Auto Transformer Tap Changing

Auto Transformer Tap Changing: Working Principle, Switching Sequence and Applications

Auto transformer tap changing is a practical method of adjusting transformer output voltage without unnecessarily interrupting the electrical supply. In power transmission and distribution networks, the load does not remain constant throughout the day. As load current changes, voltage drops across transformers, cables, feeders, and transmission lines also change. A tap-changing arrangement compensates for these variations by modifying the effective number of turns in the transformer winding.

A reactor-type on-load tap changer uses a center-tapped reactor, selector switches, and a bypass or short-circuiting switch to transfer the load from one tap to the next. The reactor limits the circulating current during the transition, allowing the tap position to change while the transformer continues supplying the connected load.

This article explains the auto transformer tap-changing working principle, switching sequence, voltage-control method, advantages, limitations, applications, maintenance requirements, and its relationship with modern on-load tap changer transformer systems.

An auto transformer tap-changing arrangement changes the effective transformer turns ratio while maintaining continuity of supply. During the changeover, a center-tapped reactor carries the load and limits the circulating current produced when two adjacent taps are temporarily connected.

Reactor-type auto transformer tap-changing arrangement for voltage regulation

What Is Auto Transformer Tap Changing?

Auto transformer tap changing is a voltage-regulation technique in which a tapped transformer winding is connected through a switching arrangement and a center-tapped reactor. By selecting different winding taps, the system changes the effective number of turns and therefore changes the transformer voltage ratio.

The basic transformer voltage ratio is:

\frac{𝑉_{1}}{𝑉_{2}} = \frac{𝑁_{1}}{𝑁_{2}}

Where:

$𝑉_{1}$ = primary voltage
$𝑉_{2}$ = secondary voltage
$𝑁_{1}$ = number of primary turns
$𝑁_{2}$ = number of secondary turns

If the selected number of secondary turns increases while the primary voltage remains constant, the secondary voltage increases. If the selected number of turns decreases, the secondary voltage decreases.

The tap changer therefore regulates voltage in discrete steps rather than through a continuous adjustment.

For a broader explanation of off-load and on-load arrangements, see Tap Changing Transformers.

Why Is Tap Changing Required?

A power system experiences changing voltage conditions because of:

Variation in consumer demand
Voltage drop along heavily loaded feeders
Transmission-line impedance
Reactive power flow
Changes in generation and network configuration
Starting of large industrial motors
Switching of capacitor banks and reactors
Renewable-energy output variations
Seasonal and daily load patterns

If the transformer ratio remains fixed, receiving-end voltage may fall below the desired level during heavy loading. Conversely, voltage may rise during light-load conditions. Tap changing corrects the transformer ratio so that the controlled-bus voltage remains within the required operating range.

An on-load tap changer, commonly abbreviated as OLTC, performs this adjustment without disconnecting the load. International application guidance covers resistor-type and reactor-type on-load tap changers as well as de-energized tap changers.

Main Components of a Reactor-Type Tap-Changing Arrangement

A traditional auto transformer or reactor transition circuit contains the following parts.

1. Tapped Winding

Several connection points are provided along the regulating winding. Each tap corresponds to a slightly different number of effective turns and therefore a different output voltage.

2. Center-Tapped Reactor

The reactor is an inductive component with a connection at the electrical midpoint. The load is connected to this midpoint in the simplified tap-changing arrangement.

During normal operation, the reactor may be bypassed or carry balanced currents. During the transition between adjacent taps, the reactor divides the load current and limits the circulating current.

3. Odd and Even Selector Contacts

One end of the reactor is connected through selector contacts to one set of taps, while the other end is connected to the alternate set. In a typical diagram:

One reactor terminal selects the odd-numbered taps.
The other reactor terminal selects the even-numbered taps.

This division makes it possible to connect two adjacent taps during the transition.

4. Bypass or Short-Circuiting Switch

A bypass switch, often marked $𝑆$ , is connected across the reactor. Under the simplified operating condition, the switch remains closed so that the reactor does not introduce an unnecessary voltage drop.

The bypass switch opens during tap transition, forcing current through the reactor so that the changeover can take place safely.

