Types of Relays

Electric Protection Relays are vital components in the protection scheme.  Protection Relays protect an equipment such as a Transformer or a Generator from internal and external faults such as overvoltage, overcurrent, earth fault, etc.

Relays have been in existence since the early years of Electrical Engineering.  Relays can be configured for instantaneous operation or delayed operation. 

There are three broad categories of relays based on the principle of functioning. 

They are Electro-Mechanical Relays, Numerical Relays and Solid State Relays

Electro-mechanical Relays

Electro-mechanical Relays are the oldest type of relays.  These relays are simple in construction.  They are easy to adjust.  These Relays consist of a disc, usually made of aluminium, which rotates when there is a fault.  The rotation occurs due to the presence of eddy currents caused by current and voltage coils. 

When a fault occurs, the rotating disc rotates and closes the alarm contacts which generate the alarm.  If the fault is severe or persistent, it closes the tripping contacts which issue the tripping command to the circuit breaker. 

The disadvantages of these relays is that the values tend to drift over time, due to the effects of heat, vibration and aging on the relay components.  These relays are gradually being replaced by numerical and solid State Relays

Numerical Relays

These relays are electronic relays.  They do not have moving parts.  In the Numerical Relay, the analog values are converted into numbers.  The alarm and the trip values are also fed into the relay and stored as digital values.    Numerical relays are also called digital relays.  The microprocessor monitors the field values and generates the alarm or the trip command.  Numerical Relays are programmable.  The behaviour and the characteristics or these relays can be programmed.  Numerical Relays are also multifunctional which means that the same relay can be used for overvoltage as well as overcurrent protection. 

Modern Numerical Relays can also communicate with protocols such as Ethernet, RS 485, etc.  They can store historical data of trends and events.  This feature will be helpful in analyzing a fault condition or a blackout.  Timestamping also enables sequential registering of events.

Static Relays

Static Relays are analog relays.  In static relays, the voltage or the current from the field is converted into rectified voltages and currents.  These values are then processed by means of op-amps, transistors, etc and the output signal is generated. 

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