Tan Delta measurements are used to provide diagnostic information about the condition of the dielectric of the test specimen.

A VLF Test set is used to apply a test voltage to the Device Under Test (DUT) and the corresponding voltages and currents that flow are accurately measured. The loss angle is measured for a range of applied test voltages and a TD value is calculated.

The measured TD value should be within an acceptable range for all test voltages depending on the construction and the age of the cable. In addition the measured TD should be consistent for the different measurements and for a range of test voltages. Any significant change at higher test voltages may indicate the onset of partial discharge activity and the cable should be subject to further investigation.

Tan Delta diagnostic measurements offer a relatively cheap and very quick and unambiguous way to determine the health of a cable. Tan Delta values taken over a period of time on the same cable can present valuable indications of the deterioration of a cable.

Tan Delta values are affected by the presence of water trees in equipment and can therefore be used to detect these before they convert into electrical trees.

Tan Delta values can be used to grade the condition of cables in order to prioritise replacement. Tan Delta values can be used in conjunction with a withstand test to mitigate the possibility of Failure On Test (FOT) should this be inconvenient to the operator.

Standards that allow the use of Tan Delta include :

• IEEE 400-2

There is a great deal of research being conducted by, for example the Cable Diagnostic Focussed Initiative (CDFI) of Neetrac which is expanding the application of these technologies.