Transformer Automatic Voltage Regulator (AVR): Voltage Setpoint, Bandwidth/Deadband, Line Drop Compensation (LDC) & Parallel Operation Master-Slave

Abstract

The automatic voltage regulator (AVR) is the control brain that commands the on-load tap changer (OLTC) to maintain the substation bus voltage within a prescribed band around a defined setpoint. While the OLTC provides the muscle, the AVR provides the intelligence — deciding when to initiate a tap change, how long to wait before the next change, and, in the case of parallel-operated transformers, how to coordinate tap positions to avoid circulating reactive current. A misconfigured AVR causes excessive tap-change operations (accelerating OLTC wear), voltage oscillations (hunting) between transformers in parallel, or failure to correct voltage deviations within acceptable timeframes. This article translates AVR configuration parameters — voltage setpoint, bandwidth/deadband, time delay, line drop compensation (LDC), and parallel-operation master-slave or circulating-current control — from control theory to practical commissioning.

1. AVR Architecture and Basic Operation

1.1 The Control Loop

The AVR measures the bus voltage (VT secondary, typically 110 V or 115 V phase-to-phase, corresponding to the rated primary voltage), compares it to the setpoint, and issues RAISE or LOWER commands to the OLTC motor drive:

V_measured < V_setpoint - bandwidth/2 → RAISE command → OLTC moves to higher tap (+1 step → increases secondary voltage)

V_measured > V_setpoint + bandwidth/2 → LOWER command → OLTC moves to lower tap (-1 step → decreases secondary voltage)

|V_measured - V_setpoint| < bandwidth/2 → No action (voltage within deadband)

1.2 Key Parameters

Parameter	Typical Setting	Function
Voltage setpoint (V_ref)	1.00-1.05 pu (e.g., 11.0 kV for an 11 kV bus)	Target voltage on the controlled bus
Bandwidth (deadband)	±0.8-1.5% of V_ref	The voltage range within which no tap change is initiated. Narrower = tighter regulation but more operations.
Time delay (T_delay)	30-60 seconds (typical first delay), 3-10 seconds (subsequent delays)	Prevents unnecessary tap changes for transient voltage deviations (motor starting, fault clearing). Longer first delay prevents tap-change "chase" after a major system event.
Line drop compensation (LDC)	R_comp (0-24 Ω) and X_comp (0-24 Ω) per phase, primary-referred	Compensates for voltage drop in the feeder impedance between the transformer and the load center
Tap step size	1.25% or 1.5% of rated voltage	Per-step voltage change
Maximum tap range	±10% to ±15%	Total regulation range

2. Voltage Setpoint and Bandwidth

2.1 Setpoint Selection

The voltage setpoint is determined by:

• The nominal bus voltage (e.g., 11.0 kV, 33.0 kV, 132 kV)
• The voltage regulation target — typically 1.00-1.03 pu for distribution buses (to allow for feeder voltage drop) and 1.00-1.05 pu for transmission buses
• The statutory voltage limits — e.g., IEC 60038: Nominal voltage ±10% for LV (<1 kV), +10%/-10% for MV (1-35 kV), ±10% typically for HV (>35 kV)

2.2 Bandwidth — The Trade-Off

Bandwidth Setting	OLTC Operations per Day (Typical 132/11 kV Substation)	Voltage Regulation Quality
±0.8% (narrow)	10-30	Tight — voltage stays within 0.8% of setpoint
±1.0% (standard)	5-15	Good — adequate for most applications
±1.5% (wide)	2-5	Loose — acceptable for lightly regulated buses

The critical constraint: The bandwidth must be larger than the per-step voltage change (tap step size). If the bandwidth = ±1.0% and the tap step = 1.25%, a single tap change will overshoot the bandwidth, causing the AVR to reverse the tap change on the next cycle — a "pumping" action that rapidly wears the OLTC. The minimum practical bandwidth is approximately 1.5-2.0× the tap step size:

Bandwidth_min ≥ 1.5 × Tap_Step_Size

For a 1.25% tap step: Bandwidth_min ≥ 1.875%. Set bandwidth to ±1.0% (total width 2.0%) → acceptable with a small margin above the minimum.

