An Improvement of Transient Current Response of Load Transformers for the Series Voltage Sag Compensator

An Improvement of Transient Current Response of Load Transformers for the Series Voltage Sag Compensator
Authors:V.V.GAYATHRI, K.SABITHA, B.HARI PRASAD

Abstract: In many countries, high-tech manufacturers concentrate in industry parks. Survey results suggest that 92% of interruption at industrial facilities is voltage sag related. An inrush mitigation technique is proposed and implemented in a synchronous reference frame sag compensator controller. The voltage sag compensator consists of a three phase voltage source inverter and a coupling transformer for serial connection. It is the most cost effective solution against voltage sags. When voltage sag happen, the transformers, which are often installed in front of critical loads for electrical isolation, are exposed to the disfigured voltages and a DC offset will occur in its flux linkage. When the compensator restores the load voltage, the flux linkage will be driven to the level of magnetic saturation and severe inrush current occurs. The compensator is likely to be interrupted because of its own over current protection. This paper proposes an inrush current mitigation technique together with a state-feedback controller for the Voltage sag compensator. 

Keywords: Voltage Sag, Flux Linkage, Inrush Current, Transformer, Power Quality, Voltage Sag Compensator.

 INTRODUCTION 

        Power quality issues have received much attention in recent years. Therefore, any power quality events in the utility grid can affect a large number of manufactures. Records show that voltage sag, transients, and momentary interruption constitute 92% of the power quality problems [1].Voltage sags often interrupt critical loads and results in substantial productivity losses. Industries have adopted the voltage sag compensators as one of the most cost-effective ride-through solutions [2]– [7], and most compensators can accomplish voltage restoration within a quarter cycles. However, the load transformer is exposed under the deformed voltages before the restoration, and magnetic flux deviation may be developed within the load transformers. Once the load voltage is restored, the magnetic flux may further drift beyond the saturation knee of the core and lead to significant inrush current. The over current protection of the compensator could be easily triggered and lead to compensation failure. Various transformer inrush reduction techniques have been presented, like controlling power-on angle and the voltage magnitude [8], or actively controlling the transformer current [9]. These methods could easily alter the output voltage waveforms of the converter, and thus, is not suitable for voltage sag compensators, which demand precise point-on wave restoration of the load voltages.

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