.In this topic, you study Classification of Protective Relays.
Protective relays can be classified depending upon different factors such as –
Principle of Operation
Ordinary Electromagnetic Relay: These relays are activated by A.C. or D.C. quantities.
Examples – moving iron, attracted armature and balanced beam type relays.
Electromagnetic Induction or Simply Induction Relay: Such relays are actuated by A.C. quantities only. It uses the principle of induction motor (whereby torque is developed by induction in a rotor) in their operation.
Electro-thermal relays
Physico-electric Relays: Example – Buchholez relay.
Static Relays: In this relay measurement is done by stationary part. It employ transistors or magnetic amplifier to obtain the operating characteristics.
Electrodynamic Relays: These relays operate on the same principles as the moving coil instrument.
On the Basis of their Application
Under Voltage, Under Current, and Under Power Relays: In these relays operation takes place when the voltage, current or power falls below a specified value.
Example — Instantaneous or induction relay.
Over Voltage, Over Current and Over Power Relays: These relays operates when the voltage, current or power rises above a specified value .e.g, instantaneous or induction relay.
Directional or Reverse Current Relays: In which operation occurs when the applied current assumes a specific phase displacement with respect to the applied voltage and the relay is compensated for fall in voltage.
Differential Relay: It is used to measure vector difference (phase/ magnitude) between two or more similar electrical quantities.
Distance Relay: It is used in transmission lines. It is based on measuring V/I ratio which measures distance between relay and fault location.
On the Basis of Time of Operation
Instantaneous Relays: In which operation takes place after small interval of time from the incidence of the current or other quantity which causes operation.
Definite Time-lag Relays: In this relay the time of operation is quite independent of the magnitude of the current or other quantity which causes operation.
Inverse Time-lag Relays: In these relays the time of operation is approximately inversely proportional to the magnitude of current or other quantity causing operation.
Inverse-definite Minimum Time-lag IDMT Relays: In these relays the time of operation is approximately inversely proportional to the smaller values of current or other quantities causing operation and tends to a definite minimum time as the value increases without limit. It shows the inverse characteristics between time and current.