5. Motor-Drive and Control System

In a practical power transformer, the tap changer is normally operated using a motor-drive mechanism. The control system may receive raise or lower commands from:

An automatic voltage regulator
A remote control panel
Supervisory control and data acquisition equipment
Local manual controls
A substation automation system

Mechanical and electrical interlocks ensure that switching operations occur in the correct sequence.

Working Principle of Auto Transformer Tap Changing

Consider a simplified arrangement having taps numbered 1 to 5. Assume that tap 1 produces the highest output voltage and tap 5 produces the lowest output voltage.

When the load increases and the controlled voltage falls, the automatic voltage-control system issues a raise command. The tap changer then moves toward a position that increases the effective number of turns or raises the regulated voltage.

When the load decreases and the controlled voltage rises above its set point, a lower command moves the tap changer in the opposite direction.

The most important design objective is not simply selecting the new tap. The tap changer must transfer the load current between taps without:

Opening the load circuit
Directly short-circuiting part of the transformer winding
Producing an uncontrolled transition current

The center-tapped reactor makes this transfer possible.

Step-by-Step Switching Sequence

Suppose the transformer is operating on tap 5 and the output voltage must be increased by moving to tap 4.

Step 1: Initial Operating Condition

The selector connected to tap 5 is closed, and the bypass switch $𝑆$ is also closed. The load receives power through the selected tap, and the reactor does not produce a significant series voltage drop in the simplified arrangement.

Step 2: Open the Bypass Switch

The bypass switch opens before the second tap is selected. The load current is now forced through part of the reactor winding.

Because the reactor has inductive impedance, a controlled voltage drop appears across the active section. However, continuity of supply is maintained.

Step 3: Connect the Adjacent Tap

The selector associated with tap 4 closes while tap 5 remains connected. At this moment, taps 4 and 5 are temporarily connected through the reactor.

A circulating current tends to flow because a voltage exists between the two adjacent tap points. The reactor presents sufficient reactance to limit this current to an acceptable value.

Step 4: Disconnect the Previous Tap

After the current has been transferred, the tap-5 contact opens. The load is then supplied from tap 4 through part of the reactor.

Step 5: Close the Bypass Switch

Finally, the bypass switch closes again. The transition is complete, and the transformer continues operating at the new voltage ratio.

The complete sequence can be summarized as:

Tap 5 \to reactor transition \to taps 5 and 4 \to tap 4

Throughout this operation, the supply remains available to the load.

Auto transformer tap-changing sequence from tap 5 to tap 4

What Happens Electrically During the Transition?

The most interesting part of a reactor-type tap changer is the temporary bridging of two adjacent taps.

Let the voltage difference between adjacent taps be $𝑉_{𝑠}$ , and let the effective transition-reactor impedance be $𝑍_{𝑟}$ . A simplified expression for the circulating current is:

𝐼_{𝑐} \approx \frac{𝑉_{𝑠}}{𝑍_{𝑟}}

Where:

$𝐼_{𝑐}$ = circulating current
$𝑉_{𝑠}$ = voltage between adjacent taps
$𝑍_{𝑟}$ = reactor impedance in the transition circuit

The actual value depends on the winding arrangement, reactor configuration, contact sequence, resistance, leakage reactance, and system conditions. The equation is therefore a conceptual approximation rather than a complete tap-changer design formula.

Without the reactor, connecting adjacent taps could effectively short-circuit a section of the regulating winding. A very high current might then flow through the contacts and winding section. The reactor prevents this by introducing sufficient impedance into the temporary circulating-current path.

At the same time, the center tap allows load current to divide between the two reactor sections during part of the transition. This arrangement maintains output continuity while the selector moves from the existing tap to the next tap.

Role of the Automatic Voltage Regulator

The automatic voltage regulator, or AVR, continuously evaluates the controlled voltage and compares the measured value with a set point.

A practical control scheme normally includes:

Target-voltage setting
Deadband
Time delay
Line-drop compensation
Raise and lower relays
Tap-position indication
Upper and lower tap limits
Operation counter
Blocking and alarm inputs

The deadband prevents unnecessary tap operations when the voltage changes only slightly. The time delay prevents the tap changer from responding to brief disturbances or temporary voltage dips.

