Next-generation, phasor-based distance relay with fault-path resistance immunity

Distance relays are commonly utilized for the protection of transmission lines. A distance relay calculates the apparent impedance between the relay location and the fault location, and uses this value to determine the approximate location of the fault. Traditional distance relays, however, are plagued with problems caused by faults through a resistance. The fault-path resistance causes the apparent impedance determined by the distance relay to appear larger than the actual impedance between the relay location and the transmission line fault location. This error in the apparent impedance may deceive the relay and cause it to misoperate. Errors due to fault-path resistance are a fundamental problem of distance relaying.

The solution to the problems caused by fault-path resistance is presented in the form of the advanced distance relay. The advanced distance relay is a computer-based relay which employs a fault locator algorithm that is immune to the effects of resistance in the faultpath. The major substance of this document is dedicated to explaining the algorithms utilized to create the advanced distance relay. A major advantage of the advanced distance relay is that it is a single-ended relay. In other words, it requires voltage and current information from only one end of the transmission line that it protects. Another advantage is that knowledge of the system equivalent source impedance is not required. While knowledge of the distribution factor phase angle, parallel line zero-sequence current, and parallel line zero-sequence impedance are not required, in the event that they are available, the relay can provide even more refined, accurate results.

The advanced distance relay prototype was written in the "C" programming language. It utilizes a sampling frequency of 1440 Hz, and includes an algorithm that eliminates DC offset in the current waveform. The relay also employs a 1/2 cycle DFT phasor calculation algorithm, which provides the data required by the per-unit fault locator equation. The implication of this information is that the relay is capable of making a “trip / block" decision in just over 1/2 of a cycle.

As mentioned, the most powerful feature of the advanced distance relay is its immunity to the effects of fault-path resistance. It is this immunity that allows the relay to overcome the aforementioned fundamental problem of distance relaying. Consequently, the advanced distance relay is a powerful addition to the tools of protective relaying.