Ex-1: - Source end O/C E/F settings calculations for radial EHV feeder.

Problem

Consider a radial 132kV line emanating from 132kV Bus-A; and terminating on 132kV Bus-B. Calculate the settings of Directional O/C E/F relay provided at Bus-A.

Solution

The line conductor size, CT ratio and fault levels at remote bus (i.e.bus-B) are as shown in above figure. Usually this O/C E/F backup relay is directional even though the line is not operating in the grid.

Calculations for O/C settings:

From table-1 thermal rating of the line is 437 A. Hence

(Or decide by referencing IDMT characteristics curve)

Relay is at source end of single circuit line hence required time of operation shall be 0.5 sec.

Optional Setting

High-Set for O/C equal to 80% of fault contribution for O/C fault

= 0.8*5000/800 = 5.0 Amp with HS delay 500 ms

 

Calculations for E/F settings:

(Or decide by referencing IDMT characteristics curve – read comments)

Optional Setting

High-Set for E/F equal to 80% of fault contribution for earth fault

= 0.8*3500/800 = 3.5 Amp with HS delay 500 ms

Comments

As now a days there is wide spread availability of calculating devices hence referencing IDMT curve for deciding time of operation of O/C E/F relay from multiple of current is became obsolete. However you have to understand and remember the respective formulae carefully.