PV Array Emulator for Testing Commercial PV Inverters

Transcription

1 ELEKTRONIKA IR ELEKTROTECHNIKA, ISSN , VOL. 19, NO. 10, 2013 PV Array Emulator for Testing Commercial PV Inverters C. Roncero-Clemente 1, E. Romero-Caaval 1, V. M. Minambres 1, M. A. Guerrero-Martinez 1, J. Gallaro-Lozano 1 1 Power Electrical an Electronic Systems Research Group (PE&ES), Elvas Avenue, University Campus. School of Inustrial Engineering of Baajoz Spain croncero@peanes.unex.es 1 Abstract This paper is evote to the esign an construction of a photovoltaic array emulator for high power applications in orer to analyze an evaluate all kin of commercial photovoltaic inverters. To valiate the propose photovoltaic emulator experimentally, a rapi prototyping tool base on xpc Target of Matlab/Simulink has been use, proviing a real-time testing environment. The propose emulator has been use to test the performance an maximum power point tracking algorithm efficiency of a commercial photovoltaic inverter by programming the esire conitions of temperature an irraiance. Inex Terms Photovoltaic systems, inverters, energy efficiency, Matlab. I. INTRODUCTION Nowaays, the power electrical system scenario is quite ifferent in comparison with the traitional configuration [1]-[2]. Several factors, such as an electrical consumption increase, the electrical market liberalization, the nee to reuce CO 2 emission, an the new technological evelopment are boosting the istribute generation (GD). Renewable istribute energy resources are becoming more an more popular (win turbines, photovoltaic (PV) plants, fuel cells, microturbines, etc) [3]. Many of them can only output DC voltage so an inverter interface has to be use. PV generation plants have increase a lot over the last years, taking an important role towars a sustainable energy system. The performance of PV panels an PV inverters an also their maintenance are keys in the pay-back time an the profitability of gri-connecte PV plants. Several control algorithms are involve in PV inverters, for instance: maximum power point tracker (MPPT) [4], anti-islaning algorithm an reactive power compensation ability. The efficiency of these algorithms must to be assure in orer to carry out their roles being essential in the overall performance [5]. Due to the high cost of the solar panels, the maximum power point (MPP) operation is necessary to maximize the output power in ifferent conitions of irraiance (W) an Manuscript receive May 21, 2013; accepte July 14, This research was supporte by Ministerio e Economía y Competitivia, Fonos FEDER, Junta e Extremaura, Consejería e Empleo, Empresa e Innovación an Fono Social Europeo. The work has been evelope uner the Project SIDER (TEC C03) an the grant BES temperature (T). This fact obliges that every commercial PV inverter must be teste uner ifferent conitions. For high power testing applications a DC power supply is har to fin an certainly a PV array coul not be available. There are some commercial PV array emulators but they present limitations in the output power an also in the number of points of the I-V panel curve reprouction. Several prototypes evelope by researchers can be foun in the literature [6] [10]. In [6] a DC power supply is programme accoring to a PV moel which increases the accuracy of the I-V curve but the same power level limitation level appears when the MPPT efficiency is teste. Other works [7] [9] use ifferent DC/DC converter topologies to obtain the I-V curves but the influence of temperature an solar irraiance is not taken into account in some of them. In other cases, the use moel to emulate the I-V curve requires some parameters which are not provie by manufacturers in the PV panel atasheet being ifficult their use [10]. The main goal of this paper is to show a regenerative PV array emulator for high power application in orer to test PV inverters. Every kin of PV panels coul be emulate by a PV moel which only uses parameters provie in atasheets. The power conversion is one by a three-phase synchronous rectifier which allows emaning sinusoial currents from the gri which are synchronize with the voltage at the point of common coupling (PCC) increasing the power quality. This emulator can be use to evaluate the performance of PV inverters an the efficiency of MPPT algorithms uner ifferent conitions of W an T. II. PV ARRAY EMULATOR TOPOLOGY In orer to achieve PV array emulation, an electronic converter working as a rectifier has been use. AC/DC family of converters constitutes the interface circuit between the electrical gri an the DC loas. With the ever increase of power quality requirements at the point of common coupling (PCC), these converters are nowaays require to achieve ifferent task such as: provie high input power factor, low current istortion [11] an fixe output voltage. Synchronous rectifier allows emaning sinusoialcurrents from the gri which are synchronize with the voltage at the PCC. Fig. 1 shows the topology use in the PV array emulator. 71

