Question 2.
A building takes a three-phase low-voltage supply nominally at 400 V. This is
provided from a 11 kV/433 V transformer with the following impedance values
referred to the low-voltage winding:
5 m$\Omega$ resistance, R, per phase;
20 m$\Omega$ reactance, X, per phase.
The building takes a load current, I, of 650 A with a displacement power factor
of 0.6.
(a) Show using a phasor diagram how voltage regulation can be
approximated by:
Voltage regulation = $IR \cos\phi + IX \sin\phi$
[30%]
(b) Calculate the transformer voltage regulation, and the resultant line
voltage at the low voltage terminals of the transformer.
[30%]
(c) Power factor improvement equipment is to be installed in the building.
Given that the power factor is improved to unity,
calculate:
(i) the new value of supply current;
[10%]
(ii) the new value of line voltage, and comment on its acceptability.
[30%]
