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

A. Kaykhosravi1, N.A.Azli2, F. Khosravi3, E. Najafi4

Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Malaysia1,2,3,4

Electrical Engineering Faculty, Kermanshah Islamic Azad University, Kermanshah, Iran3 Electrical Engineering Faculty, Gom University of Technology, Gom, Iran4

[email protected] [email protected]

Abstract—This paper deals with the employment of a single-phase 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 tap-changing 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

2012 IEEE International Conference on Power and Energy (PECon), 2-5 December 2012, Kota Kinabalu Sabah, Malaysia

978-1-4673-5019-8/12/$31.00 ©2012 IEEE 548

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 L1, L2 and two capacitors C1, C2), four IGBT power switches S1a, S1b, S2a, S2b, an Lf Cf filter, an injection transformer and the load.

Figure 3. The proposed single-phase four switches Quasi Z-Source network for sag mitigation

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 vi >0. The switches S1a and S2b are fully turned on; S1b and S2a are modulated complementary. The analysis when vi <0 is similar to vi >0. The dotted line in Fig. 4 indicates the safe-commutation switch during each particular stage. If vi <0 the switches S1b and S2a are fully turned on; S1a and S2b 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 Cf 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 4. The switching pattern of the single-phase Quasi Z-Source AC-AC converter in boost in-phase mode


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Figure 5. Operation state for Quasi Z-source converter in boost in-phase mode when vi > 0. (a) State1; (b) Commutation state when ii+iL2-if > 0; (c) Commutation

state when -ii-iL2+if > 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 in-phase with the input voltage. When the duty cycle is greater than 0.5, the output voltage is bucked/boosted and out-of-phase 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 L1=L2=1 mH, C1=C2=6.8 μF, Lf =1.4 mH, Cf =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.


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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)


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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.J130000.7123.00H87.

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