Line Protection Operate Time: How Fast Shall It Be?

An ultra-high-speed protective relay has been an important topic within the scientific community, and specifically within the power industry, for decades. The main drivers are the anticipated improvements in power system stability and power transfer capability which have become even more important with the increased penetration of renewable generation sources. The relay operate time is a relatively small part of the required improvement, since the circuit breaker interrupting time contributes the major part of the total fault clearing time. The first promising results in reducing the fault clearing time, from two or three cycles, down to one power system cycle, date back to 1976 when the relay operate time of 1.5 ms was achieved. Availability of sub-cycle breakers was anticipated in the early 1980-s. Nevertheless, almost forty years later, commercially viable 3/4 cycle breakers are still not available and their impact on the total fault clearing time has been omitted from most of the literature. The numerical relays operate time improvement has become the only goal and results are wrongly equalized with improvements in the total fault clearing time and directly correlated with improvements in power system stability. In this paper the real benefits of ultra-high-speed relay operate time are analyzed, considering the characteristics of the state-of-the-art circuit breakers and their interrupting time of 1.5-2 power system cycles (older circuit breakers are slower). A more holistic view, including interactions between protective relays and circuit breakers, as well as final benefits to utilities, is given in the paper.