On-load tap changer (OLTC) — how a regulating transformer adjusts its output voltage without interrupting the supply

The guide on the Buchholz relay covers how an oil-filled transformer is monitored against internal faults. This article covers a quite different function that many power transformers also have: actively adjusting the output voltage within a certain bandwidth, using the on-load tap changer (OLTC).

Why a transformer needs an adjustable tap

The voltage on the primary side of a transformer (for example the medium-voltage grid) varies throughout the day with the network load, and the load on the secondary side varies as well. A transformer with a fixed turns ratio would therefore deliver a secondary voltage that tracks these fluctuations, while the connected installation actually benefits from a voltage that stays within a narrow band. By adding extra taps on the high-voltage winding — each with a slightly different turns ratio — and a switch that can change between these taps, the transformer's effective ratio can be adjusted step by step to keep the secondary voltage within the desired band.

The difference between a de-energized and an on-load tap changer

A simple de-energized tap changer (DETC or NLTC) can only be adjusted while the transformer is disconnected from the network — suitable for a rare, slow adjustment (for example after a permanent change in network configuration), but unsuitable for regular regulation during operation. An OLTC, by contrast, is specifically designed to change taps during full operation, under full load current — without interrupting the supply even momentarily and without ever fully opening the winding.

How an OLTC changes taps without interrupting the current

The core problem of an on-load tap change is that the load current may never be fully interrupted at any point (that would mean an interruption of supply), yet the two taps must also never be directly short-circuited without any limitation during the transition (that would cause a heavy short-circuit current between the two taps). An OLTC solves this with two cooperating components:

The tap selector pre-selects, without carrying the load current, which two adjacent taps will be involved in the next step.
The diverter switch carries out the actual, current-carrying transition between these two pre-selected taps extremely fast (typically within a few tens of milliseconds), via an intermediate overlapping connection in which both taps briefly carry current at the same time, with a transition impedance (a resistor or reactor) in series that limits the short-circuit current between the two taps during this brief overlap to a safe value.

This keeps the load current flowing uninterrupted through at least one tap at all times, while the short-circuit current between the two involved taps stays limited during the short overlap.

Why parallel operation of two transformers makes an OLTC extra critical

The guide on transformer vector groups and parallel operation covers why two parallel-connected transformers need a matching vector group and comparable short-circuit impedance to avoid an unwanted circulating current. The same principle applies to the OLTC tap positions of both transformers: if the two transformers are not on the same effective tap, a small, permanent voltage difference arises between their secondary terminals, which drives a circulating current through the shared busbar, even when the external load on both transformers is otherwise balanced. For this reason, parallel-operated transformers with OLTCs typically use an automatic parallel control system that keeps the tap positions of both (or all) parallel units aligned, rather than letting each transformer regulate independently based on its own, locally measured voltage.

Note: the number of taps, the voltage step per tap, and the total regulation range (typically on the order of a few dozen taps with a step of a little over 1% each, for a total regulation range of typically ±10% to ±15%) vary considerably by transformer type and application; this article covers the principle, not a ready-made table for every transformer type.

Practical relevance

When inspecting or maintaining a transformer with an OLTC, it is important to follow the manufacturer's instructions for the number of switching operations between service intervals — the diverter switch is the most heavily stressed mechanical component of the transformer and wears with every tap change, mainly due to arcing and degradation of the switching oil in the separate oil compartment of the diverter switch (which is therefore normally kept separate from the transformer's main oil).

Common mistakes

Trying to adjust a de-energized tap changer (DETC/NLTC) while the transformer is still energized or loaded — this type of switch is not designed for that and can be severely damaged by an attempted on-load operation.
Not keeping two parallel-connected transformers with OLTCs on the same effective tap position — this causes a circulating current between the transformers, even without any external load imbalance.
Basing the maintenance interval of the diverter switch compartment on calendar time instead of the actual number of switching operations — the mechanical and electrical wear of an OLTC depends primarily on the number of tap changes, not on elapsed time.
Confusing the oil in the diverter switch compartment with the transformer's main oil during oil sampling or analysis — the two oils serve different functions and degrade differently, and should therefore be sampled and assessed separately.