Tap changers for power and distribution transformers · on-load (OLTC) vs de-energized (DETC), resistor vs reactor type, vacuum vs oil switching, through-current, step voltage, step capacity, Um, operating positions and change-over selectors, IEEE C57.131 and IEC 60214
A tap changer selects different connection points (taps) on a transformer winding to change its turns ratio and so regulate output voltage. Ask three things first: whether the voltage must be adjusted while the transformer carries load (on-load) or only when it is switched off (de-energized or off-circuit); the transformer's highest voltage, the current through the tap changer and the voltage per step; and how many positions and what regulating range the winding design needs.
Tap changers for power and distribution transformers, reactors and voltage regulators — on-load tap changers (resistor and reactor type, vacuum and oil switching, in-tank and bolt-on) and de-energized (off-circuit) tap changers — with motor-drive mechanisms. Complete transformers with tap changers, tap-changer test instruments and automatic voltage-regulator relays are outside this page except where compared.
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01
IEEE C57.131 sets electrical and mechanical performance and test requirements for tap changers in voltage-regulating power and distribution transformers of all voltage and kVA ratings. It covers load tap changers (LTCs), also known as on-load tap changers (OLTCs), which can change taps while the transformer is energised and carrying load, in both resistor and reactor types; and de-energized tap changers (DETCs), also known as off-circuit tap changers, which may be operated only while the transformer is not energised. It also covers aspects of the motor-drive mechanism but not the manual or automatic tap-changer control system. The 2012 edition reviewed has been superseded by C57.131-2024, and IEC 60214-2 gives application guidance for selecting tap changers designed to IEC 60214-1 or IEEE C57.131. [1]
Maschinenfabrik Reinhausen (MR), an on-load tap-changer maker, offers both oil-switching (OILTAP) and vacuum-switching (VACUTAP) designs; for VACUTAP it states: "Because the tap-change operation happens in a vacuum, our products are generally maintenance-free, which allows the life cycle costs of our customers to be drastically reduced." Its range covers in-tank, dry-type (air-insulated, for cast-resin transformers) and bolt-on assemblies, resistor-type and reactor-type switching — the reactor type with vacuum interrupters developed for North America — and small distribution OLTCs for grid-edge voltage regulation. [3]
Give the supplier the transformer type and rating, the tapped winding and its connection, on-load or de-energized operation, highest voltage for equipment Um and insulation levels, rated through-current, step voltage and number of steps or regulating range, change-over or coarse selector, oil or vacuum switching, in-tank, bolt-on or dry-type assembly, motor-drive and control interfaces, standard (IEC 60214-1 or IEEE C57.131), type-test reports, maintenance interval, and quantity.
02
MR's published key technical data show how ratings span the range. Its VACUTAP vacuum on-load tap changers reach maximum through-currents from 170 A to 2,500 A three-phase (up to 3,200 A single-phase), maximum step voltages from 500 V to 6,000 V (12,000 V in a special design), step capacities up to 10,000 kVA single-phase and 6,000 kVA three-phase (12,000 kVA in a special design), and highest voltage for equipment Um from 12 kV to 420 kV. Operating positions reach 9 to 22 without a change-over selector, up to 35 with one, and up to 107 with a multiple coarse change-over selector. Its OILTAP G oil type reaches 4,500 A single-phase and 2,000 A three-phase, 5,000 V step voltage and Um 72.5–362 kV, while its ECOTAP distribution OLTCs run at 15–100 A, 500–825 V step and Um up to 40.5 kV. [3]
Editorial note: the through-current and step voltage must cover the tapped winding's current and the voltage between adjacent taps at the highest tap, and the positions must cover the regulating range the transformer designer has set; confirm each against the transformer's design data rather than the tap changer's catalogue maximum.
