1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)
Unity Power Factor Control of Grid Connected SPV System using Instantaneous Symmetrical Component Theory Manoj Kumarl, Nikita Gupta2 and Rachana Garg3
1, 2,3 Delhi Technological University (DTU), Delhi, India E-mail:
[email protected]@gmail.com.\
[email protected] Abstract-In this paper, simulation studies for
power factor control of grid connected
generation [10, 11]. Further simulation studies are presented with linear and non-linear loads and in the last section the conclusion of the studies are presented. The total harmonic distortion (THD) reduction and power factor correction [12] at the supply side have been studied and analyzed.
unity
8 kW solar
photovoltaic (SPV) system are presented. The characteristics of photovoltaic system are studied under various load and atmospheric conditions. The six pulses for PV inverter are generated using instantaneous symmetrical component theory (ISCT) control algorithm. DC bus voltage is maintained at 800V
using
component technique
PI
theory for
controller. (ISCT)
controlling
Instantaneous
is
used
with
inverter current.
11.
symmetrical
indirect The
control
Figure 1 shows the block diagram of grid connected SPV system consisting of solar panel along with DC-DC boost converter and PV inverter. In the present studies, 8 kW power rating SPV panel is taken for grid integration. MPPT is used to extract the maximum power under variable irradiation conditions.
required
powers for linear and non-linear load are maintained thereby balancing the source currents. Simulations are performed with MATLAB Simulink version 15.
Keywords-Solar Photovoltaic (SPV); Maximum Power Point
Tracking
(MPPT);
PV
Inverter;
Instantaneous
Symmetrical Component Theory (ISCT)
I.
SYSTEM CONFIGURATION
I"
INTRODUCTION Grid
Solar energy is an llvportant source of renewable energy [1]. Also, in order to reduce the greenhouse effect from non-renewable energy resources, SPV system play an important role. The optimum power from the solar energy is the running topic from the last decades. Currently, the improvement in the efficiency of the solar panel and inverter are the most important challenges. SPV system generates DC-voItage. Due to the variation in irradiation level, power generated from SPV module is not constant [2, 3]. To increase the power extraction, DC-DC converters with MPPT controller are used along with PV inverters. DC-DC Boost converter helps in tracking the maximum power point and also increases the voItage to required voItage level [4-7]. Conventional grid Integration require DC-AC converters also known as PV Inverter along with controllers. The instantaneous symmetrical component theory [8-9] is used as the control algorithm for generation of six pulses for PV Inverter. In this paper simulation studies related to grid connected 8 kW SPV system have been presented. The paper is structured as: the first and second section describe the introduction and system details, the third section presents the design of the system under consideration followed by MPPT, DC-DC Boost converter working and parameter calculations. Next section present the inverter control using instantaneous symmetrical component algorithm for reference current 978-1-4673-8587-9/16/$31.00 ©2016 IEEE
Point of Common Coupling (PCC)
VSI Vdc
L:.i
lbi
Lei
OCBus capacitor, Cb,
Fig. I: Block Diagram of Grid Connected SPV System
The three-phase PV inverter uses six insulated gate bipolar transistor (IGBT) switches with anti-parallel diodes. The PV inverter is required to convert the DC voItage to AC voItage. The interfacing inductor (Lai, Lbi, Lei) are used with PV Inverter in order to reduce ripples in the compensating current. III. A.
DESIGN OF SYSTEM UNDER CONSIDERATION
SPV System Model
In the SPV system, the solar cell is a basic element. The solar cell is basically a P-N junction which convert the solar energy directly into DC electric power. The required power is obtained by connecting the PV-cells in [1]
1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)
series and parallel connections. In the present study, 8 kW power rating SPV panel is taken for grid integration. Three SPV modules are connected in parallel and eleven modules are connected in series for desired power. The output of the SPV array [1, 2] is given by equation (1, 2). Ipvarray = IphNp - Nplo
s [ex {q(VPV+lPVRS)} 1] Vpv+lpvR P Rp Iph = Isc * l��O [1 + (r - rref ) * a] NsAkT
_
resistance, the shunt resistance, module reference temperature, module operating temperature, and current temperature co-efficient at short circuit condition respectively. SPV system is designed for maximum 8 kW at 1000W/m2 of solar radiation, and at 25üC temperature. The parameters of SPV system are given in Appendix I. The effect of irradiation level and temperature variation on an I-V and P-V curve [3] of illuminated SPV array is shown in the Fig. 2 and 3.
(1)
_
(2)
30
B.
