What is a directional relay?

What is a directional relay?

Definition: A protection relay in which the tripping decision is dependent in part upon the direction in which the measured quantity is flowing.

How does directional protection determine the direction of current?

Directional overcurrent protection devices can achieve this requirement, albeit at extra cost. Directional IEDs determine the direction of the fault current by measuring the voltage with a voltage transformer as well as the current with a current transformer, and establishing the phase difference.

Where is directional overcurrent relay used?

Directional overcurrent relays are normally used on incoming line circuit breakers on buses which have two or more sources. They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources.

What is directional protection?

Directional protection: protection equipment capable of detecting a fault upstream or downstream (in a given direction) of its position. Earth fault (e/f) protection: protection in which the residual variable (current and/or voltage) is monitored to detect phase-to-earth fault.

What is difference between directional and nondirectional relay?

Non Directional relay: When there is fault in power system, power flows through fault. Non directional relays operate irrespective of direction of flow of current. Directional Relay: Directional relay operates when the fault is driving power to flow in particular direction. It senses the direction of current flowing.

What are the main features of directional relay?

Directional relays must have the following features:

• high speed of operation;
• high sensitivity;
• ability to operate with low values of voltage;
• adequate short-time thermal rating;
• burden must not be excessive; and.
• there should be no voltage creep and current creep.

What is directional over current protection?

Such relays are called as directional overcurrent relays. They are used in distribution system or subtransmission system where ‘ring main’ configuration is used to provide more reliability of service. Traditionally, three phase relays and one ground relay have been used to protect a feeder or a transmission line.

What is directional over current?

Directional overcurrent is the calculation of the difference between the phase angle of the current and the phase angle of the voltage. To oversimplify, if the voltage and current vectors point the same way, it’s probably forward, and if they point in opposite directions, the fault is probably reverse.

What is meant by directional overcurrent relay?

Directional Earth Fault Relay is used to protect the transformer/generator/alternator from over current fault. The relay sense the fault current in only one direction, the relay does not operate when the current in opposite direction.

What is directional overcurrent protection?

In what conditions is it necessary to use directional protection?

It is necessary to use it in the following conditions: in a system with several sources. in closed loop or parallel-cables systems. in isolated neutral systems for the feedback of capacitive current. and to detect an abnormal direction of flow of active or reactive power (generators)

Why MHO relay is called MHO relay?

It is inherently a directional relay as it detects the fault only in the forward direction. The relay is called Mho relay because its characteristic is a straight line, when plotted on an admittance diagram (G-B axes i.e., conductance – susceptance axes) as in the figure.