03
Hitachi Energy publishes ratings for its VUCG, which it describes as the smaller of its VUC vacuum on-load tap-changer types: vacuum, resistance-type, in-tank with a diverter switch, for network, special, industrial and HVDC transformers up to 800 MVA. The page states the ratings below for a tap changer placed in the neutral point of the winding and for other placements. [4]
Hitachi Energy states that the VUCG is interchangeable with its conventional UCG type by replacing only the diverter switch, that maintenance can be done in any tap position without dismantling the shaft system, and that alternative insulating fluids can be used. Its dimension-drawing table for the VUCG../C version lists insulating levels of 380, 650, 750 and 1050 kV and for selector C, switching type L and connection N, a maximum rated through-current of 450–600 A with 5 to 18 positions. [4]
Editorial calculation: against MR's VACUTAP range above, the VUCG's 3,500 V maximum step voltage sits inside MR's 500–6,000 V span, and its 1,800 A single-phase current is below MR's 2,500 A three-phase VACUTAP maximum. A transformer that needs more than 3,500 V per step or more than 800 A three-phase is outside the VUCG's published ratings, so check each maker's table against your step voltage and through-current. [4] [3]
| Rating (Hitachi Energy VUCG) | Neutral-point placement | Other placements |
|---|---|---|
| Highest voltage for equipment (Um) | 300 kV [4] | 300 kV [4] |
| Max lightning impulse (BIL) | 1050 kV [4] | 1050 kV [4] |
| Max current, 3-phase | 800 A [4] | N.A. [4] |
| Max current, 1-phase (one tap changer per phase) | 1800 A [4] | 1800 A [4] |
| Max step voltage | 3500 V [4] | 3500 V [4] |
04
Suppliers linked to this page list tap changers; each card links to the page reviewed.
Listings include 10 kV and 35 kV on-load tap changers, MZ-type on-load tap changers, and off-circuit rotary tap changers such as 3-phase 5-position and 35 kV DETC units; ask each for the standard, type-test reports, ratings and maintenance requirements.
05
This is a functional reference built from the sources cited; it is not one maker's design.
| Element | Role | Fix in the order |
|---|---|---|
| Tap selector | Selects turn ratios in distinct steps via winding taps. [2] | Positions. |
| Diverter or vacuum interrupter | Switches load current on-load. [3] | Oil or vacuum. |
| Resistor or reactor switching | The two on-load switching principles. [1] [3] | Resistor or reactor type. |
| Change-over selector | Extends positions. [3] | Reversing or coarse. |
| Motor-drive mechanism | Operates the tap changer. [1] | Drive, controls. |
06
A tap changer quotation needs values for your project. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Rated voltage (kV) | Um 12–420 kV in one maker's range; Um 300 kV, BIL 1050 kV for Hitachi Energy VUCG. [3] [4] | Um, BIL. |
| Rated current (A) | Through-current 15–4,500 A. [3] | Rated through-current. |
| Number of steps | Up to 107 positions with coarse selector. [3] | Positions, range. |
| Step voltage (V) | 500–12,000 V. [3] | Max. step voltage. |
| Switching capacity (kVA) | Step capacity up to 12,000 kVA. [3] | Step capacity. |
| Operating temperature (°C) | Not stated in reviewed sources. | Oil/ambient range. |
| Degree of protection | Not stated in reviewed sources. | Drive enclosure IP. |
| Insulation level | Set by Um. [3] | Test voltages. |
| Mechanical life | Not stated in reviewed sources. | Operations, test report. |
| Electrical life | MR on its vacuum designs: "our products are generally maintenance-free" (maker claim). [3] | Contact life, interval. |
| Contact resistance (µΩ) | Not stated in reviewed sources. | Measured values. |
| Weight (kg) | Not stated in reviewed sources. | Weight, oil volume. |
07
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Transformer: [type; MVA; voltages; tapped winding] Duty: [on-load; de-energized] Ratings: [Um; through-current; step voltage] Range: [positions; change-over selector] Design: [vacuum; oil; in-tank; bolt-on; dry-type] Drive: [motor drive; control interface] Standard: [IEC 60214-1; IEEE C57.131] Quantity: [units]
These companies list tap changers.
Lists 10 kV and 35 kV on-load tap changers and MZ-type on-load tap changers.
Product scope checked 2026-10-11
Lists off-circuit rotary tap changers including 3-phase 5-position and 35 kV DETC units.
Product scope checked 2026-10-11
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
An on-load tap changer changes taps while the transformer is energised and carrying load; a de-energized (off-circuit) tap changer may be operated only when it is not energised. [1]
IEEE C57.131 (2012 edition superseded by 2024) sets performance and test requirements; IEC 60214-2 guides selection for tap changers designed to IEC 60214-1 or C57.131. [1]
MR says: "Because the tap-change operation happens in a vacuum, our products are generally maintenance-free", reducing life-cycle costs. [3]
Hitachi Energy lists its VUCG for transformers up to 800 MVA, with Um 300 kV, BIL 1050 kV, 800 A three-phase or 1800 A single-phase, and a 3500 V maximum step voltage. [4]
Sources checked 2026-10-11. IEEE C57.131 is a published standard (scope reviewed). Maschinenfabrik Reinhausen and Hitachi Energy make on-load tap changers and publish their own ratings; Suvell also appears as a supplier on this page.
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