Dissipation/Power Factor Measurement (Tan Delta) on ...

Dissipation/Power Factor Measurement (Tan Delta) on Instrument Transformers

An insulation failure is often the reason for an instrument transformer (IT) malfunction.

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Factors which possibly accelerate the insulation's aging process:

• Moisture ingression due to broken seals
• Heat caused by the ambient temperature or sunlight

Older insulation can lead to high losses in the IT, which generates further heat. Since ITs are not actively cooled, old or degraded insulation can result in a failure or even an explosion. Fragments from the explosion represent a serious hazard for persons in the substation as well as for surrounding equipment.

Dissipation/power factor (tan(δ) delta) measurements help you determine the insulation condition. When using our testing systems, you will also be able to determine the dissipation factor at various frequencies and detect any signs of aging at an early stage. Thus, an IT can be replaced before any further damage occurs.

Why is Tan Delta so important to assess the insulation of ...

What is Tan Delta and why do we measure it?

The most common technique used to assess the insulation of primary equipment like transformers, rotating machines, cables, and circuit breakers is an AC test.

AC test is used to check the aging of the insulation and mechanical deformation. What kind of electrical insulation is used today for electrical insulation?

• Solid: Cellulose (Paper or Pressboard), cast resin, porcelain
• Liquid: mineral oils, silicone oils, synthetic compounds
• Gaseous: Sulfur Hexafluoride (SF6), air and vacuum, nitrogen

What is an electrical insulation?

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IEEE defines electrical insulation as "material or a combination of suitable non-conducting materials that provide electrical isolation of two parts at different voltages".

The advantage of measuring the Tan Delta is that this method allows you to measure the status of the WHOLE insulation system, not just a material but the electrical insulation as a whole. We need to emphasize this because there are techniques that measure the status of only one insulating material within electrical insulation systems, for example - only oil and dielectric breakdown tests.

That particular test (dielectric breakdown oil test) is also good for its purposes, but the limit of that test is that it gives you only the information of the state of the oil insulation. For a comprehensive understanding, you would want data on the entire integral insulation system, not just an element.

An example of having only one material as insulation is resin cast in dry-type transformers, while an example of complex insulation could be an insulation system of a power transformer, where materials like mineral oil, paper, and pressboard are used to insulate.

Now let us focus on the fact that we talk about two options - first: one material functions as an insulator and second: two or more different materials are used to function as an insulator.

Hence, when measuring the Tan Delta on a Power Transformer you measure the state of the insulation as a whole. What is also important to note here is why these aspects are emphasized. It is because in a Power Transformer - water - which is one of the main causes of insulation failure - usually resides in paper and pressboard (cellulose) and not in oil. Water molecules are more drawn to the cellulose insulation and if you only test the oil to see how much water you have in your insulating system, testing just the oil, will not give you the full information. That is why we do the Tan Delta test. To see the full picture.

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