Since transformer is very important capital equipment for an industry, therefore its proper maintenance is very important to allow the transformer for working with its full efficiency. Here are few tips which can be useful in maintenance of power and distribution transformers.
Under any circumstances, work on the transformer should never be done unless it is disconnected from all external electrical circuits and all windings have been properly earthed.
Naked lights and flames should be kept well away from the transformer while working.
Transformer oil is subjected to deterioration or contamination in storage as well as 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. Reason for oil deterioration and recommendations for various tests are covered in IS:1866 and is 335. The few tips given below may serve as ready reference.
The oil level should be checked at regular intervals and any leakage of oil investigated.
All leaks should be repaired as soon as possible to avoid possible trouble due to low oil level.
Samples of the oil should be taken at regular period and tested for BDV according to IS-335, BDV of oil should not be less than 60KVrms. This test should be carried out six times on the filled sample. The average value of the six results should be used.
For large transformers, it may be mentioned that the dielectric strength does not give a right indication of the condition of the oil. In this condition additional test should also be carried out.
For large transformers, it is recommended that the oil be kept under observation for acidity. If the acidity is increasing rapidly, transformer cover should be removed for checking purpose of the interior of the tank and of the core and windings. Oil should be treated or discarded, if the sludge or corrosion is there.
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.
Clean the porcelain bushing and examine them for cracks. Small chips may be ignored but any serious damage will require new porcelain which must be obtained from the manufacturer. It is recommended that some stock should be kept.
All connections should be tight. If they appear blackened, undo the connections and clean down to bright metal with emery paper.
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 with oil every two to three years.
The oil level indicator should be kept clean. The oil level is visible through a transparent material. In case of breakage immediate replacement is mandatory. When conservator is removed for cleaning, the oil gauge should be inspected and cleaned.
Silica gel breathers are fitted with a sight glass so that the colour of the crystals’ visibility is clear. The colour changes from blue to pink as the crystals absorb moisture. When the crystals get saturated with moisture, they become pink and should therefore be reactivated by heating. Oil shall be available in oil cell.
Gaskets sometimes shrink during service. It is, therefore, necessary to check the tightness of all bolts used for 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.
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 analysed 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.
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.
During yearly maintenance inspection, the level of oil in the pockets holding thermometer bulbs should be checked and the oil filled. 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.
The transformer tank cover and other parts should be inspected periodically for oil leakage, peeling of paint, or rust. Rusted portions should be properly cleaned and repainted. Oil leakage should be immediately attended to clamping bolts on gasketed joints should be tightened if required, damaged gaskets replaced. Leaks through welded joints should be stopped by welding again (this should be done without any oil in the transformer tank and radiators).
(Courtesy: Sai Electricals)