What is Voltage Regulation?
Modern power systems operate at some standard voltages. The Equipment working
on these systems is therefore given input voltages at these standard values, within
certain agreed tolerance limits. In many applications, this voltage itself may not be good
enough for obtaining the best operating condition for the loads. A transformer is interposed
in between the load and the supply terminals in such cases. There are additional drops
inside the transformer due to the load currents. While input voltage is the responsibility of
the supply provider, the voltage at the load is the one that the user has to worry about.
If the undue voltage drop is permitted to occur inside the transformer the load voltage becomes
too low and affects its performance. It is therefore necessary to quantify the drop that takes
place inside a transformer when a certain load current, at any power factor, is drawn from its
output leads. This drop is termed voltage regulation.
Vnl and Vl are no-load and load terminal voltages. This is the definition normally used
in the case of transformers, the no-load voltage being the one given by the power supply provider on which the user has no say. Hence no-load voltage is taken as the
reference.
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| Equivalent Circuit of Voltage Regulation |
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| Phasor Diagram of Voltage Regulation |
The figure given below shows the variation of full load regulation of a typical transformer as the
power factor is varied from zero power factor leading, through unity power factor, to zero
power factor lagging. The full load regulation at the unity power factor is nothing but
the percentage resistance of the transformer. It is therefore very small and negligible. Only
with low power factor loads does the drop in the series impedance of the transformer contributes
substantially to the regulation. In small transformers, the designer tends to keep the Xe very
low (less than 5%) so that the regulation performance of the transformer is satisfactory.
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| Variation of Full Load Regulation with Power Factor |