A 220/66 kV sub station has 50MVA 220/66 kV incoming Transformer feeding to 66 kV bus which has 1 No 5MVA Tx 66/0.440 kV and 1 No.10MVA Tx 66/11 kV and the total existing load is about 10 MW . I want to connect 1 No. 40MVA 66/33kV Tx on this 66 kV bus. Whether I switching on this new 40 MVA Tx will be safe and will not affect the existing Txs by insrush current of 40 MVA Tx.
Need comments. In my opinion it is not safe because of 40 MVA inrush current on only one 50 MVA main incoming Tx supplying the 66kV bus.
Probably an even better interview question. Possibly OP took the bait and figured that if the 10 MVA loading was combined with the inrush due to the 40 MVA transformer it would somehow be too close to the 50 MVA transformer rating. Answering the "somehow" is the key, the question was designed to determine someone's knowledge of transformer energizing, protective relaying and switching basics.
I don't think it should not matter. 220kV is a much stronger grid for a 40MVA transformer.
Considering the source for 66kV bus is a single 50MVA trafo, surely there will be severe voltage dip during 40MVA trafo energisation but the 220kV grid up above will ensure that the voltage dip is short lived, most likely below a sec.
Hope your under voltage trips for motors and auto bus transfer schemes in the plant are not set sensitive (typical is 2sec). Further, hope your 50MVA trafo impedance is not too high (10-12% is typical for this rating).
Thanks a lot . Sure nothing will happen to 220 kV side .At least this answer is sensible and requires further studies/calculations for voltage dip at 66 kV side to ensure safe charging without any problem.