The Application of a Quasi Z-Source AC-AC Converter in Voltage Sag Mitigation

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1 The Application of a Quasi Z-Source AC-AC Converter in Voltage Sag Mitigation A. Kaykhosravi 1, N.A.Azli 2, F. Khosravi 3, E. Najafi 4 Faculty of Electrical Engineering, Universiti Teknologi Malaysia, UTM Johor Bahru, Malaysia 1,2,3,4 Electrical Engineering Faculty, Kermanshah Islamic Azad University, Kermanshah, Iran 3 Electrical Engineering Faculty, Gom University of Technology, Gom, Iran 4 avinkaykh@gmail.com 1 naziha@ieee.org 2 Abstract This paper deals with the employment of a singlephase Quasi Z-source AC-AC converter in voltage sag mitigation application. The proposed structure overcomes the traditional structure drawbacks and has the following features; input and output voltages sharing the same ground, operating in the continuous current mode (CCM), using direct AC-AC converter, using safe-commutation strategy, capable of extending the output voltage range, improves reliability and provides the suitable output voltage with reduced harmonic. The results of a simulation study conducted on the proposed voltage sag mitigation structure have revealed its capability in compensating voltage sag. Keywords-Quasi Z-source; AC-AC converter; voltage sag compensator I. INTRODUCTION Advanced industrial systems consist of voltage sensitive elements. On the occurrence of power supply quality reduction due to voltage variations which often accrue in the form of voltage sags, these elements are prone to being destroyed and in consequent import serious interruptions in distribution systems [1]. Voltage sag is defined as the decrease in the AC voltage amplitude in a network s main frequency for a period of 0.5 cycles to 1 minute. The cause of sag is due to conditions such as atmospheric discharges, transformer energizing, short circuit or fault in the power distribution system, a major increase of load current due to the start-up power demands of many electrical loads (such as high-power rated motors, compressors, elevators, air-conditioners, etc.), turning on of motor, high power loads, soldering machines, and arc furnaces [2]. The sag phenomenon for short duration can lead to severe problem to equipment that use voltage sensitive devices, for instance in adjustable speed drives, process control equipment, and computers [3]. Fig. 1 shows an illustration of voltage sag. In solving the problems related to voltage sag, several affordable solutions have been suggested [2-14]. Some of these solutions use energy-storage component such as battery banks or large capacitors, which contribute to the rise in cost [11], [13]. For instance in [2] the significant drawback of tapchanging transformers is complex operation for fast response due to the use of large number of thyristors and poor temporary voltage rejection. Figure 1. Voltage sag Tap transformers need perpetual upkeep and the overall system is also high in complexity. Others are based on AC-AC converter topologies which do not require any energy-storage devices [2], [3], [12], [14]. Some topologies of the Dynamic Voltage Restorer (DVR) have been assessed and classified in [10]. Their main shortcomings include standby losses, component cost and the protection scheme that is typically required for downstream short circuits. Fig. 2 shows a traditional voltage sag compensation structure based on Z- source network that has been proposed in [7]. The main drawback of this structure is that it can only overcome voltage sag that is over 50%. This feature may not be useful for sensitive loads as these loads typically encounter voltage sags that are under 50%. Figure 2. Single-phase Z-source voltage sag compensator /12/$ IEEE 548

