A relay is a device which makes a measurement or receives a controlling signal in consequence of which it produces a sudden pre-determined changes in one or more electrical output circuits.
A protection relay is a relay which responds to abnormal conditions in an electrical power system and controls a circuit breaker so as to isolate the faulty section of the system with the minimum interruption to the service.
Electromechanical relays
Electromechanical relay is a conventional relay in which the measurement is performed by moveable parts. The operation of such relays is based upon the following effects of electrical current:
1. Electro-magnetic attraction
2. Electro-magnetic induction
3. Thermal effect (Heat generation, some electromechanical relays respond to gas pressure generated due to heat of arc like Buchholz relay)
Static Relays
The expansion and growing complexity of modern power system required high performance and sophisticated characteristics from protection relays. This was possible with the use of semiconductors and other components in static relays.
Numerical Relays
The Numerical relays purely work on mathematical solution of different equations. They are micro processor based relays. Tripping decisions are not made by any measuring elements but are done by micro computers who continuously calculate and monitor the system data. Main advantages of Numerical Relays are:
1. Highly economical
2. Continuous self monitoring
3. More availability
4. Less work at panel fabrication
5. High flexibility in use
6. Memory
7. Possibility of remote control
8. Possibility of downloading/uploading information to computers
Important requirements of relays are:
1. Reliability
The protection relay should be reliable. It should not fail to operate when faults and abnormal conditions, to which it is meant to respond appear. At some times it should not perform false operation.
2. Selectivity
Protection is arranged in zones, which should cover the power system completely, leaving no part unprotected.
When a fault occurs the protection is required to select and trip only the nearest circuit breakers. This is also known as “Discriminate tripping”.
A Proper coordination of the operating and tripping characteristic is essential to achieve selective tripping of protective device connected in series in the network and thus assure high level of reliability of supply required by the consumers.
Today main challenge for protection engineer is to design a very intelligent protection scheme which in case of fault would isolate the minimum part of the network. Sometime a fault on one point causes tripping of unrelated breakers resulting in unwanted tripping.
Protection engineer needs to analyze this and requires sufficient data from affected circuit to recommend the remedial steps to have better security of the system.
3. Sensitivity
The relay should have sufficient sensitivity so that it picks up under minimum fault conditions within its own protective zone. It should operate for faults at farthest end of zone under conditions of minimum generation.
4. Fast
The relay should clear the faults in shortest possible time. However, speed must not be obtained at the cost of selectivity.
5. Stability
Ability of relay to withstand the changes outside its zone.
