What are the regulations and operational precautions for voltage regulation of on-load tap-changing transformers?
On-load tap changing is a voltage regulation method that allows a transformer to adjust its output voltage by switching tap positions while operating under load. Power electronic switching components offer advantages such as frequent on/off capability, spark-free operation, and long service life, making them suitable for use as on-load tap changers in distribution transformers. This article first introduces the operational regulations for on-load tap-changing transformers, then explains their voltage regulation methods, and finally outlines key precautions for on-load tap-changing operations. Please read on with the editor for detailed information.
1.Operational Regulations for On-Load Tap-Changing Transformers
When operating an on-load tap-changing transformer, a second tap change must not be initiated until the first tap change is fully completed. Voltage, current, and other parameter changes should be closely monitored during the process.
Each tap-changing operation must be recorded in the main transformer tap-changing logbook, including the operation time, tap position, and cumulative number of operations. Records must also be maintained for all commissioning/decommissioning events, tests, maintenance activities, defects, and fault handling.
Maintenance of the on-load tap changer shall follow the manufacturer’s specifications. In the absence of such specifications, the following guidelines may be applied:
Oil samples from the tap changer compartment should be tested after 6–12 months of operation or after 2,000–4,000 switching operations.
For newly installed tap changers, the switch mechanism should be lifted out for inspection after 1–2 years of service or after 5,000 operations. Subsequent inspection intervals may be determined based on actual operating conditions.
The insulating oil in the tap changer compartment should be replaced after 5,000–10,000 operations or when the oil’s breakdown voltage drops below 25 kV.
For on-load tap changers that have remained unused or unmoved for an extended period, a full cycle of operation between the highest and lowest tap positions should be performed whenever a power outage opportunity arises.
2.Situations Where On-Load Tap-Changing Is Prohibited:
When the transformer is operating under overload conditions (except in special circumstances).
When the light-gas relay of the on-load tap-changing device has tripped and issued an alarm.
When the insulating oil dielectric strength of the tap-changing device is unqualified or the oil level indicator shows no oil.
When the number of tap changes has exceeded the specified limit.
When abnormalities occur in the tap-changing device.
When the load exceeds 80% of the rated capacity, operation of the on-load tap changer is prohibited.
3.Voltage Regulation Methods for On-Load Tap-Changing Transformers
3.1 "Boots-On" Retrofit Method
The "boots-on" approach involves opening the neutral point of the high-voltage three-phase windings of the main transformer and inserting series-connected regulating windings from a compensation transformer. The low-voltage side of the main transformer is connected in parallel with the excitation winding of the compensation transformer to achieve on-load voltage regulation. This method relies on the principle of voltage superposition: the compensator, through an on-load tap changer, maintains the high-voltage winding voltage of the main transformer within its rated range.
In this configuration, the compensator only withstands the neutral-point voltage or the N-level tap voltage (e.g., 2×OU1), requiring a relatively low insulation level. When the transformer neutral point operates under solidly grounded conditions, an insulation level of 35 kV is sufficient (we design and manufacture for 40 kV), though higher levels can be adopted based on specific operational requirements. This method requires only one additional neutral-point regulating transformer, resulting in low retrofit costs. Field modifications involving the neutral-point lead can be completed within one working day. If integrated with a major transformer overhaul, it adds virtually no extra downtime.
This method is suitable when voltage fluctuations exceed the range achievable by no-load (off-circuit) tap changers—i.e., even when the off-circuit tap changer is at its highest or lowest position, the voltage still fails to meet standards. Our neutral-point on-load tap-changing transformers provide a wide ±12% U₁ₙ regulation range. When used in conjunction with the original off-circuit tap changer, the effective regulation window can be shifted up or down more flexibly to meet actual needs and enhance the main transformer’s output capacity. The required regulation range can be customized based on site conditions, making this solution adaptable to transformers of all voltage levels. We have successfully retrofitted four main transformers using this approach. However, this method does require additional space for one extra transformer and slightly more complex primary wiring. Nevertheless, considering the short retrofit duration and cost savings, it remains an economically sound and reasonable solution.
3.2 "Backpack" Retrofit Method
The "backpack" method is a more economical and practical retrofit approach when the existing off-circuit tap changer’s range already meets local voltage fluctuation requirements. It involves disconnecting the tap leads from the original off-circuit tap changer, removing the switch, and installing a bridging-type or linear on-load tap changer in its place, with the original tap leads rerouted to the new on-load switch.
This retrofit can be completed within one major maintenance cycle. Core work (such as removing the tank cover or lifting the core) takes only one day and can be synchronized with routine core inspections; the tank or housing is modified simultaneously. The critical challenge is completing the entire retrofit within one day without exposing the core to moisture, as any delay would extend the outage and increase costs.
Additionally, since original transformers rarely include dedicated lead routing channels for such retrofits, special measures must be taken to ensure proper insulation clearances for all transformer types and to maintain ease of future maintenance (i.e., preserving original hood/core lifting procedures). We have conducted extensive research on this method, developed specialized equipment, and established a comprehensive, practical construction plan. To date, we have successfully implemented this method on five transformers, achieving all expected outcomes—confirming it as an economical and straightforward retrofit solution.
4. Precautions for On-Load Tap-Changing Operations
Tap changes must be performed step-by-step, with close monitoring of tap position, voltage, and current. After each single-step adjustment, wait at least 1 minute before proceeding to the next step.
For single-phase transformer banks or three-phase transformers with phase-separated on-load tap changers, synchronous three-phase electric operation is required; individual phase operation is generally prohibited.
When two on-load tap-changing transformers operate in parallel:
Tap changes are permitted only when the load current is at or below 85% of the transformer’s rated current.
Do not perform two consecutive tap changes on a single transformer; complete one transformer’s adjustment before operating the other.
After each tap change, check voltage and current to prevent misoperation and overloading.
During voltage-raising operations, adjust the transformer with the lower load current first, followed by the one with higher load current, to minimize circulating currents. The reverse order applies for voltage-lowering operations.
After completion, verify the current magnitude and distribution between the two paralleled transformers.
When an on-load tap-changing transformer operates in parallel with a no-load (off-circuit) tap-changing transformer, the tap position of the on-load unit should be kept as close as possible to that of the off-circuit unit.
The maximum number of tap changes allowed per day is as follows:
30 times for 35 kV transformers,
20 times for 110 kV transformers,
10 times for 220 kV transformers.
Before each tap change, verify that the difference between system voltage and the tap’s rated voltage complies with regulatory requirements.
Every tap-changing operation must be properly documented in the on-load tap changer operation logbook as required.
