Transformers have a name plate fixed on them on which rated output, rated voltages, the rated frequency, cooling method are recorded. The interesting fact is that its output rating is expressed in KVA (kilovolts ampere)rather then KW (kilowatt). This is due to the fact that the transformer rated output is limited by heating and hence losses in the transformer. The two type of losses in a transformer are core loss and ohmic losses.These losses depend on transformer voltage and current and are almost unaffected by the load pf, therefore transformer rated output is expressed in VA or in KVA and not in KW
I do not believe that this answer is correct. Power transformers transfer electrical power (energy) from one voltage level to another. Apparent power (volt-amperes) is tranferred, however, it is the load on the transformer that determines the KW/KVAR mix and the same rated transformer(KVA) could have a different KW/KVAR mix depending on the load characteristics. The transformer could care less as it supplies volt-amps, hence the transformer is rated in KVA. The ohmic losses (I2R) or other losses do not have anything to do with why a transformer is rated in KVA, but does have much to say about how the transformer is constructed.
ReplyDeleteWilliam (Bill) L. Mostia, Jr. PE
Rating of transformer is the maximum value of output it can give although it will give different KVA for different loads but the limit is rated KVA.
ReplyDeleteSecond of Load Characteristics never effect the transformer output, it is mainly the input to the primary side of transformer which alters its output.
In return this input on the primary side is effected by the load characterstics.
so how much power available in primary side is transfered to secondary side will be
input-losses
input is in KVA and so are the losses as losses are independent of load power factor, hence KVA.
for any transformer
Rated Output in KVA at primary terminals=Rated output at secondary terminals + Losses
the transformer depends on only two losses they are copperloss and core loss,the copper loss are depending on current(I)AND core loss are depending on the voltage(V) the tow terms (VI)the rating (K)
ReplyDeletetotally KVA
Both the losses of the transformer is independent of the
ReplyDeletepower factor (cos phi), which means a transformer would not
make any change in the power factor, since both the losses
viz copper loss(depends on current) and iron loss(depends
on voltage) are independent of power factor, that is why a
Transformers rating is not on kW, but on KVA
As we know that the rating of transformer in VA, means simply voltage x current. In Power System, he have to decrese or increase the voltage level accordingly current changs.. Example : i have a 5 MVA transformer at 33000/11000 KV, If I reduce my voltage by 2%, resulting it; 2% increase in current, overall the VA rating is same. If the same will feed to 11000/440 volts, we can use it up to 5 MVA, may be 50 x 100 KVA DT's used, it is simple to calculate the change in core & copper losses if required.
ReplyDeleteBut in MW or KW, we don't know about the Power Factor of load at various points n difficult to allocate the DT's.
So we always use VA in place of w.
Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again. It occurs when on account of some »inertia« in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not. Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power. It is often helpful to think of an extreme example: Imagine a use case where the only and exclusive load is a static var compensator (and such cases do exist). Would the load then be zero because the active power is zero? Most certainly not. – Caution: In this situation the voltage across the output terminals will increase with load rather than drop!
ReplyDeleteThank you so much for giving us such kind of handy content which will be most useful to me as well.... I will follow your blog always. Thanks!!! Power transformers
ReplyDeletethank for the suggestion ....nitin kushwaha
ReplyDeleteThe transformer are rated in KVA because its losses does not depend on power factor. Any equipment is rated by is output. In the case of transformers their output is only effected by the losses.
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Very nice information on transformers are rated in KVA not KW helped me in exam time thank you
ReplyDeleteTransformer ratings are known to be measured in kVA and not in kW. A transformer's rating or the rating of any electrical machine shows its ability to carry mechanical load without showing any signs of overheating.
ReplyDeletePower transformers in India | Transformer manufacturer in India
Transformers have lot of advantages that can work on an AC circuit to optimize the flow or current or voltage levels. Moreover they are the best alternatives to conventional circuits that are devoid of the usage of magnetic induction principle to control the voltage flow.
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Thank you so much for sharing this wonderful information. Power Transformers in India | Transformer Manufacturer in India
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