93017001-ejsr-27-1-15

European Journal of Scientific Research ISSN 1450-216X Vol.27 No.1 (2009), pp.152-166 © EuroJournals Publishing, Inc. 2009 http://www.eurojournals.com/ejsr.htm

Simulation of Dynamic Voltage Restorer Using Hysteresis

Voltage Control

H. Ezoji Electrical & Computer Engineering Department, Babol University of Technology

Shariati Street, Babol, Iran, P. O. Box 47135-484 E-mail: [email protected]

Tel: +98-936-3688436; Fax: +98-111-3239214

A. Sheikholeslami Electrical & Computer Engineering Department, Babol University of Technology

Shariati Street, Babol, Iran, P. O. Box 47135-484 Tel: +98-111-323-9214; Fax: +98-111-3239214

M. Tabasi

Electrical & Computer Engineering Department, Babol University of Technology Shariati Street, Babol, Iran, P. O. Box 47135-484 Tel: +98-111-323-9214; Fax: +98-111-3239214

M.M. Saeednia

Electrical & Computer Engineering Department, Babol University of Technology Shariati Street, Babol, Iran, P. O. Box 47135-484 Tel: +98-111-323-9214; Fax: +98-111-3239214

Abstract

Dynamic Voltage Restorer (DVR) is one of the custom power devices that are used as an effective solution for the protection of sensitive loads against voltage disturbances in power distribution system. The efficiency of the DVR depends on the performance of the efficiency control technique involved in switching the inverters. Unlike previous approaches, this paper presents a hysteresis voltage control technique of DVR based on bipolar and unipolar Pulse Width Modulation (PWM). The hysteresis voltage control has a very fast response, simple operation and variable switching frequency. To evaluate the quality of the load voltage during the operation of DVR, Total Harmonic Distortion (THD) is calculated with various Hysteresis Band (HB). The validity of proposed method and achievement of desired compensation are confirmed by the results of the simulation in MATLAB/ Simulink. Keywords: Dynamic Voltage Restorer (DVR), hysteresis voltage control, Total Harmonic

Distortion (THD), Hysteresis Band.

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 153

1. Introduction Power quality problems like voltage sag, voltage swell and harmonic are major concern of the industrial and commercial electrical consumers due to enormous loss in terms of time and money. This is due to the advent of a large numbers of sophisticated electrical and electronic equipment, such as computers, programmable logic controllers, variable speed drives, and so forth. The use of this equipment often requires very high quality power supplies (Ravi and Siva 2007).

Some special equipment are sensitive to voltage disturbances, especially if these take up to several periods, the circuit does not work. Therefore, these adverse effects of voltage changes necessitate the existence of effective mitigating devices. There are various solutions to these problems. One of the most effective solutions is the installation of a Dynamic Voltage Restorer (DVR). DVR is a series custom power device, which has excellent dynamic capabilities. It is well suited to protect sensitive loads from duration voltage sag or swell. A DVR is basically a controlled voltage source installed between the supply and a sensitive load. It injects a voltage on the system in order to compensate any disturbance affecting the load voltage (Ravi and Siva 2007; Banaei and Hosseini et al, 2005). Basic operating principle of a DVR is as shown in Fig.1.

In August 1996, Westinghouse Electric Corporation installed world’s first dynamic voltage restorer in Duke Power Company’s 12.47 kV substation in Anderson, South Carolina. This was installed to provide protection to an automated rug manufacturing plant. Prior to this connection, the restorer was first installed at the Waltz Mill test facility near Pittsburgh for full power tests.

Another was installed to provide service to a large dairy food processing plant in Australia (Ghosh and Ledwich, 2001).

Voltage sag/swell that occurs more frequently than any other power quality phenomenon is known as the most important power quality problems in the power distribution systems. IEEE 519-1992 and IEEE 1159-1995 describe the voltage sags /swells as shown in Fig.2.

