Before discussing about Tan Delta Test, we will find out what is Tan delta and before that we will find out what is Delta, which is also called as Loss Angle.

### What is Leakage Current in the Insulator?

A Ideal/Pure Insulator connected accross Live Portion and Earth, acts as a Capacitor and so for an Ideal Insulator, the Current passing through the Insulator (Leakage Current) has only Capacitive Component.

However, Insulator cannot be made 100% pure. Also due to the ageing of the insulator, impurities like dirt and moisture enter into it which provide the conductive path to the current. And so the Leakage Current flowing through the insulator has also a resistive component.

For an Ideal Insulator, the Leakage current current is the Capacitive Current, which leads the Applied voltage by 90 deg. But, now we know that, there is also a Resictive Component in the Leakage Current, and so the Resultant Leakage Current is the Vector sum of the Capacitive Current (Ic) and the Resistive Current (Ir).

### What is the Loss Angle?

We have seen that due to the presense of the Impurities in the Insulator, there is a Resistive Component in the Leakage Current, which introduces an Angle called as  Loss Angle or Delta between Capacitive Component and the Resultant Leakage current.

As the Impurities in the Insulator increase over the period of time, the Resistive Component of the Leakage Current increases, which inturn increases, the Loss Angle or the Angle Delta.

### What is the Tan Delta?

It is the Ratio of the Resistive Component to the Capacitive Component of the Leakage Current. It should be minimum and ideally equal to zero.

### What are different Modes of Tan Delta Test ?

UST
Test set connected for Ungrounded Specimen Test mode. This is used when specimen is isolated from earth e.g. Transformer bushing, CTs with test tap, CVTs and CB voltage grading capacitors. The test mode is often used to reduce the effect of stray capacitance losses to ground, and to reduce the effect of interference pickup from energized apparatus.
GST
Test set connected for grounded specimen test mode. This is used when specimen do not have two specific points (isolated from ground) for Tan delta measurement e.g.Transformer/Reactor winding, CTs without test tap etc.
GSTg:

Where The Guard (g) is used for measuring the Capacitance between Transformer Winding and Earth.

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