GuideTransformers11 min read
Load tap changers
A load tap changer (LTC, or on-load tap changer, OLTC) adjusts a transformer’s turns ratio while it carries load, so the secondary voltage stays within a band as the supply and the load change. It steps through about 33 positions over a range of ±10%, driven by a motor under the control of a voltage regulating relay, and it does so by switching between taps without ever opening the circuit. De-energized taps, which most distribution transformers have, do the same thing in five positions with the power off. An LTC is the one moving part in a transformer, and the part most likely to need work on a used unit.
The short version
- De-energized tap changer (DETC): ±2 × 2.5% on the primary, moved with the transformer off, to match the unit to its supply. On nearly every transformer.
- Load tap changer (LTC): typically ±10% in 32 steps of 5/8%, on the secondary, switched under load by a motor drive; on substation transformers of about 5 MVA and up, and on voltage regulators.
- The switching happens in a diverter switch through a transition impedance (a reactor in North American designs, resistors in European ones), so the load never breaks; vacuum-interrupter LTCs do the arcing in sealed bottles instead of in oil.
- A voltage regulating relay compares the secondary voltage with a setpoint and bandwidth (120 V ±1 V on the PT scale is typical) and calls for a tap after a time delay; line-drop compensation lets it hold the voltage at a point downstream.
- Maintenance is by operations: contacts are inspected or replaced every 50,000 to 150,000 operations or 5 to 7 years, and the LTC compartment’s oil is tested separately from the main tank’s.
- On a used unit, ask for the operation counter reading, the LTC maintenance history, a DGA on the LTC compartment, and a through-the-range operation test.
Taps, and the two kinds of tap changer
A tap is a connection partway along a winding. Bringing it out lets the transformer use a few more or a few fewer turns, which changes the ratio by a few percent. Nearly every transformer has taps on its high-voltage winding, because the supply is rarely exactly its nominal voltage: a 12.47 kV feeder may run at 12.1 kV at the far end or 12.9 kV near the substation, and a tap corrects the secondary back to 480 V.
| De-energized tap changer (DETC) | Load tap changer (LTC) | |
|---|---|---|
| Also called | No-load tap changer, off-circuit taps | On-load tap changer, OLTC |
| Operated | By hand, with the transformer de-energized | By a motor, under load, automatically |
| Positions | Usually 5: ±2 × 2.5% (±5%) | Usually 33: ±16 × 5/8% (±10%) |
| On which winding | High voltage | Usually the low-voltage (regulated) winding on a substation unit; the high side on some |
| How often it moves | Once, at installation, or when the supply changes | Several times a day, tens of thousands of times over its life |
| Found on | Nearly every transformer | Substation and power transformers that regulate voltage; voltage regulators |
| Maintenance | None, beyond exercising it before energizing | Scheduled, by operations count |
The DETC is a rotary switch with an external handle, padlocked in position, with a warning that it must not be operated energized. Moving it live, even by a position, arcs across the open contacts under full voltage and has destroyed transformers. The LTC is designed for the opposite: to switch under full load without a flicker.
How an LTC switches under load
The problem an LTC solves is that you can’t open the circuit to move between taps (the load would see an interruption and the mechanism an arc at full load), and you can’t simply connect two taps together (the turns between them would be shorted and carry an enormous circulating current). The answer is a transition impedance and a two-stage switch.
- Tap selector
- A multi-position switch in the main tank oil that connects to the taps. It only ever moves when no current flows through the contact it’s moving, so it doesn’t arc.
- Diverter switch
- A fast switch, in its own oil compartment, that carries the load current and transfers it from the old tap to the new one through the transition impedance. This is where the arcing happens, in a few milliseconds per operation.
- Transition impedance
- A reactor (North American practice) or a pair of resistors (European practice) connected between the two taps during the transfer, limiting the current that circulates through the shorted turns.
- Motor drive
- A geared motor in an external cabinet that moves the selector and charges a spring that snaps the diverter over; it also carries the position indicator, the operation counter, and the hand crank.
In a reactance-type changer the reactor stays in the circuit all the time, and the changer can rest with its two contacts on adjacent taps, the reactor splitting the difference: a “bridging” position halfway between taps. That’s how 16 physical taps give 32 steps, and the 5/8% step from a 1.25% tap spacing. The sequence to move one step is selector, then diverter, then selector again, and a complete operation takes a few seconds.
