QUESTIONS-1:Transformer Always Rated In kVA instead of KW ?

 As the name suggest, transformer only transfer the power from one circuit to another without changing the value of power and frequency. In other words, It can only step up or step down the value of current and voltage while the power and frequency would remain same. A general date on transformer nameplate are printed for further details, such as rating in VA, single phase / three phase (power or distribution transformer), step up / step down, connection etc.

Right to the question, In simple words,

There are two type of losses in a transformer;

• 1. Copper Losses
• 2. Iron Losses or Core Losses or Insulation Losses

Copper losses (I²R) depends on current which passing through transformer winding while Iron losses or core losses or  Insulation losses depends on Voltage. i.e. total losses depends on voltage (V) and current (I) which expressed in Volt ampere (VA) and not on the load power factor (p.f). That’s why the transformer rating may be expressed in VA or kVA, not in W or kW.

When manufactures design a transformer, they have no idea which kind of load will be connected to the transformer. The load may be resistive (R), inductive (L), capacitve (C) or mixed load (R, L and C). Its mean, there would be different power factor (p.f) at the secondary (load) side on different kind of connected loads depends on R, L and C. This way, they go for VA instead of W in case of Transformer.