Unified power flow controller (UPFC) consists of two parts the series part and parallel part. The parallel part is similar to a static synchronous compensator (STATCOM)

and there have been mature strategies for achieving its start and stop. But for the series parts

there is no effective strategy for accessing and exiting the AC grid. The series converter is charged through the parallel part firstly

and then charged to rated condition by the active charging strategy to reduce the impact of deblocking. In addition

by controlling the line current transfer between the series transformer winding and the series transformer bypass switch

the bypass switch of the series transformer can open and close at zero current

while the series converter is able to block at zero power so as to realize smooth start and stop of UPFC. The simulation results based on PSCAD show that this strategy is capable of making UPFC to start and stop without disturbance to the AC grid.