Reactance, resistance and vacuum types
| Type | Transition | Where it arcs | Typical makers | Notes |
|---|---|---|---|---|
| Reactance type | A preventive autotransformer (reactor) in the circuit continuously | In oil, in the diverter compartment | Eaton Cooper (UTT, QDT), Reinhausen RMV, Siemens, old GE LR and Westinghouse UTT | The North American standard; can bridge; slower; the reactor adds losses |
| Resistance type | Resistors in the circuit only during the transfer, for milliseconds | In oil, in the diverter compartment | Reinhausen (MR) M and V series, ABB UZ, Siemens | The European standard; fast; the resistors would burn out if the diverter stalled, so it’s spring-driven and can’t stop halfway |
| Vacuum interrupter | Reactor or resistor, with the switching contacts in vacuum bottles | Inside sealed vacuum interrupters; the oil stays clean | Reinhausen VACUTAP, Eaton Cooper vacuum types, ABB VUC | The current standard for new units; maintenance intervals of 300,000 operations and up; retrofits exist for older changers |
| Step-voltage regulator | A single-phase autotransformer with a reactance LTC built in | In oil | Eaton Cooper VR-32, Siemens, GE | Not a transformer tap changer but the same mechanism, on utility feeders; ±10% in 32 steps |
On a used unit the type sets the maintenance cost: a 1980s reactance-type changer with its arcing contacts in oil has been inspected and re-contacted several times and will need it again; a vacuum changer from the last fifteen years may never have been opened.
What controls it
A voltage regulating relay, now a microprocessor controller (Beckwith M-2001, SEL-2431, Eaton CL-7, Reinhausen TAPCON), watches the secondary voltage through a potential transformer scaled to 120 V and holds it within a band around a setpoint: 120 V ±1 V is typical, which is 480 V ±4 V on the bus. When the voltage sits outside the band for longer than the time delay (30 to 60 seconds, so that a passing dip doesn’t move the taps), the controller runs the motor one step and waits again.
- Setpoint and bandwidth
- The target voltage and how far it may drift before the changer acts. A narrow band holds voltage tightly and wears the changer faster.
- Time delay
- How long the voltage must be out of band before a tap change: the main defence against hunting and against reacting to motor starts.
- Line-drop compensation
- Settings that make the controller hold the voltage not at the transformer but at a point down the feeder, by adding a calculated drop from the measured current.
- Reverse power
- What to do when power flows backward through the transformer, as it does with generation on the low side; a controller set wrong here runs the changer to an end stop.
- Paralleling scheme
- Two LTC transformers on one bus must step together, by master-follower, circulating-current or reactance methods, or they fight each other and circulate current.
Which transformers have one
LTCs belong where the voltage has to be held without an outage: on utility substation transformers from a few MVA up, where the subtransmission voltage swings with load through the day; on generator step-up and interconnection transformers for solar and wind plants, where the grid operator requires voltage control; on industrial substation transformers feeding voltage-sensitive processes; and on the step-voltage regulators that do the same job on long feeders. A distribution transformer, pad-mount or dry-type, almost never has one, because the feeder it hangs from is already regulated upstream. On this site the LTC units are among the substation transformers, with an occasional large pad-mount built for an industrial site with its own regulation.
Maintenance, and why the LTC is where transformers fail
Utility statistics put the tap changer at the top of the list of transformer failure causes, ahead of the windings and the bushings, for the plain reason that it’s the only thing that moves. Each operation erodes the diverter contacts a little; the arc carbonizes the oil; the mechanism wears; the motor drive and its control wiring sit outdoors for decades. A changer that’s maintained on schedule lasts the life of the transformer; one that isn’t ends with a diverter that stalls between taps, which shorts the transition impedance and finishes the transformer.
| Item | Interval | What’s done |
|---|---|---|
| Operation counter and position indicator | Monthly, from the log | Count operations; a sudden rise means hunting, none at all means a stuck changer or a wrong setting |
| LTC compartment oil | Yearly, or by operations | Sample for dielectric strength, moisture and DGA; filter or replace. Read DGA against LTC limits, since acetylene is normal here |
| Motor drive | Yearly | Lubricate, check the brake, limit switches, heaters and the hand crank interlock; run through the range |
| Diverter switch internal inspection | 50,000 to 150,000 operations or 5 to 7 years (longer for vacuum types) | Drain the compartment, measure contact wear against the limit, replace contacts and transition resistors as needed, check the spring mechanism, time the operation |
| Controller | Yearly | Verify setpoint, bandwidth, delay and compensation; test with an injected voltage |
DGA on the LTC compartment follows its own rules. Because the diverter arcs by design, acetylene and ethylene are always present; what a lab looks for is the ratio of ethylene to acetylene, which rises when the contacts are overheating rather than just arcing, and sudden changes from one sample to the next. A sealed vacuum changer, by contrast, should show a main-tank-like gas picture, and acetylene there means a bottle has lost vacuum. The oil guide covers the main-tank reading.
