Voltage deviation at a substation is rarely solved by moving a tap changer until one meter looks correct. The setting must be based on the voltage that matters at the point of supply, the transformer ratio, expected load-related voltage drop, and the permitted operating range of connected equipment. A setting that corrects a low busbar voltage during peak load can create unacceptable overvoltage overnight or when local generation rises.

For an electrical substation transformer with tap changer, the practical objective is to keep the secondary voltage within the approved operating band across normal supply and load conditions, while avoiding unnecessary tap operations or unsafe adjustment practices. The right approach differs fundamentally between an on-load tap changer (OLTC), which can regulate while energized, and a no-load tap changer (NLTC), which must only be repositioned after the transformer is isolated, de-energized, and properly secured.

Start with the voltage profile, not the tap position

A tap setting changes the transformer turns ratio. It does not remove voltage drop in feeders, correct a bad meter reference, or compensate for every rapid fluctuation caused by large motors, arc furnaces, or variable renewable generation. Before changing taps, establish whether the deviation is steady, load-dependent, or intermittent.

A useful operating record should compare:

• incoming high-voltage bus voltage;
• transformer secondary bus voltage;
• voltage at the critical downstream load or feeder end;
• transformer loading, preferably as kVA or current and power factor;
• the time of day, production state, and switching configuration.

One instantaneous reading is not enough. A transformer may show a satisfactory 400 V secondary bus voltage when lightly loaded, but a remote process line can still experience low voltage when feeder current increases. Conversely, raising the transformer secondary voltage to support a distant feeder may push nearby loads above their allowed voltage under light-load conditions.

The first operating question is therefore: where is the unacceptable deviation occurring, and under what condition? Only after that can the tap changer be treated as the appropriate corrective device.

Understand the direction of the adjustment

Tap labeling can be misleading because manufacturers do not all present positions in the same operational language. Some name positions by the high-voltage winding’s rated voltage; others identify a nominal position and plus/minus percentage steps. Operators should use the transformer nameplate, approved connection diagram, and manufacturer documentation—not an assumed rule such as “higher tap always gives higher output.”

The underlying relationship is straightforward:

Secondary voltage ≈ primary voltage × secondary turns / primary turns.

If the tap position increases the effective number of primary turns, the secondary voltage falls for the same incoming voltage. If it reduces the effective primary turns, the secondary voltage rises. But the position number on the selector does not by itself confirm which of these actions is occurring.

Consider a 35 kV/0.4 kV distribution transformer with high-voltage tapping intended for nominal primary conditions such as 33 kV, 35 kV, or 38.5 kV, or with ±5% adjustment in 2.5% steps. If the actual primary supply remains below the rated value for extended periods, a ratio selected for a lower primary voltage may restore the intended low-voltage output. If the supply is consistently high, selecting the corresponding higher-primary-voltage ratio may prevent secondary overvoltage. The correct direction must always be verified against the specific rating plate.

Each 2.5% step on a 400 V nominal secondary basis represents approximately 10 V under the reference condition. That is a material change. It should not be used as a fine-control mechanism for a small and short-lived variation.

Separate transformer regulation from feeder voltage drop

Transformer impedance and downstream conductor impedance cause voltage drop as current rises. The magnitude and phase angle of that drop depend on load current, conductor resistance and reactance, transformer impedance, and power factor. A predominantly inductive load generally produces a different voltage profile from a resistive load at the same kVA.

This distinction prevents a common operating error: repeatedly increasing a transformer tap because voltage is low at the end of a feeder. Where the transformer secondary bus remains inside its required range but the remote load voltage is low only during high current demand, the principal constraint may be the feeder rather than the transformer ratio.

Possible causes include undersized cable, an excessively long feeder, poor joints, overloaded bus duct, an unfavorable network configuration, or a large reactive-power demand. A tap change may temporarily improve the remote reading, but it can overvoltage loads closer to the transformer when demand decreases. The more durable correction may involve conductor reinforcement, feeder reconfiguration, power-factor correction, or a local voltage-regulation solution.

The opposite situation also occurs. If the entire low-voltage bus rises and falls with the upstream supply, and downstream voltage drop remains proportionate, transformer tap selection or OLTC control is directly relevant.

Set a defensible target band

The target is not necessarily the nameplate voltage at every moment. It is the voltage range required by the network owner, the installation design, and the connected equipment. Motors, contactors, drives, UPS systems, rectifiers, lighting controls, and sensitive electronic loads can have different tolerance limits. A substation may supply several load classes, so the most restrictive critical load often determines the practical target.

Setpoints should also account for meter accuracy, instrument-transformer ratio and class, voltage-transformer fuse condition, and the actual measurement location. An incorrectly scaled power-quality meter can lead to an unnecessary tap intervention. Before acting on a persistent deviation, compare readings from the control system with a calibrated portable instrument at an approved test point.

For OLTC-controlled transformers, a stable voltage-regulation scheme normally requires more than a target voltage. The controller should have:

• a voltage setpoint appropriate to the controlled bus or remote point;
• a deadband wide enough to prevent hunting around the target;
• a time delay that rejects brief transients and motor-starting events;
• high- and low-voltage limits that prevent the controller from pursuing an unsuitable target;
• correct compensation settings if line-drop compensation is used;
• a defined operating mode for parallel transformers.

A deadband that is too narrow causes excessive tap movements as normal voltage noise crosses the threshold. A delay that is too short can produce the same result after switching operations or temporary load changes. Frequent operation accelerates mechanical wear and increases the importance of tap-changer maintenance. A wide deadband, however, can allow a real voltage problem to persist. Settings should be chosen from recorded voltage variation and equipment requirements, not copied from another substation.

