The main reasons for the light gas protection of transformers in operation are as follows:
(1) There is a slight fault inside the transformer, such as short circuit between turns, local heating of the iron core, heating of the oil and the wall of the transformer tank caused by magnetism, etc., which produces a small amount of gas.
(2) Air invades the inside of the transformer.
(3) Long-term oil leakage or oil seepage causes the oil level to be too low.
(4) The welding of the transformer winding joints is not firm, and the contact resistance is too large, causing heating.
(5) Two-point grounding occurs in the secondary circuit, resulting in false signals, etc.
After the light gas protection is activated, the on-duty personnel should immediately check the appearance of the transformer. For example: whether the oil level and oil color are normal; whether there is gas in the gas relay, the amount of gas and its color; check whether there is oil leakage in the transformer body and cooling system; whether there are abnormalities in the transformer load, temperature and sound. After external inspection, if no abnormality is found, the gas of the transformer should be sucked, and the laboratory personnel should verify the nature of the gas. If necessary, take oil samples for gas chromatography analysis to comprehensively determine the type of fault.
When collecting gas, two people must do it, one to operate and one to monitor. When climbing the transformer to take gas, you should be careful and keep a safe distance from the live parts. Generally, the human body should not exceed the oil storage cabinet of the transformer. In addition, when taking gas and opening the vent valve, make sure it is positive pressure. If negative pressure occurs, immediately close the vent valve and stop venting to prevent air from drowning in the transformer. The collected gas is handed over to the laboratory technician for testing and analysis. The color, quantity, flammability and chemical composition of the gas are identified. Combined with the results of gas chromatography analysis, the cause of light gas action, the nature and severity of the internal fault of the transformer are determined. The following specifically explains the relationship between the color, odor and flammability of gas and the internal fault of the transformer.
(1) If the gas is colorless, odorless and non-flammable, it means that the light gas action is caused by air leakage.
(2) If the gas is gray-black, slightly smelly and flammable, it means that it is caused by a transformer fault. It is generally caused by oil decomposition caused by discharge: At this time, oil samples should be taken for testing and analysis.
(3) If the gas is yellow and non-flammable, it indicates that there is a wooden material failure inside the transformer, such as a wooden bracket failure.
(4) If the gas is grayish white, smelly and flammable, it indicates that there is an insulation failure inside the transformer. It is worth noting that in the early stage of a transformer failure, the collected gas is usually colorless and non-flammable. In addition, the collected gas is not completely consistent with the gas composition at the fault site. This is because the oil itself has a great adsorption capacity for gas. Therefore, it is not absolutely reliable to judge the nature of the fault based solely on whether the gas has color or is flammable. The best way is to take an oil sample for gas chromatography analysis.