(P&O)
�20'-���---- C � "
o
Figure 1 shows the DC-DC boost converter [4] used in the grid connected SPV system to feed the PV power at higher voItage than the input voItage. Also, due to intermittency of the sunlight, output from SPV is not constant and dependent on irradiation and environmental temperature. To extract maximum power at every point, MPPT algorithrns are used. There are various MPPT algorithrns [5] that are based on the voItage and current signal of the SPV system which are adjusted by varying the duty cycle of DC-DC boost converter in order to achieve maximum power point. Boost converter increases the voItage level from 400 V to 800 V and perform the maximum power tracking function. The design specification of the boost converter can be computed [4, 6] using the equations (3, 4):
10 b-��� -------------------o �----�----�----�------��� o
100
200
300
400
500
100
200
300
400
500
Voltage M
10000 8000 � ';::'6000 � &4000
Voltage M
DC-DC Boost Converter and MPPT Algorithm
(3) C,infsw (4) Cb C,vrfsw Where, De is the Duty cycle given by equation (5): Vin Dc = 1 (5)
Fig. 2: I-V and P-V Characteristics for Different Irradiation Levels for 8 kW SPV Panel
Lb =
25 r---�----�---,
� focDe
Voc
and Vi, Vo, are the input and output voItages of the boost converter, t:.in is the input ripple current taken as 10% of input current, fsw is the switching frequency, t:.vr is output ripple voItage taken as 3% of the output voItage Vo and 10 is the output current. The boost converter is used with Perturb and observe (P&O) MPPT algorithm that is responsible for tracking the maximum power point. The P&O MPPT algorithm is based on perturbation to the voItage of the SPV system. In this method of perturbation, the power is compared with the previous power, and if the output power increases then the perturbation is kept in the same direction that is increment in voItage remains continuous in same direction. However, if output power decreases then the perturbation in voItage is made in the opposite direction. This process continues until the maximum power point is reached.
5
o �--�----�--�----����--� o 100 200 300 400 500 600 Voltage (V)
10000 r---�----�---, 8000
� ooo Q;
�OOO
Q.
2000 100
200
300
Voltage (V)
400
500
600
Fig. 3: I-V and P-V Characteristics for Different Temperature Levels for 8 kW SPV Panel
c.
Where, Ipvarray and Iphare the SPV array and photon currents, respectively. Ns, Np, q, k, and T are number of cells in series and parallel, the charge, the BoItzmann constant, and cell junction temperature in kelvin (K); A, Rs, Rp, Tref, T, a are the diode ideality factor, the series
PV Inverter
PV inverter is basically a three-phase DC-AC converter which consists of the six IGBT switches with anti-parallel diodes. The PV inverter is used to interface
[2]
1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)
the DC output of SPV system with an AC grid and also provide the reactive power compensation. The dc link voItage vdn interfacing inductor (Linverter), and dc link capacitor value [1, 7] are calculated using equations (6-8): (6) Vdc = (2,,[2vline)/,,[3m ,f3 m vdc (7) Linverter = 12hfsw/li
(vsb- vse - ßvsa)ia + (vse - vsa - ßVsb)ib (14) + (vsa - vsb - ßvse)ie = 0 when ß = 0, currents(ia' ib, ie) are to be in phase with voItages(vsa' vsb' VSe) which implies that the instantaneous reactive power demand of the load is supplied by the inverter. But in the case of other than zero, the source may supply or absorb the reactive power. The source should supply the active power to the load. The average power demand is given in equation (15) (15) Pavg = vsaia + vsbib + vseie Where, Pavg average active power which is obtained after fiItering the harmonics using LPF filter. The load active power can be calculated from the equation (16): (16) PI = vsaila + vsbilb + vseiJc The DC bus voItage is maintained constant using PI (proportional integral) controller, given by equation (17): (17) Ploss = KpdcVdcei + Kidcf Vdcei dt Where, Vdcei = Vde * - Vde, is the error. v de * and v de are the reference voItage and sensed voItage of DC bus respectively. Kpdc and Kidc are the proportional and integral gain of the PI controller over the DC bus voItage of PV inverter. With the help of PI controller the losses due to the switching is maintained. Hence Ploss is provided to compensate the switching [12, 13] losses in the inverter. Therefore, the total power is given by the equation (18): (18) Pavg + Ploss = vsaia + vsbib + vseiC After solving equation (9), (14) and (18), the reference current can be obtained using equation (19): ia * = [vsa - (Vsb - Vsc)ß] (Pavg + P1oss)/A ib * = [Vsb - (vsc - Vsa)/ß] (Pavg + P1oss)/A (19) ic * = [vsc - (vsa - Vsb )/ß] (Pavg + P1oss)/A where A = L(V;a+V;b + v;J and ß = 0 in the unity power factor control of grid connected SPV system. The generated reference currents are then compared with actual source currents(ia' ib, ie) using hysteresis current controller (HCC) that results in the required six pulses for controlling PV Inverter.
