Chapter 1

Chapter 1

Maintenance of Transformer Oil

Maintenance of Transformer Oil in Service

1. Transformer Oil

Transformer oil is mineral insulating oil derived from crude petroleum. It is a mixture of various hydrocarbons. It consist partly aliphatic compounds (open chain compounds) with the general formula Cn H2n+2 and Cn H2n. Many oils also contain certain aromatic compounds (closed chain or ring compounds) related to benzene, naphthalene and derivatives of these with aliphatic chains. Good transformer oil must insulate and prevent flash over of the exposed parts within the equipment and it must effectively transform the heat from the core to the radiating surface. The characteristics of new transformer oil is given in I S 335/1983.

There is no single test to judge all the qualities of the oil. Each test is significant within its limits and provides only a general information about the conditions of the oil. If the dielectric strength is low, the oil is unfit for use regardless of any other condition.

2. Tests to determine the qualities of Transformer oil

1) Dielectric Strength

2) Water Content

3) Neutralisation Value

4) Interfacial Tension

5) Dielectric Dissipation Factor

6) Test for Corrosive Sulphur in Oil

7) Test for Oxidation Stability

8) Specific Resistance (Resistivity)

9) Flash Point

10) Pour Point

11) Viscosity

12) Sludge Test

13) Dissolved Gas Analysis (DGA)

Dielectric Strength

The transformer oil under test is subjected to an AC electric field with continuously increasing voltage till the oil breaks down. The test cell shall be of glass or plastic and transparent and non-absorbent (Eg. Methyl Methacrylate vessel) with effective volume between 300 to 500 ml. The electrodes must be Copper, Brass, Bronze or Stainless steel and well polished having spherical shape with dia 12.5 mm to 13 mm. It shall have a spherical front as shown in the figure. The electrodes shall be mounted horizontally in the cell and the axis shall be immersed at a depth of approximately 40 mm. The electrode gap shall be 2.5 + 0.1 mm.

Before carrying out the test, the cell shall be cleaned by rinsing with the test oil. The sampling vessel containing the test oil shall be gently agitated to have homogeneous distribution of impurities and air bubbles to escape. The oil temperature at1he time of testing shall be around 27C. (15C to 35C preferable)

An increasing AC voltage of rated frequency is applied to the electrodes, approximately at the rate of 2 kV / Sec, starting from zero upto the value producing breakdown. The test kit will have provision for automatic switching off of the supply voltage within 0.02 second.

Alternative Shape Electrodes

BDV Test Cell

The test shall be repeated six times on the same cell filling and the arithmetic mean of the six of in results is noted as the electric strength or BDV (Break Down Voltage) of the oil under test.

The test shall be conducted in a dry place free from dust and voltage applied every time after disappearance of any air bubbles. The time intervals shall be five minutes, if the disappearance of air bubble cannot be observed.

2. Sampling of oil

Method of sampling of oil from a transformer may be as specified in I S : 6855/1973.

General precautions: When sampling oil, care shall be taken not to contaminate the oil. Outdoor sampling shall be done only during fair weather conditions. Before use, the container shall be rinsed with the oil being sampled. The operator shall not permit his hands to come into contact with the sample or internal part of the sampling equipment. The sample shall be protected from light radiation during transportation.

3. Handling of oil

Oil shall be handled with utmost care. Oil drums shall be kept under cover. Drums shall be clearly marked to show whether the oil contained is clean or dirty.

Clean oil shall be tested and treated before use. Transfer of oil to transformer shall preferably be through a transformer oil filter plant.

4. Examination of oil in service

The oil in the transformer and tap changer shall be inspected periodically as suggested in the Table below, unless otherwise specified by the suppliers.

If rapid deterioration of oil is observed, the last value may be confirmed by further tests frequently and the fact may be referred to manufacturer for advice.

Table

Sl

No

Characteristics

Equipment

Voltage

Permissible

Value

To be

Reconditioned

To be Reclaimed

Replaced

IS

1

BDV

>170 KV

70 to 170 KV

50 KV

>40 KV

>30 KV

If less than

Permissible

Value

-

6792/1972

2

Specific resistance

-cm at 27C

All

Voltages

Above

10 x 1012

Between

1 x 1012 and

10 x 1012

Below

1 x 1012

6103/1971

3

Water Content

>170 KV