Transformer overcurrent protection

[sam34]

When we size an overcurrent protection device for a transformer we always use 125% of the FLA (SEC. side) of the xfrmr. Lets use an example, we have 112.5KVA xfmr, FLA=311.6A(208V),
OCPD=1.25 x 311.6= 389.5A (400A next size-up). Now my question, We know the FLA of this XFMR is 311.6A but we also know we are using 400A OCPD. If we load our 400A MCB panel ,lets say,with 350A connected load,we know that won't trip the breaker instantly, how is the transformer is protected? and we know we are not to exceed the 311A because that will overload the Xfmr?.
any thoughts!!! Thank you

 

[bwyllie]

Re: Transformer overcurrent protection

Transformers are designed to withstand a greater percentage of the rating for a duration of time.

 

[templdl]

Re: Transformer overcurrent protection

It is of my understanding that dry type dist. xfmrs are sized based upon the load to be served, the cable sized to serve the load and the OCPD sized to protect the cable. This should all come out to that 125% sizing of the breaker.
As far as transformer overloading is concerned, will that breaker protect the transformer? My reference shows that a DTDT will deliver 200% of its NP rating for 1/2 hour, 150% for 1 hour and 125% for 4 hours w/o beong damaged provided that a constant 50% load precedes and follows the event. Take a kook at the TC curve of the breaker to see where that 200 and 150% overcurrent falls.
Also, this is not a matter of the breaker tripping instantly which is a result of the breakers magnetic element which responds to short circuit currents in the area of 10x the rating of the breaker.
IEEE std C57.12.59 should include a foot note indicating that the "Category I" damage curve may be used for 3ph transformer units from 15-500kva and 1ph units from 5-500kva.
Continuous overload capacity is not deliberately designed into the transformer because the design objective is to be within the allowed winding temperature rise with NP rating.
However, energy efficient transformers are designed with lower losses thus operating at a lower temperature rise. That in combination with a higher class of insulation allows a 115degC rise unit to carry a continuous 15% overload for example.
But remember that the overall life expectancy of the xfmr will decrease as will a transformer applied a 100% will be shorted than one that is applied at 50%.