11-08-2012, 11:20 AM
Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control
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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.
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).
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:
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
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).
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).
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).