HV Transformer Oil - Testing

Our plant (in Scotland, UK) has 10, 11KV transformers

We wish to have the transformer oil tested.

What I would like to know is:

What is the recommended test interval for a transformer ? (I have had various, conflicting answers from contractors)

What tests are necessary? We have been told DGA, & PCB testing is required.

Is there a contractor in Scotland that anyone can recommend to us ?



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To give a sensible answer it would help to know :-

(a) the size of the transformers

(b) the arrangements for the transformer to breath ie Do they have conservators or does the air above the oil simple connect to the outside atmosphere by a pipe ?

(c) How heavily loaded are the transformers ?


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John Rye

We are a US contractor that performs HV maintenance for companies. We use the NETA testing standards. NETA has a testing frequency matrix based on the type of equipment and the reliability required.Transformer oil sampling has a base frequency of 12 months . Now depending on how reliable and the condition of the equipment multiply this number by a number in the range of .25 to 2.5. Most industrial plants use the number 2.5 which means the sampling should be performed every 30 months. But depending on your equipment condition and reliability requirements you may want to use a different multiplier.

NETA recommends performing the following tests of the oil sample:

Dielectric breakdown voltage - This determines the dielectric breakdown voltage. There are two standards for this test ASTM D877-87 and ASTM D1816-84a. If ASTM D877-87 is performed the minimum nuber should be

26KV for equipment under 69KV. If ASTM D1816-84a is performed the number should be 23KV for equipment under 69KV.

Acid neutralization number - This number is an indication of sludge buildup in the transformer. New oil should have a number of 0.03. This number should not exceed 0.2.

Specific Gravity(Optional) - This is the ratio of oil to water. Water has a specific gravity of 1, oil should have a specific gravity of 0.84

- 0.91.

Interfacial Tension - This is an indication of the remaining useful life of the oil. As the ACid neutralization number goes up this number goes down. New oil in new equipment should have a number of 35 dynes/cm, min. This number should not be below 24 dynes/cm, min.

Color -This is an indication of oil contamination. Ths numbers range 0

- 8.5. New oil (Clear) is 0 - 0.5. Sevice aged oil (Yellow) is 1.0 -

2.5 and Marginal oil (Bright Yellow) is 2.5 -4.0.

Visual Condition - This test is to determine if more testing is required. The oil should be sparkling, bright and clear. If not you may want to perform other testing depending on the appearance.

Water in insulationing fluids(Required on 25KV or High Voltages) - Moisture greatly affects the paper insulation in a transformer. So it is important to know the moisture content in the transformer. These numbers are an indication of the amount of moisture in the insulation. Dry insulation will have a number of 0 - 5, moderate to wet insulation will have a number of 6 - 20.

Measure Power Factor or Dissipation Factor (Optional) - Power Factor test is the indication of power loss through the oil. New oil in new equipment (Equipment rated under 69KV) should have a %Power factor at

25C of 0.15. This number should not exceed 0.5.

All this information comes from the NETA testing standard and IEEE

62-1995 standard.

Mike McCowan Industrial Automated Systems Davenport, IA

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these guys are national, the local sales guy can take the info and prepare a quote.

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Please Reffer IEC 60076 to sort out your problum.

woodglass wrote:

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Apart from the very good recommendations already expressed, I would also refer to the manufacturers O+M manuals.

In the UK you can try

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They are very experienced in all kinds of transformer and switchgear maintenance and would be more than willing to help.

HTH Euclid

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