Distance protection relays are a specialized type of protective relay primarily used for the protection of high-voltage transmission and sub-transmission lines. Unlike overcurrent relays, which only measure current, distance relays work by measuring the impedance of a circuit, which is the ratio of voltage to current. The principle behind them is that the impedance of a transmission line is directly proportional to its length. When a fault occurs, the impedance measured by the relay drops, and the relay can calculate the approximate distance to the fault location.
This ability to "see" the distance to a fault makes them highly effective and selective, as they can quickly isolate a fault on a long transmission line without affecting other parts of the grid. Distance protection relays are also highly immune to changes in load and source impedance, which can be a problem for simpler overcurrent relays. Modern distance protection relays are microprocessor-based relays that offer a wide range of features, including multiple protection zones, directional capabilities, and advanced algorithms to distinguish between a real fault and a power swing. Their speed and accuracy make them an indispensable part of a relay protection system for long-distance power transmission, where a quick response is critical to maintaining grid stability.
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