A transformer with an on-load tap changer can adjust its turns ratio while energized and carrying load, and something has to decide when to move a tap. That job belongs to an automatic voltage control relay, associated with ANSI device number 90, which watches the busbar voltage and commands the tap changer up or down to keep it on target. This guide explains the control loop that holds voltage inside a deadband, the roles of target voltage, bandwidth, and intentional time delay, why tap-change counting matters, and how SCADA supervises the scheme and sets the target remotely.
OLTC Voltage Control (90) in one line: On-load tap changer voltage control is an automatic loop, run by an AVC relay of ANSI device class 90, that raises or lowers the transformer's tap position to hold the regulated busbar voltage close to a target. The relay compares measured voltage against the target and only acts when the error exceeds a bandwidth, or deadband, then waits an intentional time delay before commanding a tap step so it does not chase every momentary fluctuation. It counts tap operations for maintenance and can be supervised and retargeted from SCADA.
At the heart of OLTC control is a comparison. The AVC relay measures the voltage on the regulated busbar and compares it against a target voltage, the value the operator wants to hold. If the measured voltage is above target, lowering a tap reduces it; if below, raising a tap increases it. Each tap step changes the ratio by a small fixed percentage, so the relay is nudging the voltage back toward target one step at a time rather than making one large correction.
If the relay acted on the smallest deviation it would move taps constantly and wear the mechanism out, so it works within a bandwidth, also called a deadband, centered on the target. As long as the voltage stays inside the band, no action is taken. Only when the voltage drifts outside the band does the relay decide a tap change is warranted. The bandwidth is chosen a little wider than the size of one tap step, so that a single corrective step brings the voltage comfortably back inside the band rather than overshooting to the far edge and triggering a reverse correction.
The other essential ingredient is an intentional time delay. Before it actually commands the tap changer, the relay waits for the voltage error to persist beyond the bandwidth for a set time. This delay is what prevents hunting: it ignores brief voltage dips and surges from load switching or motor starts that would otherwise provoke needless tap moves, and it lets the voltage recover on its own if the excursion was momentary. Only a sustained deviation, one that outlasts the delay, results in a tap operation, so the scheme responds to real trends in voltage rather than to transient noise.
Every tap change is a mechanical operation that moves contacts under load, so the tap changer is a wearing component with a finite operation count between maintenance interventions. AVC relays therefore count tap operations, both since the last maintenance and often per day, so that the accumulated wear can be tracked and the tap changer serviced on the basis of actual duty rather than a fixed calendar. An unusually high operation count is also a diagnostic, hinting at a bandwidth set too narrow, a delay too short, or a volatile supply that is working the mechanism hard.
Where two or more transformers feed the same busbar in parallel, their tap controls must be coordinated, because if each relay independently chases voltage they can end up on different taps, driving a circulating reactive current between the transformers that serves no purpose and adds losses. Parallel control schemes, such as master-follower or circulating-current minimization, keep the transformers on compatible taps. Even a single transformer benefits from careful setting: the interplay of bandwidth and delay is what separates smooth regulation from a control that either drifts sloppily or hunts back and forth.
Hunting, the pathological case where the control repeatedly steps up then down without settling, is the failure mode the settings are chosen to avoid. It arises when the bandwidth is too tight relative to the step size, when the delay is too short to let voltage settle, or when line drop compensation is overcooked. A well-set scheme takes a step, sees the voltage land safely inside the band, and stops, whereas a badly set one overshoots and corrects endlessly, wearing the mechanism and disturbing the voltage it is meant to stabilize.
Voltage control is one of the classic functions that a SCADA system supervises rather than replaces. The AVC relay handles the fast local loop, but the target voltage itself is frequently a system-level decision: a control room may want to raise the target across a region during peak load to support voltage, or lower it to reduce demand and losses. SCADA provides the channel to send a new target voltage down to the AVC relay, so the local loop keeps doing its job against a setpoint that the wider system can adjust.
Supervision also means visibility. The tap position, the measured voltage, the target, the operation count, and whether the control is in automatic or manual are all values SCADA reads back, so an operator can confirm the tap changer is tracking, catch a control stuck at an end tap, and see a transformer that is quietly hunting. A tap position that has run to its maximum and stopped is an important signal that the transformer is out of regulating range and the busbar voltage can no longer be held.
For distributed utility and industrial sites, a cloud SCADA platform such as Merobix can pull the tap position, voltage, target, and operation counts from AVC relays across every substation into one place, trend them, and alarm on abnormal behavior such as excessive daily tap operations or a control that has reached an end tap. That lets an engineer manage voltage across a fleet of transformers, schedule tap-changer maintenance from real operation counts, and push target changes remotely, without a visit to each site to read a local relay.
An AVC relay is an automatic voltage control relay that operates a transformer's on-load tap changer to hold busbar voltage on target. ANSI device number 90 designates automatic regulating or voltage-control functions, which is the class this relay belongs to. It measures voltage, compares it to a target within a bandwidth, and commands tap-up or tap-down after an intentional time delay.
The bandwidth, or deadband, is a range around the target voltage inside which the relay takes no action, so it does not move a tap for every tiny deviation and wear out the mechanism. The time delay makes the relay wait for a voltage error to persist before acting, which ignores brief dips from load switching or motor starts. Together they prevent hunting and ensure the control responds only to sustained voltage trends.
Yes. The AVC relay runs the fast local loop, but the target voltage is often a system-level decision, so SCADA provides a channel to send a new target down to the relay. A control room can raise the target during peak load to support voltage or lower it to reduce losses, and SCADA also reads back tap position, voltage, and operation counts to supervise the scheme from a distance.
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