4160/600V, 2/2.66MVA XFMR Overcurrent Protection 5

[ElecDan]

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.

 

[waross]

If the fans are installed use the ONAF MVA.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[burntcoil]

As warros mentioned, use ONAF when fans are installed.

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.

 

[ElecDan]

Thanks a lot Waross & Burntcoil.

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?

 

[waross]

Many engineers would not use ONAF settings unless they verified that the AF fans were installed AND FUNCTIONAL.
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

 

[ElecDan]

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.

 

[prc]

Elecdan,How inrush current will change under ONAN or ONAF rating.

 

[Marmite]

Is there some confusion here on what the rating is being used for? The impedance of the transformer doesn't change with ONAN or ONAF rating. Where I come from the base MVA for the percentage impedance is the higher transformer rating. You need to check the transformer spec to check which figure is used to derive the percentage impedance.
Regards
Marmite

 

[davidbeach]

Agree with Marmite; in the ANSI world impedance is based on the self-cooled rating. The transformer is the transformer regardless of which rating is selected. Impedance, inrush, and anything else related to physical properties of the transformer remains unchanged.

 

[burntcoil]

@Warros

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.

 

[ElecDan]

Hello All,

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.

 

[davidbeach]

If the transformer is going to be operated based on the ONFA ratings, then you'd better set the relays based on the ONFA rating or you'll find yourself tripping on load. The ONFA rating is, after all, a rating of the transformer; nothing says that a transformer can have only one rating.

 

[waross]

Hello all. My comments were based on seeing ONAN/ONAF rated transformers installed without fans in anticipation of load increases in the future. My point is that even though a transformer may have an ONAF rating, it is prudent to verify that the fans are present and functional before using ONAF settings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

burntcoil,outside ANSI,when we say ONAF rating that is the nominal rating of transformer and all parameters are based on that rating and such transformers will always be provided with cooling fans.There are two design lines, use many fans of low rating or few fans of large rating.In both cases, reduction in rating due to failure of a fan is negligible and in practical operation,never an issue. Incidently ONAN/ONAF cooling is used in very large ratings - 1500 MVA 765 Kv Transformer Banks (India) and 1000 MVA 3 phase units(Sweden)