The oil deterioration of insulating oils is generally due to oxidation, especially under
conditions of prolonged high temperature operation. The acid and sludge formation proceeds slowing with operating temperatures but the rate is considerably increased if the temperature is allowed to rise above 75 o C. Experience has shown that the rate of chemical reactions is almost doubled for every 8o to 10o rise in oil temperature above 75 o C. The presence of metals, such as copper and iron, accelerates oxidation. Contamination with water and other foreign matter including varnishes, besides reducing dielectric strength of oil, promotes it's oxidation. The rate of deterioration is, therefore, affected by design of equipment by the manufacturer and the operating conditions, particularly the load cycle, ambient temperature and other atmospheric conditions.
Oxidation:
This is probably the worst type of deterioration due to the formation of acids, sludge and
water, which accompanies the chemical change. Oxygen makes contact with the oil or oil vapor in the space between the oil level and tank cover. Due to the need for breathing, a
certain amount of interchange of air with conservator takes place which lowers the area of oil surface in contact with the air and also lowers the temperature of the oil at the point of contact. Other suggested methods of reducing or entirely preventing oxidation are the use of air below type conservator in which no air is in contact with oil. Other factors affecting the oxidation are the catalytic effects of metal surfaces, such as iron and copper. The presence of electric stress, mixture of oil vapor and oxygen is also thought to have a detrimental effect as pointed out by some investigators.
Sludge Formation:
a. Sludge formation is a serious form of deterioration because it is very difficult to clean a transformer with sludge deposits without dismantling it. However, sever sludge
formation is not frequently experienced in transformer oil. Sometimes rust, scale or
dust having similar appearance may be taken for sludge.
b. If the solid material is identified as sludge (although the acidity is comparatively low),
its presence may be accounted for incomplete cleaning of the transformer following
the use of an oil which has deteriorated to the point of sludge deposition.
c. The absence of visible solid deposit does not eliminate the possibility of the existence
of sludge.
d. The appearance of precipitated sludge in a transformer needs immediate action. In bad
cases, the oil should be changed and the transformer should be cleaned immediately
after the oil has been removed to prevent hardening of sludge.
Dielectric strength:
The dielectric strength does not give a true indication of the worthiness of oil. Oil, which is
highly deteriorated, will give high dielectric strength when dry .The dielectric strength is also affected by the presence of foreign particles, such as cellulose fibers, which absorbs moisture and orient themselves across point of trees and cause arcing. The presence of oil Deterioration particles, water, and foreign contaminates results in a general overall reduction in the efficiency of the apparatus. Normal method of oil purification only maintains the dielectric strength but do not improve the deteriorated condition of the oil. It is, therefore, not advisable to rely solely on the dielectric strength of the oil by periodic test without verifying it's chemical composition
Acidity:
High acidity may not affect the dielectric strength of oil materially and there may be little or no sludge formation, but the complex petroleum acids developed in service may attack the insulations and copper of the windings. Before corrosion in a transformer is likely to take place, the presence of moisture is almost inevitable in preliminary condition, irrespective of the acidity of the oil. It is, therefore, important that both (soluble and insoluble) water should be removed which can be accomplished by vacuum dehydration.
Water soluble acids are some of the worst by products of hot oil oxidation. They result in
metals, such as the transformer tank and lid, being attacked while insulation is also
weakened. In the case of large transformers, periodical test of acidity should be carried out.
The analysis of record so maintained would indicate any dangerous increase in their values.
Acids value of 0.2 to 0.3 milligrams of KOH per gram of oil should not give trouble (unless
reached in a very short time) and will not indicate rapid deterioration of the oil. Acid value as high as 1.0 milligram of KOH per gram of oil, however, shows that the life of the oil may be limited and careful watch for change in acidity should be kept. It is unwise to allow high acid values to develop as the transformer core and windings then become contaminated to such a degree that even the most careful cleaning and washing out with new oil will not remove all traces of acidity. A new filling of oil would then tend to deteriorate at an increased rate with new, correspondingly, shortened life. Some transformers may, sometime, get badly affected that rewinding and rebuilding of the cores become necessary.
