Voltage in a power system does not remain perfectly constant. Changes in grid conditions, load demand, feeder impedance, generation output, and connected equipment can all affect the voltage supplied to a transformer.
A power transformer may therefore require a tap changer to adjust its effective winding ratio. By selecting a different tap position, the transformer changes the number of effective winding turns and adjusts the secondary-side voltage.
The main question is whether the tap position needs to be changed:
- Only after the transformer has been de-energized; or
- While the transformer remains energized and supplies the load.
An off-circuit tap changer, also known as a de-energized tap changer, is used for the first condition. An on-load tap changer, commonly abbreviated as OLTC, is used for the second.
The correct choice is not determined by voltage class alone. A 35kV power transformer does not automatically use an off-circuit tap changer, and a 110kV power transformer does not always require an OLTC. The decision should follow the actual voltage variation, load profile, operating requirements, shutdown availability, control philosophy, and applicable project specifications.
For HONLE power transformer projects, the tap-changing method should be confirmed before the winding arrangement, insulation structure, transformer tank, control system, and factory test scope are finalized.
Quick Answer: OLTC vs Off-Circuit Tap Changer
An off-circuit tap changer changes the transformer ratio only when the transformer has been completely de-energized, isolated, and made safe for adjustment. It is generally suitable when the grid voltage is relatively stable, tap changes are infrequent, and planned shutdowns are acceptable.
An OLTC changes the transformer ratio while the transformer remains energized and carries load current. It is generally selected when voltage changes frequently, the secondary voltage must remain within a controlled range, or automatic and remote voltage regulation is required.
IEC 60214-1:2014 covers performance requirements and test methods for resistor- and reactor-type OLTCs, de-energized tap changers, and their motor-drive mechanisms. IEC/IEEE 60214-2:2019 provides application guidance for selecting tap changers and coordinating them with transformer regulating windings.
Table 1: OLTC and Off-Circuit Tap Changer Comparison
| Comparison Point | Off-Circuit Tap Changer | On-Load Tap Changer |
| Transformer condition during operation | Must be de-energized | Remains energized and loaded |
| Main purpose | Infrequent ratio adjustment | Continuous or frequent voltage regulation |
| Switching under load | Not permitted | Designed for load-current transfer |
| Typical operating method | Manual or motorized | Motor-driven |
| Automatic voltage regulation | Normally not required | Can be integrated |
| Remote control | Limited or project-specific | Commonly available when specified |
| Internal construction | Relatively simple | More complex |
| Control and protection requirements | Limited | More extensive |
| Maintenance requirements | Generally lower | Higher and dependent on operating duty |
| Initial project cost | Lower | Higher |
| Typical application direction | Stable industrial or distribution systems | Utility substations and actively regulated networks |
This comparison provides an initial selection direction. The approved transformer specification and power-system study remain the basis for the final decision.
How a Transformer Tap Changer Adjusts Voltage
The transformer voltage ratio is mainly determined by the relationship between the number of turns in the primary and secondary windings.
A tap changer connects the electrical circuit to different points on a tapped winding. Moving from one tap position to another changes the number of effective winding turns and therefore changes the transformer ratio.
The tapped section is often arranged on the higher-voltage winding because the current is usually lower than on the low-voltage winding. This can reduce the current rating required for the tap-changing equipment and simplify the arrangement of the tapped turns.
However, this is not a fixed rule. The final location of the taps depends on:
- Transformer voltage ratio;
- Winding configuration;
- Rated current;
- Insulation level;
- Regulating range;
- Winding geometry;
- Short-circuit requirements;
- Transformer impedance;
- Available internal space.
A complete tap specification must define more than whether the transformer uses an OLTC or off-circuit tap changer. It should also define the total regulating range, number of tap positions, voltage change per step, principal tap, tapped winding, maximum current, control method, and parallel-operation requirements.
These parameters affect the winding arrangement, tap leads, insulation clearances, transformer dimensions, control cabinet, and factory testing. The tap changer cannot be treated as an accessory selected after the main transformer design has already been completed.
When Is an Off-Circuit Tap Changer Suitable?
