Inter-line dynamic voltage restorer control using a novel optimum energy consumption strategy

https://doi.org/10.1016/j.simpat.2006.06.001Get rights and content

Abstract

To restore the load voltage, Dynamic Voltage Restorer (DVR), which is installed between the supply and a sensitive load, should inject voltage and active power from DVR to the distribution system during voltage sag. Due to the energy storage capacity limitation of DC link, it is necessary to minimize energy injection from DVR.

The techniques of the supply voltage sag compensation in a distribution feeder are reviewed in this paper. Then a new device which is named Inter-line Dynamic Voltage Restorer (IDVR) is discussed. This device consists of two conventional DVRs which are installed in two different distribution feeders and the DC link capacitor. DVR which is installed in LV feeder operates in voltage sag compensation mode. A novel control technique minimizes the energy flow from DC link capacitor to this feeder. The DVR, which is installed in MV feeder, controls the voltage of DC link capacitor. The proposed device and its new control strategy have been modeled and simulated by PSCAD/EMTDC. The results verify the effectiveness of IDVR and the suggested control method.

Introduction

One of the most important power quality issues is voltage sag because the increasing usage of voltage sensitivity devices has made industrials processes more susceptible to supply voltage sags [1]. Voltage sag may cause tripping and shutdown of sensitive and industrial equipments and miss-operation of ASD (Adjustable Speed Drive) systems. Voltage sags of down to 70% are much more common than complete outages [2]. Dynamic voltage restorer (DVR) with energy storage can be used to correct the voltage sags [3], [4], [5], [6]. A DVR is basically a controlled voltage source installed between the supply and a critical and sensitive load. It injects a voltage to the distribution system in order to compensate any disturbance affecting the load voltage. The compensation capacity of a particular DVR depends on the maximum voltage injection ability and the real power, which can be supplied by the DVR because when DVR restorers voltage disturbances, active power should be injected from DVR to the distribution system. DVR could maintain load voltage unchanged during any kind of faults, if the capability of energy storage of DVR were infinite [3]. Energy storage devices, such as batteries or Super-conducting Magnetic Energy Storage Systems (SMES) are required to provide this active power. Considering the energy limitations of these devices, it is necessary to minimize energy injection.

Power circuit of a DVR in a distribution system is shown in Fig. 1(a). The main function of a DVR is the protection of sensitive loads from voltage sags coming from network. Therefore, the DVR is located on approach of sensitive loads. If a fault occurs on other feeders, DVR inserts series voltage, Vdvr and compensates load voltage to pre fault value. Distribution systems commonly use a delta–star or a star–star transformer. If delta–star transformer is used in distribution system, zero-sequence voltages will not propagate through the transformer when earth faults occur on the higher voltage level. Therefore, restoration of positive sequence and compensation of negative sequence voltage are necessary. As shown in Fig. 1(b) the main elements of the three-phase four-wire DVR are the energy storage system, the voltage source converter, the LC filter and the coupling transformer. The AC side of converter is connected to the line through a LC filter. Due to switching, the high order harmonics of converter must be removed by small high pass filters (represented by Ls, Cs in Fig. 1(b)). The Sinusoidal Pulse Width Modulation (SPWM) technique is commonly used to control forced-commutated converters. This method has been used in this paper too.

The inter-line dynamic voltage restorer (IDVR) is similar to the inter-line power flow controller (IPFC) which should be used in transmission system. In this device several DVRs protecting sensitive loads in different distribution systems share a common DC link energy storage [7]. In the simplest case, we consider two different voltage distribution system protected by two DVR. Low voltage (6.6 kV) DVR operates in voltage sag mitigation mode and uses the suggested minimal energy control method of the paper to inject minimum active power from DC link capacitor. In the same time medium voltage (20 kV) DVR keeps the voltage of DC link capacitor constant in order to control the active power flow from distribution system to DC link capacitor.

Section snippets

Conventional control strategies for DVR

Several control techniques have been proposed for voltage sag compensation such as pre-sag method [5], in-phase method [5] and minimal energy control [3].

It must be said that the characteristics of the sensitive load determine the control method and the compensation strategy for the DVR. For example, the linear loads are not sensitive to phase angle jump and only magnitude of voltage is dominant. The control techniques should consider the limitations of the DVR such as the voltage injection

Inter-line dynamic voltage restorer (IDVR)

The equivalent circuit on the left hand side of DVR shown in Fig. 1(a) can be presented by thevenin voltage source, Vth1and thevenin impedance, Zth1, as shown in Fig. 8.

When a fault occurs in other lines, the left hand side voltage of DVR, i.e., Vs1 drops and the DVR injects a series voltage, Vdvr1 through the transformer. The active power flow is from energy storage to the distribution system.

In the general form inter-line dynamic voltage restorer uses several DVRs in different distribution

Simulation results

To verify the efficiency of suggested control strategy for the IDVR system shown in Fig. 9 simulations carried out by the PSCAD/EMTDC [9].

The system parameters are listed in Table 1.

It is assumed that the voltage magnitude of the load bus in feeder 1, 2 must be set at 1 pu during the voltage sag conditions in feeder 1. The results of the most important simulations are presented in Fig. 12.

Left hand side voltage of DVR1, injected voltage of DVR1, load 1 voltage, load 2 voltage, injected active

Conclusion

In order to compensate voltage sag it is possible to use dynamic voltage restorer (DVR) in distribution system for a sensitive load. Due to the limit of energy storage capacity of DC link, it is necessary to minimize energy injection from DVR. In this paper the control strategies for the compensation of the supply voltage sag is reviewed presented. In addition, a new concept of restoration strategy is proposed to inject minimum energy for unbalance sags. This control strategy has been applied

References (9)

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