Voltage sag is defined as a sudden reduction of supply voltage down from 90% to 10% of nominal. According to the standard, a typical duration of sag is l0 ms to 1 minute. On the other hand, voltage swell, is defined as a sudden increasing of supply voltage up1l0% to 180% in rms voltage at the network fundamental frequency with duration from 10 ms to 1 minute. Voltage sag/swell often caused by faults such as single line-to-ground fault, double line-to-ground fault on the power distribution system or due to starting of large induction motors or energizing a large capacitor bank. Voltage sag/swell can interrupt or lead to malfunction of any electric equipment that is sensitive to voltage variations (Boonchiaml, Apiratikull et al. 2006).

Figure 1: Schematic Diagram of a Typical DVR.

154 H. Ezoji, A. Sheikholeslami, M. Tabasi and M.M. Saeednia

Figure 2: Voltage Reduction Standard of IEEE Std. 1159-1995.

There are various control methods proposed for DVR, such as the Voltage-Space Vector PWM suggested in (zhan et al, 2001). Jurado et al, (2003) present a Fuzzy Logic Control. These studies show that the transient response of the FL control is better than that of PI. A new approach to estimate symmetrical components for controlling the DVR was suggested by Marei et al (2007). Fawzi (2007) recommended the use of DVR based on hysteresis voltage control. Also, in reference (Ezoji et al, 2008) hysteresis voltage control based on unipolar pulse width modulation (PWM) has been used to control DVR. The hysteresis voltage control in terms of quick controllability and easy implementation hysteresis band voltage control has the highest rate among other control methods.

This paper presents a Hysteresis Voltage Control technique based on bipolar and unipolar PWM to improve the quality of load voltage. The hysteresis voltage control has not been investigated on DVR. The proposed method is validated through modeling in MATLAB/Simulink. The quality of the DVR and load voltage is measured using the well-known term Total Harmonic Distortion (THD). 2. DVR Power Circuit The power circuit of the DVR is shown in Fig.1. The DVR consists of mainly a three-phase Voltage-Sourced Converter (VSC), a coupling transformer, passive filter and a control system to regulate the output voltage of VSC: 2.1. Voltage Source Converter (VSC)

A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. This converter injects a dynamically controlled voltage in series with the supply voltage through three single-phase transformers to correct the load voltage. It consists of Insulated Gate Bipolar Transistors (IGBT) as switches. The switching pulses of the IGBT are the output from the hysteresis voltage controller (Perera, Salomonsson, et al. 2006). 2.2. Coupling Transformer

Basic function is to step up and electrical isolation the ac low voltage supplied by the VSC to the required voltage. In this study single-phase injection transformer is used. For three phases DVR, three single phase injection transformers can be used (Perera, Salomonsson, et al. 2006; Hannan and Mohamed, 2002).

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 155

2.3. A C-Filter

A Passive filter consists of a capacitor that is placed at the high voltage side of coupling transformer. This filter rejects the switching harmonic components from the injected voltage (Hannan and Mohamed, 2002). 2.4. Control System

The aim of the control scheme is to maintain a balanced and constant load voltage at the nominal value under system disturbances. In this paper, control system is based on hysteresis voltage control. 3. Conventional Control Strategies Several control techniques have been proposed for voltage sag compensation such as pre-sag method, in-phase method and minimal energy control (Meyer and Romaus, 2005; Godsk and Frede, 2005; Kim, 2002). 3.1. Pre-Sag Compensation Technique

In this compensation technique, the DVR supplies the difference between the sagged and pre-sag voltage and restores the voltage magnitude and the phase angle to the nominal pre sag condition.

The main defect of this technique is it requires a higher capacity energy storage device. Fig.3 (a) shows the phasor diagram for the pre-sag control strategy (Meyer and Romaus, 2005; Godsk and Frede, 2005).

In this diagram, Vpre-sag and VSag are voltage at the point of common coupling (PCC), respectively before and during the sag. In this case VDVR is the voltage injected by the DVR, which can be obtained as:

Vpre-sag = VL, VSag = VS and VDVR = Vinj |Vinj| = |V pre-sag | – |VSag| (1)

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)()()(

tan sag-pre sag-pre

sag-pre sag-pre1

SagSaginj CosVCosV

SinVθθ

θθ (2)

3.2. In-phase compensation technique

In this technique, only the voltage magnitude is compensated. VDVR is in-phase with the left hand side voltage of DVR. This method minimizes the voltage injected by the DVR, unlike in the pre-sag compensation. Fig.3 (b) shows phase diagram for the in-phase compensation technique (Godsk and Frede, 2005).

