Acceptable maximum Tan delta value for new distribution transformers 3

[Rwelagedara]

Dear All,
What shall be the acceptable tan delta values as power frequency and 20 degrees of Celsius for new small distribution transformers.
For larger power transformers there are specified maximum value of 0.5%. But when we are doing the testing in many number of small distribution transformers from many manufacturers, we are typically getting values over 0.5%.Is this normal?These small transformers are oil filled type and rated 33 kV/400 V and with porcelain type bushings.Is there any standard or guideline setting the maximum value for tan delta for these.

 

[argotier]

First of all, there is no consensus for the tan delta (or power factor) limits in transformers.

The 0.5% limit for power transformers is not stated in the transformers "design" standards, but in the ones related to field testing and maintenance:

- ANSI/IEEE C57.152 IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors recommends for new transformers PF < 0.5% and between 0.5% and 1% worthy of further inspection/analysis.

- CIGRE 445 Guide for Transformer Maintenance it only states that a typical value is 0.5% and higher than 1% is questionable.

The only reference for distribution transformers I've found is this (frankly I didn't know this existed till now): ANSI/NETA MTS Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems

In that standard the maximum value (for maintenance test) for oil filled power transformers is 1% and for oil filled distribution transformers is 2%.

Hope it helps.

 

[prc]

As per CIGRE TB761-2019 Condition Assessment of Transformers limit for new units is 0.5 % for oil filled units and 1 % for natural ester filled units. Limit for keeping in service is 2.5 % and 4 % respectively.