The nature of transformer inductive reactances

All previous references have been to shunt connected devices such as those used in normal loads, and power factor correction capacitor banks etc. The reason for this is that shunt connected equipment requires (by far) the largest quantities of reactive energy in power systems; however, series-connected reactances, such as the inductive reactances of power lines and the leakage reactance of transformer windings, etc., also absorb reactive energy.

Where metering is carried out at the MV side of a transformer, the reactive-energy losses in the transformer may (depending on the tariff) need to be compensated. As far as reactive-energy losses only are concerned, a transformer may be represented by the elementary diagram of following Figure. All reactance values are referred to the secondary side of the transformer, where the shunt branch represents the magnetizing-current path. The magnetizing current remains practically constant (at about 1.8% of full-load current) from no load to full load, in normal circumstances, i.e. with a constant primary voltage, so that a shunt capacitor of fixed value can be installed at the MV or LV side, to compensate for the reactive energy absorbed.

Reactive-power absorption in series-connected (leakage flux) reactance XL

The reactive power absorbed by a transformer cannot be neglected, and can amount to (about) 5% of the transformer rating when supplying its full load. Compensation can be provided by a bank of capacitors. In transformers, reactive power is absorbed by both shunt (magnetizing) and series (leakage flux) reactances. Complete compensation can be provided by a bank of shunt-connected LV capacitors

In practical terms, therefore, compensation for transformer-absorbed kvar is included in the capacitors primarily intended for power factor correction of the load, either globally, partially, or in the individual mode. Unlike most other kvar-absorbing items, the transformer absorption (i.e. the part due to the leakage reactance) changes significantly with variations of load level, so that, if individual compensation is applied to the transformer, then an average level of loading will have to be assumed.

Fortunately, this kvar consumption generally forms only a relatively small part of the total reactive power of an installation, and so mismatching of compensation at times of load change is not likely to be a problem.

Compensation to increase the available active power output

The installation of a capacitor bank can avoid the need to change a transformer in the event of a load increase

The same, SVG/STATCOM can avoid the need to change a transformer in the event of a load increase at LV side or MV side.