2 This paper proposes a voltage sag mitigation structure which employs a Quasi Z-source network that inherits all the benefits of the Z-source network like the buck-boost property as well as reversing or maintaining the phase angle. In comparison to the use of the Z-source network this proposed structure has these advantages: its input and output voltages share the same ground and it operates in the continuous current mode (CCM). The later contributes to the preservation of the input current sinusoidal waveform and reduction in its Total Harmonic Distortion (THD). The proposed structure also has the following features: Use of a direct AC-AC converter (provides better power factor and efficiency, low harmonic current in the line, single-stage conversion, simple topology, ease of control, smaller size and lower cost [15], [16]- [18]) Compensating voltage sag for up to 50% Use of a safe-commutation strategy (can significantly improve the performance of an AC-AC converter and makes it possible to avoid voltage spikes on the switches) Capable of extending the output voltage range Improves the reliability The next section gives the details of the proposed voltage sag mitigation structure which include its circuit operation and control method. This is followed by the results of the simulation study conducted on the proposed structure and the conclusion of the paper. II. THE PROPOSED STRUCTURE A. Circuit Operation Fig. 3 shows the proposed voltage sag mitigation structure that is mainly based on a Quasi Z-source AC-AC converter. It consists of an AC input, the Quasi Z-source AC-AC converter (comprises of two inductors L 1, L 2 and two capacitors C 1, C 2 ), four IGBT power switches S 1a, S 1b, S 2a, S 2b, an L f C f filter, an injection transformer and the load. The Quasi Z-source AC-AC converter has two types of operational state: state 1 and state 2. Fig. 5 shows the operational states in boost in-phase mode [19] of which the converter focuses on when v i >0. The switches S 1a and S 2b are fully turned on; S 1b and S 2a are modulated complementary. The analysis when v i <0 is similar to v i >0. The dotted line in Fig. 4 indicates the safe-commutation switch during each particular stage. If v i <0 the switches S 1b and S 2a are fully turned on; S 1a and S 2b are modulated complementary. Further details on the operation states are as indicated in [19]. When the proposed structure mitigates the input voltage sag in the boost in-phase mode, the Quasi Z-source converter should work in the buck/boost out-of-phase mode. Fig. 4 illustrates the switching strategy when the converter operates in the boost in-phase mode. As illustrated in Fig. 4, D refers to the equivalent duty ratio, and T is the switching period. According to [19], the voltage across the capacitor C f in the proposed structure can be calculated as follows, (1) where; is the voltage across the filter capacitor. The output voltage across the load in terms of the input voltage can be obtained as follows, (2) (3) Figure 3. The proposed single-phase four switches Quasi Z-Source network for sag mitigation Figure 4. The switching pattern of the single-phase Quasi Z-Source AC-AC converter in boost in-phase mode 549

3 Figure 5. Operation state for Quasi Z-source converter in boost in-phase mode when v i > 0. (a) State1; (b) Commutation state when i i+i L2-i f > 0; (c) Commutation state when -i i-i L2+i f > 0; (d) State2 Referring to (3), Fig. 6 shows the graph of the output voltage gain versus duty cycle for the proposed structure. Fig. 6 clearly shows the two operation regions. When the duty cycle is less than 0.5, the output voltage is boosted and inphase with the input voltage. When the duty cycle is greater than 0.5, the output voltage is bucked/boosted and out-ofphase with the input voltage. The former mode of operation has been found to be beneficial in the design of the proposed voltage sag mitigation structure. Figure 6. Output voltage gain versus duty cycle of proposed voltage sag compensator using Quasi Z-Source network B. Control Method As shown in Fig. 7 a simple feed-forward control method is used to generate the gate signals of the Quasi Z-Source AC- AC converter power devices employed in the proposed voltage sag compensator. The control method has been designed to ensure that a regulated output voltage is generated despite the occurrence of voltage sag in the input voltage. III. SIMULATION RESULTS In order to verify the functionality of the proposed voltage sag mitigation structure, simulation results were obtained using MATLAB/Simulink. The parameters that have been used in the simulation study are L 1 =L 2 =1 mh, C 1 =C 2 =6.8 μf, L f =1.4 mh, C f =6.8 μf, R=20 Ω. The switching frequency is set to 10 KHz. The input voltage is 150 V (peak)/50 Hz. Fig. 8 until Fig. 10 show the simulation results based on a resistive load (R) and for a few different sags in the input voltage. Fig. 8 shows the input voltage under different voltage sag conditions, output voltage after compensation and the compensator voltage waveform in the occurrence of voltage sag. Fig. 9 is the zoomed version of Fig. 8. As observed in Fig. 8 and Fig. 9, the output voltage does not seem to exhibit any voltage spike, demonstrating that compensation is successfully accomplished using the proposed voltage sag mitigation structure. In addition, Fig. 10 shows the harmonics spectra of the output voltage with THD generated as less than 1% indicating that the output voltage waveform is very close to sinusoidal. To show the consistency between the theoretical concept based on (3) and the simulation results, Fig. 11 depicts the duty cycle value during the sag mitigation for the proposed structure. 550

4 2012 IEEE International Conference on Power and Energy (PECon), 2-5 December 2012, Kota Kinabalu Sabah, Malaysia Figure 7. Control method of the proposed structure Figure 8. Input, output and compensator voltage under a few different voltage sag conditions Figure 9. Input, output and compensator voltage under a few different voltage sag conditions (zoomed) 551

