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On-Load Tap Changer (OLTC) Explained: Parts, Faults and Care

Most large power transformers do not have a single fixed voltage ratio. The supply voltage on a network rises and falls through the day, loads change, and a transformer that cannot adjust would let its output voltage drift outside acceptable limits. The on-load tap changer, usually shortened to OLTC, is the mechanism that solves this problem: it changes the active tap on the transformer winding while the transformer stays energised and carrying load, so the output voltage can be corrected without interrupting supply.

This guide explains what an OLTC is made of, how a tap change actually happens, what goes wrong with these mechanisms in service, and how they are looked after. It is written for engineers, technicians and asset owners who want to understand the equipment they operate, not just press the raise and lower buttons. As with everything on this site, the general guidance here never overrides the manufacturer's instructions, the asset owner's procedures, or local safety regulations. Any work inside a tap changer compartment is work on high-voltage equipment: it must only be done by competent persons, with the transformer de-energised, isolated, earthed and proved dead, following an approved method statement and permit system.

Why transformers need tap changers at all

A transformer winding has a number of taps: connection points along the winding that give slightly different turns, and therefore slightly different voltage ratios. Selecting a different tap changes the output voltage by a fixed step, typically a little over one percent per step, with a range of perhaps ten to twenty steps either side of the nominal position. On a small distribution transformer the taps are usually changed by hand with the transformer switched off, using an off-circuit tap changer. That is simple and cheap, but it cannot respond to a voltage that changes during the day, and it forces an outage for every adjustment.

Large transmission and distribution transformers, and transformers feeding fluctuating loads, need something better. The on-load tap changer changes taps under load, automatically or on command, so the voltage can be regulated continuously. It is controlled by an automatic voltage regulator that watches the output voltage and orders a tap change whenever the voltage drifts outside a set band, or it can be operated manually from a control panel or a remote control room. Because the tap changer is the only part of the transformer that moves in normal operation, it is also the part that wears out first, and it accounts for a large share of transformer maintenance effort and a large share of transformer failures. Understanding it is therefore essential to understanding transformer reliability.

The three parts of an OLTC

An on-load tap changer is not one switch but a coordinated mechanism with three main parts, each doing a different job during a tap change. Keeping their roles separate is the key to understanding both normal operation and faults.

The selector switch. The selector switch is the part that chooses the tap. It has a set of fixed contacts, one for each tap, and a moving contact assembly that rests on the tap in use. Before a tap change, the selector moves — or, in some designs, pre-selects — the next tap position. In many designs the selector does not break load current: it moves while carrying little or no current, because the diverter switch handles the current transfer. The selector's contacts still wear, especially if the mechanism is stiff or misaligned, but they are not subjected to the arcing duty of the diverter.

The diverter switch. The diverter switch is the heart of the OLTC and the part that does the hard work. During a tap change it transfers the load current from the old tap to the new tap without ever breaking the circuit, using a make-before-break sequence. For a brief moment both taps are connected together through transition resistors, which limit the circulating current that would otherwise flow between two taps at different voltages. The diverter then breaks the connection to the old tap. That break happens under load, in oil, and it produces an arc every single operation. The diverter switch therefore lives in its own oil compartment, separate from the main transformer oil, so that the carbon and gases produced by arcing do not contaminate the main tank oil. Its contacts erode with every operation, and its oil degrades faster than any other oil in the transformer.

The drive mechanism. The drive mechanism is the motor and gearing that powers the whole sequence, usually mounted on the outside of the tank or on the tap changer head. It turns the selector and diverter through a precise sequence of movements, driven by a motor, a spring mechanism, or both, and it carries the position indicator, the operation counter, limit switches, and often a hand crank for manual operation. The drive mechanism is where the control system meets the mechanics: a fault here, a slipping coupling, a failed motor or a misaligned shaft, can leave the tap changer stuck between positions, which is one of the most serious OLTC faults.

How a tap change actually happens

A tap change looks simple from the control room — press raise, the position indicator steps up by one — but inside the mechanism a careful sequence takes place in a fraction of a second. First, the drive mechanism starts the selector moving towards the next tap. In a typical design, the diverter switch then takes over: its moving contacts bridge the old and new taps through the transition resistors, so the load current is shared between the two taps for a brief instant. The diverter then breaks the old tap connection, leaving the load fully on the new tap, and the selector completes its travel and settles on the new position. Throughout the sequence the load current is never interrupted, which is the whole point of an on-load design.

