The delta tangent : a precise measurement for monitoring the insulating dans heat-carrying functions of dielectric fluides.

Explained by Martin-Charles KIMBALL-DEJOIE, Dielectric Expert of Eurofins IESPM

Tan delta vs Breakdown voltage

It is customary to evaluate the insulating capacity of a dielectric fluid using its breakdown voltage (see diagram opposite). However, this measurement requires the generating of a short circuit under conditions that are not always repeatable* if the oil is not homogeneous, which is why our experts recommend also using the Delta tangent measurement.

*In the particular case of water pollution, the breakdown voltage will be affected but repeatable.

A simple and accurate measurement

The tan delta is measured over the entire test portion. It allows, thanks to a simple
method, the determination of the precise physicochemical state of the dielectric fluid: its insulating property but also its heat transfer capacity.

Today, this analysis is mainly used for voltage transformers, but this application should be extended to a precise monitoring of oil degradation : it is very feasible and relevant in the case of power transformers.

The process is detailed below.

How is tan Delta measured ?

The delta tangent is also called the “loss factor” or “dielectric dissipation factor”.

To get a reliable result, two measurements are taken and we ensure that their difference does not exceed 15%.

The expression of the delta tangent is accompanied by the measurement of resistivity and permittivity in your analysis reports.

Conclusion 

In terms of transformer maintenance, the delta tangent can be used for predictive purposes. Indeed, the stability of this measurement makes it possible to monitor the condition of a dielectric fluid by detecting, in particular, any deterioration in its insulating capacity.

Expert’s advice

The measurement of the delta tangent is too often used as an additional test in the analysis sequences. However, our experience shows that in the context of regular monitoring, it is recommended to use a Delta tangent measurement in addition to the breakdown voltage.

> Download the file : Minute Lab – Delta tangent (PDF)

This post is also available in: French