Abstract:  ABSTRACT： There are generally excessive harmonics, reactive power and negative sequence currents in the distribution power system, which can cause hazards including poor efficiency of power grid operation, unusual heating and damage of load. Meanwhile, a large number of sensitive loads, including communications, medical and high-end manufacturing, will run out of order as long as the duration of voltage interruption or voltage sags lasts more than a few milliseconds, which may cause serious economic losses. Therefore these power quality issues need to be integrated. The shunt-unified power quality controller （S-UPQC） can solve the above two types of power quality problems respectively and the key is to realize the fast and smooth seamless switching between the two operating states. In view of the problems in the existing switching control, this paper proposes a complete seamless switching control technique for S-UPQC, including the seamless cut-in algorithm based on current rate control and the flexible cut-off algorithm based on instantaneous linear modulation, with the advantages of simpleness, quickness and reliability. Its cut-in time is less than 1 ms and there is no mutation of voltage amplitude and phase in its cut-off process. The effectiveness of the proposed method is verified by PSCAD/EMTDC electromagnetic transient simulation and physical prototype experiment based on 380 V/100 kV·A S-UPQC.

Key words:  KEYWORDS： power quality, voltage sag, shunt-unified power quality controller（S-UPQC） , seamless switching