Ex-3: - Receiving 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 Non-Directional O/C E/F relay provided at Bus-B.

Solution

The line conductor size, CT ratio and fault levels at remote bus (i.e.bus-B) are as shown in above figure and same as example-1. Hence we can adopt same settings as that of relay in example-1. However being non source bus there will not be any fault feeding from Bus-B. Hence to serve this relay as protection to 132kV Bus-B faults; we have to keep it non directional as shown in above figure. Also to protect downstream network of transformer LV side we will adopt its operational time of 350 ms instead of 500 ms.

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*5000/800 = 5.0 Amp with HS delay 350 ms

Calculations for E/F settings:

 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 350 ms

Comments

 Often it is necessary to validate the PSM and TMS settings of overcurrent and earth fault relay of radial line so as to have sufficient time discrimination with transformer HV side relay for fault on transformer HV side as well as LV side. This is explained by Example-5