DRYING OUT OF TRANSFORMER
Deterioration of insulation resistance value of transformer is mainly due to ingress of moisture into the windings and insulating materials. In order to improve the insulation resistance at site, following methods are available:
(i) Hot Oil Spraying: In this method about 7% of quantity of oil is heated up to 90-95º C separately and the hot oil is sprayed on to core and windings by means of nozzles in the form of fine spray and simultaneously the transformer is subjected to a high degree of vacuum say less than 5 m bar. The hot oil is collected at the bottom sent through a filter and reheated and sprayed. This process removes moisture from the core and windings. The oil used for spraying should be discarded.
(ii) Flushing Method: In this method, the transformer is put under hot oil circulation up to 60º C. After reaching steady temperature, the entire oil is drained quickly into a separate tank. Immediately after draining the oil, the transformer is subjected to a vacuum as per the guidelines of the manufacturer for 12 hours. During this period of application of vacuum, the transformer oil drained can be filtered to improve the quality. After 12 hours of vacuuming of the transformer, break the vacuum by means of dry nitrogen. Fill the transformer with filtered oil under vacuum. Now the oil in the transformer can be again circulated to raise the temperature of oil to 60º C. Again drain the oil, apply vacuum and repeat the process till you get a good IR value.
The diverter switch tank and the main tank should be inter-connected before the above works to equalize the pressure. Otherwise the diverter switch tank may be damaged when the vacuum is applied.
BUCHHOLZ RELAY FUNCTIONAL TEST
Tools and materials required:
Cycle pump or Nitrogen cylinder with 4 kg/cm2 pressure and connecting tubes.
Transformer shall be isolated.
Connect Nitrogen cylinder or cycle pump to the top petcock of Buchholz relay
Open the other petcock
Allow gas to enter the relay
Check and confirm alarm signal is received
Close petcock on gas supply side and release all gases trapped in relay casing
Increase the gas pressure to approx. 2 kg/cm2
Open the test petcock and allow full surge of gas to enter the relay casing
Check and confirm in control room that the Trip signal has been received
Close Buchholz relay petcocks and normalize
BUCHHOLZ GAS ANALYSIS
This is to be done only when the transformer has tripped on account of Buchholz fault or Buchholz alarm has been initiated.
The following procedure should be adopted for testing of gas accumulated in Buchholz relay of power transformers.-
(i) Switch off the transformer when the Buchholz relay alarm rings, indicating the development of an internal fault in the transformer.
(ii) Through the lateral sight hole of the Buchholz relay, the colour and quantity of the gas may be determined.
(iii) Collect a portion of the gas in the test tube and apply a lighted match stick to the test tube to test the combustibility of the gas.
If gas is not combustible, it is mere air.
(iv) Then proceed to carry out the chemical test with a simple gas tester as follows:
The gas tester consists of two glass tubes containing two different silver nitrate solutions which through passage of decomposed gases form two distinguishable precipitates. The tubes must be assembled as indicated in the sketch and tube 1 should be filled with solution prepared by dissolving 5 grams of silver nitrate (Ag NO3) in 100 cc of distilled water.
Tube 2 should be filled with solution prepared by dissolving five grams of silver nitrate (Ag NO3) in 100 cc of watery ammonia solution.
Use of the gas tester is quite simple. Each of the two glass tubes should be filled with corresponding solutions upto the marks. They should be closed by corks fitted with the connecting tubes. Then the gas tester should be screwed on to the test cock of the Buchholz relay. After opening the test cock the collected gas would flow through the solution which would indicate the nature of the fault.
If the gas causes a white precipitate in tube 1 which turns brown under the influence of light, it means the oil has decomposed. Probably a flashover has occurred between bare conductors or between one bare conductor and an earthed part of the transformer.
If the gas causes a dark brown precipitate in the solution in tube 2 it means that solid insulating material like wood, paper, cotton, etc., had decomposed producing carbon monoxide (CO). In this case a leakage in the winding causing an internal short has occurred.
If there is no sedimentation at all the gas is mere air.