(8) (PVdcC) /(2WVdripple) d Where, Vline is the line voItage, m is the modulation
Cd =
index, 4w is the switching frequency, t:.i is the 5% of input current, h is overloading factor[7] and it is taken as 1.2, W is the angular frequency and Vdrippleis the% ripple voItage taken as 3% OfVdc. IV.
INSTANTANEOUS SYMMETRIC AL COMPONENT THEORY
The reference current estimation, load balancing, harmonics reduction and power factor correction is done by PV inverter using instantaneous symmetrical component theory control algorithm [8, 9]. The block diagram for the ISCT control algorithm [10] is shown in the Fig. 4. The PCC voItage(vsa' vsb' VSe), source currents(ia' ib, ie)' load currents (ila' ilb, ile) and DC bus voItage (Vdc) of the SPV system are used for extraction of reference source current. The source currents are balanced and are given by equation (9) [10, 11]: ia + ib + ie = 0 (9) I
Vdc"
ib·
ie"
vsa
vsb
HYSTERISIS CURRENT Switching ONTROLLER Pulses (HCq
vsc
Fig. 4: Block Diagram for Control of SPV Inverter
To obtain the balanced source current, positive sequence quantities of voItage and current are to be considered. Using symmetrical component theory, positive component can be obtained using the equation (10): L{Vsa + aVsb + a2 VSe} = L{ia + aib + a2ie} + f/J (10) 0 where a = ej120 and f/J is the phase between supply voItage and current. Under the condition that the supply voItage is baIanced, the above equation can be written using the equation (11):
tan-1 (��)
=
tan-1 (�:) + f/J
V.
SIMULATION RESULTS
The grid synchronized SPV system is modeled in MATLAB using Simulink toolbox. Values of all the parameters are given in the appendix-I. The simulation resuIts of grid side voltage, grid side current, DC bus voItage, PCC voItage, PCC current, load voltage, load current, PV voItage, PV current, PV power, grid active power and grid reactive power for different loads are given in Fig. 5and Fig. 6.
(11)
Where,
A.
k1 = .J3/2 (vsb- VSe), k2 = (3/2)(vsa) k3 = .J372(ib - iJ, k4 = (ia - ib /2 - ic/2) (12)
Performance of Grid Connected SPV System with Linear Load
The performance of grid connected SPV system with linear load is shown in Fig. 5.The load is suddenly increased from 0.3sec to 0.45sec. The effect of load
(13) ß =tanf/J/,,[3 After solving equation (11), (12) and (13), it resuIts in equation (14) [11]: [3]
1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016)
demand is supplied from grid. Under all the above variable conditions, DC link and PCC voItages are maintained and grid current remains balanced. The THD level of grid current is 3.61% and THD level of load current is 30.79% as shown in Fig. 8, which is weil within the IEEE limits. The load power is the sum of grid power and the PV power in this cases also. Also unity power factor is maintained for the grid side supply system as shown in Fig. 9.
change on the various parameters can be seen. It is observed that, Vdc remains constant and source current increases to meet the load demand, but remains balanced, sinusoidal and the power factor is maintained to unity. From 0.6 sec. to 0.75 sec, irradiation of the SPV system is decreased from 1000 W/mm2 to 800 W/mm2. Under this condition, SPV power is reduced, so, the remaining load demand is supplied from the grid. Under all the above variable conditions, DC link and PCC voItages are maintained and grid current remains balanced. The THD level of grid current is 2.18% and PCC current is 1.29% as shown in Fig. 7, which is weil within the IEEE limits. The active power fed to the load is the sum of the grid power and PV power. Also unity power factor is maintained for the grid side supply system as shown in Fig. 9. Vgrld
�o Lj===L== I : �========t= :jI Ppv
0.2 Time offset: 0
0.3
0.4
0.5
0.6
Fig. 6: Performance of Grid Connected SPV System for Non-linear Load under Load Changing and Varying Irradiation Levels
0.7
FFT window: 3 of 40 cycles of selected signa l
Dl 10
Fig. 5: Performance of Grid Connected SPV System for Linear Load under Load Changing and Varying Irradiation Levels
B.