An off-circuit tap changer can only be operated after the transformer has been disconnected from all energizing sources.
It may also be described as a:
- De-energized tap changer;
- De-energized-circuit tap changer;
- DETC;
- Off-load tap changer;
- No-load tap changer.
“Off-circuit” or “de-energized” is more technically precise than “no-load.” Removing the downstream load does not necessarily mean that the transformer is no longer energized. The transformer must be disconnected, isolated, and made safe before the tap position is changed.
An off-circuit tap changer is generally suitable when:
- Incoming voltage remains relatively stable;
- Voltage adjustment is mainly required during commissioning;
- Tap changes are expected only occasionally;
- Planned shutdowns can be arranged;
- Automatic voltage regulation is unnecessary;
- The downstream network can tolerate normal voltage variation;
- A simpler transformer configuration is preferred.
Typical applications may include industrial substations with stable utility supplies, factory power distribution, agricultural facilities, pumping stations, mining projects with predictable operating conditions, and dedicated transformers supplying relatively stable loads.
Some renewable-energy step-up applications may also use off-circuit tap changers where voltage regulation is primarily managed by inverter controls, generator excitation, reactive-power equipment, or the wider grid-control system.
The project must still evaluate future operating conditions. A system that appears stable during initial operation may later experience larger load changes, additional feeders, parallel transformer operation, or stricter voltage requirements.
Advantages of an off-circuit tap changer include:
- Simpler mechanical and electrical construction;
- Lower initial cost;
- Fewer control components;
- Lower switching duty;
- Less demanding maintenance;
- Easier integration into the transformer layout.
Its main limitation is operational flexibility. It cannot correct frequent voltage changes when transformer shutdowns are difficult or unacceptable.
When Is an OLTC Required?
An OLTC changes tap position while the power transformer remains energized and supplies the connected load.
During a tap change, the load current must transfer from one winding tap to the next without creating an open circuit or permanently short-circuiting adjacent taps. The OLTC uses a controlled switching sequence to maintain current continuity and establish the new transformer ratio.
OLTCs are used to regulate transformer voltage without interrupting the load.
An OLTC should be considered when:
- Incoming grid voltage changes substantially;
- Load demand varies significantly during daily operation;
- Secondary voltage must remain within a narrow range;
- Long feeders create changing voltage drops;
- Continuous voltage regulation is required;
- Automatic voltage regulation is specified;
- Remote operation from a control room is required;
- SCADA integration is required;
- Multiple transformers operate in parallel;
- Renewable generation causes changing power-flow conditions;
- Sensitive industrial processes require a controlled supply voltage;
- Transformer shutdowns are difficult or costly.
The requirement should normally be supported by a load-flow study, voltage profile, utility specification, grid-code requirement, or clearly defined operating philosophy.
An OLTC should not be selected only because it appears more advanced. It adds mechanical switching equipment, motor-drive components, control circuits, protection functions, maintenance requirements, and cost.
It also regulates voltage only within its approved tapping range. An OLTC cannot correct every power-system problem. It does not replace the correct selection of transformer rated voltage, capacity, impedance, feeder size, reactive-power compensation, generator control, or protection settings.
Does Every 35kV Power Transformer Need an OLTC?
No. The voltage class does not determine the tap-changing method by itself.
A 35kV oil-immersed power transformer may be installed in:
- Industrial substations;
- Mining facilities;
- Cement plants;
- Agricultural power systems;
- Renewable-energy collection systems;
- Regional distribution networks;
- Utility substations;
- Infrastructure projects.
Where the incoming voltage is stable and adjustment is required mainly during commissioning or planned maintenance, an off-circuit tap changer may be sufficient.
However, an OLTC may be appropriate for a 35kV transformer when the system has:
- Significant daily load variation;
- Weak-grid voltage fluctuation;
- Long outgoing feeders;
- Frequent changes in renewable generation;
- Strict process-voltage requirements;
- Limited shutdown opportunities;
- Automatic voltage-control requirements;
- Parallel transformer operation.
