Directional comparison blocking protective relay system

Abstract

A directional comparison relaying system includes forward looking pilot relays at each terminal of a protected line segment and monitors which detect predetermined incremental changes in line currents and voltages indicative of a fault. These predetermined incremental changes in current or voltages, which can be detected before the pilot relays respond, generate local blocking signals which are transmitted to the other terminal as remote blocking signals over a two-way communications channel. Trip signals for tripping circuit breakers at the associated terminals are generated from forward fault signals produced by the pilot relays but are blocked by the presence of either a local or remote blocking signal. However, the forward fault signal cancels the local blocking signal and the corresponding remote blocking signal at the other terminal so that when both pilot relays see an internal fault, the circuit breakers at the two terminals are tripped simultaneously. Either a reverse direction distance relay or a reverse ground current relay and a zero sequence current monitor, generate a transient blocking signal which maintains the local blocking signal and its corresponding remote blocking signal at the other terminal, and blocks a trip signal during transient fault clearing in other line segments to prevent overtripping.