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How To Calculate Transformer KVa

The document explains how to calculate the rating of single-phase and three-phase transformers in kVA. It provides formulas to calculate the rating based on voltage (V) and current (I) for both single-phase (kVA = (V x I)/1000) and three-phase (kVA = (√3 x V x I)/1000) transformers. It then works through an example of a 100kVA transformer and shows that using the single-phase formula incorrectly results in a rating of 57kVA, but the three-phase formula (kVA = √3 x V x I) correctly calculates the 100kVA rating.

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Suyog Shingare
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161 views3 pages

How To Calculate Transformer KVa

The document explains how to calculate the rating of single-phase and three-phase transformers in kVA. It provides formulas to calculate the rating based on voltage (V) and current (I) for both single-phase (kVA = (V x I)/1000) and three-phase (kVA = (√3 x V x I)/1000) transformers. It then works through an example of a 100kVA transformer and shows that using the single-phase formula incorrectly results in a rating of 57kVA, but the three-phase formula (kVA = √3 x V x I) correctly calculates the 100kVA rating.

Uploaded by

Suyog Shingare
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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How to Calculate the Rating of Transformer in kVA (Single & 3 Phase)

We know that, Transformer always rated in kVA. Below are the two simple formulas to find
the rating of Single phase and Three phase Transformers.

Find Rating of Single Phase Transformer


Rating of Single Phase Transformer:
P = V x I.

Rating of a single phase transformer in kVA


kVA= (V x I) / 1000

Rating of a Three Phase Transformer


Rating of a Three Phase Transformer:
P = √3. V x I

Rating of a Three phase transformer in kVA


kVA = (√3. V x I) /1000

Wait, A question is raised here…Look at the General nameplate rating of a 100kVAtransformer.


Did you notice something????Anyway, I don’t care what is your answer but lets me try to explain.

Here is the rating of Transformer is 100kVA.


But Primary Voltages or High Voltages (H.V) is 11000 V = 11kV.

And Primary Current on High Voltage side is 5.25 Amperes.


Also Secondary voltages or Low Voltages (L.V) is 415 Volts
And Secondary Current (Current on Low voltages side) is 139.1 Amperes.

In simple words,
Transformer rating in kVA = 100 kVA

Primary Voltages = 11000 = 11kV


Primary Current = 5.25 A

Secondary Voltages = 415V


Secondary Current = 139.1 Amperes.

Now calculate for the rating of transformer according to


P=V x I (Primary voltage x primary current)
P = 11000V x 5.25A = 57,750 VA = 57.75kVA

Or P = V x I (Secondary voltages x Secondary Current)


P= 415V x 139.1A = 57,726 VA = 57.72kVA

Once again, we noticed that the rating of Transformer (on Nameplate) is 100kVA but according to
calculation…it comes about 57kVA…

The difference comes due to ignorance of that we used single phase formula instead of three phase
formula.
Now try with this formula

P = √3 x V x I
P=√3 Vx I (Primary voltage x primary current)
P =√3 x 11000V x 5.25A = 1.732 x 11000V x 5.25A = 100,025 VA = 100kVA
Or P = √3 x V x I (Secondary voltages x Secondary Current)
P= √3 x 415V x 139.1A = 1.732 x 415V x 139.1A= 99,985 VA = 99.98kVA

Consider the (next) following example.

Voltage (Line to line) = 208 V.


Current (Line Current) = 139 A

Now rating of the three phase transformer


P = √3 x V x I
P = √3 x 208 x 139A = 1.732 x 208 x 139
P = 50077 VA = 50kVA

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