3. Time Delay

3.1 First Time Delay (Definite Time)

The first time delay (T1) is the minimum time the voltage must be continuously outside the bandwidth before the AVR initiates a tap change. It prevents tap changes in response to:

• Motor starting inrush (voltage dip 3-10% for 1-5 seconds)
• Fault clearing (voltage dip to 0-30% for 100-500 ms, then recovery)
• Transformer energization inrush (voltage dip 1-5% for 0.5-5 seconds)
• Load switching transients

Typical T1 setting: 30-60 seconds. Shorter (15-30 s) for transmission voltage regulation where voltage deviations affect the entire network; longer (60-120 s) for distribution regulation where load variations are slower.

3.2 Inverse-Time Characteristic (Optional)

Some AVRs implement an inverse-time delay: for a voltage deviation that is only slightly outside the bandwidth, the delay is long (60+ seconds); for a large deviation (e.g., >5% outside), the delay is shortened (5-10 seconds). This mimics the inverse-time overcurrent protection philosophy: fast response to large deviations, slow response to small deviations.

3.3 Subsequent Time Delay

After the first tap change, if the voltage is still outside the bandwidth, a shorter subsequent delay (T2) is applied before the next tap change: Typical T2 = 3-10 seconds. The shorter delay allows the OLTC to iterate through multiple tap positions (if needed) once the initial correctness of the tap-change decision is established.

4. Line Drop Compensation (LDC)

4.1 Principle

In a substation with a long radial feeder, the voltage at the end of the feeder (at the load point) is lower than the voltage at the substation bus due to the voltage drop in the feeder impedance:

ΔV = I_load × (R_line + jX_line)

The AVR at the substation, measuring only the bus voltage, would hold the bus voltage constant while the end-of-feeder voltage sags under heavy load. LDC compensates for this by adjusting the effective voltage setpoint based on the load current:

V_effective = V_measured - I_load × (R_comp + jX_comp)

Where R_comp and X_comp are the compensation impedance settings (in Ω, referred to the VT/CT secondary or to the primary side, depending on the relay configuration). Under heavy load (high I_load):

• V_effective < V_measured → AVR perceives a voltage that is lower than the actual bus voltage → RAISE command → increases the bus voltage above the nominal setpoint → compensates for the feeder voltage drop.

Under light load (low I_load):

• V_effective ≈ V_measured → AVR holds the bus voltage near the nominal setpoint.

4.2 Setting LDC

R_comp and X_comp are set to the total feeder impedance from the substation bus to the load center (primary-referred):

R_comp = R_feeder × (CT_ratio / VT_ratio) X_comp = X_feeder × (CT_ratio / VT_ratio)

For a feeder with impedance 2 + j5 Ω (primary-referred), CT = 800/5 = 160, VT = 33,000/110 = 300:

R_comp = 2 × (160/300) = 1.07 Ω X_comp = 5 × (160/300) = 2.67 Ω

Note: The feeder impedance must be the positive-sequence impedance at the power frequency. For a cable feeder, X/R is typically 0.2-0.5 (resistance dominates at distribution voltage). For an overhead line feeder, X/R is typically 1.0-3.0 (reactance dominates at higher voltage). The correct X/R ratio ensures the compensation is accurate for both the in-phase and quadrature components of the voltage drop.

5. Parallel Operation — Master-Slave and Circulating Current Methods

5.1 The Problem

When two or more transformers are operated in parallel on the same bus, their OLTCs must maintain the same tap position. If one OLTC raises its tap and the other stays, a voltage difference appears between the transformer outputs — but the bus ties them together. The resulting circulating current flows from the higher-tap transformer through the lower-tap transformer, limited by the series impedance of the two transformers:

I_circ = (V_1 - V_2) / (Z_1 + Z_2)

Where Z_1 and Z_2 are the transformer impedances referred to the secondary side.