5 Figure 10. Harmonics spectra and THD of the output voltage Figure 11. Duty Cycle during the sag mitigation IV. CONCLUSIONS This paper has presented the application of a Quasi Z-source AC-AC converter in a new voltage sag mitigation structure. The circuit operation and control method of the proposed structure have been discussed. The results obtained from the simulation study have revealed the effectiveness of the proposed structure in mitigating various voltage sag conditions. In particular, unlike the circuit presented in [7], the proposed structure is capable of compensating voltage sag in the normal range of 0 to 50%. These results would become the basis for further investigation in the overall performance of the proposed structure as mentioned earlier which includes improving power factor and efficiency. ACKNOWLEDGMENT The authors would like to thank the Research Management Centre (RMC) of Universiti Teknologi Malaysia and the Ministry of Higher Education (MOHE) for the funding of this project through Vote Number Q.J H87. PWM-Switched Autotransformer, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 2, MARCH [4] Myung-Bok Kim, Gun-Woo Moon, and Myung-Joong Youn, Synchronous PI Decoupling Control Scheme for Dynamic Voltage Restorer against a Voltage Sag in the Power System, IEE conference, pp , [5] Choi, S.S., Wang, T.X., and Sng, E.K, Power quality enhancement in an isolated power system through series compensator, Proc. of 15th Power System Computation Conf., PSCC 05, Liege, Belgium, August, [6] T.X. Wang, S.S. Choi and E.K.K. Sng, Series compensation method to mitigate harmonics and voltage sags and s wells, IET Gener. Transm. Distrib. Vol. 1, No. 1, pp , January [7] M. K. Nguyen, Y. G. Jung, and Y. C. Lim, Voltage swell/sag compensation with single-phase Z-source AC/AC converter, in Proc. EPE 09, pp. P.1-P.8, [8] Steven M. Hietpas, Recayi Pecen, Simulation of a Three-phase AC-AC Boost Converter to Compensate for Voltage Sags, IEE conference, pp. B-4-1-7, [9] F. A. L. Jowder, Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation, IET Generation, Transmission & Distribution, vol. 3, ISS. No 6, pp , [10] NEILSEN J.G., BLAABJERGF, A detailed comparison system topologies for dynamic voltage restorers, IEEE Trans. on Ind. Application, Vol. 41, no. 5, pp , [11] D. M. Vilathgamuwa, C. J. Gajanayake, P. C. Loh, and Y.W. Li, Voltage Sag Compensation With Z-Source Inverter Based Dynamic Voltage Restorer, IEEE conference, pp , [12] S. M. Hietpas, and M. Naden, Automatic voltage regulator using an ac voltage-voltage converter, IEEE Trans. on Ind. Appl., vol. 36, no. 1, pp , January-February [13] D. O. Kisck, V. Navrapescu, M. Kisck, Single-phase unified power quality conditioner with optimum voltage angle injection for minimum VA requirement, IEEE International Symposium on Industrial Electronics, ISlE 2007, pp , [14] E. C. Aeloiza, P. N. Enjeti, L. A. Moran, O. C. Montero-Hernandez, and S. S. Kim, Analysis and design of a new voltage sag compensator for critical loads in electrical power distribution systems, IEEE Trans. on Ind. Appl., vol. 39, no. 4, pp ,2003. [15] N. A. Ahmed, K. Amei, and M. Sakui, A new configuration of singlephase symmetrical PWM ac chopper voltage controller, IEEE Trans. Ind. Electron., vol. 46, no. 5, pp , [16] F. L. Luo, and H. Ye, Research on dc-modulated power factor correction ac/ac converters, in Proc. IEEE IECON 07, pp , [17] X. P. Fang, Z. M. Qian, and F. Z. Peng, Single-phase Z-source PWM ac-ac converters, IEEE Power Electron, Letters, vol. 3, no. 4, pp , [18] Y. Tang, S. Xie, and C. Zhang, Z-source ac-ac converters solving commutation problem, IEEE Trans. Power Electron., vol. 22, no. 6, pp , [19] M. K. Nguyen, Y. G. Jung, and Y. C. Lim, single-phase Quasi Z-source AC-AC converter with safe-commutation strategy, IEEE conference, pp , REFERENCES [1] D. M. Vilathgamuwa, C. J. Gajanayake, P. C. Loh, and Y. W. Li, Voltage Sag Compensation With Z-Source Inverter Based Dynamic Voltage Restorer, IEEE Conference, pp , [2] Steven M. Hietpas, and Mark Naden, Automatic Voltage Regulator Using an AC Voltage Voltage Converter, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS,vol. 36, no. 1, pp , [3] Dong-Myung Lee, Thomas G. Habetler, Ronald G. Harley, Thomas L. Keister, and Joseph R. Rostron, A Voltage Sag Supporter Utilizing a 552