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 voltageVsbetween
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.

Percentage regulation=

Es−Vs

Vs

×100%

Es=

√

OC^2 +AC^2

Let Rse=Equivalent resistance in the secondary circuit

Xse=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 ofEs. Hence AC can be ignored and so.

Es=

√

(OC^2 )=OC=Vs+IsRsecos Ø+IsXsesin Ø

% Regulation=

(IsRsecos Ø+IsXsesin Ø)100%

Vs

(Note: See Chapter 9 for a similar expression used with cable volt-drop).

In most power transformersRseis much smaller thanXseand soRsecan be ignored in
regulation and fault level calculations.

Figure 6.4 shows the per-unit values ofRsefor typical transformers.

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