The Control Of Flux Of Transformer In Dynamic Voltage Restorer

Power quality issues have received much attention in recent years. power qualityevents in the utility grid can affect a large number of manufacturers. Survey resultssuggest that92%of interruption at industrial facilities is voltage sag related. The DynamicVoltage Restorer (DVR), based on a transformer-coupled series-connected voltage sourceinverter, is among the most cost-effective solution against voltage sags.One important issue of Dynamic Voltage Restorer (DVR) is the transformer inrush atthe start of sag compensation.The inrush current may trigger the over-current protection ofthe compensator inverter, and the compensation voltage is greatly reduced due themagnetic saturation of the coupling transformer. Oversizing the transformer is a commonapproach to avoid the inrush current. However, this would dramatically increase the sizeand weight of the sag compensation system. This paper proposes a new technique formitigating the inrush of the coupling transformer and preserving the output voltage foreffective sag compensation. The proposed method limit the inrush current down to an lowlevel through the control of voltage according to the flux linkage. The proposed ac voltagethrust and the dc voltage thrust associated with the inrush mitigation can drive the fluxlinkage of the transformer into steady state in one cycle to avoid further magneticssaturation at the starting of the compensator. The proposed method restores the loadvoltage to more than95%in the first cycle after the voltage sag is detected, and then fullrestoration afterwards. Compared to other inrush suppression techniques, it can injectmore voltage for successful ride-through.Detailed explanations of the proposed inrushmitigation technique are presented, and the effectiveness of the proposed scheme isverified by laboratory test results.The main factors resulting in magnetic bias of the output transformer of full bridgeSPWM inverter are analyzed， including the direct-current component of control system,zero floating and dynamic response of system,and so on.This paper proposes acontrol method based on the estimation of transformer flux for eliminating dc magneticflux from the series transformer of a dynamic voltage restorer. The method is proved to beeffective by simulation and laboratory test results.