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Current relay, voltage relay, differential relay and distance relay are commonly used devices in relay protection systems, and each relay has different working principles and application scenarios. Here are their working principles:
1. Current relay (Current Relay)
Working principle:
Current relay determines whether there is an abnormality by monitoring the magnitude of the current. When the current in the circuit exceeds the set value (i.e., the fault current, such as a short circuit or overload), the current relay operates, sends a trip signal, and cuts off the faulty part.
Application scenario:
Mainly used to detect excessive current (such as short circuit or overload) to protect transmission lines, motors and other equipment.
Classification:
- Overcurrent relay: operates according to overload or short-circuit current.
- Undercurrent relay: operates when the current is lower than the set value, usually used to detect current disconnection.
2. Voltage relay (Voltage Relay)
Working principle:
Voltage relay monitors the voltage in the circuit. When the voltage is higher or lower than the set threshold, the relay will operate and send a trip signal to prevent the equipment from being damaged due to overvoltage or undervoltage.
Application scenarios:
Used for the protection of generators, transformers and other electrical equipment to prevent faults caused by abnormal voltage.
Classification:
- Overvoltage relay: It operates when the voltage exceeds the set value, usually used to prevent equipment from being damaged by overvoltage.
- Undervoltage relay: It operates when the voltage is lower than the set value to prevent motors or equipment from being damaged by undervoltage.
3. Differential Relay
Working principle:
Differential relays judge faults based on the current difference between the two ends of electrical equipment or lines. When the difference between the currents at the two ends of the equipment or line exceeds the set value, it is considered that a fault has occurred, and the relay operates to send a trip signal. Under normal circumstances, the current difference between the two ends should be close to zero.
Application scenarios:
Widely used for the protection of transformers, generators and long transmission lines. It can quickly detect short-circuit faults inside the equipment, because when the fault point is inside the equipment, the current difference between the two ends will increase significantly.
4. Distance Relay
Working principle:
The distance relay determines the fault by measuring the impedance from the fault point to the location of the protection device. The impedance of the power line is proportional to the distance, so the fault distance can be estimated by measuring the impedance. When the impedance of the fault point is less than the relay setting value, the relay will operate, indicating that the fault is within the protection range.
Application scenario:
It is mainly used for the protection of transmission lines, especially in long-distance transmission lines. The distance relay can automatically cut off the fault part according to the distance of the fault point.
Classification:
- Impedance relay: Determine whether it is within the protection range by detecting impedance.
- Admittance relay: Determine the fault based on the admittance value.
- Reflection impedance relay: Comprehensively consider the changes in impedance and admittance.
In short,
- Current relay: Detect overload or short circuit through current value.
- Voltage relay: Detect overvoltage or undervoltage through voltage value.
- Differential relay: Detect internal faults of the equipment through the current difference between the two ends.
- Distance relay: Determine the fault distance by measuring impedance, especially for long transmission lines.
