4160/600V, 2/2.66MVA XFMR Overcurrent Protection
4160/600V, 2/2.66MVA XFMR Overcurrent Protection
(OP)
XFMR(D-Y): 2MVA(ONAN)
2.66MVA (ONAF)
Z% = 5.75, 2-windings
Voltage: 4160V(D)/600V(Y)
OCPD: High side has circuit breaker with protection relay (51/50). Low side also has circuit breaker with protective relay (51/50).
I know that in this application high side relay can be used for short circuit protection and low side relay can be used for transformer over load protection.
QUESTION: For OCPD settings what MVA to consider for FLA calculation? Is it 2MVA or 2.66MVA? Assuming that breakers and bus ratings are good enough for both cases.
I would appreciate any comments on this. Thanks.
2.66MVA (ONAF)
Z% = 5.75, 2-windings
Voltage: 4160V(D)/600V(Y)
OCPD: High side has circuit breaker with protection relay (51/50). Low side also has circuit breaker with protective relay (51/50).
I know that in this application high side relay can be used for short circuit protection and low side relay can be used for transformer over load protection.
QUESTION: For OCPD settings what MVA to consider for FLA calculation? Is it 2MVA or 2.66MVA? Assuming that breakers and bus ratings are good enough for both cases.
I would appreciate any comments on this. Thanks.






RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
If the fans are not installed or not operational use the ONAN MVA.
Without the fans operating the transformer may be overloaded if the ONAN MVA rating is exceeded.
Bill
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"Why not the best?"
Jimmy Carter
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
For Overcurrent and short circuit protection, I would suggest to use protection relay as primary protection for upstream and downstream of transformer and the breaker as backup protection for transformer overcurrent and short circuit.
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
I would like to explore your valuable comments as below:
I'm using ETAP for protection coordination. Thinking that ONFA is installed and operating, what MVA I should be putting in the transfomer nameplate data in the ETAP model? If I want to use max MVA then, I think, I should use 2.66MVA so that XFMR FLA can be reflected accordingly on the ETAP. Consequently I can set up my High/Low sides overcurrent settings as appropriate. Then I think I am completely ignoring ONAN MVA of the transformer, right?
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
Remember I2R.
If you use ONAF ratings to determine protection settings and the fans are not working properly then the greater I2R of the ONAF rating may damage the transformer without the protection system warning you or disconnecting the transformer.
Only use ONAF ratings if AF fans are present and functioning properly.
Without properly functioning forced air fans your transformer is an ONAN transformer.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
I completely agree with you Waross. I2R would be the thermal killer of the transformer if AF fans are not functioning. Using max MVA would also further increase transformer inrush current and thereby high side instantaneous setting might be shifting further right on the TCC.
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
Regards
Marmite
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
For the failure of fans, a watchdog contact can be used to monitor their status and the relay settings can be changed to the second group. Modern Protection relay provides settings in two group.
The fan failure will be a rare occasion. And also all of the fans will not fail at same time, that will be exteremly rare. Also If any Transformer Design Engineer can comment, because I believe the ONAF shall work upto its rated value if one of the fan fails, this point should be covered by Transformer Manufacturers.
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
Sorry for not being able to thank you earlier for helping my question answered. I agree with Marmite & David that inrush would not change unless impedance change. It was my misunderstanding/miswriting.
I'm in a situation where ONFA rating has been used to set transformer primary side relay overcurrent, and in order to satisfy the local code, some cases it exceeds bus/breaker ratings. I'm still not comfortable to use ONFA rating to set relays.
Thank you all again.
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 4160/600V, 2/2.66MVA XFMR Overcurrent Protection