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

What is the Tan Delta Test?

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A pure insulator connected across line and earth behaves like a capacitor. In an ideal insulator, the insulating material acts as a dielectric that is 100% pure. Hence, the electric current passing through the insulator will have only a capacitive component with no resistive component, leading to zero impurities.

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

In practice, insulators cannot be manufactured to be 100% pure. As insulators age, impurities like dirt and moisture infiltrate them, creating a conductive path for the current. Consequently, an electric leakage current flows from line to earth through the insulator, incorporating a resistive component.

Therefore, the resistive component of electric leakage current is relatively low for a good insulator. The health of an electrical insulator can be assessed by the ratio of the resistive component to the capacitive component, commonly referred to as tan delta (tan δ). A good insulator will demonstrate a low ratio.

Sometimes, it is 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 performed on various types of power equipment used in substations such as transformers, windings, current transformers, potential transformers, transformer bushings, cables, and generators. This testing is conducted to assess the quality of insulation and is usually combined with tests like TTR and WRM.

Reasons for doing Tan δ testing?

The primary goal of the tan delta test is to ensure the reliable functioning of transformers. Calculating the dissipation factor and capacitance values gives insights into the insulation behavior of bushings and windings.

Variations in capacitance values can indicate partial breakdowns in bushings and movements within windings. Aging of equipment and diminished insulation lead to increased energy levels, resulting in heat. The losses are determined as the dissipation factor.

With the tan delta testing method, the dissipation factor and capacitance values can be monitored at required frequency levels. This allows early detection of aging issues, enabling timely interventions.

Tan Delta Testing in Transformers

Tan Delta Testing Process

The following steps outline the tan delta testing method:

• Separate the cable, potential transformer, bushings, current transformer, and windings from the system for the test.

• Apply the minimal frequency level of test voltage to the equipment whose insulation needs analysis.

• Initially, apply normal voltage levels. If the tan delta values are satisfactory at this level, increase the voltage by 2 times.

• Record the tan delta values using the controller.

• Connect a loss angle analyzer to the tan delta calculation component to compare results at higher and normal voltage levels for accurate outcomes.

It is important to conduct the testing at very minimal frequency levels.

Testing at minimal frequency levels is recommended because higher applied voltage can reduce the capacitive reactance of the insulator device significantly. The result is that the capacitive component of current becomes larger. As the resistive component remains relatively constant, it depends on both the applied voltage level and the insulator's conductivity.

At increased frequency levels, the capacitive current is heightened, resulting in higher amplitudes for both the capacitive and resistive components. Therefore, the necessary power for the tan delta test increases, which can be unacceptable. Thus, to analyze the dissipation factor, a minimal frequency test voltage is required.

What are the Different Modes of the Tan Delta Test?

The tan delta test comprises three primary modes of power factor testing:

• GST Guard: This mode calculates the leakage current to the ground. It eliminates current leakage through red or blue leads. In comparison, in UST, the ground is considered a guard because it does not account for grounded edges. When using the UST method, the current measurement only goes through the blue or red leads, bypassing the ground lead to the AC source.

• UST Mode: This mode emphasizes insulation calculation between ungrounded leads of the equipment. Each isolation segment needs to be detached for separate analysis without other insulation connections.

• GST Mode: In this mode, both leakage pathways are assessed by the test apparatus. The current, capacitance values, UST, and GST guards together with power losses in watts should conform to the GST test parameters, giving a complete picture of the test behavior.

If the sum of GST Guard and UST does not match the GST parameters, it suggests possible issues with the test set or that test terminals may not have been designed correctly.

In summary, this article provides a comprehensive overview of the Tan Delta Test, discussing its principles, purposes, methods, and testing techniques.

We also encourage you to learn about LV to earth tests, HV to earth tests, and LV-HV tan delta testing methodologies. Keep reading KPM Technologies' TECH BLOG for updates on technological developments in the power and testing sectors.