Network voltage varies with load and with the time of day, and the transformer has to deliver its own within narrow limits. The tap changer solves this by switching between several taps of the high-voltage winding, changing the turns ratio in steps of around 1.25 %.
There are two families. The de-energised tap changer requires the transformer to be taken out of service and is set once at commissioning. The on-load tap changer, or OLTC, switches with the transformer in service, which means moving from one tap to the next without interrupting the current or short-circuiting turns: it does so with transition resistors or reactors and a fast-acting mechanism.
It is the transformer's moving part and therefore the one that fails most: around half of the failures of transformers fitted with an OLTC originate there. It is monitored through contact resistance and switching time measured at every position, through dynamic resistance signature analysis of the operation, through the temperature difference between its compartment and the main tank, and through the accumulated operation count.