2 Fig. 1. Photovoltaic array emulator topology. The electronic converter controlle to emulate the PV array is the Semikron SEMISTACK SKS 230F B8CI 190 V12. The commercial PV inverter to be teste is connecte to the DC-link of the PV array emulator. III. CONTROL STRATEGY. REFERENCE GENERATION In this section the whole control strategy to generate the references of the PV array emulator are explaine. The control strategy block iagram is epicte in Fig. 2. i DC W T V DC v DC v pcc I s,rms Fig. 2. Control strategy block iagram of the PV array emulator. A. PV Array Emulator Control Strategy The measure DC-link current (i c) is the input of the PV v pcc v V pcc pcc,rms 2 I i s,rms s array moel. By this way, the reference DC link voltage (V DC) is generate accoring to the I-V curve. In orer to generate the reference current (i s) to be emane from the gri by the PV emulator, a proportionalintegral (PI) controller calculates the reference RMS current (I s,rms) (control variable) epening on the DC voltage error. The peak value of the reference current is multiplie by three unitary waveforms synchronize with the voltage (v pcc) at the PCC. These waveforms are provie by an ajustable synchronous reference frame (ASRF) [12]. B. PV array moel In orer to reprouce the typical curve of a PV panel, a PV moel has been employe. This moel, explaine in etail in [13], is base on the I- V exponential curve efine by the information provie by the manufacturers in atasheets. I-V curves obtaine in the simulation of the panel Shell SP 150-P in ifferent conitions of W an T are shown in Fig. 3(a) an Fig. 3(b). The parameters of this commercial panel have been use to emulate the PV array uring experimental tests. a) 5 4 Current (A) ºC 50ºC 40ºC 30ºC 20ºC Voltage (V) b) Fig. 3. a) I-V curves in ifferent irraiance conitions. b) I-V curves in ifferent temperature conitions. 72

3 ELEKTRONIKA IR ELEKTROTECHNIKA, ISSN , VOL. 19, NO. 10, 2013 real-time testing environment is epicte in Fig. 5. The real time environment built for rapi prototyping is base on xpc Target from Matlab/Simulink. The PV moel, control strategy an current controller have been implemente on the host PC (which runs Matlab/Simulink, Real-Time Workshop an xpc Target toolbox). xpc Target operates with the coe generate by a C compiler from the Simulink moels in orer to evelop the real-time target application. Target application can be execute in real time once it is ownloae in the target PC from the host PC through TCP/IP. The target PC is equippe with ata acquisition Cars (DAQ s). The measurements are carrie out with hall effect sensors to measure the DC-link current an the emane current from the gri an voltage sensors to capture the DC-link voltage an voltages at PCC. To evaluate the performance of a commercial PV inverter, the power quality analyzer Fluke 435 Series II has been connecte to the power stage, which allows measuring the PV inverter performance. IV. CURRENT CONTROLLER. REFERENCE TRACKING A hysteresis ban Fig. 4 has been use as current controller in the PV array emulator to track the emane reference current. This technique compares the reference current with the measure current an epening of the sign of the error the switching signals of the IGBT s are generate. a) VI. MPPT STATIC EVALUATION In orer to evaluate commercial PV inverter efficiency, the MPPT algorithm must be evaluate. The efficiency of this algorithm an it proper operation are essentials to inject the maximum amount of energy prouce by the PV panels. The MPPT efficiency is calculate as follows (1) MPPT Ppv PMPP 100, (1) where Ppv an PMPP are the theoretical power of the PV array an the power at the MPP respectively etermine it by the commercial PV inverter. To analyze the static efficiency of a commercial inverter, an experimental test has been carrie out. The ifferent parameters use uring experimental tests are shown in Table I. b) Fig. 4. a) Hysteresis ban block iagram. b) Switching signal generation. V. PV ARRAY EMULATOR EXPERIMENTAL SETUP A block iagram of the experimental prototype an the Fig. 5. Block iagram of the experimental test setup. 73

4 TABLE I. USED PARAMETERS IN EXPERIMENTAL TESTS. Parameter Description Value Stage n p Number of panel in parallel n s Number of panel in series 12 I sc Short-circuit current (A) 4.8 V oc Open circuit voltage (V) 43.4 V mpp Voltage at MPP (V) 34 I mpp Current at MPP (A) 4.41 T Temperature (ºC) 25 W Irraiance (W/m 2 ) Array configuration PV panel parameters. Shell SP 150- P Conitions V s RMS voltage at PCC 400 Electrical gri L f Filter inuctance (mh) 6.2 Passive C 1,C 2 DC-link capacitors (mf) 28.2 elements f w Switching frequency (khz) 12 The commercial PV inverter that has been stuie is the SMA Sunny Mini Central 6000TL. Figure 6 shows the voltage at the PCC an the emane current (phase A) by the PV emulator an the proper synchronization in phase between current an voltage is emonstrate. In Fig. 7 we can see DC-link voltage an current when the MPP is achieve by the commercial inverter. Accoring to the theoretical parameters of the PV panel Shell SP 150-P provie by manufacturers at MPP an the PV array configuration, the MPP static efficiency is 98 %. VII. MPPT DYNAMIC EVALUATION In orer to analyze the ynamic MPPT efficiency an the require time to reach the MPP in variable conitions of irraiance, ifferent irraiance profiles have been implemente. First of all, the time since the MPP algorithm start to work when the commercial inverter is connecte to the electrical gri till the MPP is reache has been analyze. This evolution is shown in Fig. 8. The time that the commercial inverter requires to achieve the MPP is 9 secons. Also, a step in the irraiance from 1000 W/m 2 to 700 W/m 2 an to 1000 W/m 2 again has been one. The response of the MPP algorithm is epicte in Fig. 9. The time to reach the new MPP is 150 ms. Fig. 6. Voltage at PCC an emane current (phase A). Fig. 7. PV voltage an PV current. 74

