues to read current in the zone then all of the adjacent zones will
be tripped. Using the Zone 4 example, but this time all of the breakers provide
an open status. The Zone 4 IED would send signals to both the Zone 3 5 IEDs,
de-energizing Zones 3,5 5 (Fig.4.1). This condition ensures that the fault will be
extinguished even if the IED receives a false status from the breakers. However,
it requires large sections of the grid to be de-energized.
41


4.1.3 Reclose and Restore
Once the faulted zone had been tripped and none of the breakers failed, the
IED will the reclose and restore. Often, faults are temporary due to debris or
animals coming in contact with the cable or line. The temporary faults will
clear themselves after current flows through the unwanted ground source. The
reclose and restore mode allows the IED to autonomously restore power back
to the de-energized zone. This is done by waiting 1 second after the trip signals
have been sent. After that 1 second the IED will send close signals to all of the
breakers. If the fault has been successfully cleared, the microgrid will continue
to run normally. However, if the fault is permanent and it is detected after the
first reclose, all of the zone breakers will again be tripped and the zone will be
locked out.
4.2 Solid State Circuit Breakers
Due to the limitations of fuses and traditional circuit breakers in DC sys-
tems, a solid state circuit breaker is utilized. When selecting a solid-state circuit
breaker there are several options: GTO, IGBT, and IGCT. GTOs offer a high
blocking voltage capability and a low on stare voltage, but suffer from slower
switching speeds [26, 1]. IGBTs are widely used in the low voltage (< 1200E)
systems [25]. IGBTs offer fast interruption time (10/isec) and an ability to with-
stand short circuits [1]. The disadvantage when using a IGBT is that they suffer
from high conduction losses [1, 49, 51, 26]. IGCTs offer the lower conduction
losses of a thyristor with the turn-off capability of a transistor. Like the IGBT,
42


HJ 'J4
(a)
(b)
Figure 4.7: a)Bi-directional IGBT circuit breaker. b)Bi-directional IGCT circuit
breaker.
the IGCT offers high voltage and current ratings but does not suffer from high
conduction losses. The IGCT has a slower switching speed than the IGBT, but
when used as a circuit breaker is may not be a concern [51, 49]. In order for
the microgrid to allow power flow in either direction the IGCT-CB needs to
be bi-directional. The bi-directional IGCT-CB actually consists of two IGCTs
placed in series with one opposing the other Fig. 4.7.
4.2.1 Grounding
The magnitude of a ground fault current is dependent on the distance from
the source that the fault occurs and the resistance of the ground fault path. The
fault current from the source an converter capacitors can be given as (4.15).
Where Es is the line voltage, Req and Leq is the equivalent system resistance
and inductance, and Rc and Ceq is the equivalent series resistance and equiva-
lent capacitance of the converter capacitors. It can be seen in (4.15) that the
fault current magnitude determined by Req. The value of Req can be altered
depending on the grounding method of the system. The grounding options are:
1) solid grounding; 2) low-resistance grounding; 3) high-resistance grounding;
(4.15)
43


and 4) ungrounded [16]. Although ungrounded systems are used in some ap-
plications to avoid the effect of low-resistance pole-to-ground fault and stray
current, ungrounded systems are sensitive to changes in the grounding plane
and can be dangerous especially under abnormal fault conditions [27, 45]. The
advantages of the grounding in a DC distribution system include predictable
operating conditions, minimum voltage stress for the system components, and
easier fault detection [27]. The line-to-ground faults are the most common types
of faults in industrial distribution systems [16] and the ground fault current can
be limited by using the resistance grounding. Since the typical power electronics
converters connected in the LVDC systems cannot feed large fault currents, it
would be beneficial to reduce the fault current to an appropriate level for de-
tection and extinction. However, some protective devices are still needed even
with this low resistance grounding scheme, because the fault current cannot be
sustained [16].
It is a common practice to ground power systems at one point only and as
close to the source as possible [27]. Multiple ground points could form unnec-
essary circulating current paths. Possible grounding point for a DC system is
either one of the poles or the mid point of the bus, and it has been reported
that the balanced DC side grounding significantly reduces circulating current
compared to the AC side neutral-grounded system [27]. Although the ground
resistors can be used to detect the ground fault as well, it is not able to identify
the location of the fault because of the single ground point practices.
In this paper, a resistance grounding to the balanced DC bus mid point
has been chosen for a well-defined pole-to-ground voltages and robustness to
44


imbalance [47, 27]. The resistor was sized so that the fault current would not
drastically exceed the load current of the system. Sizing a grounding resistor
for a DC system only requires the use of Ohms law, as seen in equation 4.16
R,r = (4.16)
Where Rgr is the grounding resistor size, Es is the system voltage and If is
the desired fault current level.
45


5. Simulation Results
To verify the proposed protection scheme, computer simulations have been
performed using OrCAD/PSpice. The microgrid model can be seen in 5.1 Sim-
ulation parameters can be found in Table 5.1 The algorithm was programmed
and simulated using MATLAB and can be found in the Appendix. The Or-
CAD/PSpice figures illustrate the fault current in the affected zones; while the
MATLAB figures show the IED control signals. A negative pole line-to-ground
fault in zone 1 is simulated at 9.5msec. Fig. 5.2 shows the line-to-ground fault
current when no grounding resistance has been installed. It can be seen that the
current magnitude in the zone drastically increases from 50A of load current to
18kA.
Next, the grounding resistor was installed at the neutral point of the AC
grid converter. Fig. 5.3 illustrates the affect the grounding resisance has on the
fault current magnitude. It can be seen that the grounding resistor limited the
fault current from 18kA to just over 200A. In Fig.5.7 it can be seen that at the
event of the fault at 9.5msec the Zone 1 IED detects the fault and send trip
signals to the Zone 1 breakers. The status of the breakers can be seen in Fig.
5.6. The voltage at the AC and DC loads can be seen in Fig. 5.4.
46


Figure 5.1: OrCAD/PSpice Simulation Model
47


Table 5.1: Simulation Parameters.
Name Value
VDC 350V
Rload 1.7fl
Rline 1.6 mQ
Lline 640nH
Rlink 0.63 mVL
Llink 256nH
Rgnd 1.75ft
48


20KA
Bn aker 1 Curre ' ! Groun i Current:

Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Figure 5.2: Analytical Simulation Circuit for Line-to-Ground Fault in Zone 1
Without Resistance Grounding.
f-
Bre i kef 20 Ctirra nt
y-
r
I ireaker 2 Cui rent
Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
Figure 5.3: Analytical Simulation Circuit for Line-to-Ground Fault with Resistance
Grounding.
49


Time
Time
Figure 5.4: Voltage at AC and DC Loads during a Line-to-Ground Fault with
Resistance Grounding
50


Time

'DC Load Current ;
Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
Figure 5.5: Current at AC and DC Loads During a Line-to-Ground Fault with
Resistance Grounding
51


c
CD
a.
O
II
T3
CD
CO
_o
O
II
2
0
-2
0 2 4 6 8 10 12 14 16 18 20
Breaker 1 Status
t-------1------1------1-------1------1------1-------1------r
CD
Cl.
O
Q
"O
CD
en
Q
O
II
2
0
-2
0 2 4 6 8 10 12 14 16 18 20
Time(ms)
Breaker 2 Status
t------1------1------1------1------1-----1------1------r
i i i i
t-----------------t----------------------1------------------r
CD
Q_
o
II
§ 0
CD
to
Time(ms)
Breaker 20 Status
o
ii 0
10
12
14
16
18
20
Time(ms)
Figure 5.6: Zone 1 Circuit Breaker Status During a Line-to-Ground Fault
52


2
o
II
0
sz
o
II
-2
o
II
cz
O
II
-2
o
ii
R
c
O
II
J_______L
J_______L
I-------------1-
Breaker 1 Trip Signal
i------1-----1
i------------------1------------------1-----------------r
_L
_L
_L
8 10 12
Time(ms)
Breaker 2 Trip Signal
14
_L
_L
_L
8 10 12
Time(ms)
Breaker 20 Trip Signal
14
_L
_L
_L
16
16
_L
18
18
20
20
8 10 12 14 16 18 20
Time(ms)
Figure 5.7: Zone 1 IED Trip Signals During a line-to-Ground Fault
53


300A
[ Ground C urrent
it I
Breaker 5 C jrrent

A Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
Time
Figure 5.8: Zone 4 Current with Breaker 5 Failing to Open
A breaker failure condition was simulated in Zone 4. A negative pole line-
to-ground fault was placed in Zone 4, but breaker 5 fails to open. At this point
a breaker fail condition is recognized by the Zone 4 IED, sending a signal to
the Zone 3 IED to trip breakers 3 and 4. Fig.5.8 shows the fault current in the
zone until the breaker fail sequence is complete.In 5.9 the current in Zone 3 is
provided. Fig.5.10 and Fig.5.11 provides the status of the Zone 3 and 5 breakers,
respectively.
54


200A
100A
OA
-100A


[ Bre aker 3 Curre it
:[: aker 4 Gurrei It ;



Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
Figure 5.9: Zone 3 with Breaker 5 Failing to Open
CD
El.
O
II
T3
CD
en
_o
O
II
Q.
O
II
"O
CD
cn
Q
O
II
o
II
e
"O
CD
cn
Q
o
II
-2
2
0
-2
Breaker 5 Status
_L
_L
_L
8 10 12 14
Time(ms)
Breaker 6 Status
_L
_L
_L
8 10 12
Time (ms)
Breaker 7 Status
14
_L
_L
_L
8 10 12
Time(ms)
14
16
16
16
18
18
18
20
20
20
Figure 5.10: Zone 4 Breaker Status with Breaker 5 Failing to Open
55


2 r
0 -
-2 -
0
2 r
0 -
-2 -
0
2 r
0
2
0
2 r
0
-2
Breaker 3 Status
t-----------1---------1-----------1---------1----------1-----------1---------r
_L
2
J_________L
4 6
________I__________I__________I__________I__________I__________I
10 12 14 16 18 20
Time(ms)
Breaker 4 Status
J__________I_________I_________I__________I_________I_________I__________I__________I_________I
2 4 6 8 10 12 14 16 18 20
Time(ms)
Breaker 3 Trip Signal
1----------1---------1---------1----------1---------1---------1----------1----------1---------
J________I_________L
2 4 6
J__________I__________I__________I__________I___________I__________I
8 10 12 14 16 18 20
Time(ms)
Breaker 4 Trip Signal
i---------1----------1----------1----------1-----------1----------
J_____________I____________I____________I____________I____________I_____________I____________I____________I____________I
2 4 6 8 10 12 14 16 18 20
Time(rriis)
5.11: Zone 3 Breaker Trip Signals After Breaker 5 Fails to Open
56


700V
600V
400V
200V
0V
rrt -


AC Loi id Voitaqe










Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
700V]
600V
400V
DC Loa( i Voltage
200V
0V-
Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
Figure 5.12: DC Voltage at Loads During a Zone 4 Breaker Fail
57


Time
Time
Figure 5.13: Current at Loads During a Zone 4 Breaker Fail
58


200A





1E reaker 1 Cur ent

-k
m[) ; Ground Curt
snl *
Os 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms
Time
Time
Figure 5.14: Zone 1 Current During a Reclose and Lockout Cycle
Finally, the reclose sequence can be seen in Figs. 5.14, 5.15, 5.16 and 5.17.
Again, a fault is simulated in Zone 1 at 9.5m,sec, and the Zone 1 IED does not
detect a breaker fail condition. The IED then enters the reclose and restore
mode, and closes breaker 1, 2 and 20 at 12msec. In this case it was simulated
that the fault did not clear. Therefore, when the breakers are re-closed if the
fault is detected, the IED trips and locks out the zone breakers.
59


o
II
e
£=
o
II
o
II
£T
o
II
o
II
c
o
II
~i-------------r
J_______L
Breaker 1 Trip Signal
1 1 1 1 1 1 1 1 1 1 1 1 ri i
1 1 i i i i i
8 10 12 14
Time(ms)
Breaker 2 Trip Signal
8 10 12 14
Time(ms)
Breaker 20 Trip Signal
_L
_L
_L
8 10 12
Time (ms)
14
16
16
16
18
18
18
20
1 1 1 1 1 1 ri 1
i i
20
20
Figure 5.15: Zone 1 Reclose Cycle Trip Signals
60


2
o
II
0
sz
o
II
-2
o
II
Q
cz
O
II
-2
o
ii
£=
O
II
J_______L
J_______L
I-------------1-
Breaker 1 Close Signal
_L
_L
_L
8 10 12
Time(ms)
Breaker 2 Close Signal
14
_L
_L
_L
8 10 12
Time (ms)
Breaker 20 Close Signal
14
_L
_L
_L
8 10 12
Time (ms)
14
16
16
16
18
18
18
20
20
20
Figure 5.16: Zone 1 Reclose Cycle Close Signals
61


2
0
2
0
2
0
2
0
2
0
2
Breaker 1 Status
t-------1------1------1-------1------1------1-------1------r
2
4
t---------------------r
i <
I i
T-----------------T
2 4
t--------------r
2 4
-L
6
8 10 12 14 16 18
Time(ms)
Breaker 2 Status
h-------1-----1-------1------1-----r
i i i i i i
6 8 10 12 14 16 18
Time(ms)
Breaker 20 Status
1------1-------1------1----1------1------r~
i i i
6 8 10 12 14 16 18
Time(ms)
20
20
20
5.17: Zone 1 Breaker Status During a Reclose and Lockout Cycle
62


6. Conclusion and Future Works
With the new interest in green energy, the smart grid and distributed gen-
eration microgrids may soon become an integral part of our electric grid. DC
microgrids have proven to be a viable competitor to AC migrogrids. Protection
of the DC bus is a integral part to the DC microgrid, and must be able to isolate
faults with minimal impact to the overall system. It can be seen in Chapter 3
that the current techniques require a complete shutdown of the DC bus. This
is not suitable for critical loads.
This research proposes a new fault detection and isolation scheme for low-
voltage DC-bus microgrid system. A ring bus based microgrid system was uti-
lized. The proposed protection scheme consists of zone IEDs which are capable
of detecting abnormal fault current in the ring bus segment and isolating the
segment to avoid the entire system shutdown. The ring bus was separated into
overlapping zones with IEDs monitoring each zone has been proposed. The ring
bus allows multiple paths for power to flow when a section has been isolated.
Overlapping the zones reduced the amount of circuit breakers needed in the mi-
crogrid. The use of resistance grounding was utilized in order to limit the fault
current, to protect the source converters and also allow the IED enough time to
detect and isolate the fault. Successful fault detection and isolation was shown
using computer simulations.
63


Though the fault detection and isolation proves successful for suppressing
fault current, locating the faulted zone and isolating the zone for line-to-ground
faults, line-to-line faults will still create very large fault current. This is because
the fault consists of two sources (positive and negative) and the grouding resistor
has no influence on the fault current. Creating an algorithm or control scheme
to detect and limit a line-to-line fault is an issue that should be addressed.
Also, when a fault occurs and a source is removed from the microgrid, the
remainder of the sources must accommodate the load. Determing a real time
load flow control scheme for the microgrid would improve stability in the grid
and maximize efficiency from all of the sources.
64


APPENDIX A. MATLAB Code
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%Protection Logic%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clear all
close all
format short
clc
syms fault;
Imax=40Q;
SBl=ones(1,20) ;
SB2=ones(1,20) ;
SB3=ones(1, 20) ;
SB4=ones(1,20);
SB5=ones(1,20) ;
SB6=ones(1, 20) ;
SB7=ones(1, 20) ;
SB8=ones(1,20);
SB9=ones(1,20) ;
SBlO=ones(1,20);
SBll=ones(1,2 0) ;
SB12=ones(1,20) ;
SBl3=ones(1,20) ;
SB14=ones(1,20) ;
SB15=ones(1,20) ;
SB16=ones(1,20) ;
SB17=ones(1,20) ;
SB18=ones(1,20);
SB19=ones(1,20) ;
SB2G=ones(1,20) ;
SGC=ones(1,20);
SBC=ones(1,20);
SFC=ones(1,20);
SMC=ones(1,20);
SDC=ones(1,20);
SPC=ones(1,20) ;
SAC=ones(1,20);
STC=ones(1,20) ;
TBl=zeros(1,20) ;
TB2=2eros(1,20) ;
TB3=zeros(1,20) ;
TB4=zeros(1,20) ;
TB5=zeros(1,20) ;
TB6=zeros(1, 20) ;
TB7=zeros(1, 20) ;
TB8=zeros(1,20) ;
TB9=zeros(1, 20) ;
l.pdf
65


TB10=zeros (1,20) ;
TB11=zeros (1,20) ;
TB12 = zeros (1,20) ;
TB13=zeros (1,20) ;
TB14 = zeros (1,20) ;
TB15=zeros(1,20);
TB16=zeros(1,20);
TB17 = zeros (1,20) ;
TB18 = zeros (1,20) ;
TB19=zeros (1,20) ;
TB2 0=zeros (1,20) ;
TGC=zeros(1,20);
TBC=zeros (1,20) ;
TFC=zeros (1,20) ;
TMC=zeros(1,20);
TDC=zeros(1,20);
TPC=zeros (1,20) ;
TAC=zeros(1,20);
TTC=zeros (1,20) ;
CBl=zeros (1,20) ;
CB2 = zeros (1,20) ;
CB3=zeros (1,20) ;
CB4 = zeros (1,20) ;
CB5=zeros (1,20) ;
CB6=zeros (1,20) ;
CB7 = zeros (1,20) ;
CB8 = zeros (1,20) ;
CB9=zeros (1,20) ;
CB10=zeros (1,20) ;
CB11=zeros (1,20) ;
CB12 = zeros (1,20) ;
CB13=zeros(l,20);
CB14 = zeros (1,20) ;
CB15=zeros(1,20);
CB16=zeros(1,20);
CB17 = zeros (1,20) ;
CB18 = zeros (1,20) ;
CB19=zeros (1,20) ;
CB20=zeros(1,20);
Zone_l=zeros(1,20);
Zone_2=zeros(1,20);
Zone_3=zeros(1,20);
Zone_4=zeros(1,20);
Zone_5=zeros(1,20);
Zone_6=zeros(1,20);
Zone_7=zeros(1,20);
Zone_8=zeros(1,20);
Zone_9=zeros(1,20);
Zone_10=zeros(1,20);
Zone_ll=zeros(1,20);
2.pdf
66


Zone_12 = zeros (1,20) ;
Zone_13=zeros (1,20) ;
Zone_14=zeros(1,20);
Zone_15=zeros(1,20);
Zone_lG=zeros(1,20);
Zone_17=zeros(1,20);
Zone_18=zeros(1,20);
Zone_19=zeros(1,20);
Zone_20=zeros(1,20);
BlF=zeros(1,20);
B2F=zeros(1,20);
B3F=zeros (1,20) ;
B4F=zeros (1,20) ;
B5F=zeros(1,20);
B6F=zeros(1,20);
B7F=zeros(1,20);
B8F=zeros(1,20);
B9F=zeros(1,20);
BlOF=zeros(1,20);
BllF=zeros(1,20);
Bl2F=zeros(1,20);
Bl3F=zeros(1,20);
Bl4F=zeros(1,20);
Bl5F=zeros(1,20);
Bl6F=zeros(1,20);
Bl7F=zeros(1,20);
Bl8F=zeros(1,20);
Bl9F=zeros(1,20);
B20F=zeros(1,20);
Zone_l__RC=zeros (1,20) ;
Zone_2__RC=zeros (1,20) ;
Zone_3__RC=zeros (1,20) ;
Zone_4__RC=zeros (1,20) ;
Zone_5__RC=zeros (1,20) ;
Zone_6__RC=zeros (1,20) ;
Zone_7__RC=zeros (1,20) ;
Zone_8__RC=zeros (1,20) ;
Zone_9__RC=zeros (1,20) ;
Zone_10_RC=zeros (1,20) ;
Zone_ll_RC=zeros(1,20);
Zone_12_RC=zeros (1,20) ;
Zone_13_RC=zeros(1,20);
Zone_14_RC=zeros(1,20);
Zone_15_RC=zeros(1, 20) ;
Zone_16_RC=zeros(1,2 0) ;
Zone_17_RC=zeros(1,20);
Zone_18_RC=zeros(1, 20) ;
Zone_19_RC=zeros(1,20);
3.pdf
67


Zone_20_RC=zeros(1,20);
BF_SYM=0;
BFl_SYM=zeros(1,20);
BF2_SYM=zeros (1,20) ;
BF3_SYM= zeros (1,20) ;
BF4_SYM=zeros(1,20);
BF5_SYM=zeros(1,20);
BF6_SYM=zeros(1,20);
BF7_SYM=zeros(1,20);
BF8_SYM=zeros(1,20);
BF9_SYM=zeros(1,20);
BFlO_SYM=zeros(1,20);
BFll_SYM=zeros(1,20);
BF12_SYM=zeros(1,20);
BF13_SYM=zeros(1,20);
BFl4_SYM=zeros(1,20);
BFl5_SYM=zeros(1,20);
BF16_SYM=zeros(1,20);
BF17_SYM=zeros(1,20);
BF18_SYM=zeros(1,20);
BF19_SYM=zeros(1,20);
BF2Q_SYM=zeros(1,20);
cnt=ones(1,20);
for n=l:20
if n<=10
IBl(n)=100;
IB2(n)=200;
IB3(n)=200;
IB4(n)=50;
IB-5 (n)=150;
IBS(n)=50;
IB7(n)=100;
IBS(n)=50;
IB9(n)=150;
IB10(n)=150;
IB11(n)=50;
IB12(n)=100;
IB13(n)=100;
IB14(n)=50;
IB15(n)=150;
IBl6(n)=100;
IB17(n)=50;
IB18(n)=50;
IB19(n)=100;
IB20(n)=100;
4.pdf
68


end
if n>10 && BF_SYM==0;
IBl(n)=100;
IB2(n)=15G;
IB3(n)=150;
IB4(n)=50;
IB5(n)=10G;
IBS (n)=50;
IB7(n)=50;
IBB(n)=50;
IB9(n)=100;
IBlO(n)=100;
IB11(n)=50;
IB12(n)=50;
IBl3(n)=50;
IB14(n)=50;
IB15(n)=100;
IBIS(n)=100;
IBl7(n)=0;
IBIS(n)=lGO;
IB19(n)=10Q;
IB20(n)=100;
end
if n>lG && BF_SYM==1;
IBl(n)=100;
IB2(n)=2 00;
IBS(n)=200;
IB4(n)=50;
IB5(n)=250;
IBS(n)=5Q;
IB7(n)=10;
IBS(n)=50;
IBS(n)=SO;
IBlO(n)=60;
IB11(n)=50;
IB12(n)=10;
IBIS(n)=10;
IB14(n)=40;
IBl5(n)=50;
IBIS(n)=0;
IB17(n)=50;
IBIS(n)=50;
IB19(n)=100;
IBlO(n)=100;
end
5.pdf
69


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Source Zones%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Zone 5, Micro Turbine Source
if (IB7£n)+IB8in))~=IB9in)
Z5 CHK1=*1;
else
Z5_CHK1=0;
Zone_5(n)=Q;
Z5(n)=0;
end
if Z5_CHK1==1 && (IBS(n)+IB9(n))~=IB7(n)
Z5 CHK2=1;
else
Z5_CHK2=0;
Zone_5(n)=0;
ZB(n)=0;
end
if Z5_CHK2==1 && (IB7 (n) +IB9 (n) ) ~=IB8 (n) || IBS (n) >Irftax
Zone_5(n)=l;
Z5(n)=1;
end
%Zone 9, PV Source
if (IB13(n)+IBl4(n))~=IB15(n)
Z 9 CHK11;
else
Z9_CHK1=0;
Zone_9(n)=0;
Z 9(n)=0;
end
if Z9_CHKl=l && (IB14 (n) +IB15 (n) ) ~.=IB13 (n)
Z9 CHK2=1;
6.pdf
70


else
Z9_CHK2=G;
Zone_9(n)=0;
Z9(n)=G;
end
if Z9_CHK2==1 && (IB13(n)+IB15(n))~=IB14(n) 11 IB14(n)>Imax
Zone_9(n)=1;
Z9(n)=1;
end
%Zone 11, WTG Source
if (IB1? (n)+IB18 (n) ) ~=* IB19(n)
Z11_CHK1=1;
else
Z11_CHK1=0;
Zone_ll(n)=0;
Zll(n)=0;
end
if Z11_CHK1=1 && (IB18 (n) +IB19 (n) ) ~=IB17 (n)
Zll CHK2=1;
else
Z11_CHK2=0;
Zone_ll(n)=0;
Zll(n)=0;
end
if Z11_CHK2==1 && (IB17(n)+IB19(n))~=IB18(n) II IB18(n)>Imax
Zone_ll(n)=l;
Zll(n)=1;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Load Zones%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
7.pdf
71


%Zone 7, DC Load
if abs((IB1G(n)-IB11(n)))IB12(n) || IB11(n)>Imax
Zone_7(n)=1;
Z7 (n) =1;
else
Zone_7(n)=0;
LI(n)=0;
end
%Zone 10, AC Load
if abs((IB15(n)-IB16(n)))~=IB17(n) || IB16(n)>Imax
Zone_10(n)=l;
Z10(n)=l;
else
Zone_10(n)=0;
Z10(n]=G;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%Bi-Directional Zones%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%
%Zone 1, AC Grid Source
Gen=l; %Gen: 0=Charging, l=Generating
if Gen==l && ((IB1(n)+IB20(n))~=IB2(n))
Z1 CHK1=1;
else
Z1_CHK1=G;
Zone_l(n)=0;
Z1(n)=Q;
end
if Gen=~l && (Z1_CHK1=1 && (IB1 (n) +IB2 (n) ) ~=IB20 (n) )
Z1 CHK2=1;
else
Z1_CHK2=0;
Zone_l(n)=0;
Zl(n)=Q;
end
if Gen==l &£ (Z1_CHK2=1 && (IB2 (n) +IB20 (n) ) ~=IB1 (n) ) || IB1 (n) >Imax
8.pdf
72


Zone_l(n)=1;
disp('Zone 1);
Z1(n)=1;
end
if Gen=*=0 && ( (IB20 (n) -IB1 (n) ) ~=IB2 (n)')
Z1_CHK1=1;
elseif Gen==0
Z1_CHK1=0;
Zone_l(n)=0;
Zl(n)=Q;
end
if Gen==0 && (Zl_CHKl=l) && ( (IB2 (n) +IB1 (n) ) ~=IB20 (n) )
Z1_CHK2=1;
elseif Gen==0
Z1_CHK2=0;
Zone_l(n)=0;
Z1(n)=0;
end
if Gen==0 &£ (Z1_CHK2=1 && (IB2 (n) + IB20 (n) ) ~=IB1 (n) ) |
Zone_l(n)=l;
disp(1 Zone 1Y') ;
Z1(n)=1;
end
%Zone 3, Battery Source/Load
Batt_Stat=0; %Battery Status: 0=Charging, l=Generating
if Batt_Stat==l && (IB3(n)+IB4(n))~=IB5(n)
Z3 CHK1=1;
else
Z3_CHK1=0;
Zone_3(n)=0;
Z3 (n)=0;
9.pdf
IB1 (n)>Imax
73


end
if Batt_Stat=l && Z3_CHK1=1 && (IB4 (n) +IB5 (n) ) ~=IB3 (n)
Z3 CHK2=1?
else
Z3_CHK2=0;
Zone_3(n)=0;
Z3 (n) =0;
end
if Batt_Stat==l && Z3_CHK2=1 && (IB3(n)+IB5(n))~=IB4(n)
Zone_3(n)=1;
Z3 (n)=1;
end
if Batt_Stat==Q && (IB3 (n) -IB4 (n) ) ~=IB.5 (n)
Z3_CHK1=1;
elseif Batt_Stat==G
Z3_CHK1=G;
Zone_3(n)=0;
Z3(n)=0;
end
if Batt_Stat==0 && Z3_CHKl=l && (IB5 (n) -IB4 (ft) ) -=IB3 (n)
Z3_CHK2=1?
elseif Batt_Stat==0
Z3_CHK2=G;
Zone_3(n)=0;
Z3 (n) =0;
end
if Batt_Stat==0 && Z3_CHK2=1 && (IB3 (n) +IB5 (n) ) ~=IB4 (n)
Zone_3(n)=1;
Z3 (n)=1;
end
10.pdf
IB4(n)>Imax
IB4(n)>Imax
74


%Zone 4, Flywheel Source/Load
Flywheel_Stat=l; %Flywheel Status: 0=Charging, l=Generating
if Flywheel_5tat==l && (1B5(n)+IB6(n))-=IB7(n)
Z4 CHK1=1;
else
Z4_CHK1=0}
Zone_4(n)=0;
Z4 (n)=0;
end
if Flywbeel_5tat==l && Z4_CHKl==l && (IBS(n)+ IB7(n))*=IB5(n)
Z4_CHK2=1;
else
Z4_CHK2=Q;
Zone_4(n)=0;
Z4(n)=0;
end
if Flywheel_5tat==1 && Z4_CHK2=1 && (IBS (n) +IB7 (n) ) ~=IB6 (n)
Zone_4(n)=1;
Z4 (n)=1;
end
if Flywheei_Stat=0 && (IB5(n)-IBS(n))~=IB7(n)
Z4_CHK1=1;
el seif Flywheel_Stat~0
Z4_CHK1=Q;
Zone_4(n)=0;
Z4 (n)=0;
end
if Flywheel_3tat==G && Z4_CHK2==1 && (IB7(n)-IBS(n))~=IB5(n)
Z4_CHK2=1;
11.pdf
IB6(n)>Imax
75


elseif Flywheel_Stat==0
Z4_CHK2=G;
Zone_4(n)=0;
24 (n)=0;
end
if Flywheel_Stat==Q && Z4_CHK2=1 && (IB5 (n) + IB7 (n) ) ~=IB6 (n) | | IB6(n)>Imax
2one_4(n)=1;
24 (n) =1;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Link Zones%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if IB2(n)~=IB3(n)
2one_2(n)=1;
22 (n)=1;
else
2one_2(n)=0;
22 (n)=0;
end
if IB9(n)~=IB10(n)
Zone_6(n)=1;
26 (n) =1;
else
Zone_6(n)=0;
26(n)=G;
end
if IB12(n)-=IB13(n)
Zone_8(n)=1;
28 (n)=1;
else
Zone_8(n)=0;
28 (n)=0;
12.pdf
76


end
if IB19(n)~=IB20(n)
2one_12: (n) =1;
212(n)=1;
else
2one_12: (n) =0;
212(n)=1;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone
if Zone_l(n)==l && ZlT(n-l)==G
SB1(n)=0;
S02(n)=0;
SB2G(n)=0;
TB1(n)=1;
TB2(n) =1;
TB2Q (n)e=l;
disp('gg')
end
if 21 (n)==1
Z1T(n)=1;

else
Z1T(n)=Q;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 2%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_2(n)==l && Z2T(n-l)==0
SB2(n)=0;
SB3(n) =0;
TB2(n)=1;
TB3(n J =1;
end
if 22 (n) ==1
13.pdf
77


Z2T(n)=1;
else
Z2T(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 3%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_3(n)==1 && Z3T(n-l)==G
SB3(n)=0;
SB4(n)=0;
SB5(n)=0;
TB3(n)=1;
TB4(n)=1;
TB5(n)=1;
end
if Z3(n)==1
Z3T(n]=1;
else
Z3T(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 4%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_4(n)==1 && Z4T(n-l)==G
SB5(n) =G;
SB-6 (n) =0;
SB7(n)=0;
TB5(n)=1;
TBS(n)=1;
TB7(n)=1;
end
if Z4(n)==1
Z4T(n)-l;
else
Z 4 T(n)=0;
end
14.pdf
78


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 5%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_5(n)==l && Z5T(n-l)==0
SB7(n)=0;
SB8(n)=0;
SB9(n) =G;
TB7(n)=1;
TB8(nJ=l;
TB9(n)=1;
end
if Z5 (n)==1
Z 5-T (n) =1;
else
Z5T(n)=D;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone
if Zone_6(n)==l && Z6T(n-l)==Q
SBS(n)=0;
SB1Q(n)=0;
TB9(n)=1;
TB10 (n)e=l;
end
if ZS(n)==1

ZST(n)=1;
else
Z 6T(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 7%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_7(n)==l && Z7T(n-l)==Q
SB10(n)=0?
SB11(n)=0;
SB'12 (n) =0;
TB10(n)=l?
TBll(n)-l;
TB12(n)=l;
15.pdf
79


end
if 27(n)==1
Z7T(n)=1;
else
Z7T(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 8%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_8fn)==l && Z8T(n-l}==Q
SB'12 (n) =0;
SB13(n)=0;
TB12(n)-l;
TB13(n)=l;
end
if Z8(n)==1
Z8 T(n)=1;
else
Z8T(n)=0;
end
S % "5 % -6 % S *5 % S % S % -6 % % *5 % S % S % S S % *5 %
if Zone_9(n)==1 ss Z9T(n-1)
SB13(n)=0;
SB14(n)=0;
SB15(n)=0;
TB13(n)-l;
TB14 (n)<=l;
TB15(n)=1;
S
end
if Z9(n)==1
Z 9 T(n)=1;
else
Z9T(n)=D;
16.pdf
80


end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 10%%%%%%
if Zone_10(n)==1 && Z10T(n-l)==0
SB15(n)=0;
SB16(n)=0;
SB17 (n) *=0;
TB15(n)=1;
TB16(n)=l;
TB17(n)=l;
end
if Z10(n)==1
Z10T(n)=1;
else
Z1QT(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 11%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_ll(n)==1 && ZllT(n-l)==0
SB17(n)=0;
SB18 (n) *=0;
SB19(n)=0;
TB17(n)=l;
THIS(n)=l;
TB19(n)=1;
end
if Zll(n)==1
ZllT(n)=1;
else
ZllT(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 12%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_12(n)==1 &£ 212T(n-l)==Q
SB19(n)0;
SB20(n)=0;
17.pdf
81


IB'19 (n)=l;
TB2Q(n)=l;
end
if 212(n)==1
Z12T(n)=1;
else
Z12T(n)=0;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 1 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_l(n)==l
if BFl_SYM(n)==l
SB1(n)=1;
else
SB1(n)=0;
end
if BF2_SYM (n) ==1
SB2(n)=1;
else
SB2(n)=0;
end
if BF2G_SYM(n)==1
SB20(n)=1;
else
SB2Q(n)=0;
end
18.pdf
82


end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 2 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_2(n)==l
if BF2_SYM(n)==1
SB2(n)=1;
else
SB2(n)=0;
end
if BF3_SYM(n)==1
SB3(n)=1;
else
SB3(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 3 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_3(n)==1
if BF3_SYM(n)==1
SB3(n)=1;
else
SB3(n)=0;
end
if BF4_SYM(n)==1
SB4(n)=1;
else
SB4(n)=0;
end
19.pdf
83


if BF5_SYM(n)==1
SB5(n)=1;
else
SB5(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 4 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_4(n)==l
if BF5_SYM(n)==1
SB5 (n)=1;
else
SB5(n)=0;
end
if BF6_SYM(n)==1
SB6(n)=1;
else
SB6(n)=0;
end
if BF7_SYM(n)==1
SB7(n)=1;
else
SB7(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 5 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_5(n)==l
20.pdf
84


if BF7_SYM(n)==l
SB7(n)=1}
else
SB7(n)=Q;
end
if BF8_SYM(n)==l
SB8(n)=1;
else
SB8(n)=0;
end
if BF9_SYM(n)==l
SB9(n)=1;
else
SB9(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 6 BF_SYM%%%%%%%%%%%%%
if Zone_6(n)==l
%%%%%%%%%%%%%%%
if BF9_SYM(n)==1
SB9 (n) =1;
else
SB9(n)=0;
end
if BF10_SYM(n)==1
SB10(n)=1;
21.pdf
85


else
SB10(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 7 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_7(n)==1
if BF10_SYM(n)==1
SB10(n)=1;
else
SB10(n)=0;
end
if BF11_SYM(n)==1
SB11(n)=1;
else
SB11(n)=0;
end
if BF12_SYM(n)==l
SB12(n)=1;
else
SB12(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 8 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_8(n)==1
if BF12_SYM(n)==1
SB12(n)=1;
22.pdf
86


else
SB12(n)=0;
end
If BF13_SYM(n)==l
SB13(n)=l;
else
SB13(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 9 BF_SYM%%
if Zone_9(n)==l
if BF13_SYM(n)==l
SB13(n)=l;
else
SB13(n)=0;
end
if BFl4_3YM(n)==l
SB14(n)=1;
else
SB14(n)=0;
end
if BF15_5YM(n)==1
SB15(n)=1;
else
SB15(n)=0;
end
23.pdf
87


end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 10 BF_SYM%%%%%%%%%%%%
if Zone_lG (n)=l
if BFl5_3YM(n)==l
%%%%%%%%%%%%%%%%
SB15(n)=1;
else
SB15(n)=0;
end
if BF16_3YM(n) ==1
SB1S(n)=1;
else
SBlf (n) =Q;
end
if BF17_SYM(n)==1
SB17(n)=1;
else
SB17(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 11 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_ll(n)==1
if BF17_SYM(n)==1
SB17(n)=l;
else
SB17(n)=0;
end
24.pdf
88


if BF18_3YM(n)==1
SB18(n)=1;
else
SB18(n)=0;
end
if BF19_5YM£n)==l
SB19(n)=1;
else
SB19-(n)=0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Zone 12 BF_SYM%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone_12£n)==1
if BF19_SYM(n)==1
SB19(n)=l;
else
SB19(n)=0;
end
if BF2 G_SYM(n)==1
SB20(n)=1;
else
SB2-0 (n) =0;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%Breaker Fail Check%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if Zone l[n)=l,
25.pdf
89


Full Text

PAGE 1

FAULTDETECTIONANDISOLOATIONINLOW-VOLTAGEDC-BUS MICROGRIDSYSTEMS by JaredM.Candelaria B.S.,MetropolitianStateCollegeofDenver,2007 Athesissubmittedtothe UniversityofColoradoDenver inpartialfulllment oftherequirementsforthedegreeof MasterofScience ElectricalEngineering 2012

PAGE 2

ThisthesisfortheMasterofScience degreeby JaredM.Candelaria hasbeenapproved by Jae-DoPark,Ph.D YimingDeng,Ph.D HamidFardi,Ph.D Date

PAGE 3

Candelaria,JaredM.M.S.,ElectricalEngineering FaultDetectionandIsoloationinLow-VoltageDC-BusMicrogridSystems ThesisdirectedbyAssistantProfessorJae-DoPark,Ph.D ABSTRACT UnliketraditionalACdistributionsystems,protectionhasbeenchallenging forDCsystems.Multi-terminalDCpowersystemsdonothavetheyearsof practicalexperienceandstandardsthatACpowersystemshave.Also,thecurrentpowerelectronicdevicescannotsurviveorsustainhighmagnitudefaults. Conveterswillshutdowntoprotectthemselvesunderfaultedconditions.This makeslocatingfaultsinDCsystemdicult,andcausestheDCbustodeenergize.Afaultprotectionalgoritmandmethodforalow-voltageDC-bus microgridsystemispresentedinthisthesisinordertorevolvetheaboveissues. Themaingoaloftheprotectionmethodistodetectandisolatefaultsinthe DCsystemwithoutdegenergizingtheentireDCbus.Inordertoachievethisa ringbuswasutilizedforthemainDCbus.Thebuswasthensegmentedinto individualzoneswithsolidstatebi-directioanlswitchesusedtoisolatethezone intheeventofafault.Eachzoneismonitoredandcontrolledbyanindividual IntellegentElectricalDevice.Agroundingresistorwasaddedinordertolimit theamountofgroundcurrent.TheconceptshavebeenveriedinOrCAD/Psice andMATLAB.

PAGE 4

Thisabstractaccuratelyrepresentsthecontentofthecandidate'sthesis.I recommenditspublication. Jae-DoPark,Ph.D

PAGE 5

DEDICATION Thisthesisisdedicatedtomytwoyoungdaughters,MadilynnandKennedy.I hopethatthiscanserveasanexamplethevalueofimportanceofaneducation.

PAGE 6

ACKNOWLEDGMENT Iwouldliketorstthankmywifeforallowingmethetimetoattendedmy graduateclassesandconductresearchwhileshetookcareofourchildren.I wouldliketothankmygrandparentsforsupportingmeinpursuingthisdegree andhelpingwatchmychildrensothatIcoulddoso.Iwouldliketoacknowledge Dr.KeithMalmedalforsparkingmyinterestinpowersystemsandhirigmefor myrstjobinpowersystems.WithoutthatrstjobIwouldnotbewhereIam today.Lastly,IwouldliketothankDr.JaedoParkforhisencouragemnetand supportduringmytimeattheUniversityofColoradoDenver.Withouthim thisthesiswouldnothavebeenpossible.

PAGE 7

CONTENTS Figures....................................ix Tables.....................................xi Chapter 1.Introduction................................1 2.MicrogridSystems............................5 2.1ACvs.DC...............................5 2.2HVDCvs.LVDC............................5 2.3LCCvs.VSC..............................8 3.FaultProtectionofLow-VoltageDC-busMicrogridSystems.....10 3.1PossibleFaults.............................10 3.1.1LinetoGround............................10 3.1.2LinetoLine..............................12 3.1.3Overcurrent..............................12 3.2CurrentProtectionMethods......................13 3.2.1DCProtectionwithACDevices...................13 3.2.1.1ACCircuitBreakers........................14 3.2.1.2Fuses................................15 3.2.2DCProtectionwithDCDevices...................17 3.2.2.1IGBTCircuitBreakers.......................17 3.2.2.2ConverterEmbeddedDevices...................20 vii

PAGE 8

3.2.3Multi-TerminalSystemProtection.................23 3.2.3.1ACProtection...........................23 3.2.3.2DCProtection...........................26 3.2.4Controllers..............................29 3.2.5CurrentLimitingTechniques....................31 4.ProposedProtectionMethodandAlgorithm..............33 4.1OperationModes............................33 4.1.1FaultDetectionandIsolation....................34 4.1.2BreakerFailureDetection......................40 4.1.2.1StatusandCurrent.........................40 4.1.2.2Current...............................41 4.1.3RecloseandRestore.........................42 4.2SolidStateCircuitBreakers......................42 4.2.1Grounding...............................43 5.SimulationResults............................46 6.ConclusionandFutureWorks......................63 Appendix A.MATLABCode..............................65 References ...................................134 viii

PAGE 9

FIGURES Figure 1.1 AdamsHydroelectric3PhaseACGeneratingPlant. ..........2 2.1 ConceptualdiagramofaDC-busmicrogridsystem. ...........6 2.2VSCOperationaNormal.bPositiveLine-to-GroundFault...9 3.1 aLine-to-GroundFaultbLine-to-LineFault ..............12 3.2 BacktoBackDierentialProtection. ..................16 3.3 Two-TerminalDierentialProtection ..................16 3.4 IGBT-CBFaultBlockingCapabilities. ..................19 3.5 ETOBasedVSC-HVDCConverter. ...................22 3.6 VSC-HVDCConverterSubmodule. ...................22 3.7 Hand-ShakingMethod:aCurrentFlowDuringaFault.bFaultIsolation. 24 3.8 HandShakingMethodRe-Closing. ...................24 3.9 IGBT-CBProtection. ..........................28 3.10 OvervoltageChopperCircuit. ......................30 4.1 MicrogridProtectionZones .......................35 4.2 AlgorithmFlowChart ..........................36 4.3 Uni-DirectionalZone:NormalandFaultedCurrentFlow ........39 4.4 Bi-DirectionalZone:NormalandFaultedCurrentFlow .........39 4.5 LinkZone:NormalandFaultedCurrentFlow ..............39 4.6 ZoneControllerConnections .......................41 ix

PAGE 10

4.7 aBi-directionalIGBTcircuitbreaker.bBi-directionalIGCTcircuit breaker. .................................43 5.1 OrCAD/PSpiceSimulationModel ....................47 5.2 AnalyticalSimulationCircuitforLine-to-GroundFaultinZone1Without ResistanceGrounding. ..........................49 5.3 AnalyticalSimulationCircuitforLine-to-GroundFaultwithResistance Grounding. ................................49 5.4 VoltageatACandDCLoadsduringaLine-to-GroundFaultwithResistanceGrounding .............................50 5.5 CurrentatACandDCLoadsDuringaLine-to-GroundFaultwithResistanceGrounding ............................51 5.6 Zone1CircuitBreakerStatusDuringaLine-to-GroundFault .....52 5.7 Zone1IEDTripSignalsDuringaline-to-GroundFault .........53 5.8 Zone4CurrentwithBreaker5FailingtoOpen .............54 5.9 Zone3withBreaker5FailingtoOpen .................55 5.10 Zone4BreakerStatuswithBreaker5FailingtoOpen .........55 5.11 Zone3BreakerTripSignalsAfterBreaker5FailstoOpen .......56 5.12 DCVoltageatLoadsDuringaZone4BreakerFail ...........57 5.13 CurrentatLoadsDuringaZone4BreakerFail .............58 5.14 Zone1CurrentDuringaRecloseandLockoutCycle ..........59 5.15 Zone1RecloseCycleTripSignals ....................60 5.16 Zone1RecloseCycleCloseSignals ...................61 5.17 Zone1BreakerStatusDuringaRecloseandLockoutCycle .......62 x

PAGE 11

TABLES Table 5.1SimulationParameters.........................48 xi

PAGE 12

1.Introduction Attheendofthe19 th centurythewartoelectrifytheworldwaswaged.On onesidesatThomasEdisonwithhisDCsystemandontheotherwasGeorge WestinghouseandNikolaTeslawiththeirACsystem.InthebeginningEdison waswinningwiththeopeningofthePearlStreetStationinNewYork,1882[39]. UnfortunatelyforEdisonthetechnologytoincreaseaDCvoltagetotransmission levelwasnotyetavailable.ACsystemshadtheabilitytoreachsuchvoltages withthetechnologyofthetime.In1893WestinghouseandTeslasuccessfully supplied11MWofelectricitytotheChicagoWorld'sFair.Thenin1895the AdamsHydroelectricGeneratingPlantFig.1.1wasopenedatNiagaraFalls, servinglocalutilitiesinNiagaraFalls.By1910transmissionlineswereserving loadsinSouthernOntartio[40].ThismarkedthebirthplaceofACpowerand setinmotionit'sdominanceintheworld. EventhoughACclearlywonthewar,researchanddevelopmentintoDC didnotstop.Inthe1920'smercuryarcvalvesbecameavailableforuseinpower transmission.Inthe1950'shighpowertransistorslikethethyristorwereintroducedallowinghighervoltageandpowerratings[11,5].Theseinventionsled totheuseofHighVoltageDirectCurrentHVDCTransmission,totransmit powerovergreatdistance.HVDChasalsobeenusedasanasynchronousties betweenseparatepowergrids.ThesesystemsusingthyristorvalvesarecommonlyreferredtoasClassicalorLineCommutatedConverterLCCHDVC havebeeninplacesincethe1970's,andhavebeenavitalpartofthemodern 1

PAGE 13

Figure1.1: AdamsHydroelectric3PhaseACGeneratingPlant. 2

PAGE 14

grid. Recentlymanydistributedpowersystemshavebeenresearchedanddeveloped,especiallytomeettheneedforhighpenetrationofrenewableenergyresourcessuchaswindturbinesandphotovoltaicsystems.Thedistributedpower systemshaveadvantagessuchasthecapacityreliefoftransmissionanddistribution,betteroperationalandeconomicalgenerationeciency,improvedreliability,eco-friendlinessandpowerquality[19,14,9].Thecurrentenergypolicy ofmanygovernmentsintheworldistocompetitivelyincreasetherequirement ofthepenetrationofrenewableenergysourcesanddistributedgeneration.For instance,Californiaistryingtoincreasetheusageofrenewablegenerationup to33%by2020[54]andtheStateofColoradohassetspecicrequirementsfor distributedgenerationfromeligiblerenewableenergyresources[55]. Themicrogridsystemisasmall-scaledistributedpowersystemconsists ofdistributedenergysourcesandloads,anditcanbereadilyintegratedwith therenewableenergysources[33,42,29,41,46,25].Duetothedistributed natureofmicrogridapproach,theconnectiontothecentraldispatchcanbe removedorminimizedandinturnthepowerqualitytosensitiveloadscan beenhanced.Mostmicrogridsystemshavetheirconnecteddistributedenergy sourcesinterfacedthroughpowerelectronicsconverters.Generallytheyhave twooperationmodes:standaloneislandedandgrid-connectedoperation. MicrogridsystemscanbedividedintoAC-busandDC-bussystems,based onthebusthatthecomponentsystemssuchasenergysources,loadsandstoragesareconnectedto.AC-busbasedmicrogridsareadvantageousbecausethe existingACpowergridtechnologiescanbereadilyapplicable.Currently,allof 3

PAGE 15

thecommerciallyinstalledmicrogridsareAC[7].DCmicrogridshaveseveral advantageousoverACwhichwillbecovedinthisthesis.Oneofthemainreasons thatDCmicrogridsarenotasprevalentasACmicrogridsisthatthesystem protectionisinit'sinfancy.TheprotectionofDCsystemsisnotstandardized andoftenrequiresacompleteshutdownoftheDCbus[15,53,52]. ThisthesispresentsafaultprotectionmethodandalgorithmforalowvoltageDC-busmicrogrid.Theprimarygoaloftheproposedmethodisto detectthefaultinabussegmentbetweendevicesandisolatedthefaultedsection sothatthesystemkeepsoperatingwithoutdisablingtheentiresystem.This thesiswillprovideacomparisonofACandDCsystems,presentthetypesofDC systemsavailable,andgiveanoverviewofthecurrentDCprotectionmethods. Finally,theprotectionmethodandalgorithmwillbeprovidedwithsimulation results. 4

PAGE 16

2.MicrogridSystems 2.1ACvs.DC MicrogridsystemscanbedividedintoAC-busandDC-bussystems,based onthebusthatthecomponentsystemssuchasenergysources,loadsandstoragesareconnectedto.AC-busbasedmicrogridsareadvantageousbecausethe existingACpowergridtechnologiescanbereadilyapplicable.However,ACgrid issuesincludingsynchronization,reactivepowercontrol,andbusstabilityare inheritedaswell.DC-busbasedsystemscanbecomeafeasiblesolutionbecause microgridsaresmall,localizedsystemthatthetransmissionlossisnegligible, unlikethetraditionalpowersystemsthathavealonglineoftransmissionand distribution.Moreover,itdoesnotneedtoconsidertheACsystemissuesand systemcostandsizecanbereducedcomparedtothetypicalAC-DC-ACconversioncongurationbecauseDCpowerisgenerallyusedinthepowerelectronics devicesasamedium.AconceptualdiagramofDC-busmicrogridisshownin Fig.2.1. WhiletheadvantageousofDCmicrogridsaregreat,protectionofDCdistributionsystemshasposedmanychallengessuchasautonomouslylocatinga faultwithinamicrogrid,eectivelybreakingaDCarc,DCprotectiondevices, andcertainlythelackofstandards,guidelinesandexperience[47,15]. 2.2HVDCvs.LVDC WhenitcomestopowertransmissionsystemsHVDCisthemostprominent. HVDClineshavebeenapartoftheUnitedStatesgridforover40years.HVDC 5

PAGE 17

Figure2.1: ConceptualdiagramofaDC-busmicrogridsystem. 6

PAGE 18

lineshelptransmitbulkpoweroverlongdistancesandinterconnectingthethree gridsintheintheUnitedStates.Intraditionalpowersystemstherearethree maincomponents:generation,transmissionandloads.Generallythegeneration isalargepowerplantcoal,nuclear,etc.andislocatedoutsidelargecities, whilemostoftheloadsliewithintheselargecities.Thetransmissioncomponent deliversenergyfromthegenerationsitestotheloads.Thisishowthepower systemsaroundtheworldcurrentlywork.Thusthereasonforthedominance ofHVDCoverLVDC. Attitudesandviewsonhowthepowersystemshouldoperatehavebegin toshiftanddecentralizationofthegridisbeginningtogainattention.This wouldmeanthattraditionalloadswouldinstalllocalgenerationoftencalled independentpowerproducersIPP.Thelocalgenerationcanbeusedtosupply thelocalloads,thusdecreasingthetransmissionlosses.WhentheIPPproduces moreelectricitythantheloadconsumesitcansellthepowerbacktotheutility, andwhenproductionfallsshortthegridistheretomakeupthedierence.It isinthisapplicationthatLVDChasstartedtogainattention.Although,the electricgridisprimarilyACmanyoftheloadsinourhomesandbusinessesare DC.ThismeansthateachofthesedevicesrequiresasmallAC-to-DCconverter, whichincreaseslosses.IflocalIPP'sproduceDCpowerthattheloadsare abletousedirectlyitwoulddecreasetheseconversionlosses.Thisisespecially advantageousfordatacenterswherealmostalloftheequipmentisDC.Recently, theEuropeanTelecommunicationsStandardsInstituteETSIbegandraftinga standardon380VDCwiringforbuilding-widepowerdistribution[43]. 7

PAGE 19

2.3LCCvs.VSC CurrentlylinecommutatedconvertersLCCorclassicalthyristor-basedDC systemsholdthemarketinbulkpowertransmission.Recentlyvoltagesource converterbasedDCsystemsarebecomingmoreofacompetitorofclassical thyristor-basedDCsystems[21].NotonlyisVSCacompetitorfortransmissionbutitcanalsobeusedinmulti-terminalsystems,whichhavebecomean attractiveoptionforrenewableenergyapplicationsorfordistributioninlarge cities.Astheconverterpowerratingincreasesitmayonedayreplacethyristorbasedconverters.VSC'sareattractivebecause,unlikeclassicalconverters,no reactivepowersupportisneededtooperatethesystem.InfactVSC'scanproducereactivepower,andcontrolactiveandreactivepowerindependently[3]. ThiscontrollabilityallowsVSCconverterstooperateinsystemswithlittleor noACsupport,somethingthatclassicalconvertscannotachievewithoutexpensivesupport[13,17,48,22].VSC'sarealsoadvantageousinmulti-terminal systems.Multi-terminalsystemsconsistofthreeormoreconverterstocreate aDCnetwork.Applicationsofmulti-terminalsystemsincludedistributioninto largecities,microgrids,andevenshippowersystems[52,20,53,6].VSC'sare bettersuitedformulti-terminalsystemsasthepowerowcanbechangedby changingthedirectionofthecurrent.ClassicalDCconvertersrequiretheDC voltagepolaritytobechanged,whichcanbedicult[35,44,13]. VSCsystemsare,bydesign,vulnerabletofaultsontheDCsystems.ClassicalHVDCsystemsarenaturallyabletowithstandshortcircuitcurrents 8

PAGE 20

Figure2.2: VSCOperationaNormal.bPositiveLine-to-GroundFault. duetheDCinductorslimitingthecurrentduringfaultconditions[20,56]. WhenafaultoccursontheDCsideofaVSCsystemtheIGBT'slosecontrolandthefreewheelingdiodesactasabridgerectierandfeedthefault [52,34,23,53,57,6,56,18],asshowninFig.2.2.Thetypesoffaultspossible onaHVDCsystemareasfollows. AchallengeassociatedwiththeprotectionofVSCsystemsisthatthefault currentmustbedetectedandextinguishedveryquicklyastheconvertersfault withstandratingisgenerallyonlytwicetheconverterfullloadrating[6].Fault detectionisalsoimportant,especiallyonmulti-terminalsystems,inorderto isolatethefaultandrestorethesystemtoworkingorder. 9

PAGE 21

3.FaultProtectionofLow-VoltageDC-busMicrogridSystems 3.1PossibleFaults ForDCsystemtwotypesoffaultsexist,line-to-lineandline-to-ground,as canbeseeninFig.3.1.Aline-to-linefaultoccurswhenapathbetweenthe positiveandnegativelineiscreated,shortingthetwotogether.Aline-to-ground faultoccurswhenapathbetweeneitherthepositiveornegativepoleandground iscreated.Aline-to-groundfaultisthemostcommontypeoffault[10]. VSCsmayexperienceinternalswitchfaultsthatcancauselinetolineshort fault.Thisisaterminalfaultfordevicethatcan'tbeclearedandinmostcasesit requiresreplacementofthedevice.Hence,DCfusewouldbeproperprotection measurefor 3.1.1LinetoGround Aline-to-groundfaultgroundfaultoccurswhenthepositiveornegative lineisshortedtoground.Inoverheadlinesfaultsmayoccurwhenlightning strikestheline.Thismaycausethelinetobreak,falltothegroundandcreate fault.Inthissituationthefaultisalwayspermanentandthelinemustbe isolatedforrepair.Groundfaultsmayalsooccurbyobjectsfallingontothe line,suchastrees,providingapathtoground.Insomecaseswhenanobject causesthegroundfaultitmayfallawayfromthelineandthesystemcanbe restored.Ifthefaultpersiststhelinewouldhavetobetakenoutofserviceuntil thefaultpathcanbecleared. Undergroundcableisalmostcompletelyimmunetoline-to-linefaults,as insulation,conduitandtheearthseparatethecables.However,theycanstill 10

PAGE 22

occur.Theinsulationofthecablecanfailduetoimproperinstallation,excessive voltage/current,exposuretotheenvironmentwater,soil,etcorcableaging [56].Whenthisoccurs,thebrokeninsulationwillallowapathforcurrentto owtoground.Asthefaultpersiststheintegrityoftheinsulationisreduced causingthefaulttoworsen.Agroundfaultmayalsooccurwhenaperson inadvertentlycutsthroughoneofthelines.Thisgenerallyhappensduring constructionprojects.Ineithercasethefaultwillalwaysbepermanentandwill requireacompleteshutdownofthelineaswellasacostlyrepair. Whenaline-to-groundfaultoccurs,thefaultedpolerapidlydischargescapacitortoground.ThiscausesanimbalanceoftheDClinkvoltagebetweenthe positiveandnegativepoles.Asthevoltageofthefaultedlinebeginstofall,high currentsowfromthecapacitoraswellastheACgrid.Thesehighcurrents maydamagethecapacitorsandtheconverter[57]. 11

PAGE 23

Figure3.1: aLine-to-GroundFaultbLine-to-LineFault 3.1.2LinetoLine Asstatedbefore,aline-to-linefaultonacable-connectedsystemisless likelytooccuronthecable.Inanoverheadsystem,line-to-linefaultscan becausedbyanobjectfallingacrossthepositiveandnegativeline,theymay alsooccurintheeventofthefailureofaswitchingdevicecausingthelinesto short.Aswitchingfault,whichisindependentofhowtheconverterstationsare connectedtogether,causesthepositivebustoshorttothenegativebusinside theconverter.Aline-to-linefaultmaybeeithertemporaryorpermanent. 3.1.3Overcurrent Whileovercurrentprotectionisimportantduringline-to-lineandline-togroundfaults,itmustalsooperatewhenthesystemisbeingoverloaded.Overloadconditionsmayoccurintwo-terminalsystemswhentheloadincreasespast 12

PAGE 24

theratingoftheconverterorasaresultofafaultonanotherpartofthesystem. Forexample,ifthreeVSC'sarefeedingacommonloadandoneVSCisdropped duetoapermanentfault,theremainingtwomustsupplytheload.Thiswill resultinelevatedcurrentsthatmayoverloadtheconverters.Inthissituation theovercurrentprotectionwouldneedtooperate.Anotheroptiontoavoida widespreadblackoutwouldbetoshednon-criticalloads. 3.2CurrentProtectionMethods Although,astandardhasnotbeenagreedupononhowtobestprotect DCsystems,manymethodshavebeenproposed.Mostofthesemethodsare proposedforhighvoltagedirectcurrentHVDC.ThisisbecausemoreHVDC systemshavemadeittoconsumerproduction,andthereforehasbeenapopular researchtopic.However,theconceptsandideasarethesameforLVDCsystems. Thebelowsectionswilloverviewtheseproposedmethods. 3.2.1DCProtectionwithACDevices TraditionallyprotectionofDCsystemshasbeendonewithconventionalAC devicessuchascircuitbreakersandfuses.TheadvantagesofusingACdevices include: LessexpensivethanDCcounterparts Shorterleadtime Maturescience Morefamiliardevices ThetwooptionswhenchoosinganACdeviceiseitheracircuitbreakeror afuse. 13

PAGE 25

3.2.1.1ACCircuitBreakers PlacingACcircuitbreakersontheACsideoftheVSCisthemosteconomicalwaytoprotecttheDCsystem.Theyarecommonlyavailableandcan bereplacedinashorteramountoftime.However,ACcircuitbreakersresult inthelongestinterruptiontimeasaresultoftheirmechanicalrestrictions[52]. Currently,thebestinterruptingtimeforanACcircuitbreakeristwocycles[2]. WhenusinganACcircuitbreaker,thevoltageoftheDCcapacitorswillbe monitoredaswellasthecurrentineachDClineateachconverter.Thesevalueswillbefedbacktoastandardrelay,whichwillmonitorover/undervoltage, aswellasovercurrent.WhenaDCfaultoccurs,thecapacitorswilldischarge rapidlycausingthevoltagetodecrease.Thecurrentonthefaultedlinewill increaseovertheratedvalue.Oncetherelaysensesoneormoreoftheseconditions,itwilltripthebreaker.Inanattempttorestorethesystem,therelay willenterare-closingcycleinwhichtherelaywillclosebackinandsensethe voltageandcurrentoftheDCsystem.Ifthefaultisclearedthesystemwill returntonormal,butifapermanentfaultisdetectedtherelaywilllockoutthe breaker.Therelayidentiesapermanentfaultbythere-closingsequence.A typicalindustrystandardforre-closingonACsystemsisthattwoattemptswill bemade;thiscanbeappliedtotheVSCsystemsaswell.Aftertwoattempts withoutsuccess,therelaydeterminesthatthefaultispermanentandwillnot allowthebreakertoclose. Inback-to-backortwo-terminaltransmissionsystems,dierentialprotection maybeusedtoprotecteachconverter,asshowninFig.3.2.Thedierential relayNote:87istheANSIstandardnumberforadierentialrelaywillmeasure 14

PAGE 26

thecurrententeringtheconverteraswellasthecurrentleavingtheconverter. Ifthecurrententeringdoesnotmatchthecurrentleavingthedierentialrelay, itwilltriptheACbreaker[4,30].Inback-to-backsystemsonerelaycould alsomonitortheACcurrentatthesendingVSCaswellasthereceivingVSC. Ifthecurrentatoneenddoesnotmatchthecurrentattheotherend,the relaywouldknowafaulthasoccurredandtheVSC'swouldbetrippedoine. Inatwoterminaltransmissionsystem,tworelayswouldberequiredandthe currentreadingswouldhavetobesenttotheotherrelayviacommunication ascanbeseeninFig.3.3.Thistypeofdierentialprotectioniscommonin ACsystems[10]ascouldbeappliedtoVSCprotection.Also,compensationfor losseswouldhavetobetakenintoaccount. WhileACcircuitbreakersareinexpensive,easytouse,andwidelyused, theyalsoshutdowntheentireconverter.Thisisproblematicinthecaseof groundfaultswherethefaultedlinecouldbeisolatedandthesystemcould runmono-polarusinggroundasareturnpath[57].ACcircuitbreakersarealso inconvenientinmulti-terminalsystems,whichwillbediscussedlater. 3.2.1.2Fuses FusesontheACsidearegenerallynotagoodsolutionforprotectionof theVSC[38].Thisisbecauseafuseisathermaldevicethatisonlyallowed oneoperation.Fusesdonothavetheabilitytodistinguishwhetherafaultis temporaryorpermanent.Toafuseeveryfaultispermanentandthereforethe systemwouldnotbeabletoberestoreduntilthefusewasphysicallyreplaced. Theonlyplacethatafusemaybeanacceptablealternativeisforanon-critical load,orinareaswherespaceislimited,suchasaship.Fusesareusedfor 15

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Figure3.2: BacktoBackDierentialProtection. Figure3.3: Two-TerminalDierentialProtection 16

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protection[38],butaremainlyforACprotectionandotherDCdevicesprotect theDCline.TheDCdevicescoordinatewiththefusesuchthattheywilltrip beforethefuse.ThefusewillonlyoperateintheeventthattheDCprotection fails.Overall,fusesshouldbeusedonlywhennecessaryandallotheroptions arenotpossible. 3.2.2DCProtectionwithDCDevices WhileACdevicesareaneconomicalwaytoprotecttheDCsystem,DC devicesareabetteroptionwheneverpossible.DCprotectivedevicescanact fasterthantheirACcounterparts,aswellassectionalizelines.Thisallowsthe operationofunfaultedlinestocontinue.ThemethodsofprotectionusingDC devicesareshownbelow. 3.2.2.1IGBTCircuitBreakers AnIGBTcircuitbreakerIGBT-CButilizestheblockingcapabilityofthe solid-statedevice.LiketheotherIGBTsintheconverters,theIGBT-CBsare conguredwithananti-paralleldiode.TheonlydrawbacktotheIGBT-CBis thatitisauni-directionaldevice.ThisisillustratedinFig.3.4.Whenafault occursontheDCline,theIGBTisabletoblockthefaultcurrentrepresented bythedashedlineinFig.3.4.Ifthefaultoccursontheconverterside,the anti-paralleldiodesconductandallowcurrenttoowrepresentedbythesolid lineinFig.3.4.InthisscenariotheIGBT-CBmustrelyontheblockingofthe IGBTsintheconverter[52]. Fortwo-terminalsystems,IGBT-CBscanbeplacedateachconverterstation,oneonthepositivelineandoneonthenegativeline,ascanbeseeninFig. 3.4.FastactingDCswitchesareusedinconjunctionwiththeIGBT-CB,which 17

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isusedtoisolatethelineoncethefaultcurrenthasbeencleared.Itshouldbe notedthattheswitchcannotbreakcurrentandmayonlybeopenedoncethe faulthasbeenextinguished.AswithAC,theDCcurrentofeachlineandthe DCvoltageofeachcapacitorwillbesensed.Oncethecontrolsystemsensesa faultontheline,anappropriateIGBT-CBwillreceiveagatesignaltoblock thecurrent.OncethefaultcurrenthasbeenextinguishedthefastactingDC switcheswillopen,isolatingtheline.Todetermineifthefaultistemporary orpermanent,theDCswitchesandtheIGBT-CBwillclose.Ifthefaulthas clearedthesystemwillreturntonormaloperation.Ifthefaultisstillpresent, thelinewillbeisolatedagainandapermanentfaultwillbedetermined. TheadvantageofusinganIGBT-CBisthattheentireconverterisnot shutdowninthecaseofagroundfault.Thisallowsthefaultedlinetobe isolatedandhavethesystemcontinuetorunmono-polar.TheIGBT-CBalso opensfasterthanitsACcounterpart.ThedisadvantagetotheIGBT-CBisthat itcannotprotectagainstDCrailfaultsintherectier[38]. 18

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Figure3.4: IGBT-CBFaultBlockingCapabilities. 19

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3.2.2.2ConverterEmbeddedDevices Converterembeddeddevicesareactive,protectivecomponentsthatareinstalledinsideaVSCtodetectandisolateDCfaults.Thismethodeliminates theuseofadditionaldevices,reducingthefootprintoftheconverterstationand alsopossiblycuttingcost.However,aredesignoftheconverterisrequired.In [6],theconverterusestwoEmitterTurnOETOdevicesinananti-parallel congurationtoachievebothswitchingandprotection.TheETOhasahigher voltageandcurrentratingthanIGBT.Fig.3.5illustratestheconverterconguration.InnormaloperationtheETO'sXactastheswitchingdevices,while ETO'sYactastheanti-paralleldiode,andareconstantlyredon.Intheevent ofafaulttheETO'sXareblockedwhileETO'sYcontinuetofeedthefault. OncethefaulthasbeenidentiedaspermanenttheYETO'swillbegatedo. AnotherprotectionmethodhasreplacedthetypicalIGBTandanti-parallel diodeisacombinationswitchingdevicewiththesubmoduleshowninFig.3.6 [18,22].Theconvertersubmoduleprovidestwodierentlevelsofprotection. Therstlevelprotectstheconverterfrombeingshutdownduringaswitching devicefailure.Intheeventofaswitchingdevicefailure,thesubmodulewill closeswitch K 1 ,shortingoutthedefectivesubmodule.Thisallowstheconvertertocontinuetooperatebyusingredundantmodulesforun-alteredsystem performance.Thesecondlevelofprotectionreactsunderfaultconditions.As statedearlier,theswitchingdeviceblocksandtheanti-paralleldiodeconductsto feedthefaultunderfaultconditions.ThefreewheelingdiodesinVSC'sarenot 20

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abletowithstandlargesurgecurrents,andmaybedamagedbeforethefaultis cleared.Thesolutionin[18]istobypasstheIGBTandtheanti-paralleldiode withapress-packthyristor K 2 .Theproposedpress-packthyristorisableto withstandhighsurgecurrents,protectingtheanti-paralleldiodeuntilthefault canbecleared. Thisprotectionmethodallowstheconverteradditionalcontrolandincreases thecurrentratingoftheswitchingdevices.Italsocutsdownonthenumberof componentsrequiredforprotectionbecausetheprotectionisembeddedinthe converter.Thedisadvantageisthattheentireconvertermustbeshutdownin theeventofapermanentfault.Thisworkswellintwo-terminalsystemsbut maycauseproblemsinmulti-terminalsystems. 21

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Figure3.5: ETOBasedVSC-HVDCConverter. Figure3.6: VSC-HVDCConverterSubmodule. 22

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3.2.3Multi-TerminalSystemProtection VSCsystemsareveryappealinginmulti-terminalsystems,aspowerow canbechangednotbyvoltagepolaritybutthedirectionofthecurrent.The possibleapplicationsformulti-terminalVSCMT-VSCsystemsareusedin renewableenergyapplicationsandindistributionofpowerinmegacities.The protectionstrategiesforMT-VSCutilizebothACandDCprotection. 3.2.3.1ACProtection Asstatedpreviously,DCprotectioncanbeachievedbyusingACcircuit breakersontheACsystems.ThisstrategycanbeappliedtoMT-VSCaswell. A"handshaking"methodisproposedin[53].Thismethod,inadditiontousing ACcircuitbreakers,implementsfastactingDCswitches.Theswitchesareonly usedtoisolatelinesandcannotbreakloadorfaultcurrent.EachVSCwill receivecurrentmeasurementsfromtheirrespectiveDCswitches.Whenafault occursalloftheACcircuitbreakersassociatedwiththeMT-VSCsystemwill trip.Next,eachVSCmustdeterminewhichoneofitsrespectiveswitchesto open.Thisisdonebymeasuringthemagnitudeanddirectionofthecurrent througheachswitch.Theswitchthatwillbeselectedistheonewiththelargest positivefaultcurrent.Thehandshakingmethoddenespositiveasoutofthe nodeandnegativeintothenode.Fig.3.7illustratestheexamplesystemgiven in[53]. 23

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Figure3.7: Hand-ShakingMethod:aCurrentFlowDuringaFault.bFault Isolation. Figure3.8: HandShakingMethodRe-Closing. 24

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WhenafaultoccursonLine1,VSC1receivescurrentmeasurementsfrom SW11andSW31.VSC1sensesthatthecurrentthroughSW11ispositiveand thecurrentthroughSW31isnegative.ThroughthehandshakingmethodVSC1 opensSW11.VSC2receivescurrentmeasurementsfromSW12andSW22.Once againthecurrentthroughSW12ispositiveandthecurrentthroughSW22is negativeandswitchSW12isselected.VSC3receivescurrentmeasurement fromSW33andSW23.Thecurrentdirectionforbothswitchesismeasuredas positive.Theswitchwiththehighestmagnitudeofcurrentisselected.Atthis pointLine1,thefaultedline,isisolated,andLine3isopenatoneend.Atthis pointthesystemmustenterare-closingmode.First,alloftheACbreakers willclosebackin,re-energizingtheVSC's.Next,thefastDCswitchesofthe non-faultedDClinesmustbeclosed.TheVSC'sonlyre-closeswitcheswhen thevoltageofitsrespectivelineisnearthevoltageoftheVSCterminals.Fig. 3.8showsthere-closingmethodpresentedin[53],whereitcanbeseenthat onlySW33willbeabletore-close.DuringthefaultVSC1chosetoopenSW11, leavingSW31closed.OncetheACbreakersre-close,andtheVSC1isbackon line,Line3willrecharge.VSC3willsensetheLine3voltageandallowSW33to close.BothSW11andSW12willremainopenasLine1wasdischargedduring thefaultandbothVSC1andVSC2willsensenovoltageonLine1,therefore notallowingtheswitchestoclose. 25

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3.2.3.2DCProtection DCProtectionutilizesIGBT-CB'sandfastactingDCswitches.TheIGBTCB'scanbeplacedattheterminalsofeachVSCorattheendofeachline,as showinFig.3.9. Thevoltageofthecapacitorswillbemonitoredaswellasthecurrentthrough eachline.Whenthecurrentexceedsthemaximumsetpointandthevoltage beginstorapidlydischargetherespectiveIGBTwillbegintoblockandthefast actingDCswitchwillopenoncethefaultisextinguished.Thistypeofprotection isveryadvantageousinMT-VSCsystemsasyoucanisolateindividuallines withoutinterruptingtheentirenetwork.ThisisespeciallytrueinFig.3.9b whereeachlinehasitsownIGBT-CB.Whilethisisamoreeectivemethodof protection,itisthemostexpensiveoptionwithfurtherchallenges.UnlikeFig. 3.9acase,IGBT-CBcannotbegintoblockwhenafaultisdetectedonthe positivelinebecausetwoormorelinessplitfromthepositiveornegativenode. Sincealllinesthatareconnectedtoaparticularnodewillfeedfaultonany otherlineconnectedtothesamenode,thefaultedlinemustbedetected.Three dierentmethodstoachievethisarepresentedin[52];theyarelargecurrent change,risetime,andoscillationpattern. Thelargecurrentchangemethoddetermineswhichlinesarefaultedby comparingthecurrentmagnitudeofalllinesfeedingthefault.Thelinewith thelargestcurrentchangeinagiventimewillbechosenasthefaultedline.The risetimemethodmeasurestherisetimeoftherstwavefrontofthecurrent. 26

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Whenafaultisdetected,eachVSCwillmeasuretherisetimeofthecurrent intheirrespectivelines.Thelinewiththefastestrisetimewillbeidentiedas thefaultedline.Theoscillationpatternmethodlooksforwidepulseswithout achangeinpolarity.Thisidentiesthefaultedline. Whileisolatingthefaultisimportant,limitingtheamountoffaultcurrent isaswell.TheDClinkcapacitorscontributehighfaultcurrentsinaveryshort amountoftime.Typically,capacitorprotectionisdonewithsnubbercircuits. However,thesnubberonlylimitsthedischargerateofthecapacitor;itdoes notinterruptthedischargecurrent[6].Theideaofplacingacircuitbreaker inserieswiththecapacitorisintroducedin[38].Thetypeofcircuitbreaker chosenisaCapacitorDCCircuitBreakerCDCCB.Theadvantageofusing aCDCCBisspeed:itisaveryfastactingdevice,operatinginapproximately 10seconds.Thisfastoperationprotectsthecapacitorfromextremestressand destruction.Thevoltagewillholdbecausethecapacitordoesnotdischarge underfaultconditions.ThiscreatesashorterchargingtimewhentheVSCis putbackonline. DCprotectiondevicesnotonlyprotectagainstovercurrent,buttheycan alsoprotectagainstovervoltage.IfaconverterislostonanMT-VSCsystem thevoltageonthesystemwilldrop,butoncetheconverterisbackonlinethe voltagecanovershoot.Onemethodpresentedtomitigatethisproblemisthe implementationofachoppercircuit[34,12,8].InFig.3.10theadditionofan IGBTwithaseriesresistorcanbeseen. 27

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Figure3.9: IGBT-CBProtection. 28

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3.2.4Controllers Theprevioussectionsoverviewedthedevicesthatinterruptfaultconditions.Thissectionwillcovertheactivecontrollersthatwillattempttochange theoperationoftheVSCsunderfaultconditionsinordertokeepthesystem running.Traditionally,asinglecontrollerwilloperateduringsteadystateand faultconditions.Thiscanbeseenin[34],wheretheproposedcontrollersupplies thenormalandprotectivegatetriggerpulses.Anotheroptionistoimplement aparallelcontroller.Theparallelcontrollerisproposedtomitigateovercurrentsandovervoltages[31,32].Bothacurrentandavoltagecontrollercanbe providedinparallelconguration.Withineachoftherespectivecontrollers,a steadystateandafaultcontrollercanbeconnectedinparallelusingPIcontrollerstoregulatethecurrentandvoltage.Eachisrunningduringnormal operationbutdependingontheconditionofthesystemonewilltakecontrolof theringpulses. Overloadproblemsmaybesolvedbyimplementingsometechniquesfound inmotorcontrol[12].Atwo-terminalVSC-HVDCsystemcanbelookedatasa double-sidedconverterfeedingamotor.Whenpowerlevelsbegintoexceedthe contingencyratingofthesystemtheVSC-HVDCsystemcanenter"regenerative brakingmode,"returningthepowerbacktotheACgrid.Thisisanalternative tousingachoppercircuitastheenergyisnotdissipated,ratheritisredirected. Asmentioned,thelossofaconvertercanleadtoovervoltagesinthesystem andcanadverselyaectMT-VSCsystems.Tocombatthisproblemanadvanced 29

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Figure3.10: OvervoltageChopperCircuit. DCvoltagecontrollerADCVCisproposedin[36].ThisADCVCoperates intwostages;lowerandhigherhierarchical.Thelowerhierarchicalcontrol operatesduringnormalsystemoperation.Thislowerhierarchicalcontroller isresponsibleformaintainingactivepower,reactivepowerandDCvoltage. Thehigherhierarchicalcontrollermonitorsthesystemandonlyreactsduring transientdisturbances,i.e.,converterlossinMT-VSC.Thehigherhierarchical controllerrecognizestransientdisturbancesbychangesinthelocalvoltageand current,asthelossofaconverterwillredirectpowerowintheDCnetwork. Uponrecognitionofadisturbance,theADCVCwilltakecontrolandalterthe performanceofitsrespectiveconverterorshutdowntheconverterinorderto protectitfromharminsomecases.Theideaoflowerandhighercontrollersis similartocoordinationofprotectivedevicesonACsystems. 30

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3.2.5CurrentLimitingTechniques Thereareseveraltechniquesthatattempttolimitthecurrentundera faultedcondition.Someofthefaultcurrentlimtingtechniquesare: Superconductor PositiveTemperatureCoecient SaturaedInductor PowerElectronics Thesupercondcutortechniqueusesasuperconductorasapartofthetransmissionordistributionline.Undernormalconditionsthesuperconductorresistancewillbenearlyzero.Whenafaultedconditionisdetectedthesuperconductorwillbegintoincreaseitsresistanceinordertolimitthecurrent.This techniquerequireaspecialcoolingsystemswellasseparateprotectionforthe supercondcutor[37,24]. ThePositiveTemperatureCoecientissimilartothesuperconductor methodbutusesavariableresistorthatistemperaturecontrolled.Undernormal conditionthecurrentislowandthereforetheresistanceislow.Underfaulted conditionstheincreasedcurrentwillcauseanincreaseintemperaturethusincreasingtheresistanceoftheresistor.Whilethistechniqueislesscomplexthan thesupercondcutorithaslowcapacityforcurrentandvoltage.Thereforeunder highfaultconditionstheresistercanfail,openingthecircuit[37]. 31

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Thesaturatedinductorplacesaninductorinthelineandusesaseparate excitingcircuittodrivetheinductorintosaturation,keepingtheresistancelow. Underfaultedconditionstheinductorwillbepulledoutofsaturation,increasing resistanceanddecreasingcurrent.Thismethodrequirestheuseofalargeiron core,increasingsizeandcost.Also,theinductoronlimitstherateofriseand notthemaximumfaultlevel[37,50].Also,theincreasedinductanceintheline adverselyaectstheresponsetothefastloaddynamics. Theuseofpowerelectronicssuggestslimitingfaultcurrentbyswitchingof thebuscircuitbreakers[37,28].However,itisnoteasytoswitchlargecurrent andrequiredinductancecouldmakethedynamicresponseofnormaloperation sluggish.Also,ifafaultispermanentthenlimitingthefaultcurrentinorderto ridethroughthefaultisnotnecessary. 32

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4.ProposedProtectionMethodandAlgorithm 4.1OperationModes ThenovelprotectionmethodandalgorithmfortheDCbusmicrogridsystem isproposedinthisresearch.Unlikemayothermethods[53,52],theproposed schemedoesnotrequireacompleteshutdownofthegrid.Rather,onlythe aectedsectionofthemicrogridisisolatedandde-energized.Thisisachieved throughuseofaringbuscongurationforthemainDCbus,creatingseveral zonesofprotectionwithintheringbus,andinstallingagroundingresistorto limitthefaultcurrent.Thisisdoneforboththepositiveandnegativebus.The proposedprotectionschemecanbesplitintothreesections: FaultDetectionandIsolation BreakerFailureDetection RecloseandRestore Theringbuswillbesplitintozonesandeachzoneismonitoredbyan IntelligentElectricalDeviceIED.TheIEDwillcontinuallymonitorthecurrent throughit'sassignedbreakers.OnceafaultisdetectedtheIEDwillopenthe zonebreakers.TheIEDwillthenensurethatallofthebreakershaveopenedand thatthefaultedzoneisde-energized.Ifthezonehasnotbeende-energizedthe zoneIEDwillsendsignalstoadjacentIEDuntilthefaultisextinguished.Ifthe zonewassuccessfullyde-energizedtheIEDwillattempttorestorethefaulted zonebyreclosingthebreakers.Ifafaultisthendetectedthezonebreakersare 33

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againtrippedandthezoneisisolated.Aowchartoftheprotectionlogiccan beseeninFig.4.2. 4.1.1FaultDetectionandIsolation Themicrogridunderstudyconsists12zonesofprotection.Eachzoneis classiedbythetypeofenergydeviceithasbeenassignedto.Thezonescan besplitinto4categories:uni-directional,bi-directional,load,andlinkwhich isshowninFig.4.1.Eachzoneconsistsof2-3breakersandasectionofcable. AlocalIEDisassignedtoeachzone.TheIEDwillmonitorandcontroleach ofthebreakerswithinitsassignedzone.EachIEDisprogrammedwiththe specicsetofrulesthatdeneanormalzoneoperation.Thisisdependenton thesourcethattheIEDismonitoring.Itshouldbenotedthatduetothering busconguration,currenthasseveralpathstoow.Thereforeseveralnormal operatingconditionsmustbeaccountedfor,andtheymustallfailbeforeafault isdeclared.Oncefaulthasbeendetectedallbreakersintheaectedzoneare tripped,regardlessofthepolethefaultison.Thisisdonetokeepthemicrogrid andconvertersinbalance.Intheexamplesspecicrulesthatareneededto deneafaultareprovided.Oneruleforeachzonetypeisgiven. Oncethefaultedsegmentisisolated,theremainderofthesourcesandloads cancontinuetooperateontheringbus.Evenwithmultiplefaultedsegments, thesystemcanoperatepartiallyifthesegmentsfromthemainsourcetosome loadsareintact.Ithasbeenassumedthatthesegmentcontrollerscandetectit andopen/closeSolidStateCircuitBreakersin500 sec .Forexample,turn-o timeforanIGCTisapproximately11 sec 34

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Figure4.1: MicrogridProtectionZones 35

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Figure4.2: AlgorithmFlowChart 36

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Uni-DirectionalSourceZonesZone5Example I B 7 = I B 8 + I B 9 .1 I B 8 = I B 7 + I B 9 .2 I B 9 = I B 7 + I B 8 .3 Bi-DirectionalSourceZonesZone3Example { DischargeMode I B 3 = I B 4 + I B 5 .4 I B 4 = I B 3 + I B 4 .5 I B 5 = I B 3 + I B 4 .6 { ChargeMode I B 3 = I B 5 )]TJ/F18 11.9552 Tf 11.955 0 Td [(I B 4 .7 I B 4 = I B 3 + I B 4 .8 I B 5 = I B 3 )]TJ/F18 11.9552 Tf 11.955 0 Td [(I B 4 .9 37

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LoadZonesZone7Example I B 10 = I B 12 )]TJ/F18 11.9552 Tf 11.955 0 Td [(I B 11 .10 I B 11 = I B 10 )]TJ/F18 11.9552 Tf 11.955 0 Td [(I B 11 .11 I B 12 = I B 10 )]TJ/F18 11.9552 Tf 11.955 0 Td [(I B 11 .12 LinkZonesZone8Example I B 12 = I B 13 .13 I B 13 = I B 12 .14 38

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Figure4.3: Uni-DirectionalZone:NormalandFaultedCurrentFlow Figure4.4: Bi-DirectionalZone:NormalandFaultedCurrentFlow Figure4.5: LinkZone:NormalandFaultedCurrentFlow 39

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4.1.2BreakerFailureDetection EachzoneIEDcontinuallymonitorsthestatusopen/closedontheirrespectivezonebreakers.Undernormalconditionsthiscanbeusedforinformation purposestooperatorsofthemicrogrid.Onceafaultinsideazoneisdetected andtripsignalsaresent,theIEDwaits1secondthenentersbreakermonitoring mode.Thebreakermonitoringmodeoperatesundertwodierentconditions: StatusandCurrent Current 4.1.2.1StatusandCurrent IntheStatus/CurrentconditionsthezoneIEDagsthatatriptoallbreakershasbeensent.TheIEDthencheckstoseeifallofthebreakersareshowing aclosedstatus.IfabreakerstatusisclosedandtheIEDexpectittobeopen abreakerfailconditionissuspected.TheIEDmustalsoconrmthatcurrent continuestoowisthefaultedzone.Ifboththeclosedstatusandcurrentinthe zonearedetectedthefaultedzoneIEDwillthensendasignaltotheappropriate zonecontrollertotripitszonebreakers.Forexample,ifafaultisdetectedin Zone4,butBreaker5fails,thenasignalfromtheZone4IEDwouldbesentto theZone3IED.TheZone3IEDwouldtripitsassociatedbreakersFig.4.1.At thispointbothZones3and4havebeende-energizedandlockedout.Locking outthezonemeansthatthecontrollerswillnottrytoautomaticallyreclose andrestorethezones.Restoringthezonesafteralockoutconditionrequiresa 40

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Figure4.6: ZoneControllerConnections manualrestore,afterthefaulthadbeenremovedfromthesystem.Thisconditionrequirestwopiecesofinformation,butextinguishesthefaultwiththeleast amountofimpacttotheDCbus. 4.1.2.2Current Ifallofthebreakersinthefaultedzoneareprovidinganopenstatus,but theIEDcontinuestoreadcurrentinthezonethenalloftheadjacentzoneswill betripped.UsingtheZone4example,butthistimeallofthebreakersprovide anopenstatus.TheZone4IEDwouldsendsignalstoboththeZone35IED's, de-energizingZones3,55Fig.4.1.Thisconditionensuresthatthefaultwillbe extinguishedeveniftheIEDreceivesafalsestatusfromthebreakers.However, itrequireslargesectionsofthegridtobede-energized. 41

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4.1.3RecloseandRestore Oncethefaultedzonehadbeentrippedandnoneofthebreakersfailed,the IEDwilltherecloseandrestore.Often,faultsaretemporaryduetodebrisor animalscomingincontactwiththecableorline.Thetemporaryfaultswill clearthemselvesaftercurrentowsthroughtheunwantedgroundsource.The recloseandrestoremodeallowstheIEDtoautonomouslyrestorepowerback tothede-energizedzone.Thisisdonebywaiting1secondafterthetripsignals havebeensent.Afterthat1secondtheIEDwillsendclosesignalstoallofthe breakers.Ifthefaulthasbeensuccessfullycleared,themicrogridwillcontinue torunnormally.However,ifthefaultispermanentanditisdetectedafterthe rstreclose,allofthezonebreakerswillagainbetrippedandthezonewillbe lockedout. 4.2SolidStateCircuitBreakers DuetothelimitationsoffusesandtraditionalcircuitbreakersinDCsystems,asolidstatecircuitbreakerisutilized.Whenselectingasolid-statecircuit breakerthereareseveraloptions:GTO,IGBT,andIGCT.GTOsoerahigh blockingvoltagecapabilityandalowonstarevoltage,butsuerfromslower switchingspeeds[26,1].IGBTsarewidelyusedinthelowvoltage < 1200 V systems[25].IGBTsoerfastinterruptiontime secandanabilitytowithstandshortcircuits[1].ThedisadvantagewhenusingaIGBTisthattheysuer fromhighconductionlosses[1,49,51,26].IGCTsoerthelowerconduction lossesofathyristorwiththeturn-ocapabilityofatransistor.LiketheIGBT, 42

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Figure4.7: aBi-directionalIGBTcircuitbreaker.bBi-directionalIGCTcircuit breaker. theIGCToershighvoltageandcurrentratingsbutdoesnotsuerfromhigh conductionlosses.TheIGCThasaslowerswitchingspeedthantheIGBT,but whenusedasacircuitbreakerismaynotbeaconcern[51,49].Inorderfor themicrogridtoallowpowerowineitherdirectiontheIGCT-CBneedsto bebi-directional.Thebi-directionalIGCT-CBactuallyconsistsoftwoIGCTs placedinserieswithoneopposingtheotherFig.4.7. 4.2.1Grounding Themagnitudeofagroundfaultcurrentisdependentonthedistancefrom thesourcethatthefaultoccursandtheresistanceofthegroundfaultpath.The faultcurrentfromthesourceanconvertercapacitorscanbegivenas.15. I fault t = E s R eq )]TJ/F18 11.9552 Tf 11.956 0 Td [(e )]TJ/F20 5.9776 Tf 7.782 4.689 Td [(R eq L eq t + E s R c e )]TJ/F20 5.9776 Tf 17.746 3.258 Td [(t R c C eq .15 Where E s isthelinevoltage, R eq and L eq istheequivalentsystemresistance andinductance,and R c and C eq istheequivalentseriesresistanceandequivalentcapacitanceoftheconvertercapacitors.Itcanbeseenin.15thatthe faultcurrentmagnitudedeterminedby R eq .Thevalueof R eq canbealtered dependingonthegroundingmethodofthesystem.Thegroundingoptionsare: 1solidgrounding;2low-resistancegrounding;3high-resistancegrounding; 43

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and4ungrounded[16].Althoughungroundedsystemsareusedinsomeapplicationstoavoidtheeectoflow-resistancepole-to-groundfaultandstray current,ungroundedsystemsaresensitivetochangesinthegroundingplane andcanbedangerousespeciallyunderabnormalfaultconditions[27,45].The advantagesofthegroundinginaDCdistributionsystemincludepredictable operatingconditions,minimumvoltagestressforthesystemcomponents,and easierfaultdetection[27].Theline-to-groundfaultsarethemostcommontypes offaultsinindustrialdistributionsystems[16]andthegroundfaultcurrentcan belimitedbyusingtheresistancegrounding.Sincethetypicalpowerelectronics convertersconnectedintheLVDCsystemscannotfeedlargefaultcurrents,it wouldbebenecialtoreducethefaultcurrenttoanappropriatelevelfordetectionandextinction.However,someprotectivedevicesarestillneededeven withthislowresistancegroundingscheme,becausethefaultcurrentcannotbe sustained[16]. Itisacommonpracticetogroundpowersystemsatonepointonlyandas closetothesourceaspossible[27].Multiplegroundpointscouldformunnecessarycirculatingcurrentpaths.PossiblegroundingpointforaDCsystemis eitheroneofthepolesorthemidpointofthebus,andithasbeenreported thatthebalancedDCsidegroundingsignicantlyreducescirculatingcurrent comparedtotheACsideneutral-groundedsystem[27].Althoughtheground resistorscanbeusedtodetectthegroundfaultaswell,itisnotabletoidentify thelocationofthefaultbecauseofthesinglegroundpointpractices. Inthispaper,aresistancegroundingtothebalancedDCbusmidpoint hasbeenchosenforawell-denedpole-to-groundvoltagesandrobustnessto 44

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imbalance[47,27].Theresistorwassizedsothatthefaultcurrentwouldnot drasticallyexceedtheloadcurrentofthesystem.Sizingagroundingresistor foraDCsystemonlyrequirestheuseofOhmslaw,asseeninequation4.16 R gr = E s I f .16 Where R gr isthegroundingresistorsize, E s isthesystemvoltageand I f is thedesiredfaultcurrentlevel. 45

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5.SimulationResults Toverifytheproposedprotectionscheme,computersimulationshavebeen performedusingOrCAD/PSpice.Themicrogridmodelcanbeseenin5.1SimulationparameterscanbefoundinTable5.1Thealgorithmwasprogrammed andsimulatedusingMATLABandcanbefoundintheAppendix.TheOrCAD/PSpiceguresillustratethefaultcurrentintheaectedzones;whilethe MATLABguresshowtheIEDcontrolsignals.Anegativepoleline-to-ground faultinzone1issimulatedat9.5 msec .Fig.5.2showstheline-to-groundfault currentwhennogroundingresistancehasbeeninstalled.Itcanbeseenthatthe currentmagnitudeinthezonedrasticallyincreasesfrom50Aofloadcurrentto 18kA. Next,thegroundingresistorwasinstalledattheneutralpointoftheAC gridconverter.Fig.5.3illustratestheaectthegroundingresisancehasonthe faultcurrentmagnitude.Itcanbeseenthatthegroundingresistorlimitedthe faultcurrentfrom18kAtojustover200A.InFig.5.7itcanbeseenthatatthe eventofthefaultat9.5 msec theZone1IEDdetectsthefaultandsendtrip signalstotheZone1breakers.ThestatusofthebreakerscanbeseeninFig. 5.6.ThevoltageattheACandDCloadscanbeseeninFig.5.4. 46

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Figure5.1: OrCAD/PSpiceSimulationModel 47

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Table5.1: SimulationParameters. NameValue VDC 350 V R LOAD 1 : 7 R LINE 1 : 6 m L LINE 640 nH R LINK 0 : 63 m L LINK 256 nH R GND 1 : 75 48

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Figure5.2: AnalyticalSimulationCircuitforLine-to-GroundFaultinZone1 WithoutResistanceGrounding. Figure5.3: AnalyticalSimulationCircuitforLine-to-GroundFaultwithResistance Grounding. 49

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Figure5.4: VoltageatACandDCLoadsduringaLine-to-GroundFaultwith ResistanceGrounding 50

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Figure5.5: CurrentatACandDCLoadsDuringaLine-to-GroundFaultwith ResistanceGrounding 51

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Figure5.6: Zone1CircuitBreakerStatusDuringaLine-to-GroundFault 52

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Figure5.7: Zone1IEDTripSignalsDuringaline-to-GroundFault 53

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Figure5.8: Zone4CurrentwithBreaker5FailingtoOpen AbreakerfailureconditionwassimulatedinZone4.Anegativepolelineto-groundfaultwasplacedinZone4,butbreaker5failstoopen.Atthispoint abreakerfailconditionisrecognizedbytheZone4IED,sendingasignalto theZone3IEDtotripbreakers3and4.Fig.5.8showsthefaultcurrentinthe zoneuntilthebreakerfailsequenceiscomplete.In5.9thecurrentinZone3is provided.Fig.5.10andFig.5.11providesthestatusoftheZone3and5breakers, respectively. 54

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Figure5.9: Zone3withBreaker5FailingtoOpen Figure5.10: Zone4BreakerStatuswithBreaker5FailingtoOpen 55

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Figure5.11: Zone3BreakerTripSignalsAfterBreaker5FailstoOpen 56

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Figure5.12: DCVoltageatLoadsDuringaZone4BreakerFail 57

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Figure5.13: CurrentatLoadsDuringaZone4BreakerFail 58

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Figure5.14: Zone1CurrentDuringaRecloseandLockoutCycle Finally,thereclosesequencecanbeseeninFigs.5.14,5.15,5.16and5.17. Again,afaultissimulatedinZone1at9.5 msec ,andtheZone1IEDdoesnot detectabreakerfailcondition.TheIEDthenenterstherecloseandrestore mode,andclosesbreaker1,2and20at12 msec .Inthiscaseitwassimulated thatthefaultdidnotclear.Therefore,whenthebreakersarere-closedifthe faultisdetected,theIEDtripsandlocksoutthezonebreakers. 59

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Figure5.15: Zone1RecloseCycleTripSignals 60

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Figure5.16: Zone1RecloseCycleCloseSignals 61

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Figure5.17: Zone1BreakerStatusDuringaRecloseandLockoutCycle 62

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6.ConclusionandFutureWorks Withthenewinterestingreenenergy,thesmartgridanddistributedgenerationmicrogridsmaysoonbecomeanintegralpartofourelectricgrid.DC microgridshaveproventobeaviablecompetitortoACmigrogrids.Protection oftheDCbusisaintegralparttotheDCmicrogrid,andmustbeabletoisolate faultswithminimalimpacttotheoverallsystem.ItcanbeseeninChapter3 thatthecurrenttechniquesrequireacompleteshutdownoftheDCbus.This isnotsuitableforcriticalloads. ThisresearchproposesanewfaultdetectionandisolationschemeforlowvoltageDC-busmicrogridsystem.Aringbusbasedmicrogridsystemwasutilized.TheproposedprotectionschemeconsistsofzoneIED'swhicharecapable ofdetectingabnormalfaultcurrentintheringbussegmentandisolatingthe segmenttoavoidtheentiresystemshutdown.Theringbuswasseparatedinto overlappingzoneswithIED'smonitoringeachzonehasbeenproposed.Thering busallowsmultiplepathsforpowertoowwhenasectionhasbeenisolated. Overlappingthezonesreducedtheamountofcircuitbreakersneededinthemicrogrid.Theuseofresistancegroundingwasutilizedinordertolimitthefault current,toprotectthesourceconvertersandalsoallowtheIEDenoughtimeto detectandisolatethefault.Successfulfaultdetectionandisolationwasshown usingcomputersimulations. 63

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Thoughthefaultdetectionandisolationprovessuccessfulforsuppressing faultcurrent,locatingthefaultedzoneandisolatingthezoneforline-to-ground faults,line-to-linefaultswillstillcreateverylargefaultcurrent.Thisisbecause thefaultconsistsoftwosourcespositiveandnegativeandthegroudingresistor hasnoinuenceonthefaultcurrent.Creatinganalgorithmorcontrolscheme todetectandlimitaline-to-linefaultisanissuethatshouldbeaddressed. Also,whenafaultoccursandasourceisremovedfromthemicrogrid,the remainderofthesourcesmustaccommodatetheload.Determingarealtime loadowcontrolschemeforthemicrogridwouldimprovestabilityinthegrid andmaximizeeciencyfromallofthesources. 64

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