Line-drop compensation may be used when the objective is to regulate voltage at a remote load center rather than directly at the transformer terminals. The controller estimates feeder voltage drop using measured current and programmed resistance and reactance values.

For comparison with another remote voltage-control method, see Booster Transformer. A booster transformer injects a controllable series voltage into the line, whereas an OLTC changes the transformer ratio.

Tap Voltage and Number of Steps

If a regulating range is divided into equal steps, the approximate voltage change per tap can be expressed as:

Tap step percentage = \frac{total regulating range}{number of steps}

For example, consider a transformer having a total regulating range of 20%, extending from $- 10 %$ to $+ 10 %$ , with 16 equal steps:

Voltage per step = \frac{20 %}{16} = 1.25 %

A single tap operation would therefore change the transformer ratio by approximately 1.25%.

The exact relationship between tap position and output voltage depends on:

The winding selected for tapping
Whether the tap changer is installed on the high-voltage or low-voltage side
Additive or subtractive tap arrangement
Transformer loading
Internal voltage drop
Control-bus location

Why Are Taps Commonly Provided on the High-Voltage Winding?

Tap arrangements are often placed on the high-voltage winding because the current is lower on that side for the same transformer power rating. Lower current can reduce the current-duty requirements of switching contacts and conductors.

The high-voltage winding also generally has more turns, making it easier to obtain smaller voltage increments between adjacent taps. However, the final placement is an engineering decision influenced by insulation level, winding design, regulation requirements, tap-changer rating, manufacturing constraints, and transformer configuration.

Reactor-Type Versus Resistor-Type OLTC

Both reactor and resistor transition methods allow transformer ratio adjustment under load. IEC/IEEE application guidance specifically addresses both categories.

Reactor-Type OLTC

A reactor-type OLTC uses inductive reactance to limit current while two adjacent taps are bridged.

Typical characteristics:

Uses a center-tapped preventive autotransformer or transition reactor
Supports continuous load transfer between adjacent positions
Limits circulating current inductively
May use separate selector and switching functions
Requires correct reactor design for thermal and transition duties

Resistor-Type OLTC

A resistor-type tap changer inserts transition resistors for a short period while moving between taps.

Typical characteristics:

Uses resistors to limit transition current
Completes switching rapidly
Avoids leaving transition resistors continuously in the load circuit
Requires accurate mechanical timing
Is widely used in power-transformer applications

Modern OLTC designs may also use vacuum interrupters to perform the current switching duty. Requirements and guidance for vacuum-type tap changers are included in current tap-changer standards and application material.

On-Load Tap Changer Versus Off-Circuit Tap Changer

These two devices should not be confused.

On-Load Tap Changer

An OLTC changes the transformer tap position while load current continues flowing. The mechanism uses an impedance-based transition arrangement to prevent interruption and limit current during changeover.

De-Energized or Off-Circuit Tap Changer

A de-energized tap changer must be operated only after the transformer has been isolated and made electrically safe according to the applicable procedure.

Main Difference

The difference is not simply mechanical construction. It is the permitted operating condition:

OLTC: designed to change taps while energized and carrying load.
De-energized tap changer: operated only when the transformer is de-energized.

Operating a de-energized tap switch under load can cause severe arcing and equipment damage. Tap-changing work must therefore be performed only by qualified personnel following the transformer manufacturer’s instructions and the approved switching procedure.

Advantages of Auto Transformer Tap Changing

1. No Planned Supply Interruption for Routine Regulation

The transformer ratio can be changed while the load remains connected, improving continuity of supply.

2. Effective Voltage Regulation

The arrangement compensates for voltage variations caused by changing load and feeder conditions.

3. Controlled Transition Current

The reactor limits the circulating current when adjacent taps are temporarily bridged.

4. Suitability for Automatic Control

The tap changer can work with an automatic voltage-regulation relay and substation control system.

5. Improved Power-System Voltage Profile

Correct tap selection helps maintain bus voltage within the required operational range.

