Replace Core & Coil of existing MV dry transformer

[zemingduan]

We have a project that has an indoor unit substation dated back to 1973. The 13.2KV 3ph - 208Y/120V 3ph 500 KVA dry transformer of the unit substation is blown/ doesn't work according to the client.

The client want to just replace the core & coil of the transformer but reusing the existing enclosure. This is the name plate of the old transformer and the drawings of the new core & coil in stock.


The KVA, primary, secondary voltage ratings are the same. The impedance is a little bit different.

Is this allowed? Any concerns/issues? I thought the enclosure of the transformer is designed specifically by the manufacture for their specific core & coil. And I thought the transformer core & coil and the enclosure are listed as a whole assembly.

The drawings is only for the core & coil. It's note 6) states it meets DOE efficiency standards, and it lists the Temp rise as 150 C. Did they get the test data without the enclosure? Will different enclosures affect the efficiency, Temp rise, etc.... of the transformer since ventilation requirements may not be satisfied? And the highlighted texts only recommends instead of requires the enclosure vent opening area.

I need some knowledge and get educated here. Thank you!

 

[jim dungar]

This is a common project. 150°C temperature is a standard rating for dry type units.

Find a good local service shop and get them involved early. The biggest issue will be the bussing.

 

[Todd0x1]

off topic, but what is the MTBF on the cooling fans? Do all unit substation transformers have fans on them?

 

[David Castor]

Transformer rebuilds are pretty common. I don't see any major issues. Not sure you'll save much money over a new one, but with transformer deliveries what they are, it might make sense. Are they sure they need a new core? That adds significantly to the cost.

New core and coils will likely be more efficient than the old unit if done properly with good quality steel. Aluminum windings not as efficient as copper however. MGM builds a lot of transformers.

 

[zemingduan]

off topic, but what is the MTBF on the cooling fans? Do all unit substation transformers have fans on them?

Not sure what the MTBF refers to. Not all unit substation transformers have fan options. Some are just Ambient Air (AA)/ Air Natural(AN) cooling. Some have Forced Air Cooling(FA)/ Air Force(AF) option which you can install fans for cooling.

For this transformer you have 500 KVA rating for AA and 667KVA rating for FA if you install the fans.

 

[zbang]

Mean Time Before Failure
it's a calculated measure of when a component can be expected to fail.

 

[David Castor]

Fans are normal on big substation transformers, not as common on unit transformers. They are pretty reliable. Google Krenz & Company for the recognized top-of-the-line transformer fans. The control system (thermostat, contactor, etc) is more trouble-prone than the fans, in my experience. There will normally be temperature gauges and sensors to monitor the winding temperature.