What is Tan Delta Test ? What are the Different Modes of the Tan Delta Test?

What is the Tan Delta Test?
Tan Delta which is also termed as Dielectric Dissipation or Loss Angle or Power Factor testing method which is performed for testing of insulating oil to know the quality level of the oil. This kind of testing methodology is carried out at two temperature levels. The results that are obtained from the two tests are compared and then consideration is taken in the quality level of the coil. If the test results are good, the oil is continued in service and when the test results are not as expected, then either replacement or change in oil takes place.

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

Variation in the capacitance value, for instance, it 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.

What are the Different Modes of the Tan Delta Test?
When it comes to 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 on the device, then the current measurement is only through blue or red leads. The current flow through 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.

Principle of Tan Delta Test
When a pure insulator has a connection between the earth and the line, then it performs like a capacitor. In an ideal kind of insulator, as the insulating substance functions as a dielectric, which is totally pure, then the passage of current through the material holds only capacitive material. There will be no resistive element for the electric current that is flowing from the line to the earth via insulator as in the insulating component, there will be no presence of impurities.

In a pure capacitive material, the capacitive current precedes the voltage level by 900. As a general, the insulating material is totally pure, and even because of the aging properties of the components, the contaminations such as moisture and dirt might get added. These contaminations create a conductive path for the current. As a result, leakage current that flows from line to earth via the insulator holds resistive elements.

Therefore, it is pointless to claim that, for a good quality of insulator, this resistive element of leakage current is correspondingly minimal. In the other aspect, the behavior of an insulator might be known by the proportion of the resistive element to that of the capacitive element. For good quality of insulator, this proportion is correspondingly less and this is termed as tanδ or tan delta. In a few cases, this is also expressed as a dissipation factor. 
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