Buying a used transformer with an LTC
A used substation transformer with an LTC is a bigger decision than one without, in both directions: the changer is where the risk sits, and a well-kept one is a large part of the unit’s value. What to ask for, in order:
- The maker and type of the changer, from its own nameplate on the drive cabinet, and whether parts and service are still available for it. Reinhausen, Eaton Cooper and ABB types are supportable; some 1960s and 70s changers are not.
- The operation counter reading, with the date, and the maintenance log: when the diverter was last opened, what was replaced, the contact wear measured.
- A recent DGA and oil screen on the LTC compartment, read against LTC limits, as well as the main tank’s.
- A through-the-range test: run the changer end to end by motor and by hand, watch the position indicator and counter, and measure the winding resistance and turns ratio on every tap, which catches a contact that isn’t making. Our testing guide covers the per-tap tests.
- The controller and the drive’s condition: a modern controller is a small cost to replace; a seized motor drive on an obsolete changer is not.
Questions people ask
What is a load tap changer?
A switching mechanism inside or on a transformer that changes which tap of a winding is in the circuit while the transformer is carrying load, so the output voltage can be adjusted without an outage. It has a tap selector that chooses the next tap, a diverter switch that moves the load current to it through a transition impedance, a motor drive that operates them, and a control that decides when. The usual range is ±10% in 32 steps.
What is the difference between an on-load tap changer and a de-energized tap changer?
A de-energized tap changer (DETC, also called a no-load tap changer) is a simple rotary switch on the primary winding with five positions, moved by hand with the transformer switched off and used once, at installation, to match the supply voltage. An on-load tap changer (LTC or OLTC) is a motor-driven mechanism that switches taps automatically while the transformer carries load, many times a day, to hold the secondary voltage steady. Most distribution transformers have a DETC; substation and power transformers that regulate voltage have both.
How does a tap changer work under load without breaking the circuit?
By never opening the circuit. The tap selector first connects the next tap in parallel with the one in use, through a transition impedance that limits the current circulating between the two taps. The diverter switch then transfers the load to the new tap and disconnects the old one. In a reactance-type LTC the transition impedance is a reactor that stays in the circuit, so the changer can also rest on a bridging position between taps, which is why 16 taps give 32 steps; in a resistance type the resistors carry the current for a few milliseconds during the transfer.
Why does the LTC compartment have its own oil?
Because the diverter switch arcs every time it operates, and arcing in oil makes carbon and gases, acetylene above all. Keeping the switch in a separate compartment with its own oil keeps that contamination out of the main tank, and it means the main tank’s DGA stays meaningful. The LTC oil is tested and filtered on its own schedule, and its DGA is read against different limits, since acetylene is expected there.
How often does a load tap changer need maintenance?
By operations and by time, whichever comes first: typical makers’ figures are an inspection of the diverter switch and its contacts every 50,000 to 150,000 operations or every 5 to 7 years, with contact replacement when the erosion limit is reached, an oil change or filtering at the same time, and a motor-drive check yearly. Vacuum-interrupter LTCs stretch the interval to 300,000 operations or more, since the arc never touches the oil.
Does a pad-mount transformer have a load tap changer?
Rarely. Pad-mounts normally have a de-energized tap changer only; an LTC adds a motor drive, a control cabinet and a maintenance burden that distribution transformers don’t justify. Large pad-mounts for industrial sites and some renewable-energy step-up units are built with an LTC when the site has to regulate its voltage, and they say so in the listing.
Sources: IEEE C57.131 (load tap changers); IEEE C57.12.00 (taps and ratings); IEEE C57.104 (DGA, with its notes on LTC compartments); IEEE C57.15 (step-voltage regulators); manufacturers’ LTC manuals (Reinhausen, Siemens, Eaton Cooper, ABB).