The use of 25KV rated equipment on a 26.4KV primary? 1

[alabaza]

I have a pad mounted transformer rated: 750KVA,3 PHASE, Y-Y, 15.24/26.4KV - 277/480V. The rep. is telling me that i can use a class rating of 25KV,125KV BIL instead of 35,150KV BIL for insulation, but i am still not 100% convinced it is 26.4kv on the primary how can i get away with 25kv rated equipment?

any inputs are appreciated, thank you

 

[jghrist]

Most likely, you are OK with 125 kV BIL, but if you are concerned, you need to determine the protection margin which is determined by arrester characteristics and lead lengths, not the primary voltage rating of the transformer.

 

[alabaza]

OK, so when equipment are 25kv rated, is this 25kv to ground or line to line voltage? if to ground then i am ok otherwise my line to line voltage is 26.4kv!

 

[HornTootinEE]

If your transformer is already rated for your primary voltage, what is the concern? I guess I'm not sure what you are getting at... Are you spec'ing a transformer with those voltage ratings?

 

[alabaza]

these ratings are for the bushing, load break connectors, surge arrestors inside the transformer.

 

[odlanor]

25kV- 150kV are system nominal voltage and BIL voltage for transformer of a standard;

26.5kV- 150kV are system maximum voltage and BIL voltage of another standard;

system voltage is phase-to-phase; metering tests are normally phase-to-ground.

 

[alabaza]

the question remains can i use (bushing,load break connectors, surge arrestors rated at 25kv) knowing that the transformer is 26.5kv Y - 480V Y ?
the next standard rating for the (bushing,load break connectors, surge arrestors) is 35KV

 

[scottf]

Short answer/question is what is the BIL required for your 26.5 kV system? If the 26.5kV system is built on 125 or 150 kV BIL then that would be 25 kV class bushings, arrestors, etc...

If you require 200 kV BIL, then that would be 36 kV class equipment.

 

[odlanor]

there is no 125kV BIL for 25kV neither 26.5kV. Both are 150kV.
You have to use 25 kV class bushings, arrestors, etc...for 150kV BIL grounded system ( i guess your 25.6kV is effectively grounded system).

 

[alabaza]

it is 125kv BIL but if i get bushing or load break connectors and switches rated 25KV that is below my 26.5kv on the primary, it is like getting a disconnet switch rated 240v on a 480v system???

 

[odlanor]

125kV is not aplicable in standard for 25 or 26.5kV.
I would assumme 25 kV-150kV class bushings, arrestors, etc...

 

[alabaza]

it is, it is either 25KV-125KV BIL OR 35KV-150KV BIL , government spec. but that is still not the question

 

[jghrist]

US standard 25 kV loadbreak connectors have a maximum Ø-Ø rating of 26.3 kV, a maximum Ø-G rating of 15.2 kV, and 125 kV BIL. What is the maximum voltage of your system? Not the voltage rating of your transformer, but the maximum voltage that can be put on the loadbreak connectors. If this is less than 26.3 kV, then you are OK for 60 Hz voltage. You still have to determine what the maximum impulse voltage, based on arrester ratings and locations, to determine if 125 kV BIL gives adequate lightning protection. What is the BIL of the transformer?

 

[odlanor]

125kV BIL refers to IEC standard.