Grid Integration of Solar PV Power Generating System Using QPLL Based Control Algorithm Bhim Singh, Fellow, IEEE, Shailendra Dwivedi, Ikhlaq Hussain, Student Member, IEEE and Arun Kumar Verma, Member, IEEE Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi-110016, India Email:
[email protected],
[email protected],
[email protected] and
[email protected] Abstract—This paper deals with the grid integration of a double stage solar photovoltaic (SPV) power generating system using two-quadrature PLL (QPLL) based control, which also mitigates power quality problems in three phase, three wire distribution system. The proposed solar PV grid interfaced system consists of solar PV array, dc-dc boost converter, voltage source converter (VSC) and connected linear/nonlinear loads. The maximum power is tracked from SPV array using incremental conductance maximum power point tracking (MPPT) technique. The proposed solar PV power generating system provides load balancing, eliminates harmonics, corrects the power factor, and regulates at PCC (Point of Common Coupling) voltages under different loads. The proposed system is modeled and simulated in the MATLAB/Simulink and results are shown to validate the design and control algorithm. Keywords—Solar photovoltaic, QPLL, MPPT, control, power quality, grid interfaced.
I.
INTRODUCTION
With increasing global warming and increasing demand for electrical energy, solar photovoltaic (PV) power generating system is popular technology to overcome the power quality energy problems by integrating clean power to ac grid with improved power quality [1]. The power quality (PQ) problems are dominant in the grid due to various nonlinear loads in the distribution system. The various problems that affect the power quality are poor voltage regulation, poor power factor, and reactive power at ac mains [2]. A solar PV generating systems range from small, buildingintegrated systems or roof-top mounted to large utility-scale power plants. Today, most PV power generating systems are integrated to the grid. Many double stage solar PV systems have been reported in the literature [3-4]. In the double stage systems, a boost converter with maximum power point tracking (MPPT) is used to boost the solar PV array voltage to required dc bus voltage. Tracking the maximum power from solar PV array is also a challenging work and several methods of MPPT such as variable step incremental resistance (INR) method, Gauss-Newton technique, adaptive fuzzy, P&O, adaptive perturbation and observation, incremental conductance, etc have been reported in the literature [5]. For the integration of solar PV power generating to the grid, the various control techniques such as SRF (synchronous reference frame), power balance theory, etc have been reported in the literature [3-4]. Making the robust and fast
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control is challenging task for the reliable operation of the grid interfaced systems. In this paper, quadrature PLL (QPLL) based control algorithm is proposed for a double stage solar PV grid interfaced power generating system which is used for power factor correction (PFC), harmonics elimination, and load balancing. The proposed quadrature PLL (QPLL) based control algorithm estimates the quadrature-phase and in-phase amplitudes of the fundamental component load current [6]. The proposed system using QPLL based control algorithm is designed, modeled and its performance is simulated in MATLAB for PFC along with compensation of harmonics current and balancing of different loads. II.
DESIGN OF PROPOSED SYSTEM
The design of proposed 50 kW solar PV grid interfaced power generating system as shown in Fig. 1, is given in terms of solar PV array, dc-dc boost converter, dc bus capacitor and interfacing inductors as follows. The detailed design data of proposed system is given in Appendices. A. Design of Solar PV Array The proposed system is designed for the peak power capacity of 50 kW rated at 415 V ac grid. A solar PV module has short circuit module current (Isc) of 3.8 A and open circuit module voltage (Vocn) of 21 V. The maximum power for SPV array is given as, Pmp = (ns*Vmp)*(np*Imp) = 50 kW
(1)
Fig.1 Schematic diagram of proposed grid interfaced solar PV power generating system
where ns and np represent series and parallel strings of PV module, Vmp is the voltage of a module at MPPT, Imp is the current of a module at MPPT and Pmp is the nominal power of a module at MPPT. The Pmp is generally achieved under the condition given as, Pmp = (ns*85% of Vocn * np*85 % of Isc) = 50 kW
(2)
Thus, Imp is 3.3 A and Vmp is 17 V of each module. Considering, PV array open circuit voltage (VOCT) = 700 V. The PV modules connected in series string are estimated as, VOCT = ns * Vocn, thus ns = 700/21 = 34 Modules
(3)
Maximum current of the PV array is given as, Imp = Pmp / (0.85 * VOCT) = 84.03 A The PV modules connected in parallel string are estimated as, Imp = np*Isc, thus np = 23 Modules
(4)
Thus the array of 50 kW peak power capacity is designed with 23 modules in parallel and 34 modules in series with an PV array of 23*34 modules [7].
