A transformer tap changer is the part of the design that decides whether your secondary voltage stays usable when the primary voltage does not. Most medium-voltage distribution transformers are ordered with one of two arrangements: an off-circuit tap changer that you set once with the unit isolated, or an on-load tap changer that moves while the transformer carries current. The difference is not a small accessory choice — it changes the price, the footprint, the maintenance plan, and how the transformer behaves on a weak or heavily loaded feeder.
Short answer before the detail: choose a de-energized (off-circuit) tap changer when the supply voltage is stable and the tap is only there to match site conditions at commissioning. Choose an on-load tap changer when the incoming voltage swings through the day, when the transformer feeds a feeder with variable load and a tight voltage band, or when the load cannot be interrupted for a tap change. Everything else in this guide is how to specify that decision so the factory builds the right unit.
A tap changer selects how many turns of the high-voltage winding are in circuit. Fewer HV turns means a lower turns ratio means a higher secondary voltage. That is the whole mechanism, and it explains why a tap changer is normally fitted on the HV winding: the current there is lower, so the contacts are smaller and the insulation coordination is easier.
The relationship is inverse and roughly linear per step. If the HV winding is tapped down by 5 percent, the secondary voltage rises by about 5.26 percent, because 1 / 0.95 = 1.0526. Engineers who expect exactly 5 percent are the ones who end up with a secondary that sits slightly outside the band they promised the client.
| Tap position on a 11 kV / 0.4 kV unit | HV voltage at that tap | No-load secondary voltage | Change vs nominal |
|---|---|---|---|
| +2 (adds HV turns) | 11,550 V | 381 V | -4.8% |
| +1 | 11,275 V | 390 V | -2.4% |
| 0 (nominal) | 11,000 V | 400 V | 0% |
| -1 | 10,725 V | 410 V | +2.5% |
| -2 (removes HV turns) | 10,450 V | 421 V | +5.3% |
Two consequences follow from that table. First, a five-position ±2×2.5 percent tap changer covers roughly ±5 percent of supply variation. Second, the step size is a voltage decision, not a mechanical one — on a 400 V secondary, one 2.5 percent step is about 10 V, which is already a measurable share of a ±10 percent statutory band.
An off-circuit tap changer — also written DETC, de-energized tap changer, or manual tap changer — is a rotary selector inside the tank, operated by a handwheel or a removable handle through the tank cover. Its normal range is five positions at ±2×2.5 percent, and sometimes three positions at ±2.5 percent on small distribution units. It has no breaking capacity because it is never asked to break current: the transformer must be de-energized, isolated, and locked out before the handle is turned.
What you get for the lower cost is a part with almost nothing to wear out. There is no diverter switch, no motor drive, no separate oil compartment, no control cabinet, and no maintenance interval beyond verifying the position at commissioning and after any reconfiguration. On a 400 kVA pole-mounted or pad-mounted distribution transformer feeding a residential area, that is usually the correct engineering answer, because the supply voltage is set by the utility and the load profile does not justify continuous regulation.
What you give up is speed and range. A DETC cannot follow a daily voltage curve, and it cannot be adjusted without an outage. If the feeder is the end of a long rural line where voltage sags at evening peak, a five-position off-circuit changer will only move the problem, not solve it.
An on-load tap changer changes taps while the transformer carries load, without interrupting the supply. It does that by temporarily inserting an impedance between the two taps it is bridging, so the current never sees an open circuit. The assembly has three working parts: a selector that picks the tap, a diverter switch that makes and breaks under current, and a motor drive unit that sequences the operation and reports position.
Typical ranges are ±8×1.25 percent (±10 percent in 16 steps) on distribution and small power transformers, and ±9×1.667 percent or wider on larger units, giving 17 or 19 positions. A step takes a few seconds, and the automatic voltage relay — often called an RTCC or AVR — decides when to move based on a measured secondary voltage, a deadband, and a time delay so the changer does not hunt on short transients.
The reason OLTCs are common on transmission and sub-transmission transformers and much rarer on 400 V distribution units is economic. An OLTC adds a diverter switch with wearing contacts, a separate oil compartment that needs its own oil testing, a motor drive, a controller, and typically 15 to 30 percent to the transformer price. On a 30 MVA unit that is a rounding error against the cost of poor voltage. On a 630 kVA unit it can exceed the value of the losses it saves.
Two transition principles dominate. The resistor type inserts transition resistors for a few tens of milliseconds, makes and breaks quickly, and is the standard choice in IEC markets for medium and high voltage. The reactor type uses a preventive autotransformer or reactor to limit circulating current, breaks in oil, and remains common in North American practice. Both work; they differ in maintenance routine, oil quality sensitivity, and the spare parts your local service partner is likely to stock.