Line-drop compensation needs measured system data

Line-drop compensation (LDC) allows an OLTC controller to regulate an estimated remote voltage rather than only the local transformer bus. It can be useful where the remote feeder voltage is the true service constraint, but it depends on accurate resistance and reactance compensation values and a valid current input.

An overestimated compensation setting can drive the local bus too high at heavy load. An underestimated setting may leave the remote load undervoltage. Incorrect current-transformer polarity, phase assignment, or power-factor information can produce especially misleading regulation behavior. When a controller’s tap movements appear reasonable at the local bus but voltage complaints persist remotely, the LDC model and metering circuits deserve investigation before the target voltage is raised.

LDC is not a substitute for adequate feeder capacity. If the calculated compensation required is extreme, or if remote voltage remains poor despite a safe local bus voltage, the feeder design limitation should be addressed rather than forcing the transformer to operate at an unsuitable voltage.

Parallel transformers require coordinated control

Two or more transformers operating in parallel introduce an additional risk: poorly coordinated tap positions can create circulating current. This current does not serve the load, yet it consumes transformer capacity and can increase heating. It may appear even when the outgoing load is moderate.

Before adjusting one transformer in a parallel bank, confirm that the transformers are approved for parallel operation and that their vector groups, phase sequence, voltage ratios, impedance characteristics, and tap ranges are compatible. The operating scheme must specify whether one OLTC controller acts as master, whether controllers use circulating-current compensation, or whether another approved parallel-control method is applied.

Manual adjustment of only one unit can be particularly problematic when both are energized and tied to the same bus. An individual transformer may look correctly set in isolation but still share load poorly. Load current on each transformer, differential protection status, and tap-position indication should be reviewed after any planned change.

NLTC adjustments must be treated as planned outages

No-load tap changers are suited to correcting a sustained, predictable mismatch between the transformer’s rated ratio and the normal supply condition. They are not intended for daily or seasonal voltage trimming without a controlled outage procedure.

The 35kV/0.4kV Oil-Immersed Power Distribution Transformer, for example, uses NLTC regulation and offers high-voltage tap options around a 35 kV/400 V distribution duty. In this configuration, the tap position should be selected from the expected steady primary-voltage range and the permissible low-voltage range, then left in service unless the operating profile changes materially.

Before an NLTC position change, the transformer must be removed from service under the approved switching procedure. Isolation, lockout/tagout, absence-of-voltage verification, discharge and earthing requirements, access control, and restoration checks must follow the site’s safety rules and the equipment manufacturer’s instructions. The selector should never be moved with the transformer energized, even if the handle appears accessible. Internal arcing and severe equipment damage can result.

After the change, record the old and new positions, the reason for the adjustment, primary and secondary voltage readings before and after restoration, load level, and the approving authority. This record is essential when later voltage behavior must be interpreted, particularly where supply voltage is influenced by upstream network switching or distributed generation.

Do not diagnose a voltage problem from voltage alone

Low secondary voltage can be caused by an unsuitable tap, but it can also indicate low incoming voltage, high transformer loading, poor power factor, a connection issue, incorrect transformer configuration, or inaccurate metering. High secondary voltage may result from a high primary supply, an inappropriate ratio, capacitor switching, reverse power flow, or an OLTC control fault.

Several checks help distinguish these conditions:

• Compare phase voltages. A balanced deviation across all three phases points toward a ratio or upstream supply issue. A pronounced single-phase deviation may indicate a connection, fuse, conductor, or load-balance problem.
• Review transformer loading and temperature. A voltage reduction accompanied by high load current and rising oil temperature may reflect loading conditions, not a tap error. Tap changes do not increase the transformer’s continuous thermal rating.
• Check tap-position feedback. For OLTC equipment, a displayed position that disagrees with the mechanical indicator or controller command warrants investigation.
• Look at switching events. A deviation occurring immediately after capacitor-bank switching, feeder transfer, generator synchronization, or solar output changes may not be solved by a fixed tap position.
• Verify the reference source. Failed VT fuses, loose terminal connections, incorrect scaling, and control wiring faults can cause an automatic controller to regulate the wrong voltage.

Tap changing has limits under overload conditions

Raising the secondary voltage can increase load demand in some applications and may increase current where downstream equipment attempts to maintain constant power. It should not be assumed that correcting a low-voltage condition reduces transformer stress. The effect depends on the connected load characteristics.

For oil-immersed units, oil and winding temperatures remain central operating constraints. A copper-wound 35 kV transformer may permit a defined short-duration overload under specified conditions, but that does not justify using the tap changer to conceal a sustained capacity shortfall. Any overload allowance must be governed by the manufacturer’s thermal limits, ambient conditions, cooling arrangement, oil-temperature monitoring, and the applicable operating procedure.

Where the secondary voltage is acceptable at normal load but declines only during overload, the correct response may be load transfer, demand management, capacity reinforcement, or review of the transformer’s loading plan—not a permanent tap increase.

A practical post-adjustment check

Every tap adjustment should be followed by a period of observation across representative operating conditions. Confirm the low-voltage bus reading, upstream voltage, phase balance, load current, power factor, and the voltage at any critical remote point. For OLTC systems, review the number and direction of operations. Repeated alternating moves near the same load level indicate a deadband, delay, sensing, or compensation issue rather than a need for more aggressive regulation.

Also inspect alarms associated with OLTC motor drive mechanisms, diverter-switch condition monitoring where fitted, oil level, and protective relays. A controller that commands movement but fails to complete it can leave the transformer at an unexpected ratio. For an NLTC transformer, ensure that the documented tap position matches the physical selector position and the drawings retained for the installation.

The most reliable tap setting is therefore not the one that produces nominal voltage during a single inspection. It is the verified ratio or control setting that maintains the required voltage range over the actual operating profile, respects feeder and transformer limits, and can be traced through accurate measurements and disciplined switching records.