Page 152 - Handbook of Electrical Engineering

TRANSFORMERS 135

6.3 REGULATION OF A TRANSFORMER

Regulation is a subject that regularly occurs in power systems. Regulation is a measure of the voltage
drop in a device or circuit. It compares the volt-drop at full-load with the terminal voltage at no-load,
both of which can be obtained for a transformer from simple factory tests.
The voltage regulation of a transformer is the change in the terminal voltage V s between
no-load and full-load at a given power factor. It is usually expressed as a percentage of the rated
voltage. The phasor diagram for the single-phase transformer or one phase of three-phase transformer
is Figure 6.5.
E s − V s
Percentage regulation = × 100%
V s

2
E s = OC + AC 2
Let R se = Equivalent resistance in the secondary circuit
X se = Equivalent leakage resistance in the secondary circuit

For % regulations less than 20% it can be seen that the quadature components have little effect
on the magnitude of E s . Hence AC can be ignored and so.

2
E s = (OC ) = OC = V s + I s R se cos Ø + I s X se sin Ø
(I s R se cos Ø + I s X se sin Ø) 100%
% Regulation =
V s
(Note: See Chapter 9 for a similar expression used with cable volt-drop).
In most power transformers R se is much smaller than X se and so R se can be ignored in
regulation and fault level calculations.
Figure 6.4 shows the per-unit values of R se for typical transformers.

Figure 6.5 Phasor diagram of a loaded transformer at a lagging power factor.