The importance of partial discharge testing on power transformers

September 1, 2017

Reliably assesses insulation quality throughout development and operation

Ulrike Broniecki is specialized in the area of partial discharge testing and monitoring at OMICRON. As both an applications engineer and OMICRON Academy instructor, she helps our customers all over the world with performing PD measurements as well as analyzing and interpreting the data. We recently asked her to tell us why PD measurement is so important to ensure the safe and reliable operation of power transformers.

What types of diagnostic measurements are normally performed on power transformers and why?

Ulrike Broniecki: Power transformers are exposed to intense stresses from various sources throughout their very long service lives. This damages the electric insulation, which is very important for the safe and reliable transformer operation. A number of dielectric diagnostic methods make a crucial contribution to quality assurance and to maintaining the operation safety and reliability of power transformers, since they provide evidence about changes in the condition of the insulation. These methods typically include dielectric response measurement, frequency response analysis, insulation resistance measurement, capacitance and dissipation factor measurements and partial discharge measurement, among others.

Which of these diagnostic methods do you consider to be the most important?

Ulrike Broniecki: Compared with other dielectric diagnostic methods, the PD mea¬surement provides test engineers with very sensitive information to help them to effectively detect even the smallest weak points in the insulation system. PD can damage insulation materials in power transformer bushings and windings, leading to their eventual failure and costly outages. Therefore it is important to recognize the PD source, find it, and eliminate it when necessary.

When should partial discharge be tested on power transformers?

Ulrike Broniecki: PD measurement is a reliable and non-intrusive method that can be used anytime to diagnose the insulation condition of power transformers. It is performed to ensure quality assurance during factory acceptance testing and on-site commissioning. Once the transformer is in operation, PD activity can be evaluated over time during routine maintenance testing to make informed strategic decisions regarding the timely repair or replacement of affected components before an unexpected and costly outage occurs.

What are the challenges in measuring partial discharge?

Ulrike Broniecki: Since signals emitted from PD activity may be of low magnitude, it is crucial to use highly sensitive PD measurement and analysis equipment. This however results in a higher susceptibility to interference from electronic noise. Outside of screened laboratories, PD signals are very often superposed by noise pulses, which makes a PD data analysis more difficult for both experts and software systems. Therefore the proper minimization of disturbances is one of the main tasks when measuring PD in the field.

How are these measurement challenges best dealt with?

Ulrike Broniecki: Our MPD 600 PD measurement and analysis system allows very high sensitivity detection and measurement, using several methods of electrical noise suppression in challenging on-site conditions. For example, with the freely-selectable filtering options, the center frequency and bandwidth can be adjusted to achieve a high signal-to-noise ratio and low background noise level for reliable analysis.

When using three or more MPD 600 acquisition units, a fully digital, synchronous multi-channel PD measurement is ensured. This not only minimizes the time for which high voltage has to be applied during off-line testing and speeds up measurement time, it also enables the use of our unique separation tools, such as 3PARD (3-Phase Amplitude Relation Diagram). PD signals originating from sources of different type or location appear in separate parts of the 3PARD and can be analyzed individually.

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“PD measurement is a reliable and non-intrusive method that can be used anytime to diagnose the insulation condition of power transformers.”

(Ulrike Broniecki, Applications Engineer, OMICRON Energy Solutions GmbH)

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