Directional Overcurrent Relay (67) for Phase Fault Protection in Substation

Directional Overcurrent Relays are used for Phase Fault and Earth Fault Protection in EHV Interconnected System where there are parallel networks, Ring Main System and Parallel Connected Power Transformers, where in case of Fault, Power Flows in both the Forward and Reverse Directions.

In this Article, we are going to study only about the Directional Overcurrent Relays for Phase Fault Protection. We will be making a separate Article for Directional Overcurrent Relays for Earth Fault Protection.

What is the need to make an Overcurrent Relay Directional ?

To make a Relay Directional, we need some extra Input (Voltage), which makes it costlier than the Non-Directional Relay, still in some cases, we  use the Directional Relays.

When the Power Flow is in only One Direction, like in the case of 22 KV Radial Feeder, we use Non-Directional Relays, because if Fault Occurs on these Radial Feeders, we are certain, that the Power Flow or the Fault Current will flow only in one Direction  i.e. away from the 22 KV Bus.

But consider a case like say Interconnected EHV Network or Parallel Connected Transformers, where in case of Fault, the Power or the Current can flow in both (Forward or Reverse) Directions and will result in unwanted Trippings, if Non-Directional Relays are used.

Means if the Fault is on one Feeder/Transformer, the Parallel Feeder/Transformer will also Trip if Non-Directional Relay is used, because it feeds the Fault too.