Relay Coordination / Selectivity Studies

Relay Selectivity Studies often referred as "Co-ordination Study" is an Art with Science

One should note that Relay Co-ordination is not purely scientific, it is actually governed by constraints viz;

1. Client / Utility Protection Philosophy
2. Protection engineer preference
3. System requirements

 This study is done during:

1. System Planning Stage
2. Detail Engineering Stage / Implementation Phase of Project

During System planning phase protection zones are made and best practices in engineering are implemented to make an optimum design with required selectivity

One important thing which to be made clear is: Relay Co-ordination Study is not a backup to cover design errors in the system and wherever constraints are not fulfilled special notes to be highlighted / mentioned in study report to take necessary correction action.

Procedure: Outcome of Constraints with Rules for Selectivity

1. Establish Co-ordination Pair.
2. Summarize results of Load-flow & Short-circuit studies for given operating scenario.
3. Establish philosophy for Main-1 & Main-2 Relay Configuration and Main & Back-Up Relay Configuration.
4. Determine Pickup currents for IDMT back-up elements & Instantaneous elements
5. Establish Current / Time or Current & Time coordination for Identified Co-ordination Pair.
6. Highlight area of concerns and close out study.

These are described as below:

Co-ordination Pair:         

Instantaneous relay are not intended to be coordinated with downstream. Instantaneous relay should be immune to system transients. Instantaneous relay should have a back-up in upstream which it is coordinated with a defined coordination time interval. Pair of relays which acts as a pair to clear a fault, coordinated with each other, are termed as a “Co-ordination Pair”.

Approach for Main-1 & Main-2 Relay Configuration and Main & Back-Up Relay Configuration:

No Co-ordination required for Primary Protection functions which are grouped in Main-1 with Main-2 Relay Configuration whereas, Co-ordination is required for Primary Protection functions which are grouped in Main with Back-Up Relay Configuration