# Why do we choose apparent power rating as base in per unit analysis of a power system?

Can you please tell me the reason behind choosing the apparent power as base of power in per unit method. Why don't we choose real power instead.

• I don't get your question exactly. If it is what I think it is, it is probably because you can overload the power system without any real power taken. Apr 23, 2018 at 15:13

The capacity of the generation, transmission and distribution components is determined primarily by the apparent power. Also apparent power is always more than real power. Those factors make it generally more convenient to choose apparent power as the per-unit base. The key is convenience. There is no reason that some other base can not be selected if it is more convenient for a particular purpose. You can always work out some example problems using real power as the base and then again using apparent power and judge for yourself which is more convenient.

The reason per unit is used is to make dealing with units unnecessary. With this in mind, generation capacity, load demand, power rating of components are all listed as being in kVA or MVA. It makes more sense to use the Apparent Power as the Base value rather than Real or Reactive Power; though, if you so desired, you could use either as the base.

Convenience and patterned thinking are the main reasons to choose Apparent Power as one of the Base definitions.

For me, aparent power is the power needed for some device for running (the demanding power) and active power is the part of that which is consumed. Imagine that you have a very bad led bulb (in terms of cos phi). If cos phi is 0.5 and your led bulb consumes 3w you will need 6w of aparent power and thus, the electrical requeriments will be 6w.

To sum up consider always the aparent power.

David

That is because the copper loss in transformer or motor depends on current and iron loss. The iron loss depends on voltage. The total losses depend on VA rating that is mean its independent of the load power factor or real power.

• Use your shift key and use proper grammer Apr 23, 2018 at 21:06