Design Analysis of Low-Pass Passive Filter in Single-Phase Grid-Connected Transformerless Inverter

Transcription

1 0 IEEE First Conerence on Clean Energy and Technology CET Design Analysis o Low-Pass Passive Filter in Single-Phase Grid-Connected Transormerless Inverter Maaspaliza Azri and Nasrudin Abd. Rahim Faculty o Electrical Engineering, Universiti Teknikal Malaysia Melaka (UTeM), Melaka, Malaysia UMPEDAC Research Centre, University o Malaya, Kuala Lumpur, Malaysia maaspaliza@utem.edu.my Abstract Presented is the design analysis o a single-phase gridconnected photovoltaic-inverter low-pass-output ilter. It minimizes switching-requency current harmonics, improving output response. The inverter is H-Bridge transormerless. Switching requencies 8Hz, 4kHz, and 0kHz were compared or validation o the simulation and the experiment. Keywords - Transormerless DC-AC converter, LC ilter system. S re m a () S3 c S5 DC Filter load I. INTRODUCTION S S4 Transormerless is the norm in grid-connected voltage source inverter (SI) designs o today. SI with transormer decreases system eiciency around % to % []. A new, highly eective single-phase grid-connected transormerless inverter topology proposed comprises H-bridge inverter, diode rectiier, and auxiliary switch [] (see Fig. ). Leakage current and saety hazard issues are disadvantages o a transormerless grid-connected system []. Leakage current in transormerless inverter creates system losses and inject harmonics into grid [3]. A low-pass passive ilter is usually positioned between inverter and grid to attenuate current harmonics injected into grid [4, 5, 6, 7]. Relatively simple and needs no additional control part, its output is the ocus o this study, whose calculation method shortened the design time. II. THEORY A. SPWM technique and harmonic Sinusoidal pulse width modulation (SPWM) is an inverter controller technique in which the output voltage is controlled by the ON and the OFF states o the switches. The states are pulses obtained by comparing reerence signal re with triangular carrier signal c; ON state is when re is greater than c, OFF state is when re is less than c. Fig. illustrates the principle o SPWM. Equation () deines the modulation ratio: Figure : Topology o the proposed H-Bridge transormerless inverter re Switch pulses carrier Figure : SPWM principle Equation () deines the requency modulation ratio: carrier m () reerence Equation (3) gives the RMS (root mean square) value or output voltage o the inverter s undamental requency: This work was supported by the Post-Graduate Research Grant (PPP) University Malaya, Malaysia under grant PS9-00A 348

2 0 IEEE First Conerence on Clean Energy and Technology CET dc o, rms ma (3) r L C Hz (9) Sidebands represent the inverter s output harmonics, whose waveorms appear around the center o the switching requency in multiples o the switching requency [8]. Equation (4) deines the output current harmonics: To avoid resonance eect and ensure carrier attenuation, ilter resonance requency should be less than carrier requency. Filter THD I n ( I I n, rms ) l, rms (4) inverter L C R load B. Low-pass ilter design A simple method reducing inverter output harmonic uses low-pass passive ilters: L, LC, and LCL. Fig. 3 is the circuit or the LC low-pass passive ilter used in this study. LC ilter is second-order type o ilter, where the inductor is shunt with the capacitor. Equations (5) and (6) express LC ilter transer unction as derived rom voltage-divider rule. G jw) ( jw) LC( jw) o ( (5) in jwl / R G (6) Use o LC-ilter lowers cost and losses o the inverter system. Equation (7) is or ilter inductance. The maximum ripple current was chosen to be 5%-0% (typical value o maximum ripple current is 0% o rated current [9]). III. L Figure 3: LC ilter circuit SIMULATION RESULTS The LC ilter value resulting rom the calculation was simulated on the PSIM program; see Table or the simulation s parameter values. Fig. 4 is the uniltered outputcurrent waveorm, Fig. 5 the uniltered FFT-inverter current output. The harmonic peak o output current io at 8kHz read.a. Figs. 6 and 7 are the results or LC-iltering between the SI and the grid load; the output current harmonic peak reduced rom.a to 6.03mA. Figs. 8 and 9 are the switching requencies tested. Fig.0 the requency response o the LC ilter when used 8kHz switching requency. The cuto requency is 3.75 khz. L 8 DC ripple,max Filter capacitance is determinable by the reactive power absorbed in the ilter capacitor; Equation (8) deines it. being the reactive power actor, its value was selected to be less than 5% [0]. P rated C line rated sw (7) (8) Table : Parameter alues L L, L C R load dc sw C dc, DC-link capacitor Dead time 3.6mH.8mH uf 5ohm 60 8kHz,4kHz,0kHz 50uF 0.8u sec Equation (9) deines the resonance requency o the AC circuit: 349