VMPP D 595 * 0.2 = = 2.36 mH ΔI1 f sw (5.042 *10000)
(5)
C. Design and Selection of DC Capacitor Voltage The design of dc link voltage Vdc is given as [2],
(2
2VLL
) = (2
2 * 415
) = 713.27 ≈ 700 V
(6) 3m 3 *0.95 where VLL is the VSC ac line voltage, m is modulation index. D. Selection of AC Inductor The ac inductor (Lf ) value is calculated on the basis that current ripple ¨i, switching frequency fs, vdc and is as [2],
3mvdc 3 *0.95* 700 = ≅ 2.3mH Lf = 12hf s Δi 12*1.2*104 * (0.05* 71.43)
(7)
where ¨i, = 5% of input current, fs = 10 kHz, h is overloading factor and is taken as 1.2. Thus Lf is selected as 2.3 mH. E. Design of DC Link Capacitor The dc link capacitor value is estimated as [2],
Cdc =
( Pdc / vdc )
( 2*ω * v ) dcrip
(50*103 / 700) = = 5416.2μ F (8) ( 2*314*0.03*700)
where Ȧ is angular frequency and vdcrip is % ripple voltage considered as 3% of vdc.
CONTROL ALGORITHM
There are mainly two stages of proposed SPV system. First stage to extract the maximum power from SPV array by using dc-dc boost converter and second stage is to control a grid interfaced VSC which is also operating as a shunt aactive filter. The details of control algorithm is as follows. A. MPPT Control There are so many algorithms to track the MPP. Some are simple which are based on current or voltage feedback and some are more complicated. P&O is simple as compared to incremental conductance [5]. According to the structure of MPPT, the required parameters are voltage and current feedback signal. The maximum power point is obtained when dppv/dvpv = 0, the slope of the dppv/dvpv = 0 can be calculated by output voltage and output current. Moreover, it can be given as,
dv pv
where ǻI1 is input current ripple, and it is considered as 6 % of dc-dc boost converter inductor current I1 (PMPP/ VMPP) = 84.03 A. Thus a calculated value of ǻI1 is 5.042 A. Thus the inductance (Lb) value is selected as 2.4 mH.
vdc =
III.
dp pv
B. Design of DC-DC Boost Converter The ripple current for inductor at D = 0.2 is given as [2], Lb =
Hence estimated value of dc link capacitor Cdc is 5416.2 μF and it is selected as 7000 μF.
(n) =
p pv (n) − p pv (n − 1) v pv (n) − v pv (n − 1)
, where p pv (n) = v pv (n) i pv (n)
(9)
B. Control of VSC Fig. 2(a) and (b) shows the control algorithm for the estimation of in-phase and quadrature voltage templates and extraction of the fundamental component of load current. These fundamental components are used to extract load current real power and reactive power components. These real and reactive components of load currents are used to generate reference currents. Three phase sensed voltages (vsa, vsb, vsc), load currents (ila, ilb, ilc), and Vdc of VSC are main parameters of the control algorithm. Amplitude of PCC voltage (Vt) is estimated as, (10)
Vt = {(2 / 3)(vsa2 + vsb2 + vsc2 )}
The in phase unit templates of PCC voltage are estimated as follows, w pa =
vsa − vsb v −v v −v , w pb = sb sc , wpc = sc sa 3Vt 3Vt 3Vt
(11)
The unit templates in quadrature with grid voltages vsa, vsb and vsc are derived from wpa, wpb and wpc, as,
wqa = − wqc =
wpb 3
3wpa 2 3
+ +
wpc 3 wpb 2 3
, wqb = −
wpc 2 3
−3w pa 2 3
+
w pb 2 3
−
w pc 2 3
, (12)
1) Estimation of Fundamental Active and Reactive Components of Load Currents The fundamental component of load current is estimated as output of QPLL. Performance of the QPLL is controlled with three controlling constants given as G1, G2, and G3. The QPLL computes the magnitude, phase and frequency of the input load current signals.
