October 11, 2022

Condition assessment of transformer insulation during routine partial discharge (PD) tests and in-service monitoring

Partial Discharge (PD) measurement is the most common, non-destructive method to detect even minor insulation defects in the electrical insulation system.

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Partial Discharge (PD) measurement is the most common, non-destructive method to detect even minor insulation defects in the electrical insulation system. The use of PD measurements is generally required during type and routine testing [1], and it is recommended for on-site commissioning and in-service dielectric testing of power transformers. The PD measurement results allow users to evaluate the insulation condition and trigger the detection and localization of critical defects [2–4].

Different methods were developed to increase the sensitivity of PD measurements based on different physical characteristics of the PD processes and can be split into conventional [5] and alternative methods. Among them, electromagnetic methods are based on the measurements of PD signals from high frequencies (HF) to ultra-high frequencies (UHF) [6].

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For conventional PD measurements, the apparent charge is measured in pC. It is an integrated current impulse caused by a partial discharge, which flows through the test circuit. It allows a precise calibration. However, it needs a high signal-to-noise ratio (SNR) in the measurement circuit, which is often impossible to attain on site [2–3]. Regarding field-testing, where a very high background noise level is present, alternative methods showed to have an advantage. Measurements performed with UHF and acoustic sensors were proposed for PD detection. Until now, there are no dedicated international standards referring to these methods, as has been the case over the years for the conventional method [5].

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