VDVR = Vinj |Vinj| = |V pre - sag | – |VSag|

S inj inj V θ θ = = ∠ (3)

3.3. Energy optimization technique

Pre-sag compensation and in-phase compensation must inject active power to loads almost all the time. Due to the limit of energy storage capacity of DC link, the DVR restoration time and performance are confined in these methods. The fundamental idea of energy optimization method is to make injection active power zero. In order to minimize the use of real power the voltages are injected at 90° phase angle to the supply current. Fig.3 (c) shows a phasor diagram to describe the Energy optimization Control method (Kim, 2002).

The selection of one of these strategies influences the design of the parameters of DVR. In this paper, the control strategy adopted is Pre-sag compensation to maintain load voltage to pre fault value.

156 H. Ezoji, A. Sheikholeslami, M. Tabasi and M.M. Saeednia

Figure 3: Conventional control strategies.

(a) Pre-sag compensation technique

(b) In-phase compensation technique

(c) Energy optimized compensation technique

4. Proposed Method The main stages of the control system of a DVR include: detection of the start and finish of the sag, voltage reference generation, injection voltage generation, and protection of sensitive load. 4.1. Detection of Sags / Swell in the Supply voltage

Fitzer, Barnes and Green (2004), had been analyzed and compared several detection techniques. In this study, monitoring of Vd and Vq is used to return the magnitude and phase load voltage to the magnitude and phase reference load voltage. The control system is presented in Fig.4.

The three-phase supply voltage is connected to a transformation block that convert to rotating frame (d q) with using a software based Phase – Lock Loop (PLL). Three-phase voltage is transformed by using Park transform, from a-b-c to o-d-q frame:

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⎣

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vvv

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c

b

a

o

q

d

p (4)

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The detection block detects the voltage sag/swell. If voltage sag/swell occurs, this block generates the reference load voltage. The sag detection strategy is based on root means square (rms) of the error vector. Closed loop load voltage feedback is added, and is implemented in the frame in order to minimize any steady state error in the fundamental component (Godsk, Newman, Nielsen et al, 2004).

The injection voltage is also generated according to the difference between the reference load voltage and the supply voltage and is applied to the VSC to produce the preferred voltage, with the using the Hysteresis Voltage Control.

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 157

Figure 4: Control structure of DVR.

5. Hysteresis Voltage Control In this paper, hysteresis voltage control is used to improve the load voltage and determine switching signals for inverters gates. A basic of the hysteresis voltage control is based on an error signal between an injection voltage (Vinj) and a reference voltage of DVR (Vref) which produces proper control signals. There is Hysteresis Band (HB) above and under the reference voltage and when the difference between the reference and inverter voltage reaches to the upper (lower) limit, the voltage is forced to decrease (increase) as shown in Fig.5.

Figure 5: hysteresis band voltage control.

Figure 6: Single phase full bridge inverter.

T1 + T2 = Tc =1/fc (6) Where HB and fc are Hysteresis Band and switching frequency respectively. The HB that has

inversely proportional relation to switching frequency is defined as the difference between VH and VL (HB=VH-VL) (Zare and Nami, 2007).

In comparison with the other PWM methods, the hysteresis voltage control has a very fast response, a simple operation and a variable switching frequency, (Kale, Ozdemir, 2007).

