Effect of Regenerative Load on a Static Transfer Switch Performance

A thyristor-based static transfer switch (STS) is used to connect an alternate source of ac power to a sensitive load when the main source fails. A main criterion for performance evaluation of an STS is the transfer time from the main to the alternate source. The transfer time depends upon the commutation process of the outgoing and incoming switches, which in tum depends on the system parameters and the load characteristics. This paper investigates the effect of a regenerative load on the transfer time/total load-transfer time of an STS and identifies the worst-case scenario(s) in which maximum transfer time occurs. The IEEE STS benchmark models STS-1 and STS-2 are chosen to demonstrate the STS performance under various fault/disturbance conditions in a medium-voltage distribution system and a low-voltage experimental set-up system. The simulations are performed using the PSCAD/EMTDC package. The paper shows that a hybrid load can operate in regenerative mode depending on the load parameters and fault/disturbance characteristics.