Dynamic Winding Resistance Testing of Load Tap-Changers

What it is

Dynamic winding resistance testing is an electrical check of a load tap-changer made while the mechanism is actually operating. A steady direct current is driven through a winding, and the instrument records the resulting resistance continuously as the tap-changer moves from one tap to the next. The output is a trace over time, not a single number. Each tap change appears as a short, characteristic event in which the current path is handed from one contact to the next, usually through a transition resistor or reactor that keeps the load current flowing during the transfer. The test is applied to liquid-filled power transformers fitted with on-load tap-changers, to the load tap-changer compartment as a separate item, and to step voltage regulators, which use the same principle. It complements the ordinary winding resistance measurement, which is taken with the tap-changer at rest on each position and says nothing about what happens in between. Technicians use it to learn whether the selector, diverter and transition circuit behave as designed during the transfer.

Why it is done

A load tap-changer is one of the few moving parts in a transformer, and it carries current while it moves. Contacts wear, carbon and coking build up, springs and drive parts lose their timing, and transition resistors can age, crack or open. None of these faults necessarily changes the resistance measured at rest on a tap, yet each can interrupt the load current for an instant, produce arcing and gas, or leave a contact carrying more than its share. The consequences range from accelerated contact erosion and gassing in the compartment to a failed tap change that damages the winding or takes the unit out of service. Because the tap-changer is one of the most common sources of transformer failures and forced outages, a test that reveals its transfer behavior without opening the compartment is valuable. It helps decide whether the mechanism can be left alone, needs a closer inspection, or must be overhauled, and it gives a record that can be compared with the factory result and with later tests.

The principle

The test relies on the fact that a transformer winding behaves very differently for a steady direct current than for a changing one. Once a constant current has saturated the core, the winding is essentially a resistance, and the instrument can compute resistance from the voltage across it. When the tap-changer begins a transfer, the path through the winding changes. In a resistor-type design the load current is briefly shared through a transition resistor, so the measured resistance rises for a moment and then settles at the value of the new tap. In a reactor-type design a bridging position produces its own characteristic step. A worn or dirty contact adds contact resistance and shows as a bump or noise, a contact that bounces shows as a spike, and a break in the circuit shows as a sudden jump toward an open circuit. The shape, order and duration of these features form a fingerprint of the healthy mechanism. A fault changes that fingerprint in a way the technician can recognize, and because the same instrument views every tap change the same way, comparing traces is far more informative than reading one value.

How it is performed

The transformer is de-energized, isolated and grounded under the owner's safety procedures, and connections that could carry current or voltage into other equipment are removed. Because the test saturates the core with direct current and then interrupts it, discharge procedures matter, and the leads are handled only when the current source is off and the winding is safely discharged. The instrument is connected to the winding of interest using separate current and voltage leads at the bushings, so that lead and clamp resistance does not enter the reading. The current is raised and allowed to stabilize before the tap-changer is operated, because a steady current is what makes the trace readable. The tap-changer is then driven through its range by the normal motor drive or by the manual method the manufacturer allows, with the recorder capturing each tap change in the raise direction and then in the lower direction. The test is repeated on every phase, and the tap-changer's position indication and operation counter are noted as the work proceeds. Environment matters too: the winding temperature and the oil temperature are noted, because they affect the resistance level, and the same leads, instrument settings and current are used on repeat tests so that traces can be compared with earlier ones. After the test the current is ramped down slowly and the winding is discharged before the leads are removed.

What is recorded

The record is a set of resistance traces over time for each phase and each direction of travel, together with the source current used, the tap positions covered and the number of tap changes captured. It also holds the winding temperature, the oil temperature, the identity and calibration status of the instrument, and the number of operations counted over the course of the test. Observations about the as-found condition, such as unusual noise during an operation or signs of leakage at the compartment, are written down, along with any connection that was lifted. These details let a later reader compare a new trace against the factory result and earlier field results and decide whether any difference reflects the mechanism or only the way the test was run.

How results are judged

Results are judged by the shape of the trace, not by one limit, and this page does not give numeric criteria. The first comparison is against the factory or commissioning trace for the same unit, which acts as the reference for the expected sequence of events at each tap change. The second is comparison between phases and between the raise and lower directions, where a significant difference points to a local problem. The third is a review of the transition itself: the transfer should be orderly, with no unexplained dropouts, spikes or prolonged excursions, and with the resistance at the end of each change settling to the level expected for the new tap. The fourth is trending against earlier field tests, where gradual changes matter even if each trace still looks acceptable. Manufacturer data for the specific tap-changer model governs what is normal for its design. The scope and the circumstances under which this measurement is called for are set out in ANSI/NETA ATS-2025 for acceptance work and ANSI/NETA MTS-2023 for maintenance work, where it appears as an optional measurement for the equipment covered. For the governing wording and the values, refer to your own licensed copy of these standards.

Variations

Acceptance and maintenance applications differ in purpose. At acceptance the trace is captured to confirm that the tap-changer was not harmed in shipping and installation, and it becomes the baseline against which every later test is compared. In maintenance it is used to track wear, so repeatable setup and consistent current matter more than anything else. The design of the tap-changer changes the interpretation: resistor-type and reactor-type mechanisms, and designs with vacuum interrupters, each produce a different expected transition pattern, so the manufacturer's reference matters. Step voltage regulators, which are tested the same way, usually have simpler mechanisms and fewer positions. Units with only an off-circuit tap switch gain nothing from this test, because they are never operated under load. Dynamic measurements are normally paired with static winding resistance at each tap, turns ratio and dissolved gas analysis, which together give a fuller picture of the tap-changer and the transformer around it.