What should be the limit of tan delta value for 10MVA

The document discusses the standard limit for the tan delta value of a 10MVA, 22kV/11kV transformer. Several respondents provide information on standard limits: - FIST 3-30 specifies a limit…

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The #Tandelta test is a diagnostic tool used to assess the condition of insulation in #electrical assets. It provides real-time information about any irregularities/issues in the insulation condition. Carrying out a test is essential to measure insulation quality, enabling a maintenance engineer to decide how best to proceed with their equipment. 

Accuracy and meaningful results are vital when it comes to conducting a tan delta test, so it is essential to follow all the necessary steps in their correct order. Otherwise, you could be missing out on important information. 

Performing a tan delta test on short-length #cables can lead to high tan delta values that are not actual values; these high values are usually due to a false leakage current. In this article, I share my experience of performing a tan delta test using a #VLF source on short-length cables, and how we corrected tan delta readings by taking into account the false leakage current effect using a guard circuit connection.

This was a routine test performed during the maintenance period and during this opportunity, the effectiveness of the guard circuit in short-length cables was verified.

Cables Tested layout

Cable Details - 11kV, Single Core, 500 Sq. mm., 2003 installation, Length 44 meters, per phase 4 cables.

HVA 28TD by b2 electronics gmbh

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The instrument used - is HVA 28TD, Make b2 electronics GmbH , Austria.

Tan delta reading - without guard circuit

Test Setup for tan delta testing using HVA28TD, without guard circuit

Results

Tan delta values, with our guard circuit

Tan Delta reading - with single side guarding 

Results

Tan delta values improved by 47 %

Tan Delta reading - with both sides guarding 

Guarding is done at the far end

Results

Tan delta values improved by 90%

The actual insulation condition of the cable was finally identified. If we hadn't, it would've led us to incorrect conclusions.

Conclusion

1. For very short cables with a cable length below 100 m, the use of the guard on the far end as well as on the near end is recommended.
2. This is possible for three-phase systems or systems where there is a second connection from the far end to the near end.
3. For voltages of 15 kV and above, it is recommended to use corona shields for TD diagnostics.
4. Double-check that the cable shield and guard aren't connected in any way.
5. It is an effective method to assess the condition of short-length cables and helps to make the right decision.

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