Transformer Bushing Rating for Ungrounded or Hi Z grounded Line

[proznick]

Have a 13.8KV line, 8km long, coming from Hi-Z grounded generator and it terminates into the delta side of a 13.8kv/138kv delta-Ygrounded transformer, 138kV feeding utility. Normally 13.8kV line is Hi Z grounded at generator but should the generator trip, the line will will go to an ungrounded state and be backfed from the utility.

I am concerned about overvoltage conditions on the 13.8kV line during ground faults. Currently, the bushings on the 13.8KV delta side (7.97kV line to ground) are 15kV class rated for 10kV continuous line to ground voltage. The line to ground overvoltage during a ground fault on the unfaulted phases will increase to 13.8kV line to ground. Protection will trip line before any restrikes.


Are the bushings typically able to withstand 140% maximum continuous operating voltage (140% of 10kV) for short periods without flashing or should bushing be ugraded to withstand 13.8kV continuously?

Any thoughts on requirements for upgrading transformer bushing specs for ungrounded systems appreciated.

 

[prc]

proznick,

Dont worry, nothing need to be done.

Generally bushings of rating = or < 170 Kv are capable of withstanding rated voltage of bushing ( ie line to line volatge) continuously between phase and earth for periods not exceeding 8 hrs in 24 hours ( max 125 hrs per year)
See clause 5.1 of IEC 60137-2003 Bushings.In your case it is momentary and hence no need for concern.

 

[proznick]

Thanks PRC

R.Proznick