Protective relays detect faults in electrical circuits and trip the circuit breaker. They can respond to overcurrent, overvoltage, and frequency conditions. Modern relays use microprocessors and can estimate the distance to the fault. Distance relays are common on high voltage transmission lines.
A protective relay is a device that trips or opens a circuit breaker when it detects a fault. Monitors an electrical circuit for a variety of parameters, such as voltage and time, and trips the circuit breaker when it detects overload conditions. The parameters of an IED are selectable, unlike a changeover relay, where the parameters are fixed.
A circuit breaker in a common household circuit uses a simple bimetallic strip that bends as its temperature increases. This trips the circuit when it reaches the threshold temperature. Industrial circuit breakers, on the other hand, require a protective relay to signal the circuit breaker when to open. These types of circuit breakers have an electromagnetic coil called a trip coil that opens the circuit when activated. The protective relay activates the shunt trip if the circuit satisfies the specified overload conditions.
The design of a protective relay can be elaborate. Common components of an overload relay include induction discs, operating coils, shielded pole magnets, and solenoid operators. The first protective relays were purely electromechanical devices, but modern protective relays use microprocessors. Microprocessor-based overload relays are more accurate than electromechanical versions and typically perform more functions.
Protective relays can respond to a variety of conditions. An overcurrent condition occurs when the voltage in a circuit is higher than the desired voltage. Similarly, an overvoltage condition occurs when the circuit voltage is above the desired level. Over- and under-frequency conditions are also common types of circuit failures. Some protective relays can also estimate the distance to the fault.
An overcurrent relay is a common type of protective relay. It is typically connected to a transformer and rated at the desired maximum current. The contacts in the relay operate when the current in the circuit exceeds this level, thus breaking the circuit.
A distance relay is a general type of protective relay that senses the distance to the fault. This is the most common type of protective relay on a high voltage transmission line. A remote relay senses the current and voltage on the circuit. The impedance per unit distance on the loop is a known quantity, which allows the distance relay to calculate the distance to the fault.
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