OMICRON Magazine

Power transformers play a vital role in the electrical power grid. In order to avoid any unforeseeable outages, it is of utmost importance that the transformer and its internal components work reliably. The on-load tap changer (OLTC) is one of the essential components. By adjusting the turns ratio of the transformer, a stable system voltage is maintained in the electrical grid. The OLTC has to withstand the electrical stress as well as the mechanical and thermal stresses that are imposed by the load current. Many diagnostic tools have been developed to assess the condition of power transformers, ranging from oil analysis to static and dynamic resistance measurements (DRM). The DRM records the change in current during the switching operation. This way, the auxiliary contacts, and the commutating resistors can be evaluated, and a timing analysis can be performed. With the release of PTM 4.90, two powerful OLTC-focused features were added to the testing capabilities of the TESTRANO 600: The vibroacoustic measurement (VAM) and the three-phase OLTC Scan option. In the following examples below, we’ll take a look at both features and how they can be utilized to facilitate OLTC testing and lead to an in-depth OLTC condition assessment. 1st feature – VAM The vibro-acoustic measurement (VAM) approach is based on recording vibration patterns produced during the tap switch operation. The new measurement module VAM1 has been developed in cooperation with the OLTC manufacturer Maschinenfabrik Reinhausen in Germany. The VAM1 unit allows you to connect up to three piezo-electric sensors that are mounted on the transformer tank wall using magnetic fasteners. The vibration patterns produced during the switching operation depend on the internal design and the switching principle of the tap changer. Therefore, these patterns are considered unique for each unit and can be used as a reference for future measurements. The sensor placement plays a vital role in obtaining sufficiently strong readings. The best results are obtained when the sensor is placed as close to the OLTC as possible. The new test in PTM 4.90 supports the user by checking the data quality of the recorded traces. If it is unsatisfactory, e. g., due to weak signal strength, the software will mark the corresponding data set and suggest some troubleshooting advice. There are two options for carrying out the VAM test. The first is performing the measurement during a transformer maintenance outage (offline). In contrast, the VAM test can be conducted while the transformer remains in service (online). The findings of such a vibration analysis can then be considered when prioritizing the maintenance activity of an OLTC fleet. The raw data, sampled at 250 kHz, is transformed into the frequencytime domain using a continuous wavelet transformation. This so-called ft-diagram illustrates the various frequency components of the vibrations. It can be used to analyze the overall signal quality (signal-to-noise ratio) and identify possible external influencing factors such as disturbing noise sources. Figure 1 depicts an example of the ft-diagram, showing the start of the tap switch operation (left), the tap selector activity (middle), and finally, the diverter switch operation (right). The color coding indicates the signal strength, ranging from weak (blue) to strong (red). The signal is integrated into the frequency domain, which results in the so-called envelope curve. This 28