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FAQs Substations

What are power transformers filled with, and why?

Power transformers are filled with a pure, thin insulating oil, which serves several functions:

  • Insulation - transformer oil has many times the insulating properties of air
  • Cooling - the oil circulates throughout the transformer and keeps the coils at a safe, even, temperature. Extra cooling is often arranged by circulating the oil through radiators cooled by fans
  • Cleanliness - the oil holds particles of carbon and other impurities in suspension. These impurities are removed periodically by filtering the oil.
    Corrosion prevention - keeping the transformer full of oil prevents air and water vapour getting in, and so stops corrosion
  • Protection - developing faults in the transformer are often shown up by a surge in the oil as it circulates, or by gas bubbles in the oil. Chemical analysis of this gas can show the cause of the problem. A special device called a Buchholz relay can detect these conditions and trip the transformer, hopefully before too much damage is done.

Why are substations normally gravelled?

The gravel normally used in switchyards is selected for its low conductivity. This means that any staff in the switchyard are less likely to get a shock when electricity flows to the earth if an item of equipment develops a fault. Grass is also costly to maintain, and concrete is difficult to dig up if new cables have to be laid or other work done in the switchyard. Concrete is, however, used in areas where any oil spills have to be contained, such as around transformers.

Why do substations have electric fences around them?

Only the top wire at the top of the fence is electrified. The electric fences are used to keep animals like possums and cats out. Climbing animals can cause a lot of trouble - power cuts and equipment damage - if they get into high-voltage structures. The electric fences are powered by standard agricultural units, and so are painful, but not lethal, to humans.

Why do substations hum?

The humming comes from the transformers, and happens when the laminated steel core of each transformer vibrates at the frequency the current in it fluctuates - 50 times a second (50 Hertz, normally shown as 50 Hz). It may get louder if the load on the transformer increases. It is completely normal - if it wasn’t there it would mean the transformer isn’t working.

What are the voltage levels in substations?

Substations usually have at least two voltage levels. Most substations are supplied at 220,000 volts (220 kV) or 110 kV, but some older substations may be supplied at 66 kV (in the South Island) or 50 kV (in the North Island). If the substation is a ‘point of supply’ which feeds electricity to a local distribution network (or major industry) - and most are - it may supply that electricity at 110 kV, 66 kV, 50 kV, 33 kV, 22 kV or 11 kV. The choice of supply voltage depends on what is most economical in the circumstances.