158 H. Ezoji, A. Sheikholeslami, M. Tabasi and M.M. Saeednia

Fig.6 shows a single phase full bridge inverter that is connected in series to a sensitive load. The inverter can be controlled in unipolar or bipolar PWM method. 5.1. Hysteresis voltage control based on bipolar switching technique

In bipolar switching scheme, there are two bands (HB1) and the controller turn on and turn off the switch pairs (S1, S4 or S2, S3) at the same time to generate +Vdc or -Vdc at the output of the inverter. As shown in Fig.7, for the bipolar hysteresis voltage control technique, difference between Vinj and VRef is applied to a hysteresis controller to turn on and off the switch pairs (Sl, S4 or S2, S3). 5.2. Hysteresis voltage control based on unipolar switching technique

In the unipolar modulation, four voltage bands are used to achieve proper switching states to control the load voltage. The first upper and lower bands (HB1) are used when the output current is changed between (+Vdc & 0) or (-Vdc or 0). The second upper and lower bands (HB2) are used to change the current level Fig.8 (a). There are four switching states for switches (S1, S2) and (S3, S4) as shown in Fig.8 (b).

The switching functions of both B and C phases in bipolar and unipolar Hysteresis voltage control are determined similarly using corresponding reference and measured voltage band (HB).

Figure 7: Bipolar Hysteresis Voltage Control

(a) Out put Voltage With Lower and Higher Bands

(b) switching signals

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 159

Figure 8:Unipolar Hysteresis Voltage Control

(a) Out put voltage with two lower and higher bands

(b) Switching Signals

6. Simulation Results The proposed method is validated by simulation results of MATLAB/Simulink. The system parameters are given in the Appendix. DVR with hysteresis voltage control is applied to compensate load voltage. Here we consider two different cases. In Case 1, the unbalanced voltage sag is simulated. To demonstrate the performance of the proposed method we assumed voltage swell condition in Case 2.

In order to demonstrate the performance of the DVR using unioplar and bipolar switching technique to control, a Simulink diagram proposed as shown in Fig.9.

Figure 9: Simulation Model of DVR in MATLAB.

160 H. Ezoji, A. Sheikholeslami, M. Tabasi and M.M. Saeednia

A. Case 1

In this case, we assume that there is a 30% three-phase voltage sag with +30° phase jump in phase-a in supply voltage that is initiated at 0.1s and it is kept until 1.8 s.

Fig.10 shows the result of voltage sag compensation considering hystersis voltage control based on bipolar switching for HB1=0.005.

The outcome of voltage sag compensation considering hystersis voltage control based on unipolar switching for HB1=0.005 and HB2=0.007 is demonstrated in Fig.11.

Fig.10 (b) and Fig.11 (b) show the serial injected voltage components. Moreover, the compensated load voltage is shown Fig.10 (c) and Fig.11 (c). As it can be seen from the results, the DVR is able to produce the required voltage components for different phases rapidly and help to maintain a balanced and constant load voltage at the nominal value (400 V).

Figure 10: Simulation result of DVR response to unbalance voltage sag (HB1=0.005).

(a) Supply voltages

(b) Injected voltage, VDVR

(c) Load voltage, VL

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 161

Figure 11: Simulation result of DVR response to unbalance voltage sag (HB1=0.005, HB2=0.007).

(a) Supply voltages

(b) Injected voltage, VDVR

(c) Load voltage, VL

B. Case 2

In the second case, performance of DVR for a voltage swell condition is investigated. Here, an unbalance voltage swell with 30% three-phase voltage swell with +30° phase jump in phase-a which starts at 0.1s and ends at 1.8s is considered. The performance of DVR using hystersis voltage control is illustrated in Fig.12 considering bipolar switching for HB1=0.005.

Fig.13 shows the result of voltage swell compensation considering hystersis voltage control based on unipolar switching for HB1=0.005 and HB2=0.007.

The injected voltage that is produced by DVR to correct the load voltage is shown in Fig. 12 (b) and Fig.13 (b). As it can be seen from Fig.12 and Fig.13, DVR is able to correct the voltage swell by injecting negative three phase voltage components.

162 H. Ezoji, A. Sheikholeslami, M. Tabasi and M.M. Saeednia

Figure 12: Simulation result of DVR response to unbalance voltage sewll (HB1=0.005).

(a) Supply voltages

(b) Injected voltage, VDVR

(c) Load voltage, VL

Figure 13: Simulation result of DVR response to unbalance voltage swell (HB1=0.005, HB2=0.007).

