Detecting open phase conductors

Several papers have been written discussing high impedance fault detection (ANSI device HIZ) on overhead distribution circuits. Methods have been put forward that use relatively exotic proprietary algorithms involving relatively high sampling rates. These methods depend on the flow of at least some small arcing current in the affected phase. In most cases, the sensitivity settings of conventional ground fault protection elements such as zero sequence magnitude responsive ANSI device 50N and 51N as negative sequence magnitude responsive elements such as ANSI device 46 are limited by worst-case natural unbalance loads under heavy load conditions. These constraints make conventional zero sequence and negative sequence overcurrent elements nearly useless in detecting open phase conductors. The 46PD (pole discordance) element operates on the ratio of negative sequence to positive sequence current and therefore represents a negative sequence magnitude percentage. As a ratio, the 46PD element removes one of the limiting factors in setting the 50N/51N and 46 elements, namely the worst-case unbalance current magnitude, thereby improving detection when loads are at less-than-peak conditions.