Why is transformer rating defined in kVA?

Why is the transformer rating defined in kVA? 

Ans- A transformer, unlike a motor, has no mechanical output (expressed in kW). The current flowing through it 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 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. Similarly, the voltage that can be applied to a transformer primary winding has a limit. Exceeding this rated value will cause magnetic saturation of the core leading to distorted output with higher iron losses. It is therefore usual to express the rating of the transformer as a product of the rated voltage and the rated current (VA or kVA). This, however, does not mean that you can apply a lower voltage and pass a higher current through the transformer. The VA value is bounded individually by the rated voltage and rated current. 

Why is power transmitted at higher voltages? 

Ans- When a particular amount of power has to be transmitted over a certain distance. The following aspects need to be considered to decide the best voltage. A lower voltage they need higher size conductors to withstand the high current involved. There is a physical limitation on the size of the conductor. Also, the percentage of voltage drop may become excessive. A higher voltage will make the conductor size manageable and reduce the voltage drop (% value) but the cost of the line becomes high due to larger clearances needed. The best voltage will be one in which the total operational cost which the sum of the annualized capital cost (of the line) and the running cost due to power loss in the line is the lowest. In practice, it is found that transmitting bulk power over long distances is economical if done in the HV range. The actual voltage will vary based on the distance and quantum of power. Distribution circuits where typically the amount of power and distance involved are both lower, the best voltage is in the MV range (11, 22 or 33 kV). For the same reason, low voltage circuits are found only in local sub-distribution circuits.