(a) Supply voltages

(b) Injected voltage, VDVR

(c) Load voltage, VL

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 163

7. Discussion To evaluate the quality of the load voltage during the operation of DVR, Total Harmonic Distortion (THD) is calculated with various HB.

Table I and Table II show the obtained results for each HB1 and HB2. The HB1 and HB2 have been selected randomly.

As it can be seen, with growth of the HB1 and HB2, THD of the load voltage correspondingly raises but the effect of increasing the HB on THD of the load voltage under voltage swell is more than THD of the voltage sag. It is obvious that the THD value varies when ever HB1 and HB2 value vary or when HB1 is contented and HB2 value varies. But THD of the load voltage under the voltage swell is greater than the voltage sag case. Therefore HB value has to be selected based on the voltage sag test. Table I: THD for load voltage for various values of HB1 in bipolar hysteresis voltage control.

Hysteresis Band THD (%) HB1(v) Sag swell 0.005 0.233 0.256 0.1 0.274 0.305 5 1.662 1.814

10 3.456 3.679 15 5.753 5.915 20 8.238 8.742

Table II: THD for load voltage for various values of HB1 and HB2 in unipolar hysteresis voltage control.

Hysteresis Band THD (%) HB1(v) HB2(v) Sag swell 0.005 0.007 0.187 0.199 0.1 0.12 0.213 0.243 5 7 1.251 1.623

10 12 2.564 3.157 15 17 4.387 5.217 20 22 7.06 7.74

Table III summarizes the THD values for the constant HB1 and various HB2 for hysteresis

voltage control based on unipolar pwm. Table III: THD for load voltage for the constant values HB1 and various values HB2 for 30% voltage sag and

swell.

Hysteresis Band THD (%) HB1(v) HB2(v) Sag swell 0.005 0.007 0.187 0.199 0.005 0.1 0.34 045 0.005 5 0.75 0.98 0.005 10 1.91 2.13 0.005 15 3.23 3.42 0.005 20 4.35 5.01

For further study on the control scheme performance, the results obtained in Table II and Table

III is plotted in Fig.14 and Fig.15. It is show that in both conditions, load voltage THD in hysteresis voltage control based on

unipolar switching is less than bipolar. Move over can be seen that using HB1 less than 13 remains THD in standard region. We can say that hysteresis voltage control based on unipolar switching is more efficient and effective than bipolar switching.

164 H. Ezoji, A. Sheikholeslami, M. Tabasi and M.M. Saeednia

Figure 14: Comparison between THD of load voltage in bipolar and unipolar control in voltage sag condition.

Figure 15: Comparison between THD of load voltage in bipolar and unipolar control in voltage swell condition.

8. Conclusion This paper presents a hysteresis voltage control technique based on bipolar and unipolar Pulse Width Modulation (PWM) For Dynamic Voltage Restorer to improve the quality of load voltage. The validity of proposed method is approved by results of the simulation in MATLAB/ Simulink.

To evaluate the quality of the load voltage during the operation of DVR, THD is calculated. The result simulation shows that effect of increasing the HB on THD of the load voltage under voltage swell is more than THD of the voltage sag. Therefore HB value has to be selected based on the voltage sag test.Also it is observed that that in both conditions, load voltage THD in hysteresis voltage control based on unipolar switching is less than bipolar. Move over can be seen that using HB1 less than 13 remains THD in standard region. We can say that hysteresis voltage control based on unipolar switching is more efficient and effective than bipolar switching.

In conventional fixed band hysteresis controller, it is not impossible to determine hysteresis bandwidth and switching frequency according to system parameters. Therefore, we can focus on adaptive hysteresis band controller in our future works.

Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control 165

Appendix Table IV: Case Study Parameters. Parameters Value Supply voltage (VL-L) 400V Vdc ,Cf 200V, 500uF Series Transformer(VPh-Ph) 96V / 240V ZTrans 0.004 + j 0.008 RLoad, LLoad 31.84 Ω, 0.139 H

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