5 IX. CONCLUSIONS A PV array emulator has been propose an valiate in this paper. It reprouces any PV panel an array association with a high level of accuracy by only using the information provie by manufacturers of PV panels in atasheet. It evice is suitable for testing commercial PV inverter at high power level. Also, it is possible to program any irraiance an temperature profiles which allow evaluating MPP tracking algorithm efficiency an power energy conversion performances. Fig. 8. PV voltage an PV current evolution when the commercial inverter searchs the MPP. Fig. 9. PV voltage an PV current uring irraiance steps. VIII. PERFORMANCE EVALUATION To calculate the performance of the energy conversion on the DC-link power an the AC output power the power quality analyzer Fluke 435 Series II has been connecte to the power stage. This equipment has the capability of evaluating the power inverter performance. It simultaneously measure AC output power an DC input power for power electronics system. The obtaine performance at rate power of the inverter SMA Sunny Mini Central 6000TL is % as Fig. 10 shows. Fig. 10. Calculate performance by the power quality analyzer Fluke 435 Series II of the teste commercial inverter. REFERENCES [1] International Energy Agency. Photovoltaic Power Systems Programme, [Online]. Available: Statistic Reports, [2] A.Anrijanovits, H.Hoimoja, D. Vinnikov, Comparative review of long-term energy storage technologies for renewable energy systems, Electronics an Electrical Engineering (Elektronika ir Elektrotechnika), no. 2, pp , [3] V. Salas, E. Olias, Overview of the Photovoltaic Technology Status an Perspective in Spain, Renewable an Sustainable Energy Reviews, vol. 13, pp , [Online]. Available: [4] D. Vasarevicius, R. Martavicius, M. Pitukis, Application of artificial neural network for maximum power point tracking of photovoltaic panels, Electronics an Electrical Engineering (Elektronika ir Elektrotechnika), no 10, pp [5] F. Baumgarther, H. Schmit, B. Burger, R. Brunlinger, H. Haberlin an M. Zehner, Status an relevance of the c voltage epenency of the inverter efficiency, in 22 n European Photovoltaic Solar Energy Conf. an Exhibition, no. Session 4Do.4.6, 3 7 Sept [6] M. Valentini, A. Raucu, D. Sera, R.Teoorescu, PV inverter test setup for European efficiency, static an ynamic MPPT efficiency evaluation, Optimization of Electrical an Electronic Equipment, OPTIM th Int. Conf, May 2008, pp [7] J. M. Enrique, E. Duran, M. Sirach e Carona, J. M Anujar, M. A. Bohorquez, J. Carretero, A new approach to obtain I-V an P-V curves of photovoltaic moules by using DC/DC converters, Rec. IEEE Photovoltaic Specialist Conf., 2005, pp [8] E. Duran, J. M. Enrique, M. A Bohorquez, M. Sirach e Carona, J. E. Carretero, J. M. Anujar, A new application of the coupleinuctors SEPIC converter to obtain I-V an P-V curves of photovoltaic moules EPE Conf., Sept. 2005, pp P.1-P- 10. [9] M. C. Di Piazza, C. Serporta, G. Vitale, A DC/DC Converter Base Circuit Moel for a Solar Photovoltaic, 21 th European Photovoltaic Solar Energy Conf. an Exhibition, 4 8 Sept. 2006, Dresen, Germany, pp [10] M. Cirrincione, M. C. Di Piazza, G. Marsala, M. Pucci, G. Vitale, Real time simulation of renewable sources by moel-base control of DC/DC converters, Inustrial Electronics, ISIE IEEE Int. Symposium, pp , June July [11] B. M. Wilamowski, J. D. Irwin, The Inustrial Electronics Hanbook. Power Electronics an Motor Drives. Secon Eition [12] M. I. Milanés-Montero, E. Romero-Caaval, V. M. Minambres, F. Barrero-Gonzalez, Novel Metho for Synchronization to Disturbe Three-phase an Single-phase Systems, IEEE Int. Symposium on Inustrial Electronics, 2007, pp [13] C. Roncero-Clemente, E. Romero-Caaval, P. Roncero-Sanchez, E. Gonzalez-Romera, Comparison of Two Power Flow Control Strategies for Photovoltaic Inverters. 38 th Annual Conf. of the IEEE Inustrial Electronics Socienty IECON Montreal, Canaa, pp