Research On The Large Capacity High Reliabel And Rapid Solid State Transfer Switch

Industrial customers often suffer from supply voltage interruptions and sags due to the increase in the utilization of sensitive equipment in the process of automation and control.An effective way to improve power quality and reliability of sensitive customers is to use a solid state transfer switch.This device enables a very fast change in the supply of the customer to an alternate feeder providing adequate power conditioning for several power quality problems,such as voltage sags,swells,and interruptions.In this paper,the structure of SSTS and its operation principles are proposed.One of the most important characteristic to value SSTS is the transfer time.Thus,the detection of voltage sags is of great importance.Different voltage-detection techniques may be used in the SSTS system.This article describes three voltage-detection methods,i.e.averaging,and abc-to-dq0 transformation and the modified abc-to-dq0 transformation.This paper provides a thorough analysis of a fast gating strategy for SSTS systems.It derives analysis expressions to estimate the transfer time for different operating conditions and identifies the worst case scenarios in which maximum transfer time occurs.A fast and safe commutation of the preferred source thyristor switches for every phase depends directly on the relative position and magnitude of three variables when a voltage sag is detected.The steady state and the transient characteristics are analyzed.The fast transfer process may cause in-rush current on the load-side transformer due to the resulting DC offset in its magnetic flux as the load-transfer is completed.The in-rush current can reach 8~30 p.u.and it may trigger the over-current protections on the power feeder.This paper develops a flux estimation scheme and a thyristor gating logic based on the impulse commutation bridge SSTS to minimize the DC offset on the magnetic flux.By sensing the line voltages of both feeders,the flux estimator can predict the peak transient flux linkage at the moment of load-transfer and evaluate a suitable timing for the transfer to minimize the inrush current.Simulations using this model are performed in order to handle voltage sags based on real measurements on an actual industrial customer's supply voltage.Different phase angles and magnitudes of the two alternate source prefault voltages and different fault instances are considered.