Station service transformer(SST) selection 1

[Raj Adrika]

I have power transformer( 25MVA, Delta/wye, 138kv/4.16kv)and the Xo is connected with 160ohm , 15A for 10 sec NGR. I have metering PT ( 3 single phase units, two bushings N.S.V 15kv, 110 kv bil, H1-H2-2.4kv, SEC. 120V ratio 20:1)which I am planing to connect L-N. In such case which type of single phase 50kva SST I could select. Could I select 4.16KV Y/2.4KV, TWO BUSHINGS ( Both bushings to be same insulation)and connect L-N. Or I have to select L-L (Delta) transformer and connect VT L-N or L-G (any option).

 

[RRaghunath]

Better to connect Single Phase 50kVA SST between phases, i.e. the primary to be rated 4.16kV and connected L-L. Secondary could be 415V / 400V or 240V / 230V as required.
15A NGR for 4.16kV system suggests the system is high resistance earthed. This is OK only if the 4.16kV distribution is limited. Review.
Each PT or VT is connected L-G and star point is grounded.

 

[Kiribanda]

From the data given your 4.16kV system seems to be HRG. Therefore, you cannot & should not connect any
single phase loads. You have no choice other than using a 50kVA SST with primary voltage rated for 4.16kV nominal.