Dynamic Voltage Restorer

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Electrical Power and Energy Systems 25 (2003) 525–531
www.elsevier.com/locate/ijepes

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The DC link energy control method in dynamic voltage restorer system Il-Yop Chung*, Dong-Jun Won, Sang-Young Park, Seung-Il Moon, Jong-Keun Park Seoul National University, 130-Dong 303-Ho EESRI, Shinlim-dong, Kwanak-gu, Seoul 151-742, South Korea

Abstract
Dynamic voltage restorer (DVR), which is installed between the supply and a critical load, can restore the load voltage to the pre-fault voltage during voltage sag. To restore load voltage, DVR should inject the equivalent of dropped voltage, which represents the voltage difference between pre-fault and fault voltage through series connected transformer. Voltage restoration of DVR needs to inject active power and energy from DVR to distribution system. However, the capability of energy storage that usually consists of capacitors in DVR is limited. Therefore, it must be considered how the injection energy can be minimized and the load voltage can be made close to the pre-fault voltage. This paper describes conventional restoration techniques like pre-sag compensation, in-phase compensation, and phase advance method. In addition, this paper proposes a new concept of restoration technique to inject minimum energy during DVR compensation. The proposed method is based on the definition of voltage tolerance of the load. In the proposed method, some particular disturbances can be corrected with less amount of energy discharge than those of conventional methods. q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Power quality; Voltage sag; Custom power; Dynamic voltage restorer; Series compensation; Energy optimization

1. Introduction
Due to the development of industry, power quality has a
significant influence on high-technology equipments related to communication, advanced control, automation, precise
manufacturing technique and on-line service. For example,
voltage sag can have a bad influence on the products of
semiconductor fabrication with considerable financial
losses. Power quality problems include transients, sags,
interruptions and other distortions to the sinusoidal waveform [1]. One of the most important power quality issues is voltage sag that is a sudden short duration reduction in
voltage magnitude between 10 and 90% compared to
nominal voltage.
Recently, new technologies like custom power devices
based on power electronic concepts have been developed to
provide protection against power quality problems. Generally, custom power devices are divided by three categories such as series-connected compensator like dynamic voltage
restorer (DVR), shunt-connected compensator like distribution static compensator (DSTATCOM), and series- and shunt-connected compensator like Unified Power Quality
Conditioner (UPQC). The series-connected compensator
can restore the distorted load voltage directly by injecting * Corresponding author. Tel.: þ 82-2-886-3101; fax: þ 82-2-886-3102. E-mail address: iryop@powerlab.snu.ac.kr (I.-Y. Chung).

missing voltage while the shunt-connected compensator can
do indirectly by controlling line current. DVR is seriesconnected between the supply and a sensitive load, so that it is designed to inject the compensating voltage into
distribution line in real time [2]. Therefore, DVR can
provide an effective solution to restore voltage sag by
establishing the proper voltage quality level required by
customer.
The ideal restoration is to make load voltages unchanged.
When DVR restores voltage disturbances, active power or
energy should be injected from DVR to distribution system.
If the capability of energy storage of DVR were infinite,
DVR could maintain load voltage unchanged ideally during
any kind of faults. However, the stored energy in DVR is
limited practically by the limit of DC link capacity of DVR. Namely, DVR cannot restore the load voltage constantly
when the voltage across the DC link has gone down and...
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