Secondary protection of an MV transformer

[mull982]

I'm looking at a MV transformer application and trying to determine the best way to adequestely protect the trasformer with the breaker on the secondary side.

The transformer in question is a 1500kVA 12.47kVpri - 480Vsec with a 5.75%Z. The fuse on the primary of the transformer is a 150A rated fuse. The secondary of the transformer is connected to a 4000A rated switchear with a 4000A main breaker. I'm trying to determine what to set the long time pickup of the seconedary breaker to in order to adequately protect the transformer.

From table 450.3(A) I see that the secondary breaker can be set to 125% of xfmr FLA for "any location" and 250% of xfmr FLA for a "supervised location". I typically set the secondary protection to 125% of transformer FLA which in this case would be about 2258A however my concern is that this pickup on the secondary would not allow the use of the full capacity of the 4000A rated switchgear.

Since this transformer is locaed in a a facility with qualified electricians this probably falls under the "supervised location" category and thus would allow the secondary protection to be set up to 250% and therefore allow the breaker to be set a 4000A and allow the full capacity of the switchgear. When looking at the 4000A breaker setting on a TCC it falls below the transformer damage curve except in the breakers long time pickup region where it crosses the damge curve.

As I type this I realize that since the primary fuses are within 250% of the transfoemr FLA then no secondary protection is required for a supervised location.

I'm curious to hear how others deal with secondary protection in such applications.

 

[jim dungar]

What difference does it make, how much capacity of the switchgear is used, if the transformer 'burns up'.

Is your transformer designed to allow for 'above normal' output (e.g. does it have cooling fans, is it designed with an inherent overload amount)?

 

[JoeStillman]

How is the transformer connected to the gear? If you are using cable and you set the LTPU at 4,000, you have to use 4,000A cable. If you set the breaker for 125% of transformer capacity, you can use half the cable.

In a situation like that I would set the LTPU to 2,000A.

 

[mull982]

The transformer does have a fan cooled rating of 3000kVA which allows for a 3600A rated secondary FLA. In this case would you set the pickup to 3600A or 4000A.

The transformer is bus connected to the switchgear with 4000A bus so secondary cable protection does not need to be considered.


Jim

It sounds like you are saying that although 450.3(A) allows for seeting the secondary protection much higher than transformer rating this could still lead to transformer damage or failure.I'm used to LV secondary protection where the maximum secondary pickup can only be 125%. With MV transformer secondary protection what are the guidelines for setting the secondary breaker other than what 450.3(A) allows. Do you try to keep the pickup in the neighborhood of 1.25 to 1.5x transformer rating? Or do you try to ensure that breaker curve is completely below transformer damage curve?

 

[jim dungar]

The transformer does have a fan cooled rating of 3000kVA which allows for a 3600A rated secondary FLA. In this case would you set the pickup to 3600A or 4000A.

The transformer is bus connected to the switchgear with 4000A bus so secondary cable protection does not need to be considered.


Jim

It sounds like you are saying that although 450.3(A) allows for seeting the secondary protection much higher than transformer rating this could still lead to transformer damage or failure.I'm used to LV secondary protection where the maximum secondary pickup can only be 125%. With MV transformer secondary protection what are the guidelines for setting the secondary breaker other than what 450.3(A) allows. Do you try to keep the pickup in the neighborhood of 1.25 to 1.5x transformer rating? Or do you try to ensure that breaker curve is completely below transformer damage curve?

Your transformer is designed to allow for a continuous rating in excess of 100%, therefore it is allowable, and probably desirable, to set the LV LTPU to the higher output rating.
My point was to use the transformer output as your limiting factor not the rating of the secondary equipment.

 

[dicklaxt]

You folks are over my head on this but wouldn't load diversity be a factor and then with that applied and you were to set the secondary protection at 250 you would be far and above the normal swgr loading and kick the fans in when needed as load trending would indicate/ask for?Protective relaying could/would also be a path of protection either alert or shutdown,,,,just a couple of thoughts.

dick

 

[JoeStillman]

I am surprised that the 4000A breakeer setting puts you over the damage curve.

Are you looking at the damage curve for a 1500 kVA or a 3000 kVA? If you use the forced air rating of the transformer, 1.25xFLA is over 4000. I would use the damage curve for a 3000 kVA and set the LTPU to 4000A for this tranny, then set up some kind of fan failure alarm (thermostat or sail switch).

 

[jim dungar]

I am surprised that the 4000A breakeer setting puts you over the damage curve.

Are you looking at the damage curve for a 1500 kVA or a 3000 kVA? If you use the forced air rating of the transformer, 1.25xFLA is over 4000. I would use the damage curve for a 3000 kVA and set the LTPU to 4000A for this tranny, then set up some kind of fan failure alarm (thermostat or sail switch).

Be careful doing this.

The damage curve of the transformer does not change in regards to the short circuit portion, just because the long time area is moved.
A notation, of increased capacity, on the transformer curve is probably a better idea than selecting an entirely different curve.

 

[mull982]

Thanks for the comments guys.

I guess what I am seeing is that the NEC is a safety (code) guide and not a design guide. Although it allows for a OCPD rated up to 250% on the secondary of a transformer this is typically not a good design choice for protecting the transformer.

If we are simply talking about a self cooled transformer without any forced air cooling where do you typically set the secondary OCPD on the LV side with a transformer having a MV primary? It sounds like although the NEC allows 250% this value is never used because it does not provide adequate protection of the transformer. Do you typically use 125% for these transformers or do you use 100% in order to provide adquate secondary protection.

Thanks

 

[petersonra]

I don't know a whole lot about MV transformers but I would be inclined to set the switchgear main CB at 125%.

I am curious why they bought 4000A switchgear for an 1800A xfmr.

 

[JoeStillman]

I don't know a whole lot about MV transformers but I would be inclined to set the switchgear main CB at 125%.

I am curious why they bought 4000A switchgear for an 1800A xfmr.

He mentioned above that the transformer had two ratings - ambient air and forced air. The switchgear was probably selected on the forced air rating.

 

[Smart $]

... wouldn't load diversity be a factor...

Along that line of thinking, I'm a bit surprised that no one has asked about the calculated load. All the discussion so far is moot if the load permits setting the LTPU below the transformer damage threshold.