How To Compare The Harmonic Performance Of A Zsource Inverter To A Power Supply (Zsi)

Transcription

1 EASR - Engineering, echnology & Applied Science Research Vol. 3, o., 013, Performance Analysis of Different Control Strategies in a Z-source Inverter Byamakesh Nayak School of Electrical Engineering KII University, Bhubaneswar Orissa, India electricbkn11@gmail.com Saswati Swapna Dash Dpt of Electrical Engineering YMCA University of Science & ech. Faridabad, Haryana, India rehtoswapna@gmail.com Abstrt his paper presents the analysis for different control techniques in Z-source inverters. he voltage gain versus modulation index from simulation result is compared with the mathematical calculated voltage gain. Further detailed analysis of %HD, %harmonics of output voltage at different modulation indexes for different ing techniques of a Z-source inverter are also performed with respect to the traditional by MALAB based simulation. Keywords- Z-source inverter; shoot through; ftor; voltage gain; voltage stress; total harmonic distortion (HD) switching cycle, to hieve the same optimal harmonic performance. his paper represents the comparison of voltage gain, %HD and %harmonics at different modulation indexes using different ing techniques, in Z-source inverter. he ing techniques for the Z-source inverter used in this paper are the simple control, the maximum control, the 3 rd harmonic injected constant control and the 3 rd harmonic injected maximum control method. I. INRODUCION Conventionally two types of three phase inverters are used, the voltage source inverter () and the current source inverter (CSI). In, a capitor is used in a DC link to obtain a constant voltage source and in CSI a large inductor is used to obtain a constant current source. hese inverters have certain limitations. Commonly used Pulse Width Modulated (PWM) is used for buck (step down) DC to AC power conversion. An additional DC-DC converter is needed to the AC voltage output. he CSI is used as a inverter in DC to AC conversion. For a change in voltage range, it also requires additional buck (or ) DC-DC converters. Hence, no traditional inverter can be employed solely for buck and simultaneously, without additional DC link converters. o overcome the drawbks of traditional inverters, the newly proposed impedance source or Z-source inverter [1] can be used, which is virtually the combination of and CSI. It employs an impedance network as a DC link consisting of L and C to couple the converter bridge and the power source as shown in Figure 1. A traditional PWM 3-phase operates in eight states, six tive and two null states. In tive states, power is transported from the DC link to the AC output, whereas in null states the DC link capitor is charged from the DC source. he shootthrough state is introduced within the null state of Z-source inverter. his cannot be applied in voltage source inverter. he shoot-through state in Z-source inverter enhances the voltage at the load end. It should be inserted in such a way that, equal null intervals are again maintained at the start and end of the II. Fig. 1. Z-source inverter CONROL ECHNIQUES A. control In the sinusoidal PWM inverter, the gate pulses are obtained by comparing the 3-phase balanced sinusoidal carrier signals with triangular carrier signals. he drawbk of the method is that it cannot be used for ing of the output voltage because of the two null states in addition to the six tive states in one cycle of operations. he shoot-through should be introduced in null states by various techniques as discussed in [1-] to the output voltage. he shootthrough techniques are objectionable in sinusoidal PWM voltage source inverter. he shoot-through can be inserted in null-state if Z-source is incorporated between the input DC voltage and the inverter power circuit as shown in Figure 1. In the switching cycle, the zero state is produced when all sinusoidal waveforms are less or greater than the carrier signal. herefore, to provide shoot through in zero state the

2 EASR - Engineering, echnology & Applied Science Research Vol. 3, o., 013, other two steady signals (V p,v n ) whose amplitudes are equal to the amplitude of the sinusoidal waveform and the one is the negative of the other, are compared with triangular signal as shown in Figure. As discussed in [1-] the shoot through states are obtained in all three legs at a time, maintaining shoot through time per switching cycle as constant. he gate pulses for the simple method are shown in Figure. he voltage gain in simple constant ZSI is expressed as: G= MB= V o (1) where is the output peak phase voltage, V o is the input DC voltage, M is the modulation index and B is the ftor. he ftor is determined by following expression: 1 B= 0 () where o is the shoot-through time interval over a switching period. he duty ratio ( 0 /) decreases with the increase of M in shoot-through state. hus the voltage gain can be expressed as: M o M 1 Voltage stress [] is given by: ( ) V = BV = G 1 V (4) (3) Voltage stress in switches increases as the voltage gain increases. So, this control method cannot be implemented to obtain higher gain because of the limitation of voltage ratings of switches. herefore, other suitable control techniques can be adopted to operate the Z-source inverter at higher gains. B. Maximum control Voltage stress is minimized on switches by maximizing o and B for a given modulation index. Considering the previous method, a simple modification is required. he shoot-through state is hieved when the triangular carrier wave is either greater than the maximum curve of three sinusoidal references or smaller than the minimum of the references []. Figure 3 depicts the gate pulse for six switches. Averaging of the varying shoot-through times, the ftor, voltage gain and voltage stress are given by the following equations: 1 π B= = o 3 3M π M 3 3G V = BV = V π (5) (6) (7) he voltage stress is much lower in comparison to the simple control. Hence, the inverter can be operated at a higher voltage gain. C. hird harmonic injection control his method is opted to use the increased range of modulation indexes. Similarly to previous methods, the overshoot period can be maintained constant and variable. In third harmonic injection constant technique the sinusoidal reference signal is merged with third harmonic sinusoidal waveforms with one third amplitude of the fundamental, to generate non-sinusoidal waveforms. Constant shoot-through is introduced in zero states by comparing the triangular carrier wave with a positive, negative constant magnitude and generated non-sinusoidal sinusoidal reference waveforms as in simple control. he gate pulses are shown in Figure 4. Further, variable shoot-through times are provided considering the third harmonic injected sinusoidal reference waveforms and triangular carriers like maximum control. he gate pulses are shown in Figure 5. Fig.. Gate pulse of simple constant ZSI