Two details matter for anyone diagnosing problems. The first is timing: the whole transfer must complete within a designed window. If it is too slow, the transition resistors carry current for too long and overheat; if it is too fast or incomplete, the load can be momentarily supplied through a high resistance, or worse, the mechanism can stall mid-travel. The second is the transition resistors themselves. They are only in circuit for a fraction of a second per operation, but they carry significant current during that instant, and they heat up. Repeated rapid operations, such as hunting when a voltage regulator is badly set, can overheat them. This is why automatic voltage regulators have time delays and dead bands: to stop the tap changer from chasing every small voltage fluctuation and wearing itself out.

Some modern OLTC designs replace the mechanical diverter with vacuum interrupters, which extinguish the arc inside a sealed vacuum bottle instead of in oil. Vacuum tap changers produce far less oil contamination, need less frequent contact maintenance, and tolerate more operations between services. The principles are the same — selector, transfer, drive — but the arc duty moves from oil to vacuum, which changes the maintenance profile considerably. If you are specifying a new transformer, the choice between a conventional oil diverter and a vacuum diverter is one of the most important reliability decisions you will make.

Common OLTC faults and what they look like

Because the OLTC moves and arcs, its faults are different in character from the faults of the static parts of a transformer. The most common problems, and their usual symptoms, are worth knowing by sight.

Contact wear and coking. Diverter contacts erode with every operation, and carbon builds up on their surfaces and in the diverter oil. The symptoms are gradual: rising contact resistance, localised heating, and eventually erratic operation or failure to complete a change. Diverter oil that is black, full of carbon, or gassing heavily is a warning sign, which is why diverter oil is sampled separately from main tank oil and why its condition is a key maintenance indicator.

Drive mechanism faults. A worn motor, a slipping or broken drive shaft, a failed spring, or a misaligned coupling can leave the mechanism out of step. The classic symptom is a tap changer that starts a change and does not finish it, or a position indicator that disagrees with the actual tap position. A stuck mid-position tap changer is a serious condition: the transition resistors may be left in circuit, or the transformer may be left on an unintended tap, and the unit should not be loaded normally until the mechanism has been inspected by competent specialists.

Transition resistor failure. Transition resistors fail from overheating, often after repeated rapid operations or a stalled change. A failed resistor changes the current-sharing behaviour during a tap change, which can cause severe arcing in the diverter and rapid damage. Dissolved gas analysis of the diverter oil, and of the main oil if the compartments are not fully separated, can give early warning of abnormal arcing.

Oil and sealing problems. Leaks at the tap changer head, the drive shaft seal, or the compartment gaskets let moisture in and oil out. Moisture in the diverter compartment accelerates contact deterioration and reduces the insulating strength of the oil. Breathers and seals on the tap changer compartment deserve the same attention as those on the main tank, and any unexplained fall in the diverter oil level should be investigated, not just topped up.

Control and indication faults. Not every OLTC problem is mechanical. Faulty position transmitters, broken cables, failed limit switches, and badly set voltage regulators all produce symptoms that look like mechanical trouble: hunting, failure to respond to commands, or alarms that do not match the actual position. Before opening any compartment, check the control side: confirm the actual tap position independently where possible, check the operation counter against the log, and review recent regulator settings.

Maintaining an OLTC: what good practice looks like

OLTC maintenance is planned around operations, not just calendar time. The manufacturer's instructions give a service interval in number of operations — for example, an inspection after a set number of tap changes — and that count, read from the operation counter, is the primary trigger. A tap changer on a heavily loaded, fluctuating network may reach its operation limit in a few years, while one on a stable network may go much longer. Calendar-based checks still matter for seals, oil level, breathers and the drive mechanism, but the operation count is what drives internal inspections.

A typical maintenance visit, carried out by competent specialists with the transformer out of service and under permit, includes: recording the operation counter and the tap position; sampling the diverter oil and checking its condition; inspecting the drive mechanism, motor, gears and couplings; checking contact wear against the manufacturer's limits; measuring the timing of the change sequence where the design allows it; and checking seals, breathers and oil levels. Contacts that have reached their wear limit are replaced, not cleaned and reused beyond their limit, and the diverter oil is filtered or replaced as the manufacturer directs. After any internal work, the mechanism is operated through its full range by hand or on test, the position indicator is verified at every step, and the transformer is tested before being returned to service — including the electrical tests described in our power transformer testing and commissioning guide.

Between services, the most valuable maintenance is simply watching. Log every operation count at each visit, trend the diverter oil results, note any change in operating behaviour — slower changes, unusual noises, hunting, alarms — and act on changes early. An OLTC rarely fails without warning: the warning is usually in the oil, the counter, or the operating log, and the teams that catch it are the teams that read those records. For the wider maintenance picture, see our transformer maintenance schedule, and for the work that follows a serious OLTC fault, see our transformer overhauling guide and our guide to transformer oil processing.

Frequently asked questions

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