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What is principle of tan delta? What are its various Modes.

Updated: Jun 25, 2023

What is the Tan Delta Test?

A pure insulator when is connected across line and earth behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have a capacitive component. There is no resistive component of the current, flowing from line to earth through the insulator as in ideal insulating material, there is zero per cent impurity.

In a pure capacitor, the capacitive electric current leads the applied voltage by 90o.

In practice, the insulator cannot be made 100% pure. Also due to the aging of insulators, the impurities like dirt and moisture enter into them. These impurities provide the conductive path to the current. Consequently, an electric leakage current flowing from line to earth through the insulator has a resistive component.


Hence, it is needless to say that, for a good insulator, this resistive component of the electric leakage current is quite low. In another way, the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. For a good insulator, this ratio would be quite low. This ratio is commonly known as tanδ or tan delta.

Sometimes it is also referred to as the dissipation factor.



Thus, tan δ = IR/ IC

NB: This δ angle is known as the loss angle.



On which instruments Tan Delta testing can be done?


Tan δ testing can be done on various power types of equipment used in substations like transformers, winding, current transformer, potential transformer, transformer bushing, cables, generators. It is performed to assess the quality of insulation and is performed in combination with various test likeTTR, WRM, etc.


Reasons for doing Tan δ testing?


The main purpose of the tan delta test is to make sure of maintaining a secure and reliable functioning of the transformer. The calculation of dissipation factor and capacitance values provides the result of insulation behavior of bushings and in windings too.


Variation in the capacitance value, for instance, indicates partial kind of breakdowns in bushings and automated movement of windings. Insulation deprivation, aging of the equipment, enhancement in the energy levels is transformed into heat. The amount of losses in these is calculated as the dissipation factor.

With the tan delta testing method, one can easily know the dissipation factor and the capacitance values at the required level of frequencies. So, any kind of aging factor can be identified earlier and the corresponding action can be implemented.

As we know that “Transformers” plays a very crucial part in power, so, first of all, will discuss Tan Delta Testing in Transformers.


KPM's Tan Delta Test Set

Tan Delta Testing Process

The below process explains the method of tan delta testing in a step-by-step manner


  • The requirements necessary for this test such as cable, potential transformer, bushings, current transformer, and winding on which this testing is conducted has to be initially separated from the system.

  • The minimal frequency level of test voltage is applied along with the equipment where the insulation to be analyzed.

  • At first, normal voltage levels are applied. When the tan delta values are as expected at this voltage level, then the applied voltage level is increased by 2 times as of applied voltage.

  • The values of the tan delta are recorded by the tan delta controller.

  • To the tan delta calculating component, a loss angle analyzer is connected which compares tan delta values at higher and general voltage levels and delivers accurate results.

It has to be noted that the testing procedure to be carried out at very minimal frequency levels.


It is more recommended to conduct testing at minimal frequency levels, because when the applied voltage level is more, then the capacitive reactance of the insulator device reaches very minimal, therefore the capacitive element of the current reaches more. As the resistive element is practically constant; it is based on the applied voltage level and the insulator’s conductivity value.

Whereas at increased frequency level the capacitive current, is more, and then the amplitude of the vector amount of both the capacitive and resistive elements of the current reaches very high. So, the necessary level of power for the tan delta test would become more that seems to be not acceptable. Because of this, the power constraint for dissipation factor analysis, very minimal frequency test voltage is required.


KPM SC 100pf is a standard capacitor for calibration

What are the Different Modes of the Tan Delta Test?

When it comes to the tan delta test, there are essentially three modes of power factor testing. Those are


  • GST Guard – This calculates the amount of current leakage to the ground. This method eliminates the current leakage through red or blue leads. Whereas in UST, the ground is termed to be guard because grounded edges are not calculated. When the UST method is applied to the device, then the current measurement is only through blue or red leads. The current flow through the ground lead gets automatically bypassed to the AC source and thus excluded from the calculation.

  • UST Mode – This is employed for the calculation of insulation in between ungrounded leads of the equipment. Here the individual portion of isolation has to be separated and analyze it having no other insulation connected to it.

  • GST Mode – In this final mode of operation, both the leakage pathways are calculated by the test apparatus. The current, capacitance values, UST, and GST guards, loss in watts need to be equal to the GST test parameters. This provides the entire behavior of the test.

When the summing value of GST Guard and UST is not equal to the GST parameters, then it can be known that there is some crashing in the test set, or might the test terminal are not correctly designed.


On the whole, this is a detailed explanation of the Tan Delta Test. Here, in this article, we are completely aware of what is a tan delta test, its principle, its purpose, its methods, and its testing technique.

Also, know about what are LV to earth test, HV to earth test, and LV-HV tan delta testing methodologies and keep reading KPM Technologies “TECH BLOG” and subscribe for different technological development in the field of power sector/ testing sector/ etc







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