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elec.engnr
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Join Date: Feb 2014
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Total Load of Transformer

02/03/2014 5:58 AM

Hi,
Recently when I've visited an electrical panel room, I've noticed that the current readings in the Digi Ammeter were around 160A in each phase. Now for a voltage of 415V and current as mentioned above i.e. Ir = 160A, Iy = 158A, Ib = 145A what would be the transformer capacity (11 / 0.415kV) in kVA and what is the calculation procedure?
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kannan012
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#1

Re: Total load of transformer

02/03/2014 8:41 AM

Hi

here is the normal calculation procedure for finding KVA

Single phase amps to kVA calculation formula

The apparent power S in kilovolt-amps is equal to current I in amps, times the voltage V in volts, divided by 1000:

S(kVA) = I(A) × V(V) / 1000

3 phase amps to kVA calculation formula Calculation with line to line voltage

The apparent power S in kilovolt-amps is equal to current I in amps, times the line to line voltage VL-L in volts, divided by 1000:

S(kVA) = √3 × I(A) × VL-L(V) / 1000

Calculation with line to neutral voltage

The apparent power S in kilovolt-amps is equal to current I in amps, times the line to line voltage VL-N in volts, divided by 1000:

S(kVA) = 3 × I(A) × VL-N(V) / 1000
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elec.engnr
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#3
In reply to #1

Re: Total load of transformer

02/03/2014 10:11 AM

Hi,

Thank u for the explanation.

In the 2nd formula you've mentioned considering Line to Line voltage, the voltage would be 415 V I guess. What would be the current I(A)? I mean if we see the current reading in a digital ammeter, the reading shows different current for each phase like I(R), I(Y) and I(B). From these what would be the current I(A) that I should consider in the formula?

Kindly explain!
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RAMConsult
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#2

Re: Total Load of Transformer

02/03/2014 10:01 AM

The calculation procedure is correct, however it is quite possible that the transformer has more/less capacity than the measurements indicate.

The measurement only tells you the instantaneous transformer loading, it does not tell you the transformer capacity/rating which can easily be determined by looking at the nameplate physically on the transformer.

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elec.engnr
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#4
In reply to #2

Re: Total Load of Transformer

02/03/2014 10:15 AM

Hi,

Thanks for the reply.

Suppose I have a 11kV / 415V, 50 Hz, 500 kVA transformer with a max. rated LV current as 666 Amps, then what would be the max. current loading that is possible on each phase?

Kindly explain!
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RAMConsult
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#5
In reply to #4

Re: Total Load of Transformer

02/03/2014 10:54 AM

ee(?),

It "could" be any number up to infinity, but not for very long. Kindly explain why you believe it is other than "...max. rated LV current as 666 Amps...", and please consider changing your name from elec.engr to engr.in.training.

Alan

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elec.engnr
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#8
In reply to #5

Re: Total Load of Transformer

02/04/2014 12:25 AM

Oh I'm sorry if my question seems silly to you Alan. I am actually a engineer under training. It's just that I couldn't think of the name you suggested earlier. Thanks for your suggestion though.

Thank You.
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RAMConsult
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#9
In reply to #8

Re: Total Load of Transformer

02/04/2014 10:20 PM

Thanks for your honesty! Considering that the answer was in the question you were treated pretty well, next time do a little research and you might even get a straight answer or two.

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PWSlack
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#6
In reply to #4

Re: Total Load of Transformer

02/03/2014 2:55 PM

Er, how about 500kVA/3/voltage, continuous?

The actual current loading is, of course, restricted by the various circuit protective devices correctly connected to it. To do otherwise would be to put the integrity and longevity of the transformer at significant risk.

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67model
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#7
In reply to #4

Re: Total Load of Transformer

02/03/2014 5:46 PM

The transformer rating plate should have an impedance value, usually about 3.75 % for 50 Hz standards for the rated voltage/kVA [ which is continuous rating] you give.

If you short-circuit the secondary you could get 100%/3.75% = 26.67 times rated current as a maximum, for 3 seconds before you exceed the standard design withstand proof test.

For starting a motor, 5 times rated full-load could be acceptable for a motor terminal voltage > 80% (provided all the loads were happy with such a dip, which would be unusual because most equipment is designed to no more than +/- 10% variation from nominal).

But we will probably get tired of guessing what you are worrying about.

What you get out is proportional to what you put in.....

67model
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67model (1); elec.engnr (3); kannan012 (1); PWSlack (1); RAMConsult (3)

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