How to
Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?

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

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:

P = √3. V x
I

Rating of a
Three phase transformer in kVA

kVA = (√3. V
x I) /1000

But 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 J 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

Note: This
post has been made on the request of our Page fan Anil Vijay.

Why is transformer rating always written in KVA? We know that the unit of Power is Watt. then why don't we write transformer rating in Watts?

ReplyDeleteDear Mukesh Khatri@

DeleteRead This Article

Why Transformer rating in kVA and not in kW?

http://www.electricaltechnology.org/2012/02/why-transformer-rating-in-kva-not-in-kw.html

The two types of transformer are core loss and ohmic losses.

DeleteThe core loss depends on transformer voltage and ohmic loss depends on transformer current.As these losses depends on transformer voltage and current and are almost unaffected by the load pf, transformer rated output is expressed in VA or in KVa.

because the power factor of the load is not known so transformer rating is given in KVA

DeleteThanks

ReplyDeletedear in last consideration u shows that line to line volts is 208V then after calculation over value is came 50 KVA but actual value is 100 KVA have. plz Kindly explain it

ReplyDeleteDear Waqas Ahmed

DeleteHere are two examples.

The first one is for 100kVA which shown in the image.

The Second one is for 50kVA (where line to line voltage is 208V)

Your article looks great!Thanks for you sharing. I love it.

ReplyDeleteHigh Voltage Transformers

thanks teacher

ReplyDeleteIf we dont know the line current,how 2 calculate the rating?

ReplyDeleteIt is already given on every transformer's nameplate

DeleteThis system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and also

Deleteon most utility distribution systems.

kW=kva×pf

ReplyDeleteis it right?

Yes it is 100% right.

DeleteWhen considering the apparent power we also considered the effect of inductor and capacitor which always will be minimized by using power factor correction technique.

Points to be remembered:-

->KVA is always greater then KW

because KW only have the effect of resistor.

-> KVA=KW if and only if the power factor is one so the load is pure resistive.

-> Single Phase KVA

VI/1000

->three Phase KVA

VI*1.73/1000

->Single Phase KW

VI*PF/1000

->Three Phase KW

VI*1.73*PF/1000

(1000 if conversion factor for Kilo (10^3) & square root of 3 = 1.132051)

sir could u explain DYn-11.....

ReplyDeleteWait for the upcoming posts... Thanks

Deletehttp://en.m.wikipedia.org/wiki/Vector_group

DeleteHello Sir, Can u explain why in transformer,the power factor is not considering.? And why in others.?

ReplyDeleteCorrect me if I'm wrong ' The power factor is determined by the load, not the transformer. The PF on the load side of the transformer will be the same as it is on the primary, except at very light loads where the magnetizing current could have some effect. Having said that, the impedance in the primary circuit could effect the PF of the system. In other words, for a given load (with reactance) the system PF depends on the impedance in the primary circuit.'

Deletehi sir, how do the transformer designer calculate the turns, emf, loss, flux, area if we say we need1600KVA

ReplyDeletehi sir, how does transformer designer calculate flux, emf, loss, turns if v just say the rating eg,1600Kva if u reply with a example calculation it will be comfortable

ReplyDeleteHi Sir

ReplyDeletewhat size transformer do I need for 5A,230V Load?

dear 220kv/500kv transmission poll install in the forest.and we don't know the load and the production side.so tell the method of load and production identification?

ReplyDeletehi sir, i want to design a 500VA rating of transformer.but i dont understand how to select the v/g & c/n value? and no. of turn on each side of transformer?

ReplyDeletepls kindely rply me

Sir ,why we dont consider power factor for transfrmer but for generator or motor?

ReplyDeleteSir ,why we dont consider power factor for transfrmer but for generator or motor

ReplyDeleteDear Sir,

ReplyDeleteI want to know about how to select step down transformer rating. 415/230V and VA rating. ..?

..I mean how va can be calculated. .based on the which factors. .