- A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the faulty section from the rest of the system.
Fundamental Requirements of Protective Relays
- Selectivity: it is the ability to select the faulty section.
- Speed : speed of operation
- Sensitivity : it is the ability to sense small quantity fault current
- Reliability: it is the ability of the relay system to operate the pre-determined conditions
- Simplicity: so that it can be easily maintained
- Economy: should be cheap
Classification of Relays
According to principle and protection
- Electromagnetic attraction relay
- Electromagnetic induction type relay
- Electro-dynamic type relay
- Moving coil type relay
- Thermal type relay: these relays operate on the principle of thermal effect of electric current.
- Buchhholz relay is a gas actuated relay. This relay is used in oil immersed transformer (rating above 500KVA).
- Pick-up current: it is the minimum current in the relay coil at which the relay starts to operate.
- Current Setting:
Pick-up current = rated secondary current of C.T. *current setting
- Plug setting multiplier (P.S.M)
P.S.M. = Fault current in the relay/Pick-up current
- Time-setting multiplier:
Static relays employ thermionic valves, transistors or amplifiers
According to their function
- Main relay
- Auxiliary or supplementary relay
- Signal relay
According to applications the relays may be classified as :
- Over-voltage, over-current and Over-power relay
- Under-voltage, under-current and under-power relay
- Directional current relay
- Directional power relay
- Differential relay
- Distance relay
- Instantaneous relay : no intentional time delay is provided for operation.
- Inverse-time relay: the operating time is approximately inversely proportional to the magnitude of the actuating quantity.
- Definite time lag relay
- Inverse definite minimum Time (IDMT) relay
- Impedance relay (used for medium transmission lines)
- Reactance relay (used for small Transmission lines)
- Mho Relay (used for Long transmission lines)