As the only moving part in a power transformer, an On-Load Tap Changer (OLTC) is a frequent point of failure. Dynamic Resistance Measurement (DRM) has emerged as a vital and non-intrusive offline test to safeguard these assets — a method recently accepted and adopted by the IEEE C57.152 guide. By injecting DC current through the winding as the OLTC moves through all positions, DRM captures precise current signatures. Crucially, the connection is the same as the measurement of winding resistance. With this approach, we dive deep into how to detect slow transition times, mechanical defects, contact issues, and open circuits before they escalate.

The key advantages of dynamic tests are their proven sensitivity to a variety of dynamic defects and faults that cannot be observed in a stationary position, where standard static measurements are usually performed.

Despite these clear advantages, the majority of asset managers and maintenance teams who have adopted only the static winding resistance test as their standard procedure remain largely unaware of the critical benefits the DRM method brings. While static tests are excellent for verifying stationary contact integrity, they leave a dangerous blind spot during the transient switching phase a gap that only dynamic testing can bridge.

Static vs Dynamic Analysis

Static testing only reveals steady-state conditions. DRM shows what happens during switching.

Key Difference: If a transition resistor is damaged, SRM is blind to it because that resistor never enters the circuit while the transformer is stationary. DRM catches it instantly because it operates during the diverter switch movement.

Loose contact on the reversing switch (Phase C/Red: faulty; Phase A/Blue: good condition)

Synergy of static and dynamic analysis

Graph combined with resistance – for complete examination

Since the tap changer has two modes of operation – transition (when moving from tap to tap), and steady (in normal operation without any movement) – several operational parameters are measured and recorded.

Winding Resistance

This involves measuring static winding resistance at a specific position while the tap changer is stationary. Given that the procedure consists of two distinct steps, it is known as the step-by-step method.

Graph

The graph illustrates the transient behavior during tap changer operation, typically accompanied by a tabular representation of key numerical parameters, specifically ripple and transition time.

DV Power RMO-T and TWA series devices utilize an advanced Dynamic Resistance Measurement (DRM) method to enable non-invasive, in-tank testing of tap changers, eliminating the need to drain oil. Key benefits include single-connection simultaneous measurement (SRM and DRM), 0.1ms resolution for identifying micro-anomalies, and automated analysis of transition and ripple, facilitating proactive predictive maintenance.

September 17, 2026