Vibro-acoustic method as a fast diagnostic tool on load tap changers

Advanced technologies enable precise evaluation of On-Load Tap-Changer (OLTC) health, ensuring  grid stability and longevity through comprehensive assessment and analysis.

By combining dynamic resistance measurement (DRM) and vibro-acoustic measurement (VAM), this method offers a comprehensive analysis of On-Load Tap-Changer (OLTC) health. VAM captures vibration patterns during operation, while DRM focuses on electrical characteristics. Comparing these measurements with reference data allows for an accurate assessment of OLTC conditions. A case study comparing VAM results from sister units demonstrates efficacy  of this approach. Overall, this method  provides a sophisticated solution for  enhancing equipment reliability and  lifespan.

OLTC is crucial for maintaining a stable  voltage level in the electrical grid. With  the help of OLTC, the transformer turns ratio can be adjusted under load to  account for voltage fluctuations. This  mechanical switching process causes  heating and arcing inside OLTC.  Consequently, various components  will experience wear and tear over  their lifetime. Increasingly advanced  diagnostic tools have been developed  that can reliably analyse and assess  the condition of OLTC.

The following example discusses a condition assessment based on combining DRM and VAM. While DRM concentrates on electrical  characteristics when the load current  is switched, VAM focuses on vibration  pattern OLTC produces during  switching operation. The evaluation  of VAM results relies on comparing  them with reference data such as a  fingerprint or sister units.

Measurement Setup 

The mechanical movements and  arcing during an OLTC switching operation produce a wide frequency  range of vibrations. VAM allows these  vibration patterns to be analysed and  compared with reference data and  enables the mechanical condition  of the tap-changer to be assessed.  The signals are sampled at 250 kHz  by the measurement system using  integrated electronic piezo-electric  (IEPE) acceleration sensors. This  way, a non-invasive measurement  can be performed on the transformer.  This can even be done with the  transformer in operation throughout  the entire process.

VAM Data Evaluation 

The signal recording covers the entire  tap switch operation from motor start  to finish [2]. The strongest signal is  obtained during the diverter switch  operation. As a first post-processing  step, a wavelet transformation is  applied to the sensor signal. The  so-called envelope curve is derived  through integration in a typical  frequency range from 10 kHz to  100 kHz and subsequent Gaussian filtering. The resulting curve reflects  the energy of the raw signal in the  defined frequency range. It can be  used to compare reference data such  as factory fingerprints or data from  sister units [4]. Figure 4 illustrates  a sensor’s raw signal and the corresponding envelope curve in dB over time.

Figure 4 shows a detailed view  of the diverter switch operation  (highlighted). Attention should be  paid to shifting peaks and less so  to amplitude changes between the  reference and the actual envelope  curve to identify possible mechanical  changes in OLTC. The switching  time analysis based on the envelope  curve shows a considerably higher  tolerance for external influences,  making it much more suitable for  comparisons.

Combining VAM and DRM

The VAM and DRM measurements of OLTC represent  mechanical and electrical time  sequences respectively, and  provide an opportunity for a  complimentary evaluation. Using  the vibro-acoustic measurement, all OLTC components that produce sufficient vibrations are picked up and can be considered for evaluation. In contrast, a dynamic resistance measurement is only affected by operations that cause a change in the test current. Individual events in  the DRM curve can typically be  associated with the movement of  diverter switch contacts. Thus, this information can map parts of the envelope curve to specific steps in the switching operation, such as opening or closing contacts.

Case Study: Comparison of  Sister Units 
The VAM results of two 410 kV / 27 kV transformers are discussed in the following example. Both units have MR Oiltap® Type G diverter switch on-load tap changers. 

Since fingerprints from the units under test aren’t available, comparing the two sister units is an effective approach for assessing the results. The following evaluation will only focus on the diverter switch operation. 

The comparison of the envelope curves reveals that the vibration pattern of the two sister units matches almost perfectly Figure 9.  

Figure 9 depicts the tap switches from tap one to tap four. The markers indicate the start of the diverter switch operation. Ideally, each tap switch should be compared to its direct counterpart  with the same parity and switching direction. Adding in the DRM data (Figure 10) helps map some of the VAM peaks to specific events like the operation of the main and auxiliary contacts. 

Combining On-Load Tap Changer, Vibro-Acoustic Fingerprinting and Diagnostics with DRM and VAM optimises equipment monitoring in electrical  grids. This innovative approach enhances grid stability and  equipment reliability, contributing to the efficiency and resilience of critical infrastructure.