3 EASR - Engineering, echnology & Applied Science Research Vol. 3, o., 013, he average ftor and voltage gain in the above two cases [] are given as follows: Fig. 4. Fig. 3. Gate pulse of maximum ZSI Gate pulse for third harmonic injected constant ZSI 1 π B= = π M 3 3G V = BV = V π (8) (9) (10) he voltage gain remains the same as in the maximum control, so voltage stresses also remain the same. he only advantage in harmonic injected methods is the use of large range modulation indexes (M). III. PERFORMANCE ANALYSIS MALAB simulations were performed to verify and compare different control methods. he SIMULINK model used is shown in Figure 6. he parameters used are: L 1 =L =0.5 mh, C 1 =C = mf, V 0 =100 V (DC), Switching frequency= khz, Load: R/phase = 50 ohm, L/phase = mh. he purpose of the system is to compare the performance of the different control techniques under different modulation indexes with constant DC voltage input. A comparative analysis of voltage gain at different M is performed based on the mathematical calculation and simulation result for various control techniques. Results are shown in Figures he simulation results for voltage gain mostly matches the theoretical analysis, which verifies the above analysis and control concepts. he comparative bar graph in Figure 11 for per unit output voltage at different modulation index shows the ing of voltage with respect to. able I and the bar graph shown in Figure 1, compares the %HD of output voltage at selected modulation indexes for various control methods of ZSI with. HD in different ing methods is a little higher in comparison to, but their ranges are within the permissible limit. he maximum control technique yields higher %HD in comparison to constant technique. ABLE I. %HD COMPARISON OF OUPU VOLAGE. %HD in Different inverters Fig. 5. Gate pulse of third harmonic injected maximum ZSI

4 EASR - Engineering, echnology & Applied Science Research Vol. 3, o., 013, Fig. 6. Circuit diagram of ZSI Fig. 9. Comparison of voltage gain vs modulation index for 1/6th third harmonic injected constand ZSI Fig. 7. Voltage gain vs modulation index comparison for simple control Fig. 10. Comparison of voltage gain vs modulation index for 1/6th third harmonic injected ZSI Fig. 11. pu voltage comparison of ZSI with Fig. 8. Voltage gain vs Modulation index comparison for maximum control

5 EASR - Engineering, echnology & Applied Science Research Vol. 3, o., 013, %HD Fig. 1. M Comparison of %HD for different ZSI with at different modulation index able II represents the comparison of %age harmonics of output voltage for third, fifth and seventh order in different control methods at selected modulation index (M). he percentage harmonics of constant ZSI are at par with. But the seventh harmonic in maximum control technique is slightly higher. he simulated output voltage and current waveforms at M=0.7 with passive R-L load in third harmonics injected maximum ZSI is shown in Figure 13 for illustration. ABLE II. % HARMONICS COMPARISON OF OUPU VOLAGE. (a) 3 rd harmonics (b) 5 th harmonics Fig. 13. Simulated results of load line voltage and load current IV. CONCLUSION his paper compares the performances of the control methods of ZSI to hieve the maximum voltage output within the prescribed limit of harmonics and HD percentages. he method can hieve the minimum passive component requirement and maintain low voltage stress at the same time. his method can be suitable for controlling voltage sags and swells in drives. REFERENCES [1] F. Z. Peng, Z-source inverter, IEEE ranstions on Industry Applications, Vol. 39, No., pp , 003 [] F. Z. Peng, M. Shen, Z. Qian, Maximum control of the Z-source inverter, IEEE ranstions on Power Electronics, Vol. 0, No. 4, pp , 005 [3] P. C. Loh, D. M. Vilathgamuwa, Y. S. Lai, G.. Chua, Y. Li, Pulsewidth modulation of Z-source inverters, IEEE ranstions on Power Electronics, Vol. 0, No. 6, pp , 005 (c) 7 th harmonics