Ex-5: - Validate radial line receiving end O/C E/F settings calculations of example-3 for 3-Ph fault on 33kV bus at receiving end S/S.

Problem

With reference to example-3 let us consider Bus-B is of S/S having 2 numbers of 132/33kV 50 MVA transformers and 1 number of 132/11kV 25 MVA transformer. The calculated settings of O/C relay of 132/33 kV transformers and 3-Ph fault current for 33kV bus are as shown in figure. (a) Find out the time of O/C relay operation for relays at 50 MVA T/F HV side and 132kV incoming line (settings of O/C relay of incoming line are as calculated in example-3) and ensure that relay operational time (ROT) for incomer line is more than ROT for transformer HV side relay.

Solution

For the 33kV level fault; calculate reflected current on EHV side of the transformer. Then calculate T/F HV side relay current using CT ratio of 400/1 Amp and using formulae and as per settings of the T/F HV side O/C relay; calculated relay operation time is 498 ms. Whereas for 33kV bus faults relay operation time of 132kV line O/C relay will be 1060 ms. Important steps of calculations are as shown here under.

Comments

On observation of PSM and TMS for line and transformers it appears nearly equal. However there is very large difference in relay operation time. This is due to fact that CT secondary current for line due to fault on 33kV bus is two times less than that of transformer HV side.