6. Better Utilization of Electrical Equipment

Stable voltage supports the satisfactory operation of motors, cables, switchgear, drives, and industrial loads.

7. Remote Operation and Monitoring

Modern systems can provide tap-position indication, alarms, operation counts, and remote raise/lower control.

Limitations and Disadvantages

1. Mechanical Complexity

An OLTC contains moving contacts, drive components, interlocks, and switching elements that require inspection and maintenance.

2. Contact Wear

Current interruption and switching duty can cause contact erosion, particularly in conventional oil-switching arrangements.

3. Oil Contamination

Arcing may degrade insulating oil and produce carbon deposits in tap-changer compartments that use oil for switching.

4. Transition Reactor Heating

The reactor must withstand load-current and transition-current duties without excessive temperature rise.

5. Higher Transformer Cost

A transformer equipped with on-load voltage regulation costs more than a comparable unit with a de-energized tap switch.

6. Possibility of Tap Hunting

An unsuitable deadband or time delay may cause repeated raise and lower operations around the voltage set point.

7. Increased Maintenance Requirements

OLTC condition must be assessed using maintenance intervals and criteria appropriate to the installed design and service duty.

Common OLTC Problems and Failure Indicators

Tap-changing equipment operates repeatedly over the service life of a transformer. Potential problems include:

Worn or pitted contacts
Contact misalignment
Loose electrical connections
Drive-mechanism failure
Incorrect selector timing
Carbonized insulating oil
Excessive transition resistance or reactance
Broken springs or mechanical linkages
Motor supply failure
Position-indicator disagreement
Excessive heating
Moisture ingress
Control-relay malfunction
Tap hunting
Failure to complete a switching sequence

Warning signs may include abnormal noise, unexpected voltage steps, repeated alarms, overheating, gas accumulation, unusual dissolved-gas results, increased motor current, failure to reach the commanded position, or disagreement between local and remote position indications.

A suspected internal tap-changer defect should be handled through the site’s approved protection, alarm, isolation, and maintenance procedures. Energized equipment must not be opened or manually manipulated without authorization and proper isolation.

Inspection and Maintenance Considerations

A maintenance program may include:

Recording the tap operation count
Checking local and remote position indication
Inspecting motor-drive components
Verifying mechanical interlocks
Reviewing raise and lower control operation
Inspecting contacts where applicable
Testing insulating oil in the OLTC compartment
Checking sealing and moisture ingress
Inspecting terminal tightness
Performing dynamic resistance measurement where appropriate
Conducting transformer turns-ratio checks at selected taps
Reviewing thermographic inspection results
Analyzing alarms and abnormal operating records
Confirming correct operation of protective devices

Maintenance intervals depend on the OLTC design, number of operations, switched current, transformer loading, insulating medium, site conditions, and manufacturer recommendations. The relevant application guidance also addresses commissioning, field service, maintenance, monitoring, and safety considerations.

Applications of Auto Transformer Tap-Changing Systems

Reactor-type and other OLTC arrangements may be used in:

Power transformers
Grid substations
Distribution substations
Industrial power systems
Steel and metal-processing plants
Mining substations
Large manufacturing facilities
Voltage-regulating transformers
Step-voltage regulators
Interconnecting transformers
Renewable-energy substations
Networks with large daily load variation

The required tap-changer design depends on rated through-current, step voltage, insulation level, number of operating positions, transformer connection, expected switching frequency, overloading duty, temperature, and installation conditions.

Tap Changer, Reactive Power and Voltage Regulation

Transformer tap changing and reactive power compensation can both influence voltage, but they act differently.

An OLTC changes the transformer ratio. It does not generate reactive power. A capacitor bank, reactor, static VAR compensator, or STATCOM changes the reactive-power balance of the network.

Excessive dependence on tap changing may shift reactive-power demand or cause undesirable interactions with downstream voltage-control devices. For this reason, OLTC settings should be coordinated with:

Capacitor-bank controls
Shunt reactors
Generator excitation systems
Distribution voltage regulators
Static VAR compensators
Renewable-plant voltage controllers
Other transformers operating in parallel

For further reading, see Comprehensive Guide to Static VAR Compensators.