A newer variant replaces the oil-immersed diverter contacts with vacuum interrupters. The switching duty then happens in a sealed bottle rather than in oil, so the OLTC compartment oil stays clean far longer and the maintenance interval stretches. It costs more up front and is worth asking about on units where a tap change happens many times a day, such as a transformer feeding a steel mill, a traction rectifier, or a data center with a volatile upstream voltage.
| Parameter | Off-circuit (DETC) | On-load (OLTC) |
|---|---|---|
| Operation | Only with transformer de-energized | Under full load, no interruption |
| Typical range | ±2×2.5% (5 positions) | ±8×1.25% to ±9×1.667% (17–19 positions) |
| Step size | 2.5% | 1.25% or 1.667% |
| Wearing parts | Selector contacts only | Diverter contacts, motor drive, controller |
| Oil management | Single tank | Separate OLTC compartment, own oil tests |
| Maintenance | Inspection at commissioning, then as-found | Contact inspection and oil change on operation count |
| Controls needed | None | Motor drive, RTCC/AVR, position indication, alarms |
| Cost impact | Baseline | Typically +15% to +30% on the transformer |
| Best fit | Stable utility supply, distribution units | Variable supply, tight voltage band, critical load |
Work from the measured voltage, not from the nominal. Ask the utility or the site electrical team for a week of primary voltage logging at the connection point, and note the maximum, minimum, and the daily pattern. Then apply the decision table below.
| Site condition | Recommended | Reason |
|---|---|---|
| Primary within ±3% of nominal all day | DETC, 5 positions | Taps only compensate a fixed offset |
| Primary swings ±5% to ±10% daily | OLTC, ±8×1.25% | Needs 16 steps and automatic control |
| Long rural feeder, evening sag | OLTC plus line-drop compensation | Voltage drop is load-dependent |
| Transformer feeds a rectifier or arc load | OLTC, vacuum diverter preferred | High tap-change count wears oil contacts |
| Parallel transformers on one busbar | OLTC on both, master-follower control | Mismatched taps circulate current |
| Pad-mounted unit, no control power | DETC | No auxiliary supply for a motor drive |
If the answer is OLTC, also confirm that control power, a current transformer for line-drop compensation, and a place to mount the controller actually exist in the substation. An OLTC delivered without a working AVR is an expensive manual tap changer.
State four things on the datasheet and the factory cannot misread you: the tap range as a percentage, the number of steps, the voltage at each extreme, and the winding the changer sits on. Use the real system voltage as the centre point, not the nameplate nominal of a neighbouring project.
The tap changer is also a routine and type test item, not just a purchase line. Sequence tests, dielectric tests across the open gap, and a full tap-cycle run belong in the factory acceptance programme, and they are covered in a proper factory acceptance test plan under IEC 60076.
A DETC has no maintenance interval in any meaningful sense: verify the position at commissioning, record it, and re-verify after any work on the unit. An OLTC does have one, driven by tap-change count rather than calendar time. A distribution OLTC might see a few thousand operations a year; a transformer on an unstable industrial feeder can see tens of thousands.
When you capitalise the difference properly, the economics are clear. A 1,000 kVA distribution transformer with an OLTC might cost 20 percent more than the DETC version; that premium has to be repaid by avoided voltage complaints, avoided production loss, or avoided tap changes on a de-energized basis. On a critical process load the case is easy. On a standard commercial building it usually is not, and the DETC plus a correctly centred tap wins.
If you are still deciding the transformer itself, the wider selection logic — kVA, impedance, cooling, and vector group — sits in the power transformer guide, and the practical order-of-selection questions are laid out in how to select an oil-immersed transformer.
The tap changer question resolves to one measurement: how much does the primary voltage actually move. Stable supply, fixed offset, standard distribution duty — a five-position off-circuit changer is the honest, economical answer. Moving supply, tight voltage band, load that cannot be interrupted — specify an OLTC with a real controller, a named range, and a maintenance plan based on operation count. Get the range centred on the measured voltage and the rest of the specification follows.
If you are sizing a unit for a specific site and want the tap range checked against your voltage logs, the team at Kampa can review the datasheet against the actual feeder conditions before the winding is designed. Cost expectations by rating are covered in the oil-immersed transformer cost breakdown, and the values you should cross-check on delivery are in the nameplate ratings guide.
DETC means de-energized tap changer, also called an off-circuit tap changer. It can only be operated when the transformer is isolated and de-energized, typically with a handwheel on the tank, and normally offers five positions at ±2×2.5 percent.
No. The selector has no breaking capacity, so operating it under load causes an arc that damages contacts and can fail the winding. Isolate, lock out, change the tap, and record the new position before re-energizing.
Most medium-voltage on-load tap changers have 17 or 19 positions, typically ±8×1.25 percent or ±9×1.667 percent, giving a regulation range of about ±10 percent in 16 or 18 steps.
Because the current is lower there. Lower current means smaller contacts, less contact wear, and easier insulation coordination. The tap also produces a finer voltage resolution on the secondary when it acts on the higher turn count.
The changer itself adds negligible loss, but the extra contacts and transition impedance add a small amount. The larger practical effect is on price, footprint, and maintenance rather than on efficiency.
On operation count, not calendar time. Typical practice is a first inspection after the first few thousand operations, then contact inspection and oil treatment at intervals set by the manufacturer, with dissolved gas analysis on the diverter oil monitored in between.
