Ex-2: - Source end O/C E/F relay settings calculations for radial double circuit EHV feeder.

Problem

Consider a radial double circuit 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. Here fault current contributed by each line will be half of the total fault current.

Calculations for O/C settings:

From table-1 thermal rating of the line is 487 A. Henc.

Optional Setting

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

= 0.8*2500/800 = 2.5 Amp with HS delay 500 ms

 

Calculations for E/F settings:

Optional Setting

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

= 0.8*1750/800 = 1.75 Amp with HS delay 350 ms

Set

a.   For Overcurrent Relay MTA = +45⁰ or 45⁰ leading

b.   For Earth Fault Relay MTA = -45⁰ or 45⁰ lagging

Comments

For double circuit line; fault current get divided into two circuits. Hence for same fault levels of receiving end and same CT ratio at source end obviously TMS for double circuit line will be less with respect to that of single circuit line. Compare settings results of this example with that of example-1