Should the tap changer oil or transformer oil be changed sooner and why?
Tap changer oil - due to the number of tap changes that occur in a typical day, ie, if it is a distribution transformer, the friction and heat produced by arcs would render the dielectric property to diminish. Similarly the overheating due to currents in transformer coil would reduce the insulating strength of the transformer oil as years pass by. Hence they need regular mainteniance.
In addition to the first answer there are also pure mechanical reasons. An on-load tap changer is a very complex device that includes a lot of pieces, and involve many movements during its operation. Depending of the type and structure of the on-load tap changer, changing from one tap to another should be done sequentially. This may involve many operations per day. With time, the contact pieces may become loose, the mechanism wears, and the pressure between contacts may reduce, so increasing the contact resistance and causing hot spots to appear. There are several methods to monitor the wear of tap changer contacts, that include the measure of contact resistances and the analysis of vibration signature during tap changer operation.
A tap changer is a connection point selection mechanism along a power transformer winding that allows a variable number of turns to be selected in discrete steps. A transformer with a variable turns ratio is produced, enabling stepped voltage regulation of the output. The tap selection may be made via an automatic or manual tap changer mechanism
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