Fig. 2(a) Control algorithm for proposed system
fundamental component ilfa using sample and hold block, quadrature template (wqa), absolute block in-phase template (wpa). Similarly, from phase ‘b’ and ‘c’ load fundamental active and reactive power currents (ilpb, ilpc) and (ilfb, ilfb) are also extracted. An average value of magnitudes is estimated for load balancing and also uses to extract the 3-phase supply current as,
I lpA =
ilpa + ilpb + ilpc 3
(14)
Similarly, reactive component (IlqA) can be estimated as, Fig. 2(b) Estimation of unit templates of load current
I lqA =
The QPLL can be described in a given equation as,
ilfa = (q s sin θ + q c cosθ )
(13)
q s = ³ G1 e(t) sin θ dθ , qc = ³ G2 e(t) cosθ dθ
where θ = ³ ª¬ω0 + ³ e(t) G 3 {q s cosθ − qc sinθ }dθ º¼ dθ e(t) = ila − ilfa The controlling parameters G1, G2 and G3 control the steady state and transient behavior of the loop as well as estimate the fundamental component from the polluted load current. The values of G1, G2, and G3 are chosen as 200, 20, and 10 respectively in order to calculate fundamental load component from the load current. The magnitude of fundamental active and reactive power components is extracted from phase ‘a’ load current
ilqa + ilqb + ilqc 3
(15)
2) Estimation of Magnitude of Active Power Components of Grid Current To estimate the active power component the reference dc link Voltage v*dc and VSC dc link voltage is compared. This error voltage is given to the proportional-integral (PI) controller which is used to maintain the dc link voltage. (16) vdcer = v*dc − vdc The output of PI controller is represented as Iloss and the active current component is represented as Ip which is given by
I * p = I lpA + I loss
(17)
Thereafter, in phase components or active power components of reference instantaneous grid currents in phase of PCC voltages are calculated as,
i* psa = I * p * w
pa
, i* psb = I * p * wpb , i* psc = I * p * wpc
(18)
3) Estimation of Reactive Component of Supply Currents The terminal voltage magnitude (Vt) is calculated in (10) and the reference terminal voltage amplitude value (Vref) are fed to the voltage controller. The voltage error is estimated as, ver = V *tref − Vt
(19)
This error is given to the PI controller which gives reactive component of supply current,
I *q = − I lqA + I
(20)
qq
The reference grid currents component are calculated as,
instantaneous
i*qsa = I *q * wpc , i*qsb = I *q * wpb , i*qsc = I *q * wpc
quadrature (21)
4) Generation of Reference Source Currents The reference current can be generated by using equation (18) and (21)
i*sa = i* psa + i*qsa , i*sb = i* psb + i*qsb , i*sc = i* psc + i*qsc
(22)
By comparing reference grid currents (i*sa, i*sb, i*sc) and measured grid currents (isa, isb, isc), an error is generated which is given to the PWM current controller which generates the switching pulses. IV.
MATLAB MODELLING
The proposed solar PV grid interfaced power generating system is modeled by using MATLAB with SPS tool boxes as shown in Figs. 1. Fig. 2 explains the detail control modelling and the reference current generation using QPLL. Further an estimation of reference currents and PWM switching signal generation are achieved for the control of the combined operation of the VSC based solar PV power generating system. V.
RESULTS AND DISCUSSION
A model of solar PV grid interfaced power generating system is developed in MATLAB with the help of SPS tools for compensations of three-phase linear and nonlinear loads. The performance of QPLL based control algorithm is studied and implemented in MATLAB for PFC mode of operation under linear and nonlinear loads. The system performance demonstrates as grid voltage (vsa, vsb, vsc) grid currents (isa, isb, isc), dc link voltage (vdc) load current (iLa, iLb, iLc), active power (P), reactive power (Q) and VSC current (iinv). Here solar PV array voltage Vpv, solar PV array power and current as Ppv and Ipv are respectively. A. Steady State and Dynamic Performances of System Configuration under Linear Loads Fig. 3 shows the steady state performance of SPV grid interfaced system under linear load. The power factor on source side has been improved and is maintained near to unity. Fig. 4 shows the waveforms under dynamic condition. The load removal can be realized from 0.3 s to 0.4 s. The grid
Fig. 3 Steady state response of proposed system with linear load
currents are balanced, sinusoidal and dc link voltage is also maintained constant. B. Steady State and Dynamic Performances of System Configuration under Nonlinear Loads Under nonlinear loads, there is no demand of reactive power. So it is observed that PCC voltage falls from the rated value because of source impedance and load demands but the power factor at supply side maintain unity. Fig.5 shows the steady state performance of the proposed system under nonlinear load. The load removal is realized from 0.3 s to 0.4 s as shown in Fig. 6 and shows dynamic performance of the proposed system. The grid currents are balanced, sinusoidal and dc link voltage is also maintained constant. These results show the satisfactory performance of grid interfaced solar PV system under nonlinear unbalanced conditions. The total harmonic distortions (THD) of grid voltage, grid current, and load current are 1.43%, 3.61% and 19.13% respectively as shown in Fig. 7 and these harmonics are within the IEEE-519 standard [8]. C. Performances of Solar PV System under Different Insolation Fig. 8 shows the performance of solar PV system under different insolations. From 0.3 s to 0.4 s the solar insolation is 500 W/m2 and from 0.4 s to 0.5 s it has increased to1000 W/m2 insolation. The performance of the proposed system remains satisfactory under varying insolations. VI.
