Abstract
On-load tap changers (OLTCs) are critical components in power transformers, often subject to wear, contact degradation, and mechanical failures. While traditional static resistance measurements offer limited insight into tap changer health, Dynamic Resistance Measurement (DRM) captures transient resistance variations during tap transitions, providing a more comprehensive diagnostic tool. This article outlines the principles, test methodology, and test result analysis methods of DRM, highlighting its advantages and limitations in evaluating OLTC condition.
1. Introduction
OLTCs are essential for voltage regulation in transformers, operating under high electrical and mechanical stress. Failures such as contact wear, contamination, and timing defects often go undetected with static resistance tests. DRM addresses these limitations by measuring dynamic resistance changes during tap changes, offering a more accurate assessment of OLTC performance and identifying potential faults before they lead to failure.
2. On-Load Tap Changers and Typical Failure Modes
An OLTC changes the effective turns ratio of a transformer winding while the transformer remains energized and carries load current. This is achieved using a combination of:
- a tap selector, which chooses the next tap position, and
- a diverter switch, which transfers the current from the old tap to the new tap via transition resistors or reactors.
During a tap change, the current is temporarily commutated through different paths that include main contacts, transition contacts and limiting resistors (or reactors). The sequence occurs in a time frame of several tens to a few hundreds of milliseconds.
Typical failure modes that influence the dynamic resistance behavior include:
- increased contact resistance due to wear, pitting or contamination
- defective or incorrectly rated transition resistors
- mechanical problems in the drive mechanism causing delayed or incomplete operations
Because these phenomena mainly affect the transient current waveform, a time-resolved method such as DRM is required for early detection.
3. Test Circuit and Procedure
In practice a test current between about 5 A and 10 A is used, with the additional requirement that it does not exceed approximately 10% of the rated current of the tested winding. The measuring device should automatically detect the instant at which the tap change begins and should record the current for at least about 200 ms, so that intervals before, during and after the tap transition are all captured.
3.1 Connection of the transformer
The typical DRM circuit includes a DC source, an ammeter (or internal shunt), and appropriate connection leads. Following common recommendations:
- The three-phase winding without the OLTC is short-circuited on all three phases.
- The DC test voltage is applied to one phase of the winding that has the OLTC.
- If the primary winding is YN-connected, all phases on the secondary side may be short-circuited. For a delta-connected primary, only the corresponding phase should be shorted on the secondary; otherwise, current may circulate through more than one tap changer and defects of a single phase can be masked.
Because the short-circuited side has a finite resistance due to cables and connections, a DC voltage appears across the magnetic circuit and produces a magnetic flux in the core. After the test, the flux must be safely discharged through the shorted winding. The test set typically indicates when the discharge is complete; the short-circuit connection must not be removed earlier to avoid arcing.
3.2 Test sequence and tap coverage
The recommended procedure to achieve adequate diagnostic coverage is as follows:
- Select a reference tap
Normally the rated (neutral) tap is chosen as reference. - Operate through all taps in one direction
The OLTC is driven step-by-step through all tap positions while the DC current is recorded for each operation. Ideally the test is performed over the entire tap range. - Repeat for at least one reverse operation
At minimum, the test on one tap should be repeated in the reverse direction, because some mechanical defects are direction-dependent. - Reduced test plan (if time is limited)
Where operational constraints do not allow testing of all taps, a reduced set around the rated tap can be used. For example, if tap 10 is the rated position, transitions between taps 9–10–11–12 may be tested, including one reverse step (e.g. from tap 12 back to 11). - Test all three phases
After completing the test for one phase, the same procedure is repeated for the other two phases in a similar manner, so that inter-phase comparisons become possible.
Figure 1: DRM test wiring diagram with ALLINA T1
4. Analysis and Evaluation of DRM Results
Proper interpretation of DRM traces is the key to using the method effectively. The references describe two complementary approaches.
4.1 Qualitative waveform comparison
The simplest evaluation method is visual comparison of the current waveforms:
- between different phases at the same tap position,
- between different tap positions in the same phase, and
- between present measurements and earlier reference tests (e.g. commissioning results).
In a healthy OLTC, the shape of the current trace during the transition interval is repeatable and similar among phases. Deviations may indicate:
- Contact wear or high transition resistance – deeper or longer current dips
- Contact bounce – oscillatory or jagged behavior after the main transition
- Mechanical timing problems – extended transition duration or delay before current recovery
- Phase-specific defects – differences between otherwise identical operations in different phases
This qualitative analysis is powerful for experienced engineers and serves as the first step of evaluation.
4.2 Quantitative indices: Slope and Ripple
To support objective trend analysis and automated assessment, two numerical indicators are often derived from the DRM waveform, typically defined as in standard diagrams:
- Slope
Represents the rate of current change during the main transition interval. It is related to the overall resistance change and the speed of commutation. A higher-than-expected or irregular slope can indicate slow switching, increased contact resistance or improper operation of transition resistors. - Ripple
Represents the magnitude of oscillation or “roughness” of the current trace during the transition, after subtracting the general trend. Enhanced ripple is usually associated with unstable contact behaviour, bouncing or partial current interruption.
Figure 2: Ripple and Slope Indices for the Analysis of Dynamic Resistance Test Results of Tap Changers
For each tap and phase, the test system can calculate these indices from the sampled waveform. Interpretation is primarily comparative:
- Between phases: values of slope and ripple should be similar for the same tap. Significant differences point to a phase-specific problem.
- Over time: current values can be compared with previous test results, especially with commissioning tests. An increase in ripple or an abnormal change in slope over time may indicate progressive degradation.
It is important to remember that numerical thresholds depend on the OLTC design and test setup; therefore, trend analysis and comparison with good references are more reliable than absolute limits.
Figure 3: Tap changer Dynamic resistance test results With Alina T1
An example of a measured curve by ALLINA T1 for a transformer with specifications of 410 MVA, 420/20 kV, and an MR type VMIII tap-changer is shown in the figure below.
Figure 4: An example of a measured curve by ALLINA T1
5. Conclusion
Dynamic Resistance Measurement is a valuable tool for diagnosing the health of on-load tap changers in power transformers. By detecting transient faults during tap changes, DRM offers a more precise, non-intrusive alternative to traditional static tests. When used properly, DRM can support proactive maintenance strategies, reduce transformer downtime, and enhance operational reliability.
