How to Maintain On-Load Tap-Changing Transformers and Tap Changers?

Most tap changers adopt a resistive combined type structure, and their overall construction can be divided into three parts: the control section, the drive mechanism section, and the switching section. On-load tap changers play a significant role in improving the voltage compliance rate of power supply systems. Currently, for county-level grids powered by large transmission networks, voltage regulation is primarily achieved through on-load tap-changing transformers. This places the operation and maintenance of on-load tap-changing transformers and their tap changers in a position of critical importance.

1.Maintenance Content and Requirements

(1) Before commissioning an on-load tap changer, inspect the oil conservator: the oil level should be normal, with no oil leakage, and the control cabinet must be well protected against moisture. Manually operate one full cycle (i.e., raise and lower through all positions). The tap position indicator and counter should function correctly, the limit-position interlocks must be reliable, and the interlock between manual and electric control should also operate reliably.

(2) The Buchholz protection for the on-load tap changer should be configured such that heavy gas triggers tripping and light gas issues an alarm—identical to the Buchholz protection requirements for the main transformer body. The main transformer’s Buchholz relay is commonly referred to as the “large Buchholz,” while the on-load tap changer’s relay is called the “small Buchholz.” The Buchholz relay should be installed in a location that allows safe and convenient gas venting during operation. For newly commissioned on-load tap changers, if gas accumulates in the relay chamber, operating personnel should vent it as needed.

(3) The electric control system of the on-load tap changer must operate accurately and reliably, with all terminal connections secure. The drive motor should rotate normally in the correct direction, and the fuse rating should be set at 2 to 2.5 times the motor’s rated current.

(4) The control circuit of the on-load tap changer—including the electric operation buttons installed on the control panel and those on the tap changer control box mounted on the transformer—must be in good condition. Power indicator lights and tap position indicators should function properly, and limit-position electrical interlocks must be reliable.

(5) The electric control circuit of the on-load tap changer must include a current interlock device, set at 1.2 times the main transformer’s rated current. The return coefficient of the current relay should be ≥0.9. When automatic voltage regulation is used, the main transformer control panel must be equipped with an operation counter, and the secondary voltage disconnection lockout function of the automatic voltage regulator must operate correctly and reliably.

(6) After new installation or major overhaul, the on-load tap changer should undergo one full test cycle under no-load transformer conditions—operated remotely via the main control room buttons and manually on-site at the transformer. Only after confirming correct tap position and voltage indications, and verifying reliable electrical interlocks at limit positions, should the tap position be adjusted to the setting required by dispatch for load operation, with enhanced monitoring thereafter.

(7) Operators shall perform automatic tap-changing operations according to the voltage curve and signals issued by dispatch. After each operation, carefully observe and record changes in tap position, voltage, and current (each single-step adjustment counts as one operation).

(8) When two on-load tap-changing transformers operate in parallel, tap-changing operations are permitted only when the load current does not exceed 85% of the transformer’s rated current. Two consecutive tap changes must not be performed on a single transformer; after completing one tap change on one unit, then perform one on the other. Before paralleling an on-load tap-changing transformer with a no-load (off-circuit) tap-changing transformer, the tap position of the on-load unit must be adjusted to match or closely approximate that of the off-circuit unit, ensuring nearly identical secondary voltages. Once paralleled, tap-changing operations are strictly prohibited.

(9) When operating the on-load tap changer, operators must follow inspection protocols and observe the Buchholz relay before and after operation for any gas bubbles.

(10) If the “small Buchholz” relay issues a signal during operation, or when changing oil in the tap changer tank, tap-changing operations are prohibited, and the power isolation switch must be opened.

(11) During operation, the heavy-gas protection of the on-load tap changer’s Buchholz relay must be connected to trip. If the light-gas relay operates frequently, operators must log the events, report to dispatch, halt all operations, analyze the cause, and address the issue promptly.

(12) Oil quality monitoring and inspection intervals for on-load tap changers:

• During operation, an oil sample must be taken every 6 months for dielectric strength testing. The breakdown voltage must not be lower than 30 kV/2.5 mm. If the breakdown voltage falls between 25–30 kV/2.5 mm, automatic voltage regulation must be disabled. If it drops below 25 kV/2.5 mm, tap-changing operations must stop immediately, and oil replacement scheduled. Oil should also be replaced after 2–4 years of service or after 5,000 switching operations.

• Core inspection (lifting the tap changer core): Conduct after one year of new service or after 5,000 operations; or after 3–4 years of operation or 10,000–20,000 cumulative operations. For imported equipment, follow the manufacturer’s recommendations, preferably coordinating with the main transformer’s major overhaul.

(13) During core inspection of the on-load tap changer, the transition resistance must be measured and verified to match the manufacturer’s specified value.

(14) If “continuous stepping” (i.e., one operation causes movement across more than one tap position—commonly called “slipping”) occurs during electric operation, immediately press the “emergency trip” button on the main transformer control panel once the second tap position appears on the indicator, to cut power to the drive motor. Then manually adjust at the local control box to the correct tap position and notify maintenance personnel for prompt repair.

(15) When the voltage is too low or too high and multiple tap adjustments are needed, adjust one step at a time: press the “n+1” or “n-1” button once, wait at least 1 minute until the new tap number appears on the indicator, then press again. Repeat this process sequentially until the target voltage is reached.

2.Existing Problems and Recommendations

Based on our operational experience, the following issues with on-load tap changers require attention and resolution, along with the following recommendations:

(1) The “small Buchholz” relay of on-load tap changers is prone to oil leakage—a problem that warrants attention. It should be addressed prior to installation, and during operation, sealing gaskets should be replaced as needed.

(2) In attended substations during the early operation phase of on-load tap-changing transformers, automatic voltage regulation should be avoided initially; manual regulation is preferred. Only after a stable operational period without issues should automatic regulation be considered.

(3) In unattended substations, automatic voltage regulation can be implemented either via an automatic controller installed at the substation or through remote adjustment (“tele-adjustment”) by dispatch operators. If the grid has relatively few on-load tap-changing transformers, and to extend tap changer life by minimizing unnecessary operations, the latter (remote control by dispatch) is preferable. However, if the number of such transformers is large, voltage fluctuations are frequent and significant, and operator workload must be reduced, then installing local automatic controllers is more suitable.

(4) In older substations retrofitted later, even if there is no space on the main transformer control or protection panels to install “small Buchholz” protection, this protection should not be omitted. In fact, the “small Buchholz” relay serves as the primary protection against internal faults in the tap changer oil tank, and its importance must not be underestimated.

(5) To extend the service life of the tap changer, the number of operations should be minimized as much as possible. Tap positions should be preset based on historical voltage variation patterns and acceptable voltage ranges.