Transformers also provide the option of compensating for system regulation, as well as the regulation which they themselves introduce, by the use of tappings which may be varied either on-load, in the case of larger more important transformers, or off-circuit in the case of smaller distribution or auxiliary transformers.

Consider, for example, a transformer used to step down the 132 kV grid system voltage to 33 kV. At times of light system load when the 132 kV system might be operating at 132 kV plus 10%, to provide the nominal voltage of 33 kV on the low-voltage side would require the high-voltage winding to have a tapping for plus 10% volts.

At times of high system load when the 132 kV system voltage has fallen to nominal it might be desirable to provide a voltage higher than 33 kV on the low-voltage side to allow for the regulation which will take place on the 33 kV system as well as the regulation internal to the transformer.

In order to provide the facility to output a voltage of up to 10% above nominal with nominal voltage applied to the high-voltage winding and allow for up to 5% regulation occurring within the transformer would require that a tapping be provided on the high-voltage winding at about  13%.

Thus the volts per turn within the transformer will be: 100/87 D 1.15 approx. so that the 33 kV system voltage will be boosted overall by the required 15%. It is important to recognise the difference between the two operations described above.

In the former the transformer HV tapping has been varied to keep the volts per turn constant as the voltage applied to the transformer varies. In the latter the HV tapping has been varied to increase the volts per turn in order to boost the output voltage with nominal voltage applied to the transformer.

In the former case the transformer is described as having HV tappings for HV voltage variation, in the latter it could be described as having HV tappings for LV voltage variation. The essential difference is that the former implies operation at constant flux density whereas the latter implies variable flux density.

Except in very exceptional circumstances transformers are always designed as if they were intended for operation at constant flux density. In fixing this value of nominal flux density some allowance is made for the variations which may occur in practice.

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