For a 1.25% tap difference between two 50 MVA transformers with 12.5% impedance each:

I_circ = (0.0125 × 11,000) / (2 × 0.125 × 11,000²/50×10⁶) = 137.5 / 60.5 = 2.27 A — but this is the current flowing in a closed loop through the transformer winding impedances, equivalent to ~10% of rated current with no useful load contribution. This circulating current produces reactive power losses, heats the transformers, and reduces the available loading capacity.

5.2 Master-Slave (Master-Follower) Control

One transformer is designated the MASTER. Its AVR independently decides when tap changes are required. When the master commands a tap change, it also commands the SLAVE transformer(s) to make the same tap change:

Master issues RAISE → Master OLTC moves +1 step → Command sent to Slave → Slave OLTC moves +1 step

Advantages:

• Simple — no circulating current measurement or minimization algorithm
• All transformers always at the same tap position → no circulating current (assuming identical tap-changer step sizes and impedances)

Disadvantages:

• If slaves are not identical to the master (different impedances, different tap step sizes), the tap positions may not be perfectly matched → circulating current still possible
• Communication link required between master and slave controllers (usually a fiber-optic or RS-485 link in the substation)
• Single point of control failure — if the master AVR fails, the slaves cannot operate independently

5.3 Circulating Current (Minimizing Circulating Current) Control

Each transformer's AVR measures the reactive component of its own output current and calculates the circulating reactive current (the difference between the measured reactive current and the average reactive current of all parallel transformers). The AVR adjusts its tap position to drive the circulating current toward zero:

If transformer A has higher circulating reactive current → AVR raises its effective voltage setpoint (or lowers, depending on the current direction) → tap change to balance

Advantages:

• No communication required between AVRs — each operates independently based on its own current measurement
• Compensates for non-identical transformers (different impedances, different tap step sizes)
• Redundant — failure of one AVR does not disable the others

Disadvantages:

• Tap positions may differ between transformers (the algorithm compensates for this — the position difference is intentional to balance the reactive current)
• Stability: The circulating current control loop interacts with the voltage control loop — incorrect gain settings can cause hunting (alternating tap changes between transformers)
• Requires CTs on each transformer with Class 0.5 or better accuracy for reactive current measurement

FAQ

Q: What bandwidth should I set for the AVR?

The standard recommendation: bandwidth = 1.5-2.0× the OLTC tap step size. For a 1.25% step, set bandwidth = ±1.0% (2.0% total width). For a 1.5% step, set bandwidth = ±1.25% (2.5% total width). The minimum bandwidth (tighter regulation) must be ≥1.5× the step size to prevent pumping. If tighter regulation than this is required, the solution is a smaller tap step (e.g., 0.625% step) — not a narrower bandwidth — but this increases the number of tap positions and the OLTC complexity. Setting the bandwidth too narrow causes the most common AVR misconfiguration: "hunting" — the OLTC cycles up and down through one or two tap positions repeatedly, generating 100+ unnecessary operations per day.

Q: When should I use LDC?

LDC is required when the substation bus feeds a long radial feeder (>2-3 km at distribution voltage, longer at transmission voltage) and the voltage at the load end of the feeder varies significantly with load current. Without LDC, the AVR holds the bus voltage constant, and the end-of-feeder customers experience voltage that sags under heavy load. With LDC, the bus voltage is raised under heavy load to compensate. LDC is particularly important for: (1) rural distribution feeders (long lines, high impedance), (2) industrial feeders with large, intermittent loads (crusher motors, arc furnaces), and (3) any feeder where the voltage drop from no-load to full-load exceeds 3%. For short feeders (<1 km) or substations directly supplying an urban load center, LDC is typically not required (set R_comp = X_comp = 0).

Q: How do I verify that the AVR is correctly controlling the OLTC after commissioning?

