Inrush current based on XFMR rating

[Meinkaun]

Hi,...

I am working on a basic feeder protection involving a new 10/12.5 MVA ONFA 69/13.8 KV transformer. The %Z of transformer is 6.4%. I am using ETAP for simulation. Now,
When I have to input the value of inrush current, the typical value is 12 times I(full load current). Now what I am wondering, this full load current is based on 10 or 12.5 MVA because 12.5 MVA is forced air but steady state rating.

The other issue I am wondering about is that inrush current depends upon the voltage and current cycle.zero point.... That means, is there a possiblity the current might exceed 25XI(FL) for 1/2 cycle and 12XI(FL) for 6 cycle?

I would appreciate your comments.

Thanks

 

[davidbeach]

Everything is based on the transformer's self cooled rating (10MVA in your case) and the additional cooling just allows more load current without changing any of the electrical parameters of the transformer. "Typical" inrush is typically maximum inrush, closed into just right (wrong?) to achieve that value, at nominal voltage plus some margin. If you run the transformer above that voltage it will more readily saturate and you will see more inrush. If you aren't using anything that is current limiting, you probably don't need to worry about the 1/2 cycle value.

 

[slavag]

Dear All.
For my education only.
Why need for protection setting calculation of alone xfr used some SW? With today's protection relay's are included options as:
doubling of setting by inrush current
blocking of inst. stage by inrush current detector.
2'd harm. restraint, filtering, etc.
More or less we have rules: small xfr 16In, big 12In = inrush current. SC current = In/%Z ( but of course nominal, that means according to 10MVA).It's all, not so much. For grading with upstream and downstream device, yes, we need some SW.
Regards.
Slava

 

[cranky108]

slavag,

Although modern relays have many features that coordinate with inrush, a transformer at the size of 10MVA may be protected with Fuses at a much lower cost than that of breakered arrangment. (No breaker, No relays).
Then we are at a position where we need to coordinate the fuse with the transformer inrush.

 

[slavag]

Hi Cranky108.
I meant something other.
Why I need some simulation SW for calculate inrush current.
It's some standard data, may be today we have new dry type xfr. with smalest inrush current, but actually most of us use 16In, 12In for the 0.1sec and 25In for the 0.01sec.
Regards.
Slava.

 

[cranky108]

Hmmm,
I don't know that there's a difference between a dry type transformer and an oil transformer. Same magnetics, as I don't think the oil changes that. There maybe some difference based on the amount of iron. I know there is a difference based on the Copper vs. Aluminum, but that's not what you ment.
The IEEE tables I've always looked at are for oil transformers. That's probally a good question for the manufacturers.

 

[slavag]

I don't know also, I think, someone wrote aboit it, may be it's my mistake. I try check previous posts on this topic.
Regards.
Slava

 

[slavag]

Please see.
thread238-203689
differnt only is, for small trafos ( Q is what ia a small)
recommended use 16In

 

[ppaya]

Typicaly, you will put transformer base FLA (on ONAN rating) into your software for inrush current of 12 times FLA for 100 mSEC. Now, it is not strictly accurate to do so. Here's why: transformer inrush current consist of 2 components, first one is magnetizing current (which is for larger distribution/industrial transformers such as in your case 12xFLA - compare this to overhead small transformers where this is going to be significantly higher inrush in first3-4 cycles up to 25-30xFLA) and second component is load inrush component. Now, this second component is not an issue if you are energizing station transformer with secondary breaker/recloser in open condition (no load) , but it IS an issue when energizing transformer with secondary breaker in closed position, such as when you lose voltage on the primary of the transformer due to upstream reclosing (this is momentary interruption).
So, technically speaking this combined inrush is potentially always higher then 12xFLA and depends on how much load your closing-on. It may be in range 14xFLA for 100mSEC.
Reason why we dont' use this for ANSI point? probably because we have enough margin already when we say that transformer supply is infinite.

 

[ppaya]

O, yes for the second part of your question, the answer is NO. Your 10MVA Tx will never go that high.