1. Overcurrent Relay
The overcurrent relay, commonly known as CO, is a protective device that activates when the current in an electrical circuit exceeds a predetermined threshold. It is primarily used for overload and short-circuit protection in power systems. The most widely used type is the induction overcurrent relay, which consists of an electromagnet and a rotating aluminum or copper disc. This disc rotates due to electromagnetic induction, and the movement is controlled by the magnitude of the current flowing through the system.
Working Principle:
The induction overcurrent relay operates by using the secondary current from a current transformer to create a magnetic field within the relay. This magnetic field induces rotation in the disc, but only when the current exceeds the set value on the current plate. This mechanism allows the relay to respond to abnormal conditions and initiate protective actions such as tripping a circuit breaker.
2. Overvoltage Relay
The overvoltage relay, abbreviated as OV, is designed to trip a circuit breaker when the system voltage rises above 120% of its rated value. This helps protect electrical equipment from damage caused by excessive voltage. The structure and operating principle of the inductive overvoltage relay are similar to those of the overcurrent relay, with the main difference being the configuration of the coil used to detect voltage changes.
3. Undervoltage Relay
The undervoltage relay, denoted as UV, has a construction similar to the overvoltage relay. However, it is designed to activate when the voltage drops below a certain level. When voltage is applied, the internal contacts and rotating components respond accordingly to trigger protective actions.
4. Grounding Overvoltage Relay
The grounding overvoltage relay, also known as the grounding alarm relay (GR), is used in ungrounded three-phase three-wire systems. It is connected to a grounding transformer in an open-delta configuration to monitor the zero-sequence voltage, allowing it to detect ground faults effectively.
5. Ground Overcurrent Relay
The ground overcurrent relay, abbreviated as GCR, is specifically designed for high-voltage line grounding protection. Its primary functions include:
6. Selective Grounding Relay
The selective grounding relay, also called the directional grounding relay (DG), is used in non-grounded systems for distribution line protection. It can be applied to both overhead lines and cable systems. This relay detects zero-sequence current through a grounding voltage transformer. In the event of a ground fault, it identifies the faulty line and triggers an alarm, allowing for selective disconnection while maintaining power to unaffected circuits.
7. Phase Loss Relay
The phase loss relay, often referred to as OPR or PHR, is used to detect and respond to single-phase operation in a three-phase system. If one of the phases is disconnected, the motor may run in a single-phase condition, leading to overheating and potential damage. The relay ensures that the circuit is quickly interrupted to prevent such failures.
8. Ratio Differential Relay
The ratio differential relay (RDR) is used for differential protection in transformers, AC motors, and alternators. Unlike traditional overcurrent relays, it prevents false trips caused by external faults or imbalances in current transformer characteristics. By comparing the current on both sides of the protected equipment, it provides accurate and reliable protection against internal faults.
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