5. A 415V to 11 kV transformer has a rating of 200 kVA. The winding resistance and leakage reactance when referred to the primary are 0.014 Ω and 0.057 Ω respectively.
(a) Determine the % regulation of the transformer at 0.8 power factor lagging.

(b) In designing a particular 415V to 11 kV, 200 kVA transformer, the primary winding resistance is to be 10 mΩ. Find the maximum winding resistance of the secondary winding if the transformer is to have 2% regulation at unity power factor.

Attempt at solutions
(a) (200*10^3) / (415^2) * (0.014* 0.8 +0.057 * √(1-0.8^2)) * 100% =
5.27%

(b) P.F = 1
Using the reflected impedance equation:

The primary resistance is determined by the secondary load resistance (Rl) and the turns ration i.e R'p =(1/n)^2*Rl . If we solve for Rl since the turns ration is 415/11k =0.037, 0.01=((415/11000)^2)*Rl , Rl= 7.026 ohms. Which for a step up transformer makes sense. Not sure where power factor at unity and 2% Vreg come into play.

At unity power factor, 2% = (200000/415^2)*R'p*100 , R'p=0.017

Any help with (b) please

 

Well for 2% regulation at UPF your total primary referred resistance is limited to 17 milli-ohms. You allow 10 milli-ohm of this total for the primary winding. The remainder (7 milli-ohms) you refer back to an equivalent secondary (11 kV side) resistance.

 

Cheers mate