The on-load tap changer (OLTC) is a crucial component in power transformers, and its primary function is maintaining voltage in electrical networks. Moving the position of the tap changer makes it possible to adjust the transformer ratio to the respective situation in the network. This switching process places an electrical, thermal, and mechanical load on the tap changer components, such as the switch contacts, that can cause wear and tear over the life span of the switch.

Developmental work has and continues to center around increasingly advanced diagnostic instruments that can reliably assess the OLTC state. Meanwhile, conventional electric diagnostic methods are focused on static and dynamic resistance measurements and circuit time analyses. This makes it possible to calculate the change in current during the switching operation, providing insights into the OLTC timing and potential power outages.

New vibro-acoustic measurement (VAM)

In addition to these established approaches, vibro-acoustic measurement (VAM) is another method used for tap changers. VAM uses piezoelectric sensors to record the vibrations during the switching operation. The tap changer generates a series of characteristic vibration patterns during a tap-changing operation. As these patterns vary by type and model, they act as fingerprints for every tap changer. Degradation in mechanical integrity or faults in the OLTC can result in a modification to these waveforms, enabling conclusions to be drawn regarding the condition of the unit.

The VAM method is a comparative process in which the measurement results can be compared to previously recorded reference data. The reference data can be recorded before the unit leaves the factory, during commissioning, or be taken from an identical sister unit. Piezoelectric sensors are used during the recording process. They are attached to the cover of the tap changer or the transformer's boiler wall with a magnetic fastening.

The measurement can be performed during maintenance or while the power system operates. Conducting it during operation makes it possible to detect mechanical changes in the tap changer early enough and adjust the maintenance strategy for the affected unit.

Fewer blind spots during diagnosis

If the transformer has been shut down for maintenance, the vibro-acoustic and dynamic resistance measurements can be performed simultaneously. Combining the DRM and VAM data sets can compensate for any diagnostic weaknesses in the respective approaches. Combining the electrical DRM and mechanical VAM data will form a complementary set that provides a detailed assessment of the tap changer’s condition.

(Time: 16:12)
In this episode of "Energy Talks," VAM experts Dr. Karsten Viereck from Maschinenfabrik Reinhausen and Christoph Engelen explain the vibro-acoustic measurement (VAM) method in depth.

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