Most power quality problems in distribution systems are related to voltage sags. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. Dynamic voltage restorers (DVRs) have been proposed to provide higher power quality. Currently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to operate and to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor is used, which makes it impractical for DVRs to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback, an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper, a brief introduction of the basic DVR principle will be given. Next, three standard control strategies will be compared and an optimized control strategy is developed in this paper. Finally, experimental results using a medium-voltage 10-kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

Optimized Control Strategy for a Medium-Voltage DVR—Theoretical Investigations and Experimental Results

This paper proposes a new connection for a unified power quality conditioner (UPQC) to improve the power quality of two feeders in a distribution system. A UPQC consists of a series voltage-source converter (VSC) and a shunt VSC both joined together by a common dc bus. It is demonstrated how this device is connected between two independent feeders to regulate the bus voltage of one of the feeders while regulating the voltage across a sensitive load in the other feeder. Since the UPQC is connected between two different feeders (lines), this connection of the UPQC will be called an interline UPQC (IUPQC). The structure, control and capability of the IUPQC are discussed in this paper. The efficacy of the proposed configuration has been verified through simulation studies using PSCAD/EMTDC

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Interline Unified Power Quality Conditioner

Recently utilities are continuously seeking cost-effective and accurate power quality (PQ) improvement techniques in order to achieve customer satisfaction. This paper intends to provide a comprehensive review on the status of topological aspects of techniques used to improve the power quality in distribution network for the researchers, designers and engineers working in this area. This review helps to select a PQ improvement technique that suits a specific application in terms of technical and economical aspects. More than 300 research publications on the state of the art of PQ improvement techniques have been rigorously analyzed, classified and listed for quick reference.

Topological aspects of power quality improvement techniques: A comprehensive overview

A comprehensive review of dynamic voltage restorers

In this paper, design aspects of various passive components and switching dynamics of a voltage source inverter (VSI) for compensating unbalanced and nonlinear load are presented. The design method for VSI to track desired reference currents smoothly, is illustrated. By using the proposed method, it is possible to design the passive components of different VSI topologies used for load compensation. The results are supported by detailed simulation studies on a three-phase four-wire compensated system using PSCAD 4.2.1. The design steps are also verified by conducting experiments using a prototype model developed in the laboratory.

An Investigation on Design and Switching Dynamics of a Voltage Source Inverter to Compensate Unbalanced and Nonlinear Loads

Voltage deviations, often in the form of voltage sags, can cause severe process disruptions and result in substantial economic loss. The dynamic voltage restorer (DVR), a custom power device, has been proposed to protect sensitive loads from the effects of voltage sags on the distribution feeder. One of the main factors which limits capabilities of a particular DVR in compensating long-duration voltage sags is the amount of stored energy within the restorer. This paper proposes a new concept of interline dynamic voltage restoration (IDVR) where two or more DVRs in different feeders are connected to a common dc link. While one of the DVRs compensates for a voltage sag, the other DVRs connected to a common dc Link replenish the dc-link energy storage. A current-mode control strategy is incorporated into the IDVR system in both working modes, voltage sag compensation and power flow control. The reference voltage for power flow control mode is derived from instantaneous active current calculated using the dc-link voltage error signal. Simulation results are presented for a simple system in order to demonstrate the efficacy of the proposed IDVR system.

Interline dynamic voltage restorer: a novel and economical approach for multi-line power quality compensation

The dynamic voltage restorer (DVR), a custom power device, has been proposed to mitigate voltage sags, thereby protecting sensitive loads. The maximum amount of real power that the DVR can provide to the load is a deciding factor in determining the capability of the restorer. This paper proposes a concept of interline dynamic voltage restoration (IDVR) in which several DVRs in different feeders are connected to a common DC-link energy storage. The cost of installation of a group of DVRs protecting various parts of a power system can be reduced by the sharing of a common energy storage system. While one of the DVRs carries out voltage compensation during sags, the other DVRs can replenish energy to the DC-link to maintain the DC-link voltage at a specific level. A closed-loop controller that consists of an inner current loop and an outer voltage loop is incorporated into the IDVR system in both the voltage control and the power flow control modes. Simulation results are presented to verify the efficacy of the proposed IDVR design.

Interline dynamic voltage restorer: an economical way to improve interline power quality

Highly automated production processes are particularly susceptible to voltage disturbances in the power system. Series compensation in the form of dynamic voltage restorer (DVR) is used to mitigate these voltage disturbances. The compensation capability of a particular DVR topology depends on its voltage injection ability and maximum energy storage. This paper presents a new DVR circuit topology which has the ability to mitigate long duration voltage sags with comparatively small energy storage capacitors. The proposed DVR is equipped with a line-side current forced reversible rectifier that helps to maintain the DC-link voltage under voltage sag conditions. A closed loop controller that consists of an inner current loop and an outer voltage loop has been incorporated into the DVR inverter to maintain the load voltage at a desired level. Simulation and experimental results are presented to demonstrate the efficacy of proposed DVR multiloop controller for various voltage sags.

Control and analysis of a new dynamic voltage restorer circuit topology for mitigating long duration voltage sags

The dynamic voltage restorer (DVR) provides a technically advanced and economical solution to voltage-sag problem. As the voltage-restoration process involves real-power injection into the distribution system, the capability of a particular DVR topology, especially for compensating long-duration voltage sags, depends on the energy storage capacity of the DVR. The interline DVR (IDVR) proposed in this paper provides a way to replenish dc-link energy storage dynamically. The IDVR consists of several DVRs connected to different distribution feeders in the power system. The DVRs in the IDVR system share a common energy storage. When one of the DVR compensates for voltage sag appearing in that feeder, the other DVRs replenish the energy in the common dc-link dynamically. Thus, one DVR in the IDVR system works in voltage-sag compensation mode while the other DVRs in the IDVR system operate in power-flow control mode. In principle, IDVR can operate effectively when constituent DVRs are electrically (not necessarily physically) far apart. Closed-loop load voltage and current-mode-control techniques are used as the control strategy in the two modes of operation. Experimental results obtained for a laboratory prototype of the IDVR are presented to show the effectiveness and the efficacy of the proposed IDVR system to improve power quality

A Novel Technique to Compensate Voltage Sags in Multiline Distribution System : The Interline Dynamic Voltage Restorer

The increased use of voltage-sensitive equipment in the industrial sector has made industrial processes more susceptible to supply voltage quality problems in the form of voltage sags. Series-connected voltage source converter (VSC)-based devices have been gaining popularity in protecting sensitive loads against voltage sags. This paper presents a novel transformerless dynamic series compensator (DSC) to mitigate long-duration voltage sags effectively. A line-side controllable rectifier connected to the proposed DSC maintains the dc-link voltage at a specific value while the DSC compensates voltage sags. The DSC is controlled by a closed-loop voltage and current mode controller which provides better damping and dynamic performance than that of in open-loop controller. The closed-loop controller consists of an outer voltage loop and an inner current loop derived from filter capacitor current. Performance of the proposed DSC is tested under varied voltage sag conditions, and simulated and experimental resul...

H.M. Wijekoon

Papers

Interline dynamic voltage restorer: a novel and economical approach for multi-line power quality compensation

Interline dynamic voltage restorer: an economical way to improve interline power quality

Control and analysis of a new dynamic voltage restorer circuit topology for mitigating long duration voltage sags

A Novel Technique to Compensate Voltage Sags in Multiline Distribution System : The Interline Dynamic Voltage Restorer

A novel dynamic series compensator with closed-loop voltage and current mode control for voltage sag mitigation