On some aspects of power system stabilizer blocking logic

This paper summarizes and discusses operational cases where the PSS blocking should be activated and also presents a different scenario in which the PSS should be blocked. More specifically, the paper is concerned with the operation of the PSS of a generator which is part of a power system experiencing a frequency stability problem caused by a severe real power unbalance. The paper discusses in detail the impact of the slow, but large-magnitude system frequency variations on the operation of PSS structures, which use the frequency signal for the synthesis of the stabilizing signal. It is demonstrated that under some circumstances PSS blocking during a frequency instability incident might not be an acceptable option. To address this case, a new PSS operational logic, referred to as `PSS de-sensitizing´ is proposed. It is shown that with the PSS de-sensitizing it is possible to operate the PSS during all operational conditions without compromising the small-signal stability of the power system. The discussion is exemplified by realistic examples inspired by field experience.