Power Transformer Oil Insulation Deterioration

Monitor the condition of insulating oil is a proactive means of detecting equipment problems in the early stages. However insulating oil can begin to deteriorate through relatively normal operating conditions and become a potential hazard to the proper operation of equipment.

This bulletin outline 4 common tests which are used to determine the degree of deterioration of insulating oil. These tests are Dielectric strength, neutralization number (acidity), resistivety and water content. The following section provides information regarding the IEC recommended values for each test and recommended test frequencies.

Information

Dielectric Strength Testing

The absorption of moisture by transformer insulating oil has a direct bearing on the transformer's insulation performance. Moisture content is normally checked by measuring the dielectric strength. It can also be checked by measuring an oil sample's resistively, tan 8, or actual water content.

The measurement of dielectric strength should be made with an oil tester rated according to the oil to be measured and using IEC electrodes. The following table provides the allowable KV ratings for dielectric testing. The frequency recommended for these tests by the IEC are as follows:
            - After filling or refilling,
            - Prior to energizing,
            - Then after 12 months and subsequently every 2 years after that,
            - Tap changer neutral point every 6 years.

Category of Transformer	Limits
> 420 Kv	> 50 Kv/2.5 mm
> 170 Kv up to 420 Kv	> 50 Kv/2.5 mm
> 72.5 Kv up to 170Kv	> 40 Kv/2.5 mm
Up to 72.5 Kv	> 30 Kv/2.5 mm
Neutral point of tapchanger	> 25 Kv/2.5 mm

Neutralization number (Acidity) Testing

The neutralization number is important because the level of acidity in the oil determines the rate of sludge buildup, -and sludge hinders cooling efficiency. Less than 0.2 inhibits sludge formation, 0.5 allows sludge buildup, and more than 0.5 accelerates sludge development.

According to IEC; for all transformers the neutralization number must not be greater than 0.5 mg Koh/g, at this point the oil would have to be processed. The frequency for this test is every 6 years as long as the test results do not exceed 0.3 mg Koh/g. Readings above this amount indicate the oil must be monitored closer and tested more often. The table below is an example taken from a Hitachi transformer manual and follows the guide set by IEC.

Neutralization number	Evaluation
less than 0.2	Good
0.2 to 0.5	Acceptable, but oil should be replaced soon
More than 0.5	Bad. Oil should be replaced immediately

Resistivity Testing

The resistively of the insulating oil is directly related to the insulation performance of the transformer. Resistively, such as insulation resistance, decreases with transformer temperature.

IEC recommends the following frequencies for conducting resistivity tests:

• After filling and refilling,
• Prior to energizing,
• Then after 12 months and subsequently every 6 years after that.

The following table provides the IEC acceptable limit for resistivity  

Category Of Transformer	Limits Ohms-cm
	20 degree C	90 degrees C
420 Kv	> 2 x 10 13	> 1 x 10 11
170Kv up to 420Kv	> 2 x 10 13	> 1 x 10 11
72.5Kv up to 170Kv	> 6 x 10 12	> 2 x 10 11

Testing For Water Content

            Measure the water content of the oil by using the Karl Fischer method

• Note: The Karl Fischer method detects the existence of water in the organic solvent. It calls for titration of the sample liquid by dissolving the sample oil in absolute methyl alcohol, then introducing the dissolved sample by measured amounts into the Karl Fischer reagent, which is a solution of iodine, sulfur dioxide, pyridine, and absolute methyl alcohol with a reddish brown color. The color of the reagent becomes light yellow if water exists in the sample liquid. The end point is found visually or by potentiometric titration.

The table below provides the IEC limits for water content which are only applicable where the acidity values are > 0.1 mg KOH/g.

Category Of Transformer	Limits	Frequency of Tests
> 420 Kv > 170 Kv up to 420 Kv	< 20 mg/Kg	After filling or refilling, Prior to energizing, Then after 12 months, subsequently in conjunction with dissolved gas analysis testing.
> 72.5 Kv up to 170 Kv	< 20 mg/Kg	After filling or refilling, Prior to energizing, Then after 12 months, subsequently every 6 years or in conjunction with dissolved gas analysis testing.
Up to 72.5 Kv	No free moisture at room temperature	Only when breakdown voltage approaches the rejection level.