What is Tan delta test and how can it prevent the damages in power system

What is Tan delta

It is a dissipation factor testing also called as loss angle is the method used for testing cables to know the quality of the cable insulation, by this test we could determine the remaining life of the cable and when to replace the cable

The electrical properties of the system can change due to the age and continuous electrical stress, so we need to measure the electrical properties such as capacitance or other properties regularly in order to ensure the operational reliability of H.V insulation systems so by this we can avoid breakdowns

Electrical capacitance and tan delta test system


Tan delta test is needed for

  • HV bushings
  • Power transformers
  • Generators
  • Power capacitors
  • H.V cables

What are water trees and how it will create insulation failure

It is a small tree-shaped channel which is found within the insulation of the cable which is caused by the presence of moisture when there is a presence of an electric field in this tree-shaped moisture channels it will lead to the inception of partial discharge

Thus it will lead to the formation of electrical trees, which will lead to the insulation failure by using tan delta test we could know the extent of water tree damage

How tan-delta tester operates

Tan delta test system includes a power voltage test source, standard capacitor and a set of cables, it uses the principle of three winding differential transformer on high permeability metalcore, which will shield it from electromagnetic and electrostatic influences. It has a built-in battery power null indicator which will make the system suitable for operations in workshop and factories, high voltage switchyard… it is suitable for both ground and ungrounded objects

If the cable insulation is free from defects like water trees and other faults then it has the properties of a perfect capacitor, which is very similar to the parallel plate capacitor

In a perfect capacitor, the voltage and current are 90 degrees phase-shifted and current through the insulation is capacitive, if there are any impurities present in the insulation the resistance of the insulation is decreased and it will result in the flow of resistive current through the insulation. So the current will not be in 90 degrees phase-shifted, the extent which it varies from the normal phase shift will determine the contamination level of the insulation

How is the test performed

The cable which is to be tested must be de-energized and isolated at each end by using a VLF hipot and the test voltage is applied to the cable while the tan delta controller takes the measurement. To move further in testing the applied voltage is increased to a normal line to ground operating voltage, the tan delta measurements at higher voltage are compared to the lower voltage

Difference between insulation and dielectric

Insulation is the ability of the medium to resist the flow of current it is a poor conductor, whereas dielectric is a medium or a material which has specific measurable properties such as dielectric strength, dielectric constant, dielectric loss and power factor.

Advantages of Tan delta test

  • System reliability will be enhanced
  • Apparatus damages could be decreased
  • Enhanced personnel safety
  • Loss of revenue can be minimized