Arcing under Oil:
Any burning or arcing under oil will produce carbon particles. which may settle on the
winding surfaces may produce of low resistance path. Burning will also give rise to Cracking of the oil, which produces little more volatile members of the same group of compounds. The presence of free acetylene, as might collect in the chamber of a Buchholz Relay would indicate cracking of the oil In view of the above, the testing of oil for a acidity, sludge and dielectric strength so as to
find out, if it has deteriorated so that corrective action can be taken, which is utmost
important.
SAMPLING OIL FORTESTING:
The dielectric strength (break down voltage) of oil is greatly affected by the minute traces of
certain impurities, particularly moisture. It is, therefore, very important, that the samples be
handled with utmost care to avoid contamination.
a. Samples should be normally taken on dry and clean day so as to avoid contamination
of the oil with moisture.
b. Samples shall be taken from a tank only after it has been allowed to settle for a
minimum of sixteen hours. If the tank is in the open samples may be taken in dry
weather only.
c. It is desired to obtain a sample of that part of the oil in a tank which is in the worst
condition, since water is heavier than oil, the sample must be taken from the bottom
of the tank.
d. Draw off enough oil trough a sampling valve to ensure that the sample will be
composed of oil from the tank and not from the area immediately adjacent to the
sampling valve. This oil must not be drawn off into sample container.
e. Draw off small quantity of oil into the sample container. After using this to rinse the
container. discard it. Then draw up the required quantity of oil for sampling purposes
being careful to have the oil flow into the bottle without it touching the fingers .till
bottle is full and seal in a suitable manner. Do not open the bottle again except in a
warm room after allowing it to reach room temperature.
f. Tag the sample jar, giving particulars of the apparatus from which the sample is
taken, serial no, etc and whether from bottom middle or top of the tank, also date and
name of the person taking the sample.
g. Sample taken from barrels should be taken by means of glass or brass tube which
should be cleaned and dried before use. This tube should be lowered into the barrel
with the thumb over the top and until it is about ½ from the bottom the barrel. The
thumb is removed, allowing the tube to fill and then replaced in order to lift out the
sample. Do not touch the bottom of the tube. The first sample should be allowed to
waste, in order to rinse out the tube
INSPECTION OF SAMPLES:
Colour & Odour give useful information, such as
a. Cloudiness in oil may be due to suspended moisture or suspended solid matter, such
as iron oxide or sludge. Oil of a muddy colour usually contain moisture. The moisture
may be detected by crackle rest.
b. Dark brown colored oil may indicate the presence of dissolved asphaltenes.
c. A green colour indicates the presence of dissolved copper compound and rapid
deterioration of the oil may be expected.
d. The information thus obtained shall be noted for record purposes.
TEST ON OIL SAMPLES:
a. DIELECTIRC STRENGTH TEST:
The test shall be performed in accordance with the method described in Appendix ‘E’
of IS:335.1953 specification for insulating oil for transformers and switchgear(low
Viscosity type) oil shall with stand a test voltage of at least 30 kV for one minute
without breakdown. If the sample fails, a test shall be performed on another sample. If
that also fails, the oil shall be suitably treated.
Frequently transient sparking suggests the presence of foreign matter, for example,
moisture, material, carbon particles etc.
A higher minimum proof voltage may be desirable of certain apparatus, particularly
apparatus of very high voltage type of apparatus of which a higher of voltage is
recommended by the manufacturer.
b. CRACKLE FREE TEST FOR WATER:
The test shall be performed in accordance with the method described in Appendix ‘F’
of I.S:335-1953 . Specification on Insulating oil for transformers and switchgear(low
viscosity type) . If the first sample fails to pass the test, two more samples shall be
tested. Both shall pass the crackle test for the oil to be considered satisfactory. In case
of failure, the oil shall be treated suitably./ This test is only qualitative and is not a
substitute for the laboratory test for dielectric strength.
c. ACIDITY:
1. The acidity of oil shall be determined by any of the methods 1 and 2 given in
Appendix D of IS 0 1866-1961.
Where circumstances do not justify precise determination of acidity, a simple form of
test by the method 3 given in Appendix B may be carried out on the site to find out
whether the acidity has acceded a predetermined value.
2. The recommended limits for acidity are as follows:
d. SLUDGE TEST:
The traces of solid matter in oil samples may be examined according to Appendix B
of IS – 1866-1961 to determine whether they consist of oxidation sludge or any other
foreign matter.