1 The oil level of the transformer is too high or too low
The oil level of the transformer varies with the amount of oil inside the transformer, the level of oil temperature, the change of the load on the transformer, and the change of the ambient temperature. In addition, the oil leakage and water leakage of the transformer will also affect the change of the oil level. The volume of the oil pillow is generally about 10% of the transformer volume. If the oil level is too high, it is easy to cause oil spills and waste. If the oil level is too low, when it is lower than the upper cover of the transformer, the lead wire of the transformer will be exposed to the air, which will reduce the insulation strength and may cause internal flashover. At the same time, due to the increased contact area between oil and air, the dielectric strength of oil decreases rapidly. When the oil level is low and encounters light transformer load, power failure or low temperature in winter, the oil level will drop rapidly, which may cause the light gas relay to act.
If the staff finds that the oil level is too low and the oil level cannot be seen in the oil level gauge, they should refuel immediately. The oil shortage is less severe, i.e. the oil level cannot be seen at night. When you can see the oil level during the day, you should continue to strengthen the observation and then deal with it. In large transformers cooled by forced oil circulating water, if the oil level is found to decrease. The reason should be found out immediately and check whether there are oil splashes in the water, so as to prevent water seepage in the oil and endanger the insulation of the transformer, because since the oil seeps into the water, the water may seep into the oil. The normal oil level must be maintained, so the operator must always check the indication of the oil level gauge. When the oil level is too high (such as summer), try to drain the oil, and when the oil level is too low (such as winter), try to refuel to maintain the normal oil level and ensure the safe operation of the transformer.
2 The oil temperature continues to rise
If the oil temperature of the transformer continues to rise during daily operation, it means that there is a serious fault inside the transformer, mainly due to overheating of the iron core or short circuit between turns of the winding. The overheating of the iron core is caused by the insulation damage of the eddy current or the through-hole screw used to clamp the iron core. The eddy current will cause the iron core to overheat for a long time and cause the insulation damage between the silicon steel sheets. At this time, the iron loss increases and the oil temperature rises. The insulation damage of the feedthrough screw causes a short circuit between the feedthrough screw and the silicon steel sheet. At this time, a large current flows through the feedthrough screw, which makes the screw overheat, causing the temperature of the oil to gradually reach the ignition point, causing the iron core to overheat, melt, and even weld together. . In this case, the transformer should be powered off in time to avoid fire or explosion.
For the above faults, under normal circumstances, the protection device of the transformer will act to automatically trip the circuit breakers on both sides of the transformer. If the protection does not act for some reason, the transformer should be manually disabled immediately and repaired by the maintenance personnel. If there is a backup transformer and time permits, put the backup transformer into operation first.
3 Failures due to damage to insulating materials
The case of winding inter-turn insulation failure: poor heat dissipation due to long-term overload, long-term operation of the transformer, and aging of insulating materials resulting in inter-turn short circuit: due to multiple short-circuits at the transformer outlet, the winding is deformed by force and the inter-turn insulation is damaged; Moisture enters the transformer, causing the inter-turn insulation to be damp and short-circuit; at the reinforced section of the high-voltage winding or the end of the low-voltage winding, due to the expansion of the overall insulation, the oil passage is blocked, and the insulation overheating and aging cause inter-turn short circuit.
The main insulation and lead wires fail: moisture enters the transformer, causing the main insulation to break down due to moisture; due to oil leakage, the transformer is seriously short of oil, and the lead wires are partially exposed to the air, reducing the insