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A Control algorithm for the stable operation of islanded microgrid with dynamic loads

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Title:
A Control algorithm for the stable operation of islanded microgrid with dynamic loads
Creator:
Jeong, Hwanmin
Donor:
Matthew Mariner
Place of Publication:
Denver, CO
Publisher:
University of Colorado Denver
Publication Date:
Language:
English

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Degree:
Master's ( Master of science)
Degree Grantor:
University of Colorado Denver
Degree Divisions:
Department of Electrical Engineering, CU Denver
Degree Disciplines:
Electrical engineering
Committee Chair:
Park, Jaedo
Committee Members:
Radenkovic, Miljoe
Dey Satadru

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University of Colorado Denver
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Auraria Library
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Full Text
A CONTROL ALGORITHM FOR THE STABLE OPERATION OF ISLANDED
MICROGRID WITH DYNAMIC POWER LOADS
by
HWANMIN JEONG BS, Chosun University, 2015
A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Master of Science Electrical Engineering
2018


This thesis for the Master of Science degree by Hwanmin Jeong has been approved for the Department of Electrical Engineering Program
by
Jaedo Park, Chair Miloje Radenkovic Satadru Dey
‘Date’ July 28, 2018
n


Jeong, Hwanmin (M.S., Electrical Engineering)
A Control Algorithm for the Stable Operation of Islanded Microgrid with Dynamic Power Loads
Thesis directed by Associate Professor Jaedo Park
ABSTRACT
An isolated DC microgrid is simulated with photovoltaic (PV) as the main source to resistive dynamic DC loads along with Battery. The PV generators have nonlinear I-V and P-V characteristics. To enhance the conversion efficiency of PV arrays, it requires maximum power point tracking (MPPT) control. The perturb and observe (P&O) method is implemented to track the maximum power point. The algorithm regulates the battery charge, hold and discharge operations by DC-DC bidirectional converter depending on the power of PV and load. The different load power was applied to approve the proposed power management strategy such as PV supplying the load and charging the battery and PV-Battery both supplying the load. The output parameters such as power, voltage and current and the power flow are plotted and analyzed.
The form and content of this abstract are approved. I recommend its publication.
Approved: Jaedo Park
m


DEDICATION
This file is dedicated to my family, who support me to finish writing my thesis and support me for the Master program.
IV


ACKNOWLEDGMENTS
My deep gratitude goes first to Professor Jaedo Park, who expertly guided me through my master program and support me to finish this thesis. My appreciation also extends to my colleagues in my laboratory and my office. Gadi Ogbogu, Md Habib Ullah, Bhanu Shankar, Hector Campos and Muhanand Alaraj helped sustain a positive atmosphere and encouraged me all the time. I appreciate for their golden heart.
v


TABLE OF CONTENTS
CHAPTER
I. INTRODUCTION ....................................................... 1
II. DC MICROGRID CONFIGURATION......................................... 3
2.1 PV Generator.................................................. 5
2.2 Battery...................................................... 10
2.3 Power Electronics Converters................................. 11
2.3.1 DC/DC Boost Converter .................................. 11
2.3.2 DC/DC Bidirectional Converter........................... 14
2.3.3 Non-Isolated Bidirectional DC-DC Converter.............. 15
2.4 Proportional-Integral (PI) Control........................... 16
III. SYSTEM MODELING AND CONTROL METHODS............................ 18
3.1 PV System Modeling........................................... 18
3.1.1 Estimating the Maximum Power............................ 18
3.1.2 MPPT Modeling........................................... 20
3.2 Battery System Modeling...................................... 21
3.3 Dynamic Load Modeling ............................... 24
3.4 Control Logic ............................................... 25
IV. SIMULATION AND RESULTS........................................... 26
4.1 Description of Simulation Circuit............................ 26
4.2 Current Flow................................................. 27
4.3 Bus Voltage ................................................. 29
4.4 Power........................................................ 30
4.4.1 PV Output Power......................................... 30
4.4.2 Battery Output Power.................................... 31
4.4.3 Load Power.............................................. 32
4.5 Battery SOC.................................................. 33
vi


4.6 Duty Cycle...................................... 34
V. CONCLUSION.......................................... 37
REFERENCES............................................. 38
vii


TABLES
TABLE
2.1 PV module datas.............................................. 7
2.2 Battery parameters........................................... 11
2.3 Effects of controllers Kp and Ki on a closed-loop system..... 16
viii


FIGURES
FIGURE
2.1 Microgrid configuration................................................ 3
2.2 An Example of PV-based Islanded microgrid.............................. 4
2.3 PV cell equivalent circuit............................................. 5
2.4 V-I characteristic..................................................... 7
2.5 P-V characteristic..................................................... 8
2.6 P&O flow chart......................................................... 9
2.7 Simplest battery equivalent circuit................................... 10
2.8 Boost Converter Circuit............................................... 11
2.9 Switch on and D oh.................................................... 12
2.10 Switch oh and D on.................................................... 12
2.11 Bidirectional DC-DC Converter Structure............................... 14
2.12 Non-isolated Bidirectional DC-DC Converter Circuit................. 15
2.13 PI Controller in time domain.......................................... 16
3.1 Maximum Power Reference Block Diagram................................. 19
3.2 PV Circuit with Control Block Diagram................................. 20
3.3 P&O Controller........................................................ 20
3.4 Battery circuit with controller....................................... 21
3.5 Controller for S2 of Bidirectional DC-DC Converter.................... 22
3.6 Controller for SI of Bidirectional DC-DC Converter.................... 22
3.7 Switching controller depending on the load power demands.............. 22
3.8 Battery Control Flow Chart............................................ 23
3.9 Dynamic load blockdiagram............................................. 24
3.10 Dynamic load blockdiagram............................................. 24
4.1 Schematic of islanded microgrid developed in Simulink ................ 26
4.2 Microgrid model with node and currents................................ 27
IX


4.3 (a)PV output current (b)Battery output current (c)Load current . . 28
4.4 Bus voltage.......................................................... 29
4.5 PV output power with power reference................................. 30
4.6 Battery output power................................................. 31
4.7 (a)Power reference demand (b) Load power demand...................... 32
4.8 Battery SOC.......................................................... 33
4.9 Switch 2 duty cycle ................................................. 34
4.10 Switch 1 duty cycle ................................................. 35
4.11 Pulse wave........................................................... 36
x


I. INTRODUCTION
Modern society relies on a secure supply of energy. Due to the increment of demand for reliability and quality issues of power system, the concern for primary energy availability of current electrical transmission is rapidly increasing [1], Microgrid is a cluster of electricity sources, controllable loads and energy storage. The performance of ac microgrids during the past decade has been remarkably improved. At the same time, dc microgrid has been considered as more attractive for various uses due to the simpler control structure, better compliance with many electronics, and higher efficiency for the distribution [2], Also, there are no issues such as power quality, frequency regulation, and reactive power, which make a simpler control system [3]. The disconnection of distributed generators to distribution networks is the challenges in power system [4],
Microgrid can be operated with two different modes: an islanded mode and a grid-connected mode. The islanded microgrid is separated from the main grid. The stability and operation of the system are determined by the generators in the microgrids. Since the distributed generators based on renewable energy and the energy storage systems are integrated by power electronic equipment, the control and operation of the islanded microgrid is challenging. Therefore, various analyses are required to determine if the islanded microgrid works in a stable situation. Photovoltaic system is usually programmed to operate according to the maximum power point tracking algorithm due to V-I characteristics and P-V characteristics. Meanwhile, the role of energy storage system is to regulate the bus voltage and maintain the power balance between supply and demand by charging and discharging processes [5].
In this thesis, the islanded microgrid with dynamic load is designed. Chapter II introduces the configurations of the dc microgrid such as photovoltaic system, battery and power electronics converters. Then, in Chapter III, the configurations and PI controller are mathematically modeled. Also, the algorithm for the stable operation
1


of islanded microgrid is introduced. Chapter IV shows the voltage, current, power and State-of-Charge (SOC) plots. Finally, Chapter V addresses the conclusions.
2


II. DC MICROGRID CONFIGURATION
Microgrids comprise the low voltage(LV) distribution networks with distributed energy resources (DER) that are diesel generators, photovoltaic (PV), wind power, etc with different kind of loads and storage devices that are batteries, capacitors, etc. Microgrid systems can be stably operated in a self-governing way if the systems are disconnected to the grid, or in a non self-governing way if the systems are interconnected from the main grid.
Gas / Diesel
Figure 2.1: Microgrid configuration.
DC microgrid systems compared with AC microgrid systems have the potential advantages: higher efficiency due to less conversion steps, better stability since DC system has no synchronization problem, better power quality as reactive power is not
3


transferred in DC system, and higher reliability because fewer components will have higher mean time between failures (MTBF).
In an islanded microgrid, the utility is disconnected and the microgrid is operated with sources within the microgrid. Since renewables are intermittent and the loads can change continuously, the source-load balancing in a microgrid is a challenge; as there is no inertia in a power system which consists of only renewable energy sources. Rotating generators and energy storage are used for managing the demand variation.
Power Energy
PV - generator Optimizer Controller Facility - load
Battery - storage
Figure 2.2: An Example of PV-based Islanded microgrid.
4


2.1 PV Generator
Figure 2.3 shows the equivalent circuit of PV cell. In the equivalent circuit, a current source which has the amount of Iph, an ideal diode along with parallel resistance and series resistance as the sheet resistance and the contact resistance on the surface of PV cell.
/
Figure 2.3: PV cell equivalent circuit.
The output current of the PV is given by
Ieq = Isc ~ Id ~ hh, (2.1)
Also, the diode current is shown as
l(^/pvJr I Rs)
ID = I0(e AksT _i), (2.2)
The resistant current is shown as
Vr>
Ish = (2.3)
Dsh
Thus, when the number of parallel and series PV cell are considered, the output current of PV array can be expressed by
5


Ipv = Nplsc ~ NpIo(e NsAkB
cl(VpvJrI Rs) NqAknT
(2.4)
where:
Np is the number of parallel PV cells in the PV array.
Ns is the number of series PV cells in the PV array.
Vpv is output voltage of the PV array.
Ipv is output current of the PV array. q is the electronic charge.
I sc is the light-generated current.
Ia is the saturation current.
A is a dimensionless number. k is the Boltzman constant.
T is the temperature in Kelvin.
The more details of I sc and IQ can be shown with equations 2.5 and 2.6.
Tref is the base temperature 298 [k], Isco is the short-circuit current on the base temperature, S is irradiation, J is temperature coefficient of Isc, Eg is band energy gap, A is temperature coefficient of IQ. PV module example was used in Matlab Simulink . On Matlab, temperature and irradiation value can be applied. The parameter of PV generator is shown in the table 2.1.
The characteristic of V-I and P-V from Matlab Simulink are shown in Figures 2.4 and 2.5. To supply the maximum power, MPPT control is designed. MPPT control
Cs'c — + J{T - Tref)
(2.5)
(2.6)
6


Table 2.1: PV module datas
Parameters Values
Maximum Power 213.15 W
Voltage at Pmax 29 V
Current at Pmax 7.35 A
Voltage with open circuit 36.3 V
Current with short circuit 7.84 A
Temperature coefficient of Voc -0.36099 %/deg.C
Temperature coefficient of Isc 0.102 %/deg.C
has diverse methods: constant voltage tracking method, incremental conductance method, variable step size method, and perturb and observe method. The P&O method is one of the most commonly used methods in practice.
Array type: ISoltech 1STH-215-P; 7 series modules; 5 parallel strings
Figure 2.4: V-I characteristic.
7


Array type: ISoltech 1STH-215-P; 7 series modules; 5 parallel strings
Figure 2.5: P-V characteristic.
The P&O algorithm operates by perturbing the increase or decrease of the array terminal voltage and comparing the PV output power with the previous value. If the array terminal voltage changes and the PV output power increases, the PV array operating point is controlled in that direction. Otherwise the PV array operating point moves in the opposite direction.
The logic of the P&O algorithms and the flowchart are explained in Figure 2.6. The voltage of PV array is perturbed every cycle. This algorithm keeps perturbing the operating voltage by a small increment of voltage AV , and this resulting change in AP.
AV = V(k) - V(k — 1), (2.7)
AP = P(k) - P(k-l), (2.8)
8


Figure 2.6: P&O flow chart.
If AP is positive value, the operating voltage needs to move in the same direction of the increment. In contrast, if AP is negative value, the operating voltage needs to move in the opposite direction of the increment. Thus, when the maximum power point is reached, the output power makes oscillation around the maximum [6].
9


2.2 Battery
The MATLAB model of Lithium-Ion battery is used for simulation. A zero-time-constant circuit as shown in Figure 2.7 is the simplest equivalent circuit model of the Li-Ion battery. This model represents the static behavior of the system. The SOC is defined in equation 2.9.
SOC= Cc^rrent 100%, (2.9)
C/«//
where Ccurrent is the amount of current charge availability in the cell and Cfuu is the capacity in the same cell [7].
Rs
Figure 2.7: Simplest battery equivalent circuit.
The temperature in practice effects on the discharge behavior. The chemical activity decreases and the internal resistance of the battery increases when the temperature is below 25 Celsius degrees. On the contrary, the capacitance of the charging decrease when the temperature is over 25 Celsius degrees. In this paper, the temperature is not considered for the ideal condition of the battery. Thus, the battery is charged when the current direction is to the battery without any loss. Also, the battery discharges when the current flows to the bus.
The battery model is a lithium-acid battery with a nominal voltage of 170 V. The simulation parameters used in Simulink are shown in Table 2.2. The initial SOC is set as 80%.
10


Table 2.2: Battery parameters.
Parameters Values
Nominal voltage 170 V
Rated capacity 200 Ah
Initial state of charge 80 %
2.3 Power Electronics Converters
2.3.1 DC/DC Boost Converter
The boost converter is to step up the output voltage from the input voltage [8]. The converter typically uses a MOSFET in parallel with a diode as a transistor switch.
Figure 2.8: Boost Converter Circuit.
11


iL
Figure 2.9: Switch on and D off.
When the MOSFET is on as shown Figure 2.9, the current is
ipk io AO
( 0,, V’rran ) Tq,
L
iL
Figure 2.10: Switch off and D on.
And when the MOSFET is off as shown Figure2.10, the current is
(2.10)
12


ipk i-o — —
(Vout — Vin + Vp)T0ff L
(2.11)
where Vd is the voltage drop across the diode, Varans is the voltage drop across the transistor.
By deriving equations 2.10 and 2.11, Vout can be solved
(bin Vrran)Tan ___ (Vout Vin T V]j^)T0ff
L
L
(2.12)
V,nTan — VrransTon — V0utT0ff — VinT0ff + V]jT0ff (2-13)
VinTan + V,nT0ff — V0UtT0ff + VrransTon + Vr)T0ff (2-14)
Vin ~ VrransD — (Vout + Vd)( 1 ~ D)
(2.15)
t t Vin Vrrans D
Vout = ^ Jyj VD
If the voltage drops are negligible, Vout can be expressed
(2.16)
Vr.
Vir
out
1 - D
From equation 2.17, the duty cycle can be shown as
(2.17)
D = 1 -
Vir
Vr.
out
(2.18)
Thus, the output voltage is related to the duty cycle of the pulses directly.
13


2.3.2 DC/DC Bidirectional Converter
The basic concept of most bidirectional DC-DC converter can be illustrated as the generic circuit structure in the Figure 2.11, which characteristics power flows in forward and backward direction. The bidirectional DC-DC converter is used for charging and discharging energy storages in general. The bidirectional DC-DC converter operates as two types of converters; boost type converter and buck type converter [9]. The boost type converter has energy storage placed on the low voltage side, and the buck type converter has energy storage placed on the high voltage side.
Forward Power Flow
' >
+
11
V1
12
Bidirectional V2
DC-DC Converter
+
<1
Backward Power Flow
Figure 2.11: Bidirectional DC-DC Converter Structure.
The power flow in bidirectional DC-DC converter is defined by the direction of the current. To carry the current on both directions, the bidirectional DC-DC converter is implemented with a unidirectional semiconductor switching devices such as MOSFET and IGBT in parallel with a diode. Basically, there are two types of the bidirectional DC-DC converters, non-isolated and isolated converters [10] [11].
14


2.3.3 Non-Isolated Bidirectional DC-DC Converter
The buck and boost type DC-DC converters are used in the transformer-less non-isolated power conversion systems without any isolation between input and load [12]. Even though the isolated conversion system between the source and load sides is attractive from high frequency conversion applications but non-isolated conversion system is much more attractive from the efficiency, cost, weight and size.
Figure 2.12: Non-isolated Bidirectional DC-DC Converter Circuit.
The topology of the non-isolated bidirectional DC-DC converter is shown in the Figure 2.12. When the current flows from the left side to the right side (high voltage to low voltage) by controlling the duty cycle for two switches, the bidirectional DC-DC converter operates as the buck converter. On the contrary, when the current flows from the right side to left side, the bidirectional DC-DC converter operates as the boost converter.
15


2.4 Proportional-Integral (PI) Control
The transfer function of the PI controller can be expressed by equation 2.19.
G(s) = Kp+Ij (2.19)
The basic block diagrams of PI controller with time domain is shown in the Figure 2.13.
Figure 2.13: PI Controller in time domain.
The effect of Kp is to reduce the rise time and reduce the steady state error, but never eliminate. The effect of Ki is to eliminate the steady-state error. The effects of each of controllers Kp, Ki on a closed-loop system are shown in the Table 2.3.
Table 2.3: Effects of controllers Kp and Ki on a closed-loop system.
Closed Loop response Rise time overshoot settling time Steady-state error
KP decrease increase small change decrease
/c small change increase increase eliminate
Even though the effects can be defined, but Kp and Ki are dependent of each
other. Thus, these correlations might not be accurate. For this reason, to determine
16


the values for Kp and Ki} this table should be used as a reference.
The Pi-controller parameters of non-isolated bidirectional DC-DC converter are set by the Ziegler-Nichols tuning method [13]. The controller design steps by the Ziegler-Nichols tuning method are as follows [14].
First, the proportional gain Kp and the integral gain Ki are set at zero. Second, the value of Kp increase step by step while Kj is fixed at zero until the waveform of the voltage output is close to the reference value. Last, the value of Ki increase while Kp is set at which the second step found.
17


III. SYSTEM MODELING AND CONTROL METHODS
3.1 PV System Modeling
3.1.1 Estimating the Maximum Power
Osterwalds method is one of the most used classical methods that provides statistic factory results [15]. This method is using the solar panel testing conditions (STC).
Pmax = Pmax,STC~X,---[1 ~ j(TC ~ 25)] (3.1)
^STC
where:
Pmax is a cell maximum power (IT),
Pmax,stc is a cell maximum power in STC (IT),
G is the solar irradiance (IT/m2),
Gstc is the solar irradiance in STC (W/m2),
Tc is the cell temperature,
7 is the cell maximum power temperature coefficient (%/deg.C).
Coefficient 7 is expressed as
7
hoc,7 I sc, 7
hoc Tc
Thus, the PV module power can be shown as
(3.2)
Pmaxg — NsNpPmax (3.3)
Ns and Np are series and parallel cells number of PV module. In this thesis,Pmaxg based on the Table 2.1 is calculated. Also, this power is used as the power reference for the P&O method.
18


Figure 3.1: Maximum Power Reference Block Diagram.
In the simulation, the maximum power reference block diagram is shown in the Figure 3.1.
The maximum power reference based on the PV parameters from Table 2.1 can be calculated by equation 3.4.
Pmaxref = 7*5* 213.15(IT) *
0.102 (%/deg.C) * 7.84(A)
1000(W/m2)
1000(W/to2)
(1
—0.36099 (%/de(/. (7)
36.3(D)
* (25deg.C — 25deg.C))
(3.4)
Thus,
Pmaxref is found as 7460.25 W.
19


3.1.2 MPPT Modeling
The block diagram of the Figure 3.2 represents PV array, a boost type power converter and an MPPT control module for the DC/DC boost converter.
Ipv
Figure 3.2: PV Circuit with Control Block Diagram.
MPPT block includes P&O block controller. P&O algorithm operates by the increase or decrease of the PV output voltage, and the comparison with the PV output power and the previous value.
Figure 3.3: P&O Controller.
20


The P&O method block diagram shown in the Figure 3.3 is built based on the P&O flow chart [6]. In the block diagram, PWM is added to generate the duty cycle to the switch of the converter.
3.2 Battery System Modeling
The block diagram of the Figure 3.4 represents Battery, a bidirectional DC-DC converter, PI controller and the algorithm to choose the charging signal or discharging signal.
Figure 3.4: Battery circuit with controller.
Both the Figures 3.5 and 3.6 show two PI controllers for S2 and 51. For all the PI controllers, the gains, Kp and Ah, are decided by the Ziegler-Nichols tuning method [13]. The error between the voltage reference and the output voltage is converted by Kp and Ah for the voltage control. After the PI controller for the voltage, the current reference is created. The error between the current reference and the battery current is converted by Kp and Ah for the current control. Then, the duty cycle is made by PWM with the output signal from the PI controller for the current, cl.
21


Figure 3.5: Controller for S2 of Bidirectional DC-DC Converter.
Figure 3.6: Controller for SI of Bidirectional DC-DC Converter.
The block diagram shown in the Figure 3.7 decides whether to send the duty cycle of 51 or 52 to operate the MOSFET based on the subtraction with the PV output power and the load power demand.
Figure 3.7: Switching controller depending on the load power demands.
The flow chart to control the battery between charging and discharging is represented in the Figure 3.8. If the load power demand is higher than the PV output power, 51 is constantly switching and 52 is OFF. On the contrary, if the PV output power is higher than the load power demand, 51 is OFF and 52 is constantly switching.
22


START
Figure 3.8:
Battery Control Flow Chart.
23


3.3 Dynamic Load Modeling
The topology of the dynamic load is shown in the Figure 3.6. The load voltage needs to be measured to calculate the specific current reference to generate as much as the power reference requires depend on the load voltage. The current reference can be expressed by equation 3.5.
I ref
Vload,
T/2
vloadf
Pr
ref
(3.5)
Figure 3.9: Dynamic load blockdiagram.
The filter block diagram in the Figure 3.8 removes the noise of the load voltage. The output value of the filter is Vioacif.
Figure 3.10: Dynamic load blockdiagram.
Thus, the power that dynamic load demands can be controlled is estimated in equation 3.6.
24


Plead = ^ (3-6)
iref
If power reference is set as 5000 W, the current reference should be around 22.7 A since the bus voltage is fixed as 220 V.
3.4 Control Logic
While managing the microgrid power, the bus voltage tends to change itself according to the power demand on the load side or the output power PV generates. When the load power increase or the output power PV generates decrease, the bus voltage drops. On the contrary, when the load power decrease or the output power PV generates increase, the bus voltage rises [16]. The bus voltage is controlled as 220 V by the bidirectional DC-DC converter. This is done by varying the duty ratio with PI control loop. When the power demand on the load side is higher than the power PV generates, the bidirectional DC-DC converter operates as the boost converter since the input voltage of the battery is lower than the bus voltage and the battery discharges the insufficient amount of power to the load. Similarly, when the power PV generates is higher than the load demand power, the bidirectional DC-DC converter operates as the buck converter and the battery charges the remain power after the load consume.
25


IV. SIMULATION AND RESULTS
4.1 Description of Simulation Circuit
Figure 4.1 depicts the simulation model for an islanded DC microgrid comprised of photovoltaic, battery and dynamic load. The operation of the proposed islanded DC microgrid is studied under two modes. During the first mode, the demand of dynamic load is set as 9, 500VF from simulation time 0 to 20 seconds. The PV module operates at the insolation level of l,000WTn2. The PV module and the battery both supply the load. The second mode operates from simulation time 20 to 40 seconds. During the second mode, the PV module supplies the load as well as charges the battery.
Dynamic Load
----*G
----â–º Tc_________
Maximum Power Reference
Bidirectionacl DC-DC Converter
Power Measurement
Figure 4.1: Schematic of islanded microgrid developed in Simulink
26


4.2 Current Flow
At node 1, by checking the sign of the measured currents, the current flow can be observed.
Node 1
Dynamic Load
Power Measurement
Figure 4.2: Microgrid model with node and currents
As we can see the measured current plots shown in Figure 4.3 during the first mode from simulation time 0 to 20 seconds, the current from PV flows to the load and the current from battery flows to the load too. During the second mode, the current from PV flows to the load and to the battery from simulation time 20 to 40 seconds.
27


100
80
60
40
g 20
<5 0
° -20 -40 -60 -80 -100
10 20 30
Time(Sec)
100 -80 60 40
g 20,
S o
° -20
-to
-60
-80
-100
10 20 30
Time(Sec)
100 â–  80 60 40 :
g 20 â– 
cD 0 ° -20 -to -60 -80 -100
10 20 30
Time (Sec)
Figure 4.3: (a)PV output current (b)Battery output current (c)Load current
By calculating with the below equation 4.1, we can see that the load current substitutes both PV and Battery currents
Hoad — IPV
L Bait-
(4.1)
28


4.3 Bus Voltage
The below plot shows that the bus voltage is regulated as 220V. Basically, the bus voltage is controlled by the bidirectional DC-DC converter. When the dynamic load demand changed from 9, 5001V to 5, 500, there is an overshoot about 8V, which removed soon by the converter controller.
500
450
400
350
^ 300 (D
^250
4—1
> 200
150
100
50
0
0 5 10 15 20 25 30 35 40
Time (Sec)
Figure 4.4: Bus voltage
— — — — —




...... V.





29


4.4 Power
4.4.1 PV Output Power
The PV output power is controlled by P&O method. The real power has an oscillation, but reaches the power reference. Although the dynamic load demand changes, PV generates the stable power. On the Figure 4.5, the blue color is the output power of PV and the red color is the power reference.
10000 9000 8000 7000 —. 6000 | 5000
O
4000 3000 2000 1000 0
0 5 10 15 20 25 30 35 40
Time (Sec)
Figure 4.5: PV output power with power reference
— 1









I
30


4.4.2 Battery Output Power
When the first mode turns to the second mode, the output power sign of the battery changes from positive value to negative value because the current of battery is measured with only one direction. During the first mode, the battery discharge to supply the dynamic load. So, the sign of the output power is positive value. During the second mode, the sign of the output power is negative, which means the battery is charging.
1
0.8 0.6 0.4 0.2
| 0
o
-0.2 -0.4 -0.6 -0.8 -1
0 5 10 15 20 25 30 35 40
Time (Sec)
Figure 4.6: Battery output power
x 10^
31


4.4.3 Load Power
There is an oscillation since the dynamic load is basically controlled by the current. As we can see, the load demand power reaches the reference value. When the reference value changes, the load demand power also changes to the reference value. On the Figure 4.7, (a) shows the power reference demand and (b) shows the dynamic load power demand.
Time(Sec)
|
<1)
I
0-
Time(Sec)
Figure 4.7: (a)Power reference demand (b) Load power demand
32


SOC(%)
4.5 Battery SOC
During the first mode, the value of SOC decrease, which means the battery is discharging. During the second mode, the value of SOC turns to increase, which means the battery is charging.
Figure 4.8: Battery SOC
33


4.6 Duty Cycle
By the battery control algorithm, when the power demand changes at 20 seconds, the switch 2 stopped operating as the duty ratio turned to 0.
2 |" '"i 'i r f 'v 'i v
1.8 -1.6 -1.4 -1.2 [
0 5 10 15 20 25 30 35 40
Time (Sec)
Figure 4.9: Switch 2 duty cycle
The switch 1 started operating as the duty ratio turned from 0 to which the duty cycle was created by the battery controller.
34


2
1.8
1.6
1.4
1.2 o +—■
s 1
&
D 0.8
0.6 0.4 0.2 0
0 5 10 15 20 25 30 35 40
Time (Sec)
Figure 4.10: Switch 1 duty cycle
PWM uses a rectangular pulse wave whose pulse width is modulated resulting in the variation of the average value of the waveform. The Figure 4.11 shows a pulse wave.
35


Time(Sec) X10'3
Figure 4.11: Pulse wave


V. CONCLUSION
This thesis presents the energy management strategy for a PV based microgrid with battery system and dynamic load. The proposed islanded microgrid model is tested for maintaining regulated bus voltage and for managing power flow through various simulation case studies. The simulation was performed in Matlab-Simulink environment.
The model is stable to run with any demand of the load since the bus voltage is well regulated.
The model developed in this thesis, such as PV system and islanded microgrid model, are useful to perform the dynamic analysis of islanded microgrid. For the next step, the performance of battery and control system based on these models might be investigated.
37


REFERENCES
[1] Nikos Hatziargyriou. The Microgrids Concept, pages 344-. Wiley-IEEE Press, 2013.
[2] Tomislav Dragicevic, Xiaonan Lu, Juan C Vasquez, and Josep M Guerrero. Dc microgridspart i: A review of control strategies and stabilization techniques. IEEE Transactions on povjer electronics, 31(7):4876-4891, 2016.
[3] Robert S Balog and Philip T Krein. Bus selection in multibus dc microgrids. IEEE Transactions on Power Electronics, 26(3):860-867, 2011.
[4] RA Walling, Robert Saint, Roger C Dugan, Jim Burke, and Ljubomir A Ko-jovic. Summary of distributed resources impact on power delivery systems. IEEE Transactions on povjer delivery, 23(3): 1636-1644, 2008.
[5] K. D. Hoang and H. H. Lee. Accurate power sharing with balanced battery state of charge in distributed dc microgrid. IEEE Transactions on Industrial Electronics, pages 1-1, 2018.
[6] Seok-II Go, Seon-Ju Ahn, Joon-Ho Choi, Won-Wook Jung, Sang-Yun Yun, and II-Keun Song. Simulation and analysis of existing mppt control methods in a pv generation system. Journal of International Council on Electrical Engineering, 1(4):446-451, 2011.
[7] Fida Saidani, Franz X Hutter, Rares-George Scurtu, Wolfgang Braunwarth, and Joachim N Burghartz. Lithium-ion battery models: a comparative study and a model-based powerline communication. Advances in Radio Science: ARS, 15:83, 2017.
[8] Barhoumi Abdelhamid, Laajimi Radhouane, and Azouzi Bilel. Real time implementation of perturb and observe algorithm and pi controller for dc/dc converter. In Sciences and Techniques of Automatic Control and Computer Engineering (STA), 2017 18th International Conference on, pages 520-526. IEEE, 2017.
[9] Y-S Lee and Y-Y Chiu. Zero-current-switching switched-capacitor bidirectional dc-dc converter. IEE Proceedings-Electric Power Applications, 152(6): 1525— 1530, 2005.
[10] Oscar Garcia, Pablo Zumel, Angel De Castro, and A Cobos. Automotive dc-dc bidirectional converter made with many interleaved buck stages. IEEE Transactions on Power Electronics, 21 (3) :578—586, 2006.
[11] Felix A Himmelstoss. Analysis and comparison of half-bridge bidirectional dc-dc converters. In Power Electronics Specialists Conference, PESC’94 Record., 25th Annual IEEE, volume 2, pages 922-928. IEEE, 1994.
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[12] Hirofumi Matsuo, Wenzhong Lin, Fujio Kurokawa, Tetsuro Shigemizu, and Nobuya Watanabe. Characteristics of the multiple-input dc-dc converter. IEEE Transactions on Industrial Electronics, 51 (3) :625—631, 2004.
[13] YangQuan Chen, Derek P Atherton, et al. Linear feedback control: analysis and design with MATLAB, volume 14. Siam, 2007.
[14] Brigitte Hauke. Basic calculation of a boost converter’s power stage. Texas Instruments, Application Report Nouember, pages 1-9, 2009.
[15] F Almonacid, C Rus, P Perez-Higueras, and L Hontoria. Calculation of the energy provided by a pv generator, comparative study: conventional methods vs. artificial neural networks. Energy, 36(l):375-384, 2011.
[16] Smita Sinha, Avinash Kumar Sinha, and Prabodh Bajpai. Solar pv fed standalone dc microgrid with hybrid energy storage system. In Computer Applications In Electrical Engineering-Recent Aduances (CERA), 2017 6th International Conference on, pages 31-36. IEEE, 2017.
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Full Text

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ACONTROLALGORITHMFORTHESTABLEOPERATIONOFISLANDED MICROGRIDWITHDYNAMICPOWERLOADS by HWANMINJEONG BS,ChosunUniversity,2015 Athesissubmittedtothe FacultyoftheGraduateSchoolofthe UniversityofColoradoinpartialfulllment oftherequirementsforthedegreeof MasterofScience ElectricalEngineering 2018

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ThisthesisfortheMasterofSciencedegreeby HwanminJeong hasbeenapprovedforthe DepartmentofElectricalEngineeringProgram by JaedoPark,Chair MilojeRadenkovic SatadruDey `Date'July28,2018 ii

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Jeong,HwanminM.S.,ElectricalEngineering AControlAlgorithmfortheStableOperationofIslandedMicrogrid withDynamicPowerLoads ThesisdirectedbyAssociateProfessorJaedoPark ABSTRACT AnisolatedDCmicrogridissimulatedwithphotovoltaicPVasthemainsourceto resistivedynamicDCloadsalongwithBattery.ThePVgeneratorshavenonlinear I-VandP-Vcharacteristics.ToenhancetheconversioneciencyofPVarrays,it requiresmaximumpowerpointtrackingMPPTcontrol.Theperturbandobserve P&Omethodisimplementedtotrackthemaximumpowerpoint.Thealgorithm regulatesthebatterycharge,holdanddischargeoperationsbyDC-DCbidirectional converterdependingonthepowerofPVandload.Thedierentloadpowerwas appliedtoapprovetheproposedpowermanagementstrategysuchasPVsupplying theloadandchargingthebatteryandPV-Batterybothsupplyingtheload.The outputparameterssuchaspower,voltageandcurrentandthepoweroware plottedandanalyzed. Theformandcontentofthisabstractareapproved.Irecommenditspublication. Approved:JaedoPark iii

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DEDICATION Thisleisdedicatedtomyfamily,whosupportmetonishwritingmythesisand supportmefortheMasterprogram. iv

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ACKNOWLEDGMENTS MydeepgratitudegoesrsttoProfessorJaedoPark,whoexpertlyguidedmethrough mymasterprogramandsupportmetonishthisthesis.Myappreciationalsoextends tomycolleaguesinmylaboratoryandmyoce.GadiOgbogu,MdHabibUllah, BhanuShankar,HectorCamposandMuhanandAlarajhelpedsustainapositive atmosphereandencouragedmeallthetime.Iappreciatefortheirgoldenheart. v

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TABLEOFCONTENTS CHAPTER I.INTRODUCTION...............................1 II.DCMICROGRIDCONFIGURATION....................3 2.1PVGenerator..............................5 2.2Battery..................................10 2.3PowerElectronicsConverters......................11 2.3.1DC/DCBoostConverter....................11 2.3.2DC/DCBidirectionalConverter.................14 2.3.3Non-IsolatedBidirectionalDC-DCConverter.........15 2.4Proportional-IntegralPIControl...................16 III.SYSTEMMODELINGANDCONTROLMETHODS............18 3.1PVSystemModeling..........................18 3.1.1EstimatingtheMaximumPower................18 3.1.2MPPTModeling.........................20 3.2BatterySystemModeling........................21 3.3DynamicLoadModeling........................24 3.4ControlLogic..............................25 IV.SIMULATIONANDRESULTS........................26 4.1DescriptionofSimulationCircuit....................26 4.2CurrentFlow...............................27 4.3BusVoltage...............................29 4.4Power...................................30 4.4.1PVOutputPower........................30 4.4.2BatteryOutputPower......................31 4.4.3LoadPower............................32 4.5BatterySOC...............................33 vi

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4.6DutyCycle................................34 V.CONCLUSION.................................37 REFERENCES...................................38 vii

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TABLES TABLE 2.1PVmoduledatas.............................7 2.2Batteryparameters............................11 2.3Eectsofcontrollers K p and K i onaclosed-loopsystem........16 viii

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FIGURES FIGURE 2.1Microgridconguration..........................3 2.2AnExampleofPV-basedIslandedmicrogrid..............4 2.3PVcellequivalentcircuit.........................5 2.4V-Icharacteristic.............................7 2.5P-Vcharacteristic.............................8 2.6P&Oowchart..............................9 2.7Simplestbatteryequivalentcircuit....................10 2.8BoostConverterCircuit..........................11 2.9SwitchonandDo............................12 2.10SwitchoandDon............................12 2.11BidirectionalDC-DCConverterStructure................14 2.12Non-isolatedBidirectionalDC-DCConverterCircuit..........15 2.13PIControllerintimedomain.......................16 3.1MaximumPowerReferenceBlockDiagram...............19 3.2PVCircuitwithControlBlockDiagram.................20 3.3P&OController..............................20 3.4Batterycircuitwithcontroller......................21 3.5ControllerforS2ofBidirectionalDC-DCConverter..........22 3.6ControllerforS1ofBidirectionalDC-DCConverter..........22 3.7Switchingcontrollerdependingontheloadpowerdemands......22 3.8BatteryControlFlowChart.......................23 3.9Dynamicloadblockdiagram.......................24 3.10Dynamicloadblockdiagram.......................24 4.1SchematicofislandedmicrogriddevelopedinSimulink.......26 4.2Microgridmodelwithnodeandcurrents................27 ix

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4.3aPVoutputcurrentbBatteryoutputcurrentcLoadcurrent..28 4.4Busvoltage................................29 4.5PVoutputpowerwithpowerreference.................30 4.6Batteryoutputpower..........................31 4.7aPowerreferencedemandbLoadpowerdemand.........32 4.8BatterySOC...............................33 4.9Switch2dutycycle...........................34 4.10Switch1dutycycle...........................35 4.11Pulsewave................................36 x

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I.INTRODUCTION Modernsocietyreliesonasecuresupplyofenergy.Duetotheincrementofdemandforreliabilityandqualityissuesofpowersystem,theconcernforprimaryenergy availabilityofcurrentelectricaltransmissionisrapidlyincreasing[1].Microgridisa clusterofelectricitysources,controllableloadsandenergystorage.Theperformance ofacmicrogridsduringthepastdecadehasbeenremarkablyimproved.Atthesame time,dcmicrogridhasbeenconsideredasmoreattractiveforvarioususesdueto thesimplercontrolstructure,bettercompliancewithmanyelectronics,andhigher eciencyforthedistribution[2].Also,therearenoissuessuchaspowerquality, frequencyregulation,andreactivepower,whichmakeasimplercontrolsystem[3]. Thedisconnectionofdistributedgeneratorstodistributionnetworksisthechallenges inpowersystem[4]. Microgridcanbeoperatedwithtwodierentmodes:anislandedmodeanda grid-connectedmode.Theislandedmicrogridisseparatedfromthemaingrid.The stabilityandoperationofthesystemaredeterminedbythegeneratorsinthemicrogrids.Sincethedistributedgeneratorsbasedonrenewableenergyandtheenergy storagesystemsareintegratedbypowerelectronicequipment,thecontrolandoperationoftheislandedmicrogridischallenging.Therefore,variousanalysesarerequired todetermineiftheislandedmicrogridworksinastablesituation.Photovoltaicsystem isusuallyprogrammedtooperateaccordingtothemaximumpowerpointtracking algorithmduetoV-IcharacteristicsandP-Vcharacteristics.Meanwhile,theroleof energystoragesystemistoregulatethebusvoltageandmaintainthepowerbalance betweensupplyanddemandbycharginganddischargingprocesses[5]. Inthisthesis,theislandedmicrogridwithdynamicloadisdesigned.ChapterII introducesthecongurationsofthedcmicrogridsuchasphotovoltaicsystem,battery andpowerelectronicsconverters.Then,inChapterIII,thecongurationsandPI controlleraremathematicallymodeled.Also,thealgorithmforthestableoperation 1

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ofislandedmicrogridisintroduced.ChapterIVshowsthevoltage,current,power andState-of-ChargeSOCplots.Finally,ChapterVaddressestheconclusions. 2

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II.DCMICROGRIDCONFIGURATION MicrogridscomprisethelowvoltageLVdistributionnetworkswithdistributed energyresourcesDERthataredieselgenerators,photovoltaicPV,windpower, etcwithdierentkindofloadsandstoragedevicesthatarebatteries,capacitors, etc.Microgridsystemscanbestablyoperatedinaself-governingwayifthesystemsaredisconnectedtothegrid,orinanonself-governingwayifthesystemsare interconnectedfromthemaingrid. Figure2.1:Microgridconguration. DCmicrogridsystemscomparedwithACmicrogridsystemshavethepotential advantages:highereciencyduetolessconversionsteps,betterstabilitysinceDC systemhasnosynchronizationproblem,betterpowerqualityasreactivepowerisnot 3

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transferredinDCsystem,andhigherreliabilitybecausefewercomponentswillhave highermeantimebetweenfailuresMTBF. Inanislandedmicrogrid,theutilityisdisconnectedandthemicrogridisoperated withsourceswithinthemicrogrid.Sincerenewablesareintermittentandtheloads canchangecontinuously,thesource-loadbalancinginamicrogridisachallenge;as thereisnoinertiainapowersystemwhichconsistsofonlyrenewableenergysources. Rotatinggeneratorsandenergystorageareusedformanagingthedemandvariation. Figure2.2:AnExampleofPV-basedIslandedmicrogrid. 4

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2.1PVGenerator Figure2.3showstheequivalentcircuitofPVcell.Intheequivalentcircuit, acurrentsourcewhichhastheamountof I ph ,anidealdiodealongwithparallel resistanceandseriesresistanceasthesheetresistanceandthecontactresistanceon thesurfaceofPVcell. Figure2.3:PVcellequivalentcircuit. TheoutputcurrentofthePVisgivenby I eq = I SC )]TJ/F19 11.9552 Tf 11.955 0 Td [(I D )]TJ/F19 11.9552 Tf 11.955 0 Td [(I sh ; .1 Also,thediodecurrentisshownas I D = I o e q V pv + IR s Ak B T )]TJ/F15 11.9552 Tf 11.955 0 Td [(1 ; .2 Theresistantcurrentisshownas I sh = V D R sh ; .3 Thus,whenthenumberofparallelandseriesPVcellareconsidered,theoutput currentofPVarraycanbeexpressedby 5

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I pv = N P I SC )]TJ/F19 11.9552 Tf 11.955 0 Td [(N P I o e q V pv + IR s N S Ak B T )]TJ/F15 11.9552 Tf 11.955 0 Td [(1 )]TJ/F19 11.9552 Tf 11.955 0 Td [(N P V + IR s R sh ; .4 where: N P isthenumberofparallelPVcellsinthePVarray. N S isthenumberofseriesPVcellsinthePVarray. V pv isoutputvoltageofthePVarray. I pv isoutputcurrentofthePVarray. q istheelectroniccharge. I SC isthelight-generatedcurrent. I o isthesaturationcurrent. A isadimensionlessnumber. k istheBoltzmanconstant. T isthetemperatureinKelvin. Themoredetailsof I SC and I o canbeshownwithequations2.5and2.6. I SC = I SCO S 1000 + J T )]TJ/F19 11.9552 Tf 11.955 0 Td [(T ref ; .5 I o = AT 3 e )]TJ/F20 7.9701 Tf 6.586 0 Td [(E g =nKT ; .6 T ref isthebasetemperature298[k], I sco istheshort-circuitcurrentonthebase temperature, S isirradiation, J istemperaturecoecientof I sc , E g isbandenergy gap, A istemperaturecoecientof I o .PVmoduleexamplewasusedinMatlab Simulink.OnMatlab,temperatureandirradiationvaluecanbeapplied.TheparameterofPVgeneratorisshowninthetable2.1. ThecharacteristicofV-IandP-VfromMatlabSimulinkareshowninFigures2.4 and2.5.Tosupplythemaximumpower,MPPTcontrolisdesigned.MPPTcontrol 6

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Table2.1:PVmoduledatas ParametersValues MaximumPower213.15W VoltageatPmax29V CurrentatPmax7.35A Voltagewithopencircuit36.3V Currentwithshortcircuit7.84A TemperaturecoecientofVoc-0.36099%/deg.C TemperaturecoecientofIsc0.102%/deg.C hasdiversemethods:constantvoltagetrackingmethod,incrementalconductance method,variablestepsizemethod,andperturbandobservemethod.TheP&O methodisoneofthemostcommonlyusedmethodsinpractice. Figure2.4:V-Icharacteristic. 7

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Figure2.5:P-Vcharacteristic. TheP&Oalgorithmoperatesbyperturbingtheincreaseordecreaseofthearray terminalvoltageandcomparingthePVoutputpowerwiththepreviousvalue.If thearrayterminalvoltagechangesandthePVoutputpowerincreases,thePVarray operatingpointiscontrolledinthatdirection.OtherwisethePVarrayoperating pointmovesintheoppositedirection. ThelogicoftheP&OalgorithmsandtheowchartareexplainedinFigure2.6. ThevoltageofPVarrayisperturbedeverycycle.Thisalgorithmkeepsperturbing theoperatingvoltagebyasmallincrementofvoltage 4 V,andthisresultingchange in 4 P. 4 V = V k )]TJ/F19 11.9552 Tf 11.955 0 Td [(V k )]TJ/F15 11.9552 Tf 11.955 0 Td [(1 ; .7 4 P = P k )]TJ/F19 11.9552 Tf 11.955 0 Td [(P k )]TJ/F15 11.9552 Tf 11.956 0 Td [(1 ; .8 8

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Figure2.6:P&Oowchart. If 4 Pispositivevalue,theoperatingvoltageneedstomoveinthesamedirection oftheincrement.Incontrast,if 4 Pisnegativevalue,theoperatingvoltageneeds tomoveintheoppositedirectionoftheincrement.Thus,whenthemaximumpower pointisreached,theoutputpowermakesoscillationaroundthemaximum[6]. 9

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2.2Battery TheMATLABmodelofLithium-Ionbatteryisusedforsimulation.Azero-timeconstantcircuitasshowninFigure2.7isthesimplestequivalentcircuitmodelofthe Li-Ionbattery.Thismodelrepresentsthestaticbehaviorofthesystem.TheSOCis denedinequation2.9. SOC = C current C full 100% ; .9 where C current istheamountofcurrentchargeavailabilityinthecelland C full isthe capacityinthesamecell[7]. Figure2.7:Simplestbatteryequivalentcircuit. Thetemperatureinpracticeeectsonthedischargebehavior.Thechemical activitydecreasesandtheinternalresistanceofthebatteryincreaseswhenthetemperatureisbelow25Celsiusdegrees.Onthecontrary,thecapacitanceofthecharging decreasewhenthetemperatureisover25Celsiusdegrees.Inthispaper,thetemperatureisnotconsideredfortheidealconditionofthebattery.Thus,thebattery ischargedwhenthecurrentdirectionistothebatterywithoutanyloss.Also,the batterydischargeswhenthecurrentowstothebus. Thebatterymodelisalithium-acidbatterywithanominalvoltageof170V.The simulationparametersusedinSimulinkareshowninTable2.2.TheinitialSOCis setas80%. 10

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Table2.2:Batteryparameters. ParametersValues Nominalvoltage170V Ratedcapacity200Ah Initialstateofcharge80% 2.3PowerElectronicsConverters 2.3.1DC/DCBoostConverter Theboostconverteristostepuptheoutputvoltagefromtheinputvoltage[8]. TheconvertertypicallyusesaMOSFETinparallelwithadiodeasatransistorswitch. Figure2.8:BoostConverterCircuit. 11

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Figure2.9:SwitchonandDo. WhentheMOSFETisonasshownFigure2.9,thecurrentis i pk )]TJ/F19 11.9552 Tf 11.955 0 Td [(i o = 4 i = V in )]TJ/F19 11.9552 Tf 11.955 0 Td [(V Tran T on L .10 Figure2.10:SwitchoandDon. AndwhentheMOSFETisoasshownFigure2.10,thecurrentis 12

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i pk )]TJ/F19 11.9552 Tf 11.955 0 Td [(i o = 4 i = V out )]TJ/F19 11.9552 Tf 11.956 0 Td [(V in + V D T off L .11 where V D isthevoltagedropacrossthediode, V Trans isthevoltagedropacrossthe transistor. Byderivingequations2.10and2.11, V out canbesolved V in )]TJ/F19 11.9552 Tf 11.955 0 Td [(V Tran T on L = V out )]TJ/F19 11.9552 Tf 11.955 0 Td [(V in + V D T off L .12 V in T on )]TJ/F19 11.9552 Tf 11.955 0 Td [(V Trans T on = V out T off )]TJ/F19 11.9552 Tf 11.955 0 Td [(V in T off + V D T off .13 V in T on + V in T off = V out T off + V Trans T on + V D T off .14 V in )]TJ/F19 11.9552 Tf 11.955 0 Td [(V Trans D = V out + V D )]TJ/F19 11.9552 Tf 11.955 0 Td [(D .15 V out = V in )]TJ/F19 11.9552 Tf 11.955 0 Td [(V Trans D )]TJ/F19 11.9552 Tf 11.955 0 Td [(D )]TJ/F19 11.9552 Tf 11.955 0 Td [(V D .16 Ifthevoltagedropsarenegligible, V out canbeexpressed V out = V in 1 )]TJ/F19 11.9552 Tf 11.956 0 Td [(D .17 Fromequation2.17,thedutycyclecanbeshownas D =1 )]TJ/F19 11.9552 Tf 15.167 8.088 Td [(V in V out .18 Thus,theoutputvoltageisrelatedtothedutycycleofthepulsesdirectly. 13

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2.3.2DC/DCBidirectionalConverter ThebasicconceptofmostbidirectionalDC-DCconvertercanbeillustratedasthe genericcircuitstructureintheFigure2.11,whichcharacteristicspowerowsinforwardandbackwarddirection.ThebidirectionalDC-DCconverterisusedforcharging anddischargingenergystoragesingeneral.ThebidirectionalDC-DCconverteroperatesastwotypesofconverters;boosttypeconverterandbucktypeconverter[9]. Theboosttypeconverterhasenergystorageplacedonthelowvoltageside,andthe bucktypeconverterhasenergystorageplacedonthehighvoltageside. Figure2.11:BidirectionalDC-DCConverterStructure. ThepowerowinbidirectionalDC-DCconverterisdenedbythedirectionofthe current.Tocarrythecurrentonbothdirections,thebidirectionalDC-DCconverteris implementedwithaunidirectionalsemiconductorswitchingdevicessuchasMOSFET andIGBTinparallelwithadiode.Basically,therearetwotypesofthebidirectional DC-DCconverters,non-isolatedandisolatedconverters[10][11]. 14

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2.3.3Non-IsolatedBidirectionalDC-DCConverter ThebuckandboosttypeDC-DCconvertersareusedinthetransformer-less non-isolatedpowerconversionsystemswithoutanyisolationbetweeninputandload [12].Eventhoughtheisolatedconversionsystembetweenthesourceandloadsides isattractivefromhighfrequencyconversionapplicationsbutnon-isolatedconversion systemismuchmoreattractivefromtheeciency,cost,weightandsize. Figure2.12:Non-isolatedBidirectionalDC-DCConverterCircuit. Thetopologyofthenon-isolatedbidirectionalDC-DCconverterisshowninthe Figure2.12.Whenthecurrentowsfromtheleftsidetotherightsidehighvoltage tolowvoltagebycontrollingthedutycyclefortwoswitches,thebidirectionalDCDCconverteroperatesasthebuckconverter.Onthecontrary,whenthecurrent owsfromtherightsidetoleftside,thebidirectionalDC-DCconverteroperatesas theboostconverter. 15

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2.4Proportional-IntegralPIControl ThetransferfunctionofthePIcontrollercanbeexpressedbyequation2.19. G s = K p + K i s .19 ThebasicblockdiagramsofPIcontrollerwithtimedomainisshownintheFigure 2.13. Figure2.13:PIControllerintimedomain. Theeectof K p istoreducetherisetimeandreducethesteadystateerror,but nevereliminate.Theeectof K i istoeliminatethesteady-stateerror.Theeectsof eachofcontrollers K p , K i onaclosed-loopsystemareshownintheTable2.3. Table2.3:Eectsofcontrollers K p and K i onaclosed-loopsystem. ClosedLoopresponseRisetimeovershootsettlingtimeSteady-stateerror K p decreaseincreasesmallchangedecrease K i smallchangeincreaseincreaseeliminate Eventhoughtheeectscanbedened,but K p and K i aredependentofeach other.Thus,thesecorrelationsmightnotbeaccurate.Forthisreason,todetermine 16

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thevaluesfor K p and K i ,thistableshouldbeusedasareference. ThePI-controllerparametersofnon-isolatedbidirectionalDC-DCconverterare setbytheZiegler-Nicholstuningmethod[13].Thecontrollerdesignstepsbythe Ziegler-Nicholstuningmethodareasfollows[14]. First,theproportionalgain K p andtheintegralgain K i aresetatzero.Second, thevalueof K p increasestepbystepwhile K i isxedatzerountilthewaveformof thevoltageoutputisclosetothereferencevalue.Last,thevalueof K i increasewhile K p issetatwhichthesecondstepfound. 17

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III.SYSTEMMODELINGANDCONTROLMETHODS 3.1PVSystemModeling 3.1.1EstimatingtheMaximumPower Osterwaldsmethodisoneofthemostusedclassicalmethodsthatprovidesstatisticfactoryresults[15].ThismethodisusingthesolarpaneltestingconditionsSTC. P max = P max;STC G G STC [1 )]TJ/F19 11.9552 Tf 11.955 0 Td [( T c )]TJ/F15 11.9552 Tf 11.955 0 Td [(25].1 where: P max isacellmaximumpower W , P max;STC isacellmaximumpowerinSTC W , G isthesolarirradiance W=m 2 , G STC isthesolarirradianceinSTC W=m 2 , T c isthecelltemperature, isthecellmaximumpowertemperaturecoecient% =deg:C . Coecient isexpressedas = V oc; V oc I sc; I sc .2 Thus,thePVmodulepowercanbeshownas P MAXG = N s N p P max .3 N s and N p areseriesandparallelcellsnumberofPVmodule.Inthisthesis, P MAXG basedontheTable2.1iscalculated.Also,thispowerisusedasthepowerreference fortheP&Omethod. 18

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Figure3.1:MaximumPowerReferenceBlockDiagram. Inthesimulation,themaximumpowerreferenceblockdiagramisshowninthe Figure3.1. ThemaximumpowerreferencebasedonthePVparametersfromTable2.1can becalculatedbyequation3.4. P maxref =7 5 213 : 15 W 1000 W=m 2 1000 W=m 2 + )]TJ/F15 11.9552 Tf 9.299 0 Td [(0 : 36099% =deg:C 36 : 3 V 0 : 102% =deg:C 7 : 84 A deg:C )]TJ/F15 11.9552 Tf 11.956 0 Td [(25 deg:C .4 Thus, P maxref isfoundas7460 : 25 W . 19

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3.1.2MPPTModeling TheblockdiagramoftheFigure3.2representsPVarray,aboosttypepower converterandanMPPTcontrolmodulefortheDC/DCboostconverter. Figure3.2:PVCircuitwithControlBlockDiagram. MPPTblockincludesP&Oblockcontroller.P&Oalgorithmoperatesbythe increaseordecreaseofthePVoutputvoltage,andthecomparisonwiththePV outputpowerandthepreviousvalue. Figure3.3:P&OController. 20

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TheP&OmethodblockdiagramshownintheFigure3.3isbuiltbasedonthe P&Oowchart[6].Intheblockdiagram,PWMisaddedtogeneratethedutycycle totheswitchoftheconverter. 3.2BatterySystemModeling TheblockdiagramoftheFigure3.4representsBattery,abidirectionalDC-DC converter,PIcontrollerandthealgorithmtochoosethechargingsignalordischarging signal. Figure3.4:Batterycircuitwithcontroller. BoththeFigures3.5and3.6showtwoPIcontrollersfor S 2and S 1.ForallthePI controllers,thegains, K p and K i ,aredecidedbytheZiegler-Nicholstuningmethod [13].Theerrorbetweenthevoltagereferenceandtheoutputvoltageisconvertedby K p and K i forthevoltagecontrol.AfterthePIcontrollerforthevoltage,thecurrent referenceiscreated.Theerrorbetweenthecurrentreferenceandthebatterycurrent isconvertedby K p and K i forthecurrentcontrol.Then,thedutycycleismadeby PWMwiththeoutputsignalfromthePIcontrollerforthecurrent, d . 21

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Figure3.5:ControllerforS2ofBidirectionalDC-DCConverter. Figure3.6:ControllerforS1ofBidirectionalDC-DCConverter. TheblockdiagramshownintheFigure3.7decideswhethertosendtheduty cycleof S 1or S 2tooperatetheMOSFETbasedonthesubtractionwiththePV outputpowerandtheloadpowerdemand. Figure3.7:Switchingcontrollerdependingontheloadpowerdemands. TheowcharttocontrolthebatterybetweencharginganddischargingisrepresentedintheFigure3.8.IftheloadpowerdemandishigherthanthePVoutput power, S 1isconstantlyswitchingand S 2isOFF.Onthecontrary,ifthePVoutputpowerishigherthantheloadpowerdemand, S 1isOFFand S 2isconstantly switching. 22

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Figure3.8:BatteryControlFlowChart. 23

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3.3DynamicLoadModeling ThetopologyofthedynamicloadisshownintheFigure3.6.Theloadvoltage needstobemeasuredtocalculatethespeciccurrentreferencetogenerateasmuch asthepowerreferencerequiresdependontheloadvoltage.Thecurrentreference canbeexpressedbyequation3.5. I ref = V load V 2 loadf P ref .5 Figure3.9:Dynamicloadblockdiagram. ThelterblockdiagramintheFigure3.8removesthenoiseoftheloadvoltage. Theoutputvalueofthelteris V loadf . Figure3.10:Dynamicloadblockdiagram. Thus,thepowerthatdynamicloaddemandscanbecontrolledisestimatedin equation3.6. 24

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P load = V load I ref .6 Ifpowerreferenceissetas5000 W ,thecurrentreferenceshouldbearound22 : 7 A sincethebusvoltageisxedas220 V . 3.4ControlLogic Whilemanagingthemicrogridpower,thebusvoltagetendstochangeitselfaccordingtothepowerdemandontheloadsideortheoutputpowerPVgenerates. WhentheloadpowerincreaseortheoutputpowerPVgeneratesdecrease,thebus voltagedrops.Onthecontrary,whentheloadpowerdecreaseortheoutputpower PVgeneratesincrease,thebusvoltagerises[16].Thebusvoltageiscontrolledas 220 V bythebidirectionalDC-DCconverter.Thisisdonebyvaryingthedutyratio withPIcontrolloop.Whenthepowerdemandontheloadsideishigherthanthe powerPVgenerates,thebidirectionalDC-DCconverteroperatesastheboostconvertersincetheinputvoltageofthebatteryislowerthanthebusvoltageandthe batterydischargestheinsucientamountofpowertotheload.Similarly,whenthe powerPVgeneratesishigherthantheloaddemandpower,thebidirectionalDC-DC converteroperatesasthebuckconverterandthebatterychargestheremainpower aftertheloadconsume. 25

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IV.SIMULATIONANDRESULTS 4.1DescriptionofSimulationCircuit Figure4.1depictsthesimulationmodelforanislandedDCmicrogridcomprisedof photovoltaic,batteryanddynamicload.TheoperationoftheproposedislandedDC microgridisstudiedundertwomodes.Duringtherstmode,thedemandofdynamic loadissetas9 ; 500 W fromsimulationtime0to20seconds.ThePVmoduleoperates attheinsolationlevelof1 ; 000 Wm 2 .ThePVmoduleandthebatterybothsupply theload.Thesecondmodeoperatesfromsimulationtime20to40seconds.During thesecondmode,thePVmodulesuppliestheloadaswellaschargesthebattery. Figure4.1:SchematicofislandedmicrogriddevelopedinSimulink 26

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4.2CurrentFlow Atnode1,bycheckingthesignofthemeasuredcurrents,thecurrentowcan beobserved. Figure4.2:Microgridmodelwithnodeandcurrents AswecanseethemeasuredcurrentplotsshowninFigure4.3duringtherst modefromsimulationtime0to20seconds,thecurrentfromPVowstotheload andthecurrentfrombatteryowstotheloadtoo.Duringthesecondmode,the currentfromPVowstotheloadandtothebatteryfromsimulationtime20to40 seconds. 27

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Figure4.3:aPVoutputcurrentbBatteryoutputcurrentcLoadcurrent Bycalculatingwiththebelowequation4.1,wecanseethattheloadcurrent substitutesbothPVandBatterycurrents I load = I PV + I Batt : .1 28

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4.3BusVoltage Thebelowplotshowsthatthebusvoltageisregulatedas220 V .Basically,the busvoltageiscontrolledbythebidirectionalDC-DCconverter.Whenthedynamic loaddemandchangedfrom9 ; 500 W to5 ; 500,thereisanovershootabout8 V ,which removedsoonbytheconvertercontroller. Figure4.4:Busvoltage 29

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4.4Power 4.4.1PVOutputPower ThePVoutputpoweriscontrolledbyP&Omethod.Therealpowerhasan oscillation,butreachesthepowerreference.Althoughthedynamicloaddemand changes,PVgeneratesthestablepower.OntheFigure4.5,thebluecoloristhe outputpowerofPVandtheredcoloristhepowerreference. Figure4.5:PVoutputpowerwithpowerreference 30

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4.4.2BatteryOutputPower Whentherstmodeturnstothesecondmode,theoutputpowersignofthe batterychangesfrompositivevaluetonegativevaluebecausethecurrentofbattery ismeasuredwithonlyonedirection.Duringtherstmode,thebatterydischargeto supplythedynamicload.So,thesignoftheoutputpowerispositivevalue.During thesecondmode,thesignoftheoutputpowerisnegative,whichmeansthebattery ischarging. Figure4.6:Batteryoutputpower 31

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4.4.3LoadPower Thereisanoscillationsincethedynamicloadisbasicallycontrolledbythecurrent.Aswecansee,theloaddemandpowerreachesthereferencevalue.Whenthe referencevaluechanges,theloaddemandpoweralsochangestothereferencevalue. OntheFigure4.7,ashowsthepowerreferencedemandandbshowsthedynamic loadpowerdemand. Figure4.7:aPowerreferencedemandbLoadpowerdemand 32

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4.5BatterySOC Duringtherstmode,thevalueofSOCdecrease,whichmeansthebatteryis discharging.Duringthesecondmode,thevalueofSOCturnstoincrease,which meansthebatteryischarging. Figure4.8:BatterySOC 33

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4.6DutyCycle Bythebatterycontrolalgorithm,whenthepowerdemandchangesat20 seconds , theswitch2stoppedoperatingasthedutyratioturnedto0. Figure4.9:Switch2dutycycle Theswitch1startedoperatingasthedutyratioturnedfrom0towhichtheduty cyclewascreatedbythebatterycontroller. 34

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Figure4.10:Switch1dutycycle PWMusesarectangularpulsewavewhosepulsewidthismodulatedresultingin thevariationoftheaveragevalueofthewaveform.TheFigure4.11showsapulse wave. 35

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Figure4.11:Pulsewave 36

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V.CONCLUSION ThisthesispresentstheenergymanagementstrategyforaPVbasedmicrogrid withbatterysystemanddynamicload.Theproposedislandedmicrogridmodelis testedformaintainingregulatedbusvoltageandformanagingpowerowthrough varioussimulationcasestudies.ThesimulationwasperformedinMatlab-Simulink environment. Themodelisstabletorunwithanydemandoftheloadsincethebusvoltageis wellregulated. Themodeldevelopedinthisthesis,suchasPVsystemandislandedmicrogrid model,areusefultoperformthedynamicanalysisofislandedmicrogrid.Forthenext step,theperformanceofbatteryandcontrolsystembasedonthesemodelsmightbe investigated. 37

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REFERENCES [1]NikosHatziargyriou. TheMicrogridsConcept ,pages344{.Wiley-IEEEPress, 2013. [2]TomislavDragicevic,XiaonanLu,JuanCVasquez,andJosepMGuerrero.Dc microgridsparti:Areviewofcontrolstrategiesandstabilizationtechniques. IEEETransactionsonpowerelectronics ,31:4876{4891,2016. [3]RobertSBalogandPhilipTKrein.Busselectioninmultibusdcmicrogrids. IEEETransactionsonPowerElectronics ,26:860{867,2011. [4]RAWalling,RobertSaint,RogerCDugan,JimBurke,andLjubomirAKojovic.Summaryofdistributedresourcesimpactonpowerdeliverysystems. IEEE Transactionsonpowerdelivery ,23:1636{1644,2008. [5]K.D.HoangandH.H.Lee.Accuratepowersharingwithbalancedbattery stateofchargeindistributeddcmicrogrid. IEEETransactionsonIndustrial Electronics ,pages1{1,2018. [6]Seok-IIGo,Seon-JuAhn,Joon-HoChoi,Won-WookJung,Sang-YunYun,and II-KeunSong.Simulationandanalysisofexistingmpptcontrolmethodsinapv generationsystem. JournalofInternationalCouncilonElectricalEngineering , 1:446{451,2011. [7]FidaSaidani,FranzXHutter,Rares-GeorgeScurtu,WolfgangBraunwarth,and JoachimNBurghartz.Lithium-ionbatterymodels:acomparativestudyanda model-basedpowerlinecommunication. AdvancesinRadioScience:ARS ,15:83, 2017. [8]BarhoumiAbdelhamid,LaajimiRadhouane,andAzouziBilel.Realtimeimplementationofperturbandobservealgorithmandpicontrollerfordc/dcconverter. In SciencesandTechniquesofAutomaticControlandComputerEngineering STA,201718thInternationalConferenceon ,pages520{526.IEEE,2017. [9]Y-SLeeandY-YChiu.Zero-current-switchingswitched-capacitorbidirectional dc{dcconverter. IEEProceedings-ElectricPowerApplications ,152:1525{ 1530,2005. [10]OscarGarca,PabloZumel,AngelDeCastro,andACobos.Automotivedc-dc bidirectionalconvertermadewithmanyinterleavedbuckstages. IEEETransactionsonPowerElectronics ,21:578{586,2006. [11]FelixAHimmelstoss.Analysisandcomparisonofhalf-bridgebidirectionaldc-dc converters.In PowerElectronicsSpecialistsConference,PESC'94Record.,25th AnnualIEEE ,volume2,pages922{928.IEEE,1994. 38

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[12]HirofumiMatsuo,WenzhongLin,FujioKurokawa,TetsuroShigemizu,and NobuyaWatanabe.Characteristicsofthemultiple-inputdc-dcconverter. IEEE TransactionsonIndustrialElectronics ,51:625{631,2004. [13]YangQuanChen,DerekPAtherton,etal. Linearfeedbackcontrol:analysisand designwithMATLAB ,volume14.Siam,2007. [14]BrigitteHauke.Basiccalculationofaboostconverter'spowerstage. Texas Instruments,ApplicationReportNovember ,pages1{9,2009. [15]FAlmonacid,CRus,PPerez-Higueras,andLHontoria.Calculationofthe energyprovidedbyapvgenerator.comparativestudy:conventionalmethods vs.articialneuralnetworks. Energy ,36:375{384,2011. [16]SmitaSinha,AvinashKumarSinha,andPrabodhBajpai.Solarpvfedstandalonedcmicrogridwithhybridenergystoragesystem.In ComputerApplicationsInElectricalEngineering-RecentAdvancesCERA,20176thInternational Conferenceon ,pages31{36.IEEE,2017. 39