3 0 IEEE First Conerence on Clean Energy and Technology CET 4kHz,mA Figure 4: Uniltered inverter output current Figure 8: LC-iltered (4kHz) FFT-inverter output current 8kHz,.A 0kHz,mA Figure 5: Uniltered FFT-inverter output current Figure 9: LC-iltered (0kHz) FFT-inverter output current 3.75 khz Figure 6: LC-iltered output current Figure 0: LC-iltered requency response, r = 3.75kHz 8kHz, 6.03mA I. EXPERIMENT RESULTS Figure 7: LC-iltered (8kHz) FFT-inverter output current A prototype was built to test the theoretical operation o the LC ilter design. An experiment was set up in which the same parameters as those o the simulations were used. Control o the prototype was by TMS30F8 DSP. Fig. shows the S-S5 switching patterns, generated or the H-Bridge transormerless inverter topology. Figs. and 3 are experiment results or the output inverter current tested with, and without, LC ilter. alues o switching requency were also tested on the prototype. Figs. 4, 6, and 8 are the output voltages and 350

4 0 IEEE First Conerence on Clean Energy and Technology CET currents measured, showing output voltage against decreased switching requency. Figs. 5, 7, and 9 are the THD results or each o the switching requencies. Grid-injected harmonics was ound to be the least at 8kHz switching requency. Fig. 0 is THD versus modulation ratio (ma) curve or the various switching requencies. The proposed low-pass passive ilter was designed and calculated or 8kHz switching requency. THD was ound to decrease little and slowly with increases in modulation ratio. The THD at 8kHz switching requency thus bettered those at the other switching requencies. Fig. is the system eiciency, which, at 8kHz switching requency, was ound to be 88%. Figure 3: LC-iltered output voltage and output current Figure 4: Inverter voltage and current outputs, s=0 khz Figure : Switching patterns Figure : Uniltered inverter output Figure 5: Inverter output-current THD, s=0 khz 35

5 0 IEEE First Conerence on Clean Energy and Technology CET Figure 6: Inverter voltage and current outputs, s=4 khz Figure 9: Inverter output-current THD, s=8 khz Figure 0: THD curves o the various switching requencies Figure 7: Inverter output-current THD, s=4 khz Figure : Eiciency o the H-Bridge transormerless inverter, measured against various switching requencies Figure 8: Inverter voltage and current outputs, s=8 khz. CONCLUSION Outputs o an LC low-pass passive ilter or a singlephase grid-connected transormerless inverter have been presented. Calculations made on equations were simulated on PSIM, a program that not only acilitated study and analysis o the basic ilter, but also shortened the ilter s design time. 35

6 0 IEEE First Conerence on Clean Energy and Technology CET Experiment results veriied the proposed design or the LC low-pass passive ilter as satisying the below-5% output THD requirement at 8kHz switching requency. ACKNOWLEDGMENT The authors thank University o Malaya or the Post- Graduate Research Grant (PPP), PS9-00A unding o this project. REFERENCES [] T. Kerekes, R. Teodorescu, P. Rodriguez, G. azquez, E. Aldabas, A new high-eiciency single-phase trasormerless P inverter system, IEEE Transactions on Industrial Electronics,olume 58, Issue, pp. 84-9, January 0. [] J. Selvaraj and N.A. Rahim, Multilevel Inverter or Grid-Connected P System Employing Digital PI Controller, IEEE Transactions on Industrial Electronics, olume 56, Issue, pp , January 009. [3] O. Lopez, F.D. Freijedo, A.G. Yepes, P. Fernandez-Comesaa, J. Malvar, R. Teodorescu, J. Doval-Gandoy, Eliminating Ground Current in a Transormerless Photovoltaic Application, IEEE Transactions on Energy Conversion, olume 5, Issue, pp. 40, March 00. [4] Khaled H.Ahmed, Stephen J. Finney and Barry W. Williams, Passive Filter Design or Three Phase Inverter Interacing in Distributed Generation, Electrical Power Quality and Utilisation Journal,olume 3, No., 007. [5] Hanju Cha, u T.K, Comparative Analysis o Low Pass Output Filter or Single-Phase Grid-Connected Photovoltaic Inverter, Applied Power Electronics Conerence and Exposition (APEC) 00, Twenty-Fith Annual IEEE, issue -5, pp. 659, Feb. 00. [6] J. Kim, J. Choi, H. Hong, Output LC Filter Design o oltage Source Inverter Considering the Perormance o Controller, International Conerence on Power System Technology, olume 3, pp. 659, Dec [7] P.A. Dahono, A. Purwadi, Qamaruzzaman, An LC Filter Design Method or Single-Phase PWM Inverters, International Conerence on Power Electronics and Drive Systems, pp. 57, Febuary 995. [8] Daniel W. Hart, Introduction to Power Electronics,997,Prentice Hall. 353