All About Electrical Relays, Principles, Classification and Requirements

Protective relays
Relay is a device by means of which an electric circuit can be controlled by the change in the same circuit or in other circuit. The primary function of the protective relay is to sense the fault in the system, compare the signal so obtained with  the reference signal under normal conditions of operation and amplify the error signal such that the trip coil of the circuit breaker is energized and faulty section of the system is disconnected from the rest of the system.Under normal conditions of the system, a protective relay is required to keep itself alert such that in case of any emergency, the action may  be taken almost instantneously.It should remain silent under normal condition. Under certain operating condition, the power system components are required to carry more than rated current on a temporary basis. During peak load condition, the power system components arc intentionally over-loaded. Under such conditions of operation, often known as abnormal condition, it is not necessary to disconnect the section from the system. When the operation parameters are unduly strained, the protective relays are required to energize the alarms such that proper precautions are taken.

The Basic Principles of Relays
In electromechanical relays, there are one or more coils, movable elements, contact system, etc. The operation of such relays depends on whether the operating torque/force is greater than the restraining torque/force.The relay operates, if the net force, F in equation given below is positive. Where F is the net force, Fo is the operating force and Fr is the restraining force. In other words, the relay operates only if the operating force is greater than the restraining force. In electromechanical relays, the operating torque is produced by electromagnetic attraction/electromagnetic induction/thermal effects of electric current. The restraining torque is given by springs.

The Classification of Relays
Protective relays are classified as follows according to their construction and the principle of operation.
1. Electromagnetic relays These are actuated by Direct current or Alternating current quantities.
2. Electomagnetic induction relays
3. Electothermal relays
4. Static relays these employ transistors or magnetic amplifiers to obtain the operating characteristic.
5. Electrodynamic relays these are operate on the same principle as moving coil instruments.
6. Under-voltage, under-current and under-power relays.
7. Over-voltage, over-current and over-power relays
8. Direntional or reverse current relays
9. Differential relays
10. Distance relays

The Basic Requirements of Protective System
The efficient protective relaying system should posses the following characteristics:
1. Speed 
Minimum voltage and minimum fault time operate.
2. Selectivity
Maximum continuity of service by disconnecting the faulty part of the system.
3. Sensitivity
Capability of operating reliably under the actual desired conditions.

Protective Relays
These are the devices that detect abnormal conditions in electrical circuits by measuring the electrical quantities which are different under normal and faulty conditions. Due to abnormal conditions, voltage, current, phase and frequency may change. After detecting the fault, the relay operates to complete the trip circuit which results in the opening of the circuit breaker and isolating the faulty circuit.