On ships, not only transformers, but also generators, protection devices etc., are mostly rated in kVA. A motor does mechanical work and thus has mechanical output expressed in kW. A transformer is a static device, which does not perform any mechanical work. But the main functions are stepping down and stepping up of voltage ratings. Invariably, while stepping up/down the voltage, it also steps down/up the current inversely.
Thus the rating of a transformer can only be expressed as a product of Volts and Amperes (V *A)
The current flowing through the transformer can vary in power factor, from zero PF lead (pure capacitive load) to zero PF lag (Pure inductive load) and is decided by the load connected to the secondary. The conductor of the transformer winding is rated for a particular current beyond which it will exceed the temperature for which its insulation is rated irrespective of the load power factor.
The maximum voltage which the primary winding can be subjected to, has also a maximum limit. If the applied voltage to the primary winding exceeds the maximum rated value, then this will cause magnetic saturation of the core leading to distorted output with higher iron losses.
Thus considering both the above ratings, it is usual for transformers to be rated in VA. It can further be understood as product of voltage (V) and Current (A). But this does not mean that one can apply a lower voltage and pass a higher current through the transformer contributing to the rated VA value. The VA value is bounded individually by the rated voltage and rated current.
All electrical equipments in connection with generation, transmission, distribution of a.c. power such as alternators, transformers, switch gear, cables etc are rated on k VA basis.
We know that
Cos φ = kW/ k VA.
k VA = kW/ Cos φ.
It is evident from the above equation that the larger the Power factor, the smaller is the k VA requirement of the machinery. Therefore at low power factors, the K VA rating of the equipment has to be made more, making the equipment larger and expensive.
Thus k VA rating is so important and it is assigned at the design stage itself.
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