Ex-4: - Receiving end O/C E/F relay settings calculations for radial double circuit EHV feeder.

Problem

Consider a 20 km 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-B.

 Solution

The line conductor size, CT ratio and fault levels at bus-B are as shown in above figure. Here for calculation purpose consider a fault on line but very near to bus-B. In this case 50% fault current will be contributed by line under consideration. Considering this fault current Relay setting calculations are performed as previous. 

Calculations for O/C settings:

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

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 350 ms

 

Calculations for E/F settings:

 Set

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

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

Optional Setting

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

= 0.8*1500/800 = 1.5 Amp with HS delay 350 ms

Comments

In this case for remote bus that means bus-A fault there will not be any contribution from bus-B; as bus-B is not having any source. Hence fault current considered for calculations of O/C and E/F setting at receiving end is the current contributed by the line for the fault very near to bus-B. Obviously for double circuit radial line this fault current will be half of receiving end bus total fault current.