Many electrical equipment failures stem from the degradation of insulation over time. Routine testing and proactive maintenance can help predict and prevent such failures. Variations in the normal capacitance of insulation may signal problems like moisture infiltration, short circuits, or open circuits within the capacitance system. Tan delta, also referred to as dissipation factor, and power factor assessments can reveal several conditions related to the insulation of diverse electrical apparatus:

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• Degradation from chemical reactions influenced by time and heat, which can include localized damage from overheating.
• Contamination from water, carbon deposits, dirt, and other harmful substances.
• Severe insulation failure through cracks and surface damage.

Interpreting these measurements relies on expertise, guidance from equipment manufacturers, and observations of the following factors:

• Comparative measurements on the same unit over different time periods.
• Comparative assessments between identical units or similar components tested under similar conditions concurrently, such as several transformers or individual windings from a three-phase transformer.
• Results from different testing levels on the same unit part; significant increases in the slope of the dissipation factor versus voltage curve suggest the initiation of ionization at a specific voltage.

If a dissipation factor exceeds standard values, it could indicate issues as described above. Notably, if the factor fluctuates considerably with voltage until reaching a stable point, ionization is suspected. If this stability is below the operational threshold, ionization can worsen during operation, leading to further insulation degradation. Additionally, capacitance increments (evidenced by increased charging current) may occur above the stabilization voltage due to ionization processes bridging voids.

Moreover, a rising dissipation factor alongside a notable increase in capacitance is often a sign of excessive moisture in the insulation. In contrast, an increase in dissipation factor alone might stem from thermal damage or other contamination sources.

To ensure accurate measurements, it is crucial that components such as bushing and pothead surfaces, terminal boards, etc., are clean and dry. Any leakage along terminal surfaces can contribute to insulation losses and provide misleading readings regarding the insulation's condition.

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Evaluation of Tan Delta Test Kit

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Tan Delta Test Kit Insights

Tan Delta Test Kit Insights

power

(Electrical)

(OP)

20 Jun 06 09:31

1. Can anyone please suggest how to determine the current rating of a Tan-Delta test kit for evaluating a 16.5kV, 260MVA Generator / 400/16.5kV 260MVA transformer (both HT and LT)?

2. Regarding transformer bushing tan-delta testing, why should we avoid applying the rated voltage to the bushing?

Thank you in advance,

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Continue reading for responses below.

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RE: Tan Delta Test Kit Insights

electricpete

(Electrical)

20 Jun 06 10:02

2 '
2A - You are primarily limited by your test set's specifications, commonly at 10kVAC.
2B - It's vital to compare your results with standard tests typically conducted at voltages of 10kV or lower.
2C - It's been some time since I reviewed the testing setup but I recall two variations in test taps. If you can apply full line-to-ground voltage, ensure that you are not overloading parts of the insulation system. Perhaps someone else can clarify this for us.

1 - As I remember, the Doble test set has a limit of 300 milliamps at 10kV test voltage (3kVA). If the tested device draws more current than the test set can handle, you may need to lower the test voltage to comply with the test set's limits.2 ' 2A - Given the constraints of your test set, it's usually 10kVAC.2B - It's essential to align your findings with standard test benchmarks, which are generally below 10kV.2C - It's been quite some time since I examined the connections for testing. I remember two varieties. If you can indeed apply the full voltage, take care not to push the insulation system too hard. Perhaps another knowledgeable person can better assist on this.

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RE: Tan Delta Test Kit Insights

edison123

(Electrical)

21 Jun 06 05:42

I utilize 12 kV inductors when connected in parallel with the generator/motor windings, enabling my 100 mA TD kit to reach up to 3 Amps.

I employ 12 kV inductors in parallel with the generator/motor windings to boost my 100 mA TD kit to 3 Amps.

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RE: Tan Delta Test Kit Insights

power

(Electrical)

(OP)

21 Jun 06 08:03

The first question is clarified.
Let's address the second query.
Appreciate it...

RE: Tan Delta Test Kit Insights

prc

(Electrical)

22 Jun 06 02:47

No problem. You can indeed apply the full rated voltage to the bushing and measure the tan delta at that level. However, you would need a standard capacitor rated for that voltage to perform the tan delta calculation. The Doble kit typically includes a 10 kV capacitor for measurements conducted at that standard. In fact, manufacturers of bushings measure tan delta at the rated voltage as well as at full test voltage.

RE: Tan Delta Test Kit Insights

power

(Electrical)

(OP)

23 Jun 06 09:09

Thank you, Mr. PRC. I've noted on various platforms that the Doble test for HV bushings is often performed at 2.5kV, although the Doble kit supports testing up to 10kV. Is there a specific rationale for this?

RE: Tan Delta Test Kit Insights

prc

(Electrical)

24 Jun 06 01:01

The Doble kit is equipped with a 12 kV power supply, which dictates that site tests are performed at lower voltages. If higher testing voltages are necessary, additional equipment like a standard capacitor and supplementary accessories from Doble will be required.
The choice of 2.5 kV may relate to the availability of benchmark test data at that voltage. Certain utilities have established testing standards around 10 kV.

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