Transformer testing

Industry insight: Hussain Al-Juffairi, application engineer at Megger, specialising in power transformer diagnostics and testing, explains how reliability of your power network can be enhanced.

Transformers are one of the most mission critical components in the electrical grid. The need for reliable diagnostic methods drives the world’s leading experts to evaluate new technologies that improve reliability and optimise the use of the power network.

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Insulation Diagnostics
Modern technology and developments in signal acquisition and analysis techniques have provided new tools for transformer diagnostics. Of particular interest are dielectric response measurements where insulation properties of oil-paper systems can be investigated.

Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than a decade ago and has been thoroughly evaluated in a number of research projects and field tests with good results.

DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for moisture assessment.

Since the modeling theory contains influence of temperature, DFR and modeling can be used to calculate the temperature dependence of the insulation system.

Winding resistance
Measuring a transformer’s DC resistance from one external terminal to another can reveal a great deal of information about the transformer. In addition to the obvious faulted winding (i.e., an open winding or shorted turn), more subtle problems can be detected.

The DC current, in addition to flowing through the winding, must also flow through the off-load ratio adjusting switch (RA switch), the on-load ratio adjusting switch (load tap changer or LTC), as well as numerous welded and mechanical connections. Hence, the integrity of all these components can be verified.