You can also review Excitation Control to understand how generator field current contributes to terminal-voltage regulation.

Parallel Transformer Operation

When two transformers operate in parallel, uncoordinated tap positions can produce circulating current and unequal reactive-power sharing.

Important considerations include:

Compatible voltage ratios
Similar impedance characteristics
Correct phase relationship
Tap-position coordination
Master-follower control
Circulating-current control
Suitable load-sharing logic
Prevention of simultaneous conflicting operations

Two independently acting voltage controllers may respond differently to the same network disturbance. A coordinated control strategy is therefore needed to stop one transformer from raising its tap while another lowers its tap.

The Per Unit System in Power System Analysis is useful when modelling transformer voltage ratios, impedances, load flow, and multi-voltage networks.

Practical Engineering Example

Consider an 11 kV controlled bus that falls to 10.78 kV during heavy loading.

The percentage voltage deviation is:

% Δ 𝑉 = \frac{11.00 - 10.78}{11.00} \times 100

If each tap step changes the voltage by approximately 1.25%, one raise operation may not completely restore the nominal value, while two raise operations may produce a total nominal correction of approximately 2.5%.

The voltage regulator does not necessarily issue both commands immediately. The actual decision depends on:

Controller set point
Deadband
Time delay
Measured load current
Line-drop compensation
Permitted tap range
Parallel-transformer control
Existing system voltage

This example shows why OLTC performance depends not only on the mechanical tap changer but also on proper control settings and power-system coordination.

Auto Transformer Tap-Changing Sequence Summary

The reactor-type switching process can be remembered in five stages:

The transformer operates on the existing tap.
The bypass switch opens.
The adjacent tap is connected through the reactor.
The original tap is disconnected.
The bypass switch closes at the new tap position.

This sequence maintains load continuity and prevents an uncontrolled short circuit between adjacent winding taps.

Frequently Asked Questions

What is the purpose of an auto transformer tap changer?

The purpose is to regulate transformer output voltage by changing the effective winding turns ratio. In an on-load arrangement, this adjustment occurs without disconnecting the supplied load.

Why is a reactor used during tap changing?

The reactor limits the circulating current that appears when two adjacent taps are temporarily connected during the transition.

Can taps be changed when the transformer is carrying load?

Taps can be changed under load only when the transformer is equipped with an OLTC designed for that duty. A de-energized tap changer must not be operated on load.

Does an OLTC interrupt the supply?

A correctly operating OLTC transfers current between adjacent taps without intentionally interrupting the supply.

What is circulating current in a tap changer?

Circulating current is the current driven by the voltage difference between two adjacent taps while those taps are temporarily bridged during changeover.

What controls the OLTC?

The OLTC may be controlled by an automatic voltage regulator, local controls, remote control equipment, or a substation automation system.

Does a tap changer improve power factor?

A tap changer primarily changes voltage ratio. It does not directly provide power-factor correction or reactive-power compensation.

What causes frequent OLTC operations?

Possible causes include a narrow deadband, short time delay, fluctuating loads, unstable system voltage, inappropriate line-drop compensation, renewable-generation changes, or poor coordination with other voltage-control equipment.

What standards apply to transformer tap changers?

IEC 60214-1 addresses performance requirements and test methods, while IEC/IEEE 60214-2 provides application guidance for tap changers. The documents cover reactor-type and resistor-type OLTCs as well as de-energized tap changers.

Conclusion

Auto transformer tap changing provides a practical means of regulating transformer voltage while maintaining continuity of supply. The method uses tapped windings, selector contacts, a center-tapped reactor, and a bypass switch to move the load from one voltage position to the next.

During the transition, two adjacent taps may be connected temporarily. The reactor limits the resulting circulating current and prevents direct short-circuiting of the tapped winding section. Once the load has transferred to the new position, the old tap is disconnected and the bypass switch restores the normal current path.

For electrical engineering students, transformer designers, and substation engineers, understanding this sequence is essential because the OLTC is both a voltage-control device and a mechanically active transformer component. Proper selection, control coordination, monitoring, and maintenance are necessary for dependable operation of the transformer and the wider power system.

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