CONCLUSION
The proposed grid interfaced solar PV system using QPLL based control algorithm has been found quite suitable for mitigating the power quality problems with improved power quality. The QPLL based control algorithm has been observed
Fig. 4 Dynamic response of proposed system with linear load
linear and nonlinear loads under varying insolations and total harmonic distortion of grid current within IEEE-519 standard. APPENDICES A. Solar PV Data Vocn = 21 V, Isc = 3.8 A, Vmp = 17 V, Imp = 3.3 A., No. of series cell in each module Ns = 36, ns = 34, np = 23. B. DC-DC Boost Converter Parameters D = 0.2, Lb = 2.36 mH, Fswb = 10 kHz, [5]. C. Parameters for VSC Vs = 415 V, f = 50 Hz, fs = 10 kHz, Vdc= 700 V, Cdc = 7000 ȝF, L = 2.24 mH, dc voltage controller: Kpd = 3, Kid =2, line impedance: Ls = 2 mH, Rs = 0.05 ȍ, linear load: Series combination of 13 ȍ, 100 mH, nonlinear load: single phase diode bridge rectifiers with L=200mH, R = 15 ȍ and ripple filter: Cf = 5 ȝF, Rf = 5 ȍ. ACKNOWLEDGMENTS Authors are extremely grateful to Department of Science and Technology (DST), Govt. of India, for aiding this work under Grant Number: RP02583. REFERENCES
Fig. 5 Steady state response of proposed system with non linear load
robust in nature and worked well for mitigating power quality problems. The proposed algorithm has been found to provide acceptable performance characteristic in UPF mode. The performance of the solar PV system found satisfactory under
[1] [2]
F. A. Farret and M. G. Simoes, Integration of alternative sources of energy, Hoboken, New Jersey: John Wiley & Sons, Inc., 2005. N. Mohan, T. M. Undeland, and W. P. Robbins, Power electronics: converters, applications and design, 3rd ed. New Delhi, India: John Wiley & sons Inc., 2009.
Fig. 6 Dynamic response of proposed system with non linear load
(a)
(b)
(c) Fig. 7 Harmonic spectra of proposed system (a) grid voltage (b) grid current and (c) load current. [3]
[4]
[5]
A. K. Verma, B. Singh, and D. T. Shahani, “Grid interfaced solar photovoltaic power generating system with power quality improvement at AC mains,” in IEEE ICSET, 24-27 Sept. 2012, pp. pp. 177-182. B. Singh, D. T. Shahani, and A. K. Verma, “Power balance theory based control of grid interfaced solar photovoltaic power generating system with improved power quality,” in IEEE Int. Conf. Power Electron., Drives Energy Systems (PEDES), 2012, pp. 1-7. B. Subudhi and R. Pradhan, “A Comparative Study on Maximum Power Point Tracking Techniques for Photovoltaic Power Systems,” IEEE Trans. Sustainable Energy, vol. 4, no. 1, pp. 89-98, Jan. 2013.
Fig.8 Performance of solar PV system under different insolation with non linear load [6]
[7]
[8]
M. K.-Ghartemani, H. Karimi, and M. R. Iravani, “A magnitude/Phaselocked loop system based on estimation of frequency and inphase/quadrature-phase amplitudes,” IEEE Trans. Ind. Electron., vol. 51, no. 2, April 2004. M. G. Villalva, J.R. Gazoli, and E.R. Filho, “Comprehensive approach to modelling and simulation of photovoltaic arrays,” IEEE Trans. Power Electron, vol. 24, no. 5, pp. 1198-1208, May 2009. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” IEEE Std. 519-1992, 1993.
