Transformer Overload protection

[HamidEle]

In NEC and CEC, transformer overload protection is not mentioned anywhere. If we look at TCC curves, overload zones always seem to have a problem with protection by using Thermal magnetic breakers, even though we follow the CEC or NEC rules to set the settings, especially for low voltage small dry type transformers. I talked to one of engineers in town. He said the tranformer will be protected so long as we set as per CEC. I want to hear your experiences , especially for small dry type transformers.
Thanks.

 

[davidbeach]

In general you will not protect the transformer damage curve unless you oversize the transformer. You aren't likely to get very many through faults at this level. For internal faults you will have lost the transformer but should clear soon enough to limit the collateral damage. (All that per the NEC, don't know about the CEC.)

 

[HamidEle]

If we just look at transformer damage curve and circuit breaker curve, the transformer can't be protected by the cicuit breaker. However, as matter of the fact, I applied 0.58 shift factor(made the curve to the left) due to Delta/Star connection of the transformer. But for overload protection and three phase fault protection, the transformer damage curve shouldn'y be shifted by 0.58 factor. My understanding is, we can't protect the transformer internal phase-ground fault within low fault zones.

 

[npower0073]

From what i have seen until now the overload protection of a transforemr is by means of monitoring the oil situation with bucholz, gas anlysis units, pressure relief devices. Another 'overload'protection is by having correct operation of the cooling system. Usually the circuit beakers at the primary and secondary of the transformers winding have overload protection curves but it is difficult to adjust an overload protection to a pass through device like a trasnforemr in comparison with a motor or generator.

 

[rbulsara]

Where did you look in NEC? Read NEC article 450. Permissible overcurrent protections for both primary and secondary sides of both LV and MV tranformers are clearly indicated.

 

[davidbeach]

npower, the things you mention are really irrelevant for "low voltage small dry type transformers" asked about by the OP. That may apply in other situations.

Thermal-magnetic circuit breakers have way too much tolerance in their settings to truly and tightly protect the transformer, coordinate with upstream and downstream devices, meet the NEC (and presumably CEC) requirements, and allow the transformer to be used at full nameplate rating. The consensus standard of care is to meet the NEC sizing requirements.

As the transformers become larger and the protective devices more sophisticated it becomes easier to meet all of the protective objectives.

 

[HamidEle]

Essentially, the CEC requirements are simillar to NEC requirements.
As matter of fact, as Davidbeach indicated, thermal-magnetic circuit breakers won't protect small transformers very well. So caution should be taken to select thermal-magnetic cicuit breakers for the protection purposes of the transformers. There are always some cases where the equipment is not fully protected, so long as we abide by the codes.

 

[sslobodan]

I think that the Siprotec or similar microprocessor device should suffice. It goes hand to hand with current, voltage measurement and differential current protection. That is the all he needs for his work and successful measurement. Differential current is signal for short circuit with ground (1,2,3 phases) and current with voltage will protect transformer from overload according to settings range. I usually use this at 10KV/0.4KV transformer application (European system) but like I said there is nothing different with US network because only voltage and frequency differs, principles are the same. I doubt that anyone would go in new application on old mechanical relays (price) so I believe that should answer your question.

P.S. Such protection goes for all voltage ranges, and from type of transformer you can have more protections recommended from manufacturer, like ventilators that are going on and off depending of the temperature, water cooling for oil cooling, Buchholz relay for gas detection .... There are many aspects, try using google for them.
(oil transformer protection filetypedf)

 

[sslobodan]

You can find some basic principles here but It's not too comprehensive

http://www.geindustrial.com/multilin/journals/Transformer_Protection.pdf

This one explains usage of circuit breakers (yes they are necessary as cheap way of protecting network when everything else fails)

http://www.sandc.com/edocs_pdfs/EDOC_027579.pdf

and this one is I believe all most of us will ever need for such application.

http://www.basler.com/downloads/transfguide.pdf