SITE INSTRUCTIONS FOR TRANSFORMERS (PART 2)

For proper operation and getting maximum performance from transformer it is essential that a reasonable amount of attention and maintenance should be taken care. Here we are giving some important steps which will be useful for site maintenance of transformer:   

Safety Precautions:-

a) Under any circumstances, work on the transformer should never be undertaken unless it is disconnected from all external electrical circuits and all windings have been solidly earthed.

b) Naked lights and flames should be kept well away from the transformer while working.

Maintenance Steps:

a) OIL

Transformer oil is subjected to deterioration or contamination in storage and in service. Accordingly, a periodic treatment to maintain it in fit condition is required, and eventually, it may have to be replaced by new oil. The causes of oil deterioration and recommendations for various tests are comprehensively covered in IS:1866-1978. Reference shall be made to the same regarding the maintenance of quality oil in service .The few tips given below may serve as ready reference.

1. The oil level should be checked at frequent intervals and any excessive leakage of oil investigated.  

2. All leaks should be repaired as quickly as possible so as to avoid possible trouble caused by low oil level.

3. Oil for topping up should comply with IS:335 and should preferably be from the same source as the original oil because the oil refined from different crudes may not be completely miscible and may separate into layers. Furthermore, there may be a greater tendency to form acidity or sludge in a mixture than in oil from single source of supply. Used oil shall not be mixed. New oil may be added as make up only, not exceeding about 10%.

4. Samples of the oil should be taken at regular intervals and tested for BDV When tested in accordance with IS-335, electrical strength (BDV) should not be less than 60 kV RMS.  This test should be carried out six times on the filled sample cell.  The arithmetic mean of the six results should be used.

5. For large transformers, it may be mentioned that the dielectric strength does not give a true indication of the deteriorated condition of the oil. Even oil, which is highly deteriorated, may give a high dielectric strength, If dry Normal method of oil purification only maintain the dielectric strength, but do not give an indication of the deteriorated condition of the oil. It is therefore inadvisable, to rely solely on the dielectric strength of the oil by periodic tests. In addition to chemical tests other tests as per IS: 1866-1978 should also be carried out.

6. For large transformers, it is recommended that the oil be kept under observation for acidity. If the acidity is increasing rapidly, or exceeds limits the cover should be removed to ascertain the condition of the interior of the tank and of the core and windings. Oil should be treated or discarded, if the sludge or corrosion is evident. Advice should be obtained from the manufacturer.

When transformer is opened or the IR values are low, it shall be filtered for 2 passes and sealed after topping up.  Then the transformer is cooled down to ambient temperature, the oil level shall be reduced to normal fill level.

b) BUSHINGS

Clean the porcelain bushing and examine them for cracks and chips.  Very slight chips may be ignored but any serious damage will require new porcelain which must be obtained from the manufacturer.  It is recommended to have a spare in stock.

c) External Connections

All connections should be tight. If they appear blackened of corroded, undo the connections and clean down to bright metal with emery paper.  

d) Conservator and Oil Level Gauge

Conservators are so arranged that the lower part acts as a sump in which any impurities entering the conservator will collect.  A valve is fitted at the lowest point of the conservator for draining and sampling.  The inside of the conservator should be cleaned or flushed with oil every two to three years.  A removable end is provided for this purpose. 

The oil level indicator should be kept clean. The oil level is visible through a transparent material.  In case of breakage immediate replacement is essential.  When conservator is stripped for cleaning, the mechanism of the oil gauge should be inspected and cleaned.

e) Breather

Silica gel dehydrating breathers are fitted with a sight glass so that the color of the crystals may be seen.  The color changes from blue to pink as the crystals absorb moisture.  When the crystals get saturated with moisture they become predominantly pink and should therefore be reactivated by heating.  Oil shall be available in oil cell. 

f) Gaskets :

Gaskets sometimes shrink during service.  It is, therefore, necessary to check the tightness of all bolts fastening gasketed joints.  The bolts should be tightened evenly round the joints to avoid uneven pressure.  Leaking gaskets should be replaced as soon as the circumstances permit.

g) Buchholz Relay

Routine operation and mechanical inspection tests should be carried out at one and two yearly intervals respectively.  During operation if gas is found to be collecting and giving alarm, the gas should be tested and analyzed to find out the nature of fault.  Sometimes, it may be noticed that the gas collecting is only air.  The reasons for this may be that the oil is releasing any absorbed air due to change in temperature.  The absorbed air is released in initial stages only when no vacuum is applied during filling of oil.  The internal faults can be identified to a great extent by a chemical analysis of gas. Buchholz relay also give alarm / trip due to the oil level falling below the buchholz level.

h) Explosion Vent 

The diaphragm which is fitted at the exposed end of the vent should be inspected at frequent intervals and replaced, if damaged.  Failure to replace the diaphragm quickly may allow the ingress of moisture, which will contaminate the oil.  If the diaphragm has broken because of a fault in the transformer, an inspection shall be carried out to determine the nature and cause of the fault.

i) Temperature Indicators

At each yearly maintenance inspection, the level of oil in the pockets holding thermometer bulbs should be checked and the oil replenished, if required.  The capillary tubing should be fastened down again if it has become loose.  Dial glasses should be kept clear and if broken, replaced as soon as possible to prevent damage to the instrument.  Temperature indicators should be calibrated with standard thermometer immersed in hot oil bath if found to be reading incorrectly.

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