For example, a 35kV transformer supplying a stable factory load may not need continuous regulation. A similar voltage-class transformer connected to a weak grid, long feeder, or variable renewable-energy system may have a very different operating requirement.
HONLE’s 35kV oil-immersed power transformer solutions are applied in industrial, distribution, substation, renewable-energy, and other project-based power systems. The tap-changing arrangement therefore needs to follow the actual system conditions rather than a fixed assumption based on the 35kV rating.
Does Every 110kV Power Transformer Require an OLTC?
OLTCs are common in many 110kV step-down substation transformers, but they are not automatically required in every 110kV application.
A 110kV transformer is often installed at an influential point in the power network, including:
- Utility substations;
- Power plants;
- Transmission interfaces;
- Large industrial substations;
- Regional grid connections;
- Renewable-energy substations.
Voltage changes at this level may affect several downstream feeders or large groups of connected loads. Continuous regulation may therefore be necessary, especially where the transformer supplies a changing network load.
However, different transformer functions follow different voltage-control strategies.
A generator step-up transformer may rely partly on generator excitation control. An industrial interconnection transformer may operate within a stable voltage system. A special step-up transformer may only require infrequent ratio adjustment.
The final decision should therefore consider:
- Whether the transformer is step-up or step-down;
- Normal and extreme system voltages;
- Load variation;
- Generator or inverter controls;
- Reactive-power equipment;
- Utility voltage-control requirements;
- Parallel operation;
- Required secondary-voltage range;
- Shutdown availability.
HONLE publicly lists both 35kV and 110kV oil-immersed power transformer solutions for larger industrial, utility, infrastructure, power-generation, and substation applications.
Key Parameters for Transformer Tap Changer Selection
Writing only “OLTC required” or “off-circuit tap changer required” is not sufficient for transformer design or quotation.
The following information should be confirmed before the transformer configuration is finalized.
Table 2: Information Required for Tap Changer Selection
| Required Information | Why It Matters |
| Transformer function | Confirms whether the unit is step-up, step-down, interconnecting, or generator-connected |
| Rated capacity | Determines winding current and tap changer through-current |
| HV and LV rated voltage | Establishes the transformer ratio |
| Normal system voltage | Defines the expected operating point |
| Minimum and maximum system voltage | Helps determine the required regulating range |
| Tapped winding | Determines tap changer voltage and current duty |
| Total tapping range | Defines the available voltage adjustment |
| Number of positions | Determines regulation resolution |
| Voltage per step | Influences the accuracy of voltage control |
| Load profile | Shows whether voltage adjustment will be occasional or frequent |
| Expected operating frequency | Influences OLTC switching and maintenance duty |
| Automatic regulation | Determines whether an automatic voltage regulator is required |
| Remote control | Defines control-room or SCADA interfaces |
| Parallel operation | Requires coordinated tap positions and control logic |
| Site conditions | Temperature, altitude, humidity, dust, and corrosion can affect the design |
| Applicable standards | Defines performance, testing, documentation, and accessory requirements |
| FAT scope | Confirms which tap positions and control functions must be tested |
A larger tap range is not automatically better. Increasing the regulating range or reducing the step size may increase the number of taps, winding complexity, tap-lead quantity, selector requirements, control positions, testing scope, transformer dimensions, and cost.
The selected range and step size should provide useful voltage control without adding unnecessary complexity.
How the Tap Changer Affects the Power Transformer Design
The difference between an OLTC and an off-circuit tap changer extends beyond the switching device itself.
Winding arrangement
Tap sections and leads must be integrated into the winding design. The manufacturer needs to consider current distribution, insulation between tap sections, additional conductor length, short-circuit forces, lead support, losses, and clearances.
The selected tap positions can also influence transformer impedance, losses, winding current, voltage regulation, and flux conditions. The technical specification should therefore clarify whether guaranteed performance values apply only at the principal tap or at additional specified positions.
Transformer tank and layout
An off-circuit tap changer generally has a limited effect on the transformer tank and external arrangement.
An OLTC may require additional space for the switching unit, motor drive, control cabinet, inspection access, position indication, piping, protective devices, and associated wiring.
