Short Answer:

Protective relays are special electrical devices used to detect faults in power systems and quickly disconnect faulty parts to prevent damage. These relays sense abnormal conditions like overcurrent, under-voltage, or short circuits and send a signal to circuit breakers to open the circuit. They help in maintaining safety, system stability, and reducing damage during faults.

There are many types of protective relays, and each one is designed for a specific type of protection. Common types include overcurrent relay, differential relay, distance relay, earth fault relay, and under/over voltage relay. The selection of relay depends on the type of equipment and fault expected in that part of the power system.

Detailed Explanation:

Types of Protective Relays

Protective relays are one of the most important safety devices in any electrical power system. Their main job is to continuously monitor electrical quantities like current, voltage, frequency, or power and take action if these quantities go beyond normal limits. When a fault is detected, the relay sends a signal to the circuit breaker to disconnect the faulty section, thus avoiding damage to equipment and ensuring system reliability.

Over the years, many types of protective relays have been developed for different protection needs. These relays can be classified in different ways such as based on function, construction, or the principle of operation.

1. Overcurrent Relay

An overcurrent relay works when the current in the circuit goes above a set limit. It is the most basic type of relay used to protect lines, transformers, and motors. These relays can be of two types:

• Instantaneous overcurrent relay – operates immediately when current exceeds the limit.
• Time-delay overcurrent relay – operates after a fixed time delay if overcurrent continues.

2. Earth Fault Relay

This relay detects faults between phase and ground. When an earth fault occurs, it sends a signal to trip the breaker. It is very important for protecting transformers, motors, and cable systems from ground faults which can otherwise cause fire or damage.

3. Differential Relay

A differential relay works by comparing the current entering and leaving a part of the system. If the difference between them is large (indicating internal fault), the relay operates. This is commonly used in transformers, generators, and large motors. It is very fast and accurate.

4. Distance Relay (Impedance Relay)

Distance relays are mainly used in transmission lines. They measure the impedance between the relay and the fault point. If the measured impedance is less than a preset value, it means the fault is close, and the relay operates. These relays are very useful in high-voltage lines where fault location matters.

5. Under/Over Voltage Relay

These relays operate when the voltage in the system drops too low or rises too high. Low voltage may harm motors, and high voltage can damage insulation. These relays help to disconnect equipment during unsafe voltage conditions.

6. Frequency Relay

Frequency relays are used to protect systems from abnormal frequency changes. If the frequency of the power system drops below or goes above safe limits, the relay operates. These are useful in generation stations and grid protection.

7. Reverse Power Relay

This relay is used to protect generators. If a generator starts absorbing power instead of generating it (reverse power), the relay operates and disconnects it. This protects the prime mover like diesel engine or turbine.

8. Thermal Relay

Thermal relays are used mostly in motor protection. They measure the heat produced due to overload and operate if the temperature rises too much. These are also called overload relays.

9. Buchholz Relay

Used only in oil-immersed transformers, the Buchholz relay detects gas formation due to internal faults. It is placed between the transformer tank and conservator. It is a mechanical relay and very reliable for transformer protection.

10. Pilot Relays

Pilot relays are used in schemes like pilot wire protection. They work using communication links between two substations and detect faults based on current flow at both ends.

Conclusion

There are many different types of protective relays, each serving a special purpose in the electrical power system. Whether it’s overcurrent, voltage imbalance, or ground fault, each relay helps detect problems early and disconnect faulty equipment. Choosing the right relay type depends on the nature of the system and the kind of fault expected. Protective relays are essential for the safe and reliable working of modern power networks and are a critical part of power system protection.