# What is directional over current relay?

Jul 20, 2019

## What is directional over current relay?

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.

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.

### 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 are the problems of directional relays?

Directional relays use voltage as the polarizing quantity. When three-phase faults occur close to the relay bus, the available voltage becomes nearly zero and this creates a problem in estimation of the fault direction. The capacitor coupling voltage transformer subsidence transients add to this problem.

#### Why directional relay is used?

The relay will then pick up for power flowing in one direction through the circuit and will reset for the opposite direction of power flow. If a single-phase circuit is involved, a directional relay is used having maximum torque when the relay current is in phase with the relay voltage.

What is meant by 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.

## What is the difference between directional and non directional relay?

The relay operates when the fault current exceeds the pickup current. For Directional Over current relay, the fault current can flow in both the directions through the relay either forward or reverse, depending upon fault location.

How does a directional relay work?

A directional relay uses an additional polarizing source of voltage or current to determine the direction of a fault. Directional elements respond to the phase shift between a polarizing quantity and an operate quantity.

### How do you test a directional relay?

Directional Overcurrent Relay Test Plan Summary

1. Connect all currents and voltages.
2. Choose the fault type to apply.
3. Apply nominal balanced three-phase voltages.
4. Cut the fault voltage in half.
5. Raise the fault current more than 110% of the pickup setting.

Why do we use directional 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 meant by non directional 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. For example, breaker at generator end. So we use a non-directional relay. It has to operate in fault conditions irrespective of direction of power flow.

When to use directional overcurrent relay ( 67 ) testing?

In the previous post about Directional Overcurrent relay (67) testing ( Finding the Direction in Directional Overcurrent Relays ), we reviewed Directional Overcurrent protection from a system perspective to enhance the descriptions in The Relay Testing Handbook: Principles and Practice.

## How is the direction of a relay determined?

This relay is designed to protect the same transmission line from the other direction. The current enters the non-polarity mark of the CT, and the relay determines that current is leaving the transmission line; or the reverse direction.

What are the tests for a relay circuit?

Determine current supervision pickup. Determine time delays. Test all inputs and outputs. Test all used initiate inputs and all used outputs. A relay with either a definite or inverse time characteristic that functions when the current in an a-c circuit exceed a predetermined value. Determine minimum pickup.

### Is the test current in the reverse direction?

Notice that our test current is right on the edge of the reverse direction. This means that it is a coin toss whether the relay will operate or not. We can ensure the relay always operates by setting the faulted current to a fault angle that would happen in the system.