Protection Relay

Protective Relay

             A protective relay is an intelligent device which responds to abnormal conditions in an electrical power system, to control a CB so as to isolate faulty section of the system, with the minimum interruption to service.

Desirable Qualities

•                 Reliability
•                 Selectivity
•                 Speed
•                 Discrimination
•                 Security
•                 Sensitivity

1) Reliability            It should not fail to operate in the event of fault.

2) Selectivity           It should identify and isolate the faulty portion of the system.

3) Speed                 Fault should be cleared as quickly as possible.

                Total Fault Clearing time = Relay operation time + CB operation time.

4) Discrimination      It should discriminate between different types of faults.

5) Security              It should not operate when it is not suppose to operate.

6) Sensitivity          The smallest value of actuating quantity  at which the protection starts operating  in relation with the minimum value of  fault current in the protected zone.

Types of Relays

Electromechanical Relays            

These relays depend on operating  force of electromagnet and retarding  force of spring.

Static Relays                                      

               These relays use semiconductor  devices like Transistors, diodes and  Op-Amps.

Microprocessor based Relays     

               These relays use microprocessors with digital computational techniques and memory features.

Electromagnetic Relay

Static Relay

mp based Relay