Step-by-step verification: (1) With the transformer at nominal voltage and the OLTC at the principal tap, set the AVR to MANUAL mode. (2) Raise the tap by one step from the AVR panel. Verify the OLTC operates, the tap position indicator increments, and the bus voltage increases by one step (measure at the VT secondary). (3) Lower the tap by one step from the AVR panel. Verify. (4) Set the AVR to AUTO mode. (5) Adjust the setpoint slightly above the measured voltage (e.g., if the bus is at 11.0 kV and the setpoint is at 11.0 kV ±0.5%, raise the setpoint to 11.1 kV). Wait for the time delay. Verify the AVR issues a RAISE command. (6) Restore the setpoint and verify the AVR returns the bus voltage to nominal. (7) Test the "tap change in progress" interlock — while the OLTC is moving between taps, the AVR should not issue any commands until the tap change is complete. (8) Verify the maximum/minimum tap position limit switches prevent the AVR from commanding beyond the OLTC range.

Q: What is the difference between master-slave and circulating current control for parallel transformers?

Master-slave is a position-matching scheme — all transformers are forced to the same tap position. It is simple and effective when the transformers are identical (same impedance, same tap step size, same rating). Circulating current control is a current-balancing scheme — each transformer independently adjusts its tap to minimize circulating reactive current, resulting in possibly different tap positions. It is more robust for non-identical parallel transformers but requires more careful tuning (gain settings, integration time constant) to avoid instability. The rule of thumb: use master-slave for identical parallel transformers; use circulating current control for non-identical parallel transformers or when a communication link between AVRs is not feasible.

Q: Can the AVR cause voltage instability (hunting) between parallel transformers?

Yes. Voltage hunting between parallel-operated OLTCs is a well-known instability. The mechanism: Transformer A perceives the voltage as low (but the average bus voltage is actually acceptable because Transformer B is at a slightly higher tap) → Transformer A raises its tap → now Transformer B perceives the voltage as high → Transformer B lowers its tap → Transformer A perceives the voltage as low again → cycle repeats. Mitigations: (1) Set different time delays for the two AVRs (e.g., T1_A = 30 s, T1_B = 45 s) — the AVR with the shorter delay acts first, bringing the voltage within the bandwidth before the second AVR's timer expires, (2) add a counter that limits the number of tap changes per time interval (e.g., max 5 tap changes per 10 minutes), and (3) if the OLTCs support it, use a digital parallel-operation controller that coordinates the tap positions through a communication link.

Q: How often should the AVR settings be reviewed?

The AVR settings should be reviewed: (1) at commissioning — verify all parameters against the design specification and the transformer's OLTC data, (2) whenever the network configuration changes (new feeder added, parallel transformer reconfigured), (3) whenever the load profile changes significantly (new large industrial customer, feeder reconfiguration), and (4) annually as part of the substation maintenance cycle. The tap-change counter should be read monthly and trended — a sudden increase in daily tap-change count indicates either a system condition change (new fluctuating load, new wind/solar farm on the feeder causing voltage fluctuations) or an AVR misconfiguration (bandwidth too narrow, time delay too short). Either way, the AVR settings need to be revisited.

References / Standards

Reference	Title
IEC 60214-1:2014	Tap-changers — Part 1: Performance requirements and test methods
IEC 60214-2:2004	Tap-changers — Part 2: Application guide
IEC 60038:2009	IEC standard voltages
IEEE C57.131-2012	IEEE Standard Requirements for Tap Changers
IEC 61000-3-7:2008	Electromagnetic compatibility (EMC) — Assessment of emission limits for fluctuating loads

*Authored by Du Fu, Production Engineer at ZY POWER. The AVR is the interface between the transformer and the grid's voltage quality — a correctly configured AVR provides stable voltage to customers while protecting the OLTC from unnecessary wear; an incorrectly configured AVR creates operational problems that cascade from the substation to the customer meter.*