Power & Distribution Transformers

The standard power transformer is “on-load tap changing” (OLTC) to ensure that the secondary distribution voltage is within limits although the primary voltage may change. Tap changer range is usually from 15% above normal to 5% below, and has up to 22 steps, step one giving the highest voltage and step 22 the lowest. Some older transformers, however, have a different range, and the numbering of the taps is different, in some cases step one gives the lowest voltage.

There are two controls that govern the voltage for this equipment they are:

  • An “automatic voltage regulating relay”(AVR), which determines a ‘base’ voltage for the distribution busbar,
  • A “line drop compensator”, which varies the voltage up or down according to the load.

For example, if the load is high the distribution voltage should be higher to offset the increased voltage drop. These controls are not altered during normal operating. The tap changer can also be operated manually.

If there is more than one transformer in a substation each unit is usually operated independently, but they must not be operated in parallel automatically unless there is a master-slave relationship fitted to each transformer.  Without this control the two transformers, if switched to automatic, could probably take up different tap positions. They would not share the load equally and excessive circulating current may flow between them and cause serious damage by overheating the windings.

When transformers are run in parallel the fault level on the secondary side of the transformer is increased considerably.


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