cu E
cu c Ol
Performance of Grid Connected SPV System with
Uj
Non-linear Load
The performance of grid connected SPV system with non-linear load is shown in Fig. 6. The load is suddenly increased from 0.3 sec to 0.45 sec. The effect of load change on the various parameters can be seen. It is observed that, Vdc remains constant and source current increased to meet the load but remains balanced, sinusoidal and power factor is maintained to unity. From 0.6 sec to 0.75 sec irradiation of the SPV module is decreased from 1000 W/mm2 to 800 W/mm2• Under this condition, SPV power is reduced, so, the remaining load
0 -10 0.2
0.21
0.22
0.23
Time(s)
0.24
0.25
F _ n_da � =_2 _.18 . T_ H_D _ __� _ °� ._ 5 _ � _ � _ � _ � 1_6 4 Z) = _ H (5 0 � ta_ I� m en cu 100 �"u
C
Q) E cu -0 C
� o
Ol
:ll1
[4]
50
0
0
5
10
Harmonie order
15
20
1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016) � öT----,---�--�----�--�--�---,
FFT window: 3 of 40 eyeles of selected signal
20 ci> '" E (ijc 0 Cl (jj -20
200
0.2
0.21
0.22
0.23
0.24
-200
0.25
Time (5) Fundamental (50Hz) = 27.98 • THD= 1.29%
20 Q) � 15 -0 c � 10 0 5 ::R Cl 0 :2:'" 0 ]c
-� �----�--�--�----�--�--�--� 0.16 0.18 0.2 0.22 0.24 0.26 0.28 Time ellset:
0
200
5
10
15
Harmonie order
20 o
Fig. 7: Waveform and Harmonie Analysis for Grid Current (Igrid) and PCC Current (Ipcc) for Linear Load
-200
FFT window: 3 of 40 cycles of selected signal
10 g> 5 E (ij 0 c -0 -5 -10 L-�� 0.2 0.21 Cl
-400 L-�__-'-__-'-__---,-____'--__-'-__-'-__-J 0.16 0.18 0.2 0.22 0.24 0.26 0.28
Time offset:
-L���____���____-J
0.22
0.23
0.24
Time(s) Fundamental (50Hz) = 11.87
VI.
0.25
�
10
Cl
_I •
� 0
o
•
1_
5
10
15
APPENDIX-I
20
Data for SPV system: Maximum power of array=8 kW, Short circuit current of module= 8.32 A, open circuit voItage of module = 44.6 V, Cells per module=72, series resistance (Rs) =0.42018 n, shunt resistance (Rsh) =0.42018 n, Module current at MPP=7.78 A, Module voItage at MPP=36, q= 1.602 x 10° ° ° 19 C, k =1 . 38 x 10-23 J K1 , Tref = 25 C' T=25 c to 70 C , and K = 0.01 re. Data for DC-DC boost converter: D=0.5, L=0.02H, C=500J.lF, fs=25 kHz Data for PV inverter: Vd = 800 V, Linverter=7 mF, Cd = 3000 J.lF. Data for different loads: • Linear load-RL load is 20 kVA, additional linear load-RL is 22.36 kVA. • Nonlinear load-universal bridge with 1011, 100mH RL load, and additional non-linear load is universal bridge with 1011, 100mH.
FFT window: 3 of 40 eycles of seleeted signal
: ; , � " , m r , I: I : 0.2
0.21
0.22
c-
:2:'"
'
0
5
0.23
0.24
0.25
Time(s) H,) - 3 06 , rHO- 30 79%
10
Harmonie order
15
I
CONCLUSION
Simulation studies are performed on the SPV system integrated with conventional grid. These studies are performed under various load and irradiation conditions. Under all the conditions, the DC link voItage and power balance is maintained. The THD in source side current is weil within IEEE standard and unity power factor of grid side is maintained.
THD= 3.61%
20�-r----�--�--�------�-------, •
� 15
-0 c
0
Fig. 9: In Phase Grid Side Voltage and Current for Linear and Nonlinear Load Conditions
____
]c
0
I
20
Fig. 8: Waveform and Harmonie Analysis for Grid Current (Igrid) and Load Current (ILoad) for Non-linear Load
[5]
1st IEEE International Conference on Power Electronics. Intelligent Control and Energy Systems (ICPEICES-2016) Bhim Singh, Shailendra Owivedi, Ikhlaq Hussain and Arun kumar, "Grid Integration of Solar PV Power Generating system Using QPLL based Control A1gorithm", 2014 IEEE. [8] A. Ghosh and A. Joshi, "A new method for load balancing and power factor correction in power distribution system, " IEEE Trans. Power Delivery, Vol. 15, No. I, Jan. 2000, pp. 4I7-422. [9] Arindam Ghosh and Gerard Ledwich, "Load Compensating OSTATCOM in Weak AC Systems", IEEE Trans. Power Delivery, vol.I8, no. 4, pp. 1302-1309, Oct 2003. [10] Sunil kumar and Bhim Singh "Control of 4-Leg VSC Based OSTATCOM using ModVified Instantaneous Symmetrical Component theory, "2009 third international conference on Power
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