This can affect:
- Transformer dimensions;
- Total weight;
- Oil volume;
- Transport conditions;
- Maintenance clearances;
- General Arrangement drawing;
- Site foundation and installation layout.
Control system
An OLTC installation may require:
- Local raise and lower control;
- Remote raise and lower commands;
- Manual and automatic operating modes;
- Tap-position indication;
- Operation counter;
- Upper and lower limit switches;
- Motor protection;
- Alarm and trip contacts;
- Automatic voltage regulator;
- SCADA communication;
- Parallel-control signals.
These functions should be confirmed before the transformer control cabinet is manufactured.
Protection and monitoring
Depending on the selected OLTC construction and project specification, the arrangement may include motor-drive protection, oil-level indication, pressure- or flow-related protection, control-supply supervision, temperature monitoring, mechanism alarms, and remote status signals.
The exact accessory scope depends on the OLTC design and project requirements. It should not be assumed that every accessory is automatically included.
Automatic Voltage Regulation and Parallel Operation
An OLTC does not automatically regulate voltage unless it is connected to an appropriate controller.
The automatic voltage regulator measures the controlled voltage and compares it with the target setting. When the voltage remains outside the permitted range for the specified delay time, the controller commands the OLTC to raise or lower the tap position.
The control philosophy may include:
- Target voltage;
- Permitted voltage band;
- Operating delay;
- Upper and lower tap limits;
- Line-drop compensation;
- Manual or automatic selection;
- Local or remote selection;
- Alarm conditions;
- Blocking conditions;
- Parallel transformer logic.
The voltage band and operating delay must be coordinated with the network.
A very narrow band or short delay may cause unnecessary tap operations in response to temporary voltage changes. A very wide band or long delay may allow the system voltage to remain outside the desired range for too long.
Parallel transformer operation requires additional attention. Transformers operating in parallel should have compatible voltage ratios, vector groups, impedances, tap ranges, and step sizes.
If one transformer moves to a different effective ratio from another, circulating current may flow between them even when the external load has not increased. Coordinated tap control may therefore require master–follower logic, circulating-current control, or communication between controllers.
Testing and Maintenance Considerations
An off-circuit tap changer normally operates infrequently and has a relatively low switching duty. Its inspection scope generally focuses on mechanical engagement, contact condition, tap position, lead connections, continuity, and correspondence with the nameplate.
An OLTC performs a more demanding switching function and requires a structured maintenance plan. The maintenance interval may depend on:
- OLTC construction;
- Number of operations;
- Switched current;
- Load-current waveform;
- Insulating medium;
- Contact condition;
- Motor-drive condition;
- Site environment;
- Monitoring results;
- Equipment instructions.
Even vacuum-switching OLTCs still contain mechanical, control, drive, sealing, and selector components that require appropriate inspection. OLTC technology should therefore not be described as completely maintenance-free. Official OLTC service guidance also treats maintenance and inspection as necessary for both oil- and vacuum-switching equipment.
During transformer factory acceptance testing, the agreed tap-changing functions should be verified.
For an off-circuit tap changer, typical checks may include:
- Mechanical movement through the required positions;
- Correct position indication;
- Mechanical locking;
- Electrical continuity;
- Voltage ratio at specified taps;
- Winding resistance at specified taps;
- Nameplate correspondence.
For an OLTC, the agreed functional checks may include:
- Local operation;
- Remote operation;
- Tap-position indication;
- Operation counter;
- Limit switches;
- Motor direction and current;
- Electrical interlocks;
- Alarm and trip contacts;
- Automatic control simulation;
- Manual emergency operation.
The FAT report should clearly identify the principal, maximum, minimum, and intermediate tap positions used during transformer testing.
Common Tap Changer Selection Mistakes
Selecting an OLTC without a voltage study
An OLTC adds flexibility, but it also adds cost, controls, protection, and maintenance requirements. Its need should be supported by the actual power-system conditions.
Selecting an off-circuit tap changer where shutdowns are impractical
An off-circuit tap changer cannot solve frequent operating-voltage problems if the transformer cannot be taken out of service.
