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What is a protective relay and How do protective relays work?

News June 01, 2020


What is a protective relay

In a power system power reliability is an important factor and a power system is comprised of generators, transformers, distribution circuits, etc. in order to get a proper and uninterrupted power the power system failure must be quickly determined and the failed part must be disconnected from the power system. So there won’t be any interruption of power service if we detect the fault quickly. Protective relays are devices that are very useful in the power system. This device will detect and disconnect the faulty system. The protective relay will detect the faulty section and isolate it from the rest of the system. A relay can be considered as a switch that can be open and close circuits electromechanically and electronically.

What is the function of a protective relay and why protective relay is used

The function of a protective relay is to remove the faulty section in a power system so there won’t be any power supply interruption and the equipment will be protected. A relay can detect the abnormal conditions in an electrical circuit, it is achieved because of the constant measurement of the electrical quantities. There are certain electrical quantities that may change in case of fault conditions they are voltage, current, frequency, phase angle.

What is the purpose of protective relay

To isolate the faulty section from the rest of the power system

To detect the faulty section and to give a signal to the circuit breaker

By using a low current high current system is controlled

Protection of the equipment in the power system

Constant monitoring of the operational condition of the equipment

To detect defective lines or devices or other power system conditions

What is the difference between fuse and relay

There are different settings for relay and they can be reset, well it is not possible in case of the fuse. The fuse only has a specific characteristic for a certain type. Like a relay, fuses cannot be reset they need to be replaced.

How to select a protective relay and what are the required characteristic of a protective relay

A relay should have certain qualities such as selectivity, speed, sensitivity, reliability, simplicity, and economy. If all this quality is present in a relay then it can be selected and installed so that the relay would work properly and efficiently.

Selectivity – it is the capability of the protective system to detect the faulty part and disconnecting it from the rest of the system

Speed – the protective relay must be able to disconnect the faulty section from the rest of the system very quickly otherwise there is a chance for the damage of the electrical equipment because of the fault current.

Sensitivity – it is the ability to detect the fault or abnormal condition in a protective zone.

Reliability – it is the ability of the protective system to work under the predetermined conditions, so there should not be any false tripping or no failure to trip

Simplicity – if the protective system is simple then it can be easily maintained. If the device is simple then the reliability will be high.

Economy – Economical factors must be considered while selecting a protective system if the apparatus which needs to be protected is very important so we must not consider this.

Why a system needs protection and what would happen if there is no protective relay

There no fault-free system

A fault-free system can’t be built, and if we try to build one it would be costlier

Mostly the system fault is caused by the breakdown of insulation and it could happen by system aging, lightning, etc.

If there is no protection, the equipment could damage so the loss of revenue, it can cause fire hazard and loss of life

How do protective relays work and what is relay principle

There are different types of protective relays and some of them are different in their operation method such as an electromechanical relay does its operation by either magnetic attraction or by magnetic induction. Armature type relay has a pivoted lever supported on a hinge or knife edge pivot, which carries a moving contact. Armature relays could work in either AC or in DC. Relay works as a protective switch and they are used in power systems, but in certain cases relays are used as sensing devices such as in higher power applications, so in case of any abnormal condition the relay would sense it and sends the signal to the circuit breaker. Protective relay senses the fault position and gives a signal to the circuit breaker, the circuit breaker will disconnect the faulty portion after receiving the signal


The above image shows the connection of the relay and we can see the primary winding of the current transformer is connected to the line which is to be protected. The secondary winding of the current transformer is connected to the relay coil, and there is a tripping circuit it could be AC or DC and there is a circuit breaker. So in the case of a short circuit at point F, there is a huge hike in the current flow, and this current would reach the relay coil and the relay will operate due to this by closing its contact. So due to this the breaker trip circuit will close and thus the circuit breaker opens and disconnects the faulty section.

How protective relays are connected in a power system

The protective relays are connected by current and voltage transformer in a power system. Current and voltage transformer is used in a circuit to measure the current and voltage. So the relay would detect the changes or variations in voltage, power, etc, so if there is any fault the relay will isolate that part from the rest of the system.

What is the difference between directional and non-directional relay

Non-directional relays can detect faults in any direction while the directional will only detect the faults in front of relay. Mostly non-directional relays are used because, it can detect more faults than the directional ones and they are used in the radial systems.


What are the types of protective relay

Types of the protective relay according to their construction and operation

  • Electromechanical relay

  • Static relay

  • Numerical relay

Classification based on the speed of operation

  • Instantaneous relay

These relays operate very quick, so there is no time delay in this type

  • Time delay relay

In this type, a time delay is occurred because of relays decision time and also because of the trip action

High-speed relays

This type of relay operates within milliseconds

Classification of protective relays according to their functions

  • Overcurrent relay

  • Impedance relay

  • Directional relays

Other types of relays

Differential relay

  • Pilot relay

  • Control relays

  • Pneumatic relay

  • Snap acting relay

  • Microprocessor-based relay

  • What are the advantages of using protective relay

System failure could be detected and disconnects the faulty section from the remaining system

Mitigating the effect of failures after they occur. The risk of fire is reduced, and there is less danger to personnel and other high voltage systems.

What are the applications of protective relay

  • Generator protection

  • Protection of large motor

  • Bus zone protection

  • Transformer protection

  • Transmission line protection by phase comparison current carrier protection

  • Feeder protection by pilot wire differential protection

  • Short circuit and earth fault protection

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