Measurement, localisation, and monitoring of partial discharges on a power transformer
PD is a local electrical breakdown of a weak region within the electrical insulation system, resulting in fast current impulses.
byUdo Ranninger and Michael Krüger
Introduction
Power transformers, a vital element of the electrical grid, are subjected to different levels of electrical, thermal, mechanical, and chemical stress during service. To ensure reliable and safe operation, it is critical to continuously assess the ageing of the system insulation during a transformer’s life cycle. Partial discharge (PD) measurements are a non-destructive tool which allows for measurement, assessment, and localisation of weak spots in complex insulation systems. PD measurements on power transformers are typically carried out during the manufacturing process as a part of quality assurance, after onsite installation, and are used as a tool for condition-based maintenance for matured assets.
PD is a local electrical breakdown of a weak region within the electrical insulation system, resulting in fast current impulses. These electrical signals are often accompanied by other physical effects, such as pressure waves, electromagnetic signals, chemical effects or optical effects [1]. PD measurements of the different effects using conventional and unconventional tools and combining the findings will lead to a more meaningful assessment.
Case study – PD measurement on a 300 MVA oil-filled transformer
The high-voltage (HV) bushings of the 220 kV and the 110 kV windings had to be disassembled for transportation of a 300 MVA transformer. After mounting the bushings at the new substation, the bushing domes had to be refilled with oil. Due to the horizontal-oriented bushings, this had to be done very carefully to avoid gas bubbles. To ensure a proper filling, partial discharge (PD) measurements have been performed.
Fig. 1 shows the 300 MVA transformer with the horizontal 220 kV and 110 kV bushings and a small step-up transformer with 24 / 0.4 kV for exciting the 300 MVA transformer with a diesel-powered generator.
Electrical PD measurement
The test setup was performed in accordance with IEC 60270 [2], simultaneously decoupling the PD and AC signals at the measuring taps of all 220 kV and 110 kV bushings. Fig. 2 shows the setup of the MPD 800 PD detection instrument at the 220 kV bushings 1U and 1V. The signals of both bushing taps can be directly connected to one MPD 800 detector without using an additional coupling device. Fig. 3 shows the overall PD test setup.
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