Defining only the total tap range
The specification must also define the number of positions, voltage per step, principal tap, tapped winding, current duty, and operating method.
Adding the OLTC after the transformer design is complete
The OLTC affects the windings, insulation, tank arrangement, control system, accessories, testing, transport, cost, and production schedule.
Assuming an OLTC can correct every voltage problem
It cannot compensate for an incorrect rated voltage, excessive feeder drop, unsuitable conductor size, inadequate reactive-power control, or conditions outside its regulating range.
Ignoring expected operating frequency
An OLTC operating many times per day has a different service duty from one operating only occasionally.
Overlooking parallel transformer requirements
Uncoordinated tap movement can create unequal transformer ratios and circulating current.
How HONLE Configures the Tap-Changing Arrangement
HONLE manufactures oil-immersed distribution transformers and 35kV and 110kV power transformers for industrial systems, substations, power generation, infrastructure, renewable-energy projects, and other project-based electrical applications.
For a power transformer requiring voltage adjustment, the tap changer is reviewed together with:
- Rated capacity;
- Transformer function;
- HV and LV voltage;
- System voltage variation;
- Load profile;
- Total tap range;
- Number of positions;
- Step size;
- Vector group;
- Percentage impedance;
- Insulation level;
- Short-circuit duty;
- Cooling method;
- Control requirements;
- Site conditions;
- Applicable standards;
- Factory test requirements.
Where an off-circuit tap changer meets the system requirement, the transformer can retain a simpler control and maintenance arrangement.
Where continuous voltage regulation is required, HONLE coordinates the OLTC with the tapped winding, transformer tank, motor drive, control cabinet, automatic voltage regulator, position indication, alarm contacts, remote signals, and maintenance access before production.
The choice is not made only according to whether the transformer is rated 35kV or 110kV. It follows the actual network conditions and operating requirements of the project.
Providing a single-line diagram, transformer capacity, voltage ratio, system-voltage range, required tap range, load profile, control method, parallel-operation data, applicable standards, and FAT requirements allows the transformer and tap-changing arrangement to be reviewed as one coordinated system.
Frequently Asked Questions
Is an OLTC necessary for every 110kV power transformer?
No. OLTCs are common in many 110kV step-down substations, but the requirement depends on transformer function, voltage variation, grid-control strategy, shutdown availability, and the approved project specification.
Can an off-circuit tap changer be operated when the transformer has no load?
Removing the downstream load is not sufficient. The transformer must be completely de-energized, isolated, and made safe before the tap position is changed.
Does an OLTC change the power-system frequency?
No. An OLTC changes the transformer winding ratio and adjusts voltage. It does not convert 50Hz to 60Hz or otherwise change the system frequency.
Is a larger tap range always better?
No. A larger range can increase winding, insulation, switching, control, testing, and cost requirements. The range should follow the system voltage study.
Can an OLTC be operated remotely?
Yes. Remote operation can be provided when the motor drive, control cabinet, position indication, communication interface, and SCADA signals are included in the project specification.
Conclusion
The decision between an OLTC and an off-circuit tap changer is ultimately determined by how the power system needs to regulate voltage during operation.
An off-circuit tap changer is appropriate where voltage conditions are stable, adjustment is infrequent, and planned transformer shutdowns are acceptable. It provides a simpler structure, lower initial cost, and less demanding maintenance requirements.
An OLTC is appropriate where voltage changes frequently, continuous regulation is required, shutdowns are impractical, or automatic and remote control forms part of the substation operating philosophy. It offers greater operating flexibility but also adds switching equipment, controls, protection, maintenance requirements, and transformer-design complexity.
For HONLE 35kV and 110kV power transformer projects, the tap-changing method should be confirmed before the winding, insulation structure, transformer tank, control cabinet, accessories, and factory test schedule are finalized.
Send HONLE your single-line diagram, rated capacity, voltage ratio, system-voltage range, required tap range, step size, load profile, parallel-operation information, control requirements, applicable standards, and FAT scope. Our team can review the transformer and tap-changing arrangement according to the actual operating conditions of your project.