Voltage Regulation Equipment
Almost all transformers incorporate some means of adjusting their voltage ratio, by adding or removing tapping turns.
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| Transformer Voltage Regulation (Off-Load Tap Changer, On-Load Tap Changer and AVR) - photo credit: ABB |
This adjustment may be made by an on-load tap-changer, or by means of an off-circuit tap-changer, or by the selection of bolted link positions with the transformer disconnected and grounded.
The degree of sophistication of the system of tap selection depends on the frequency with which it is required to change taps and the size and importance of the transformer.
Let’s discuss now the following ways of voltage regulation. First two are performed on the transformer, while third way is on the relay system in the power substation.
1. Off Circuit Tap Changer
The off-circuit tap-changer is of rather simple design, giving connection to a selected tap in the winding. As the name says it is designed only to be operated only when the transformer is de-energised.
The contact pressure may occur to be retained by some kind of spring arrangement and then some vibration is possible.
In off-circuit tap-changers operating on the same tap position for years the contact resistance then may slowly increase due to local degradation and oxidation of material in the contact point. Heating will take place resulting in a build up of pyrolytic formed carbon, which will increase the contact resistance further, and also reduce the cooling.
Ultimately a runaway situation is reached and the transformer will probably trip on gas actuated protection or worse – a step short circuit occurs.
To avoid this it is vital that the tap changer is operated, off circuit, through its complete range a few times during regular routine maintenance to wipe the contact surfaces clean before returning it back to the selected tapping.
Of course the same advice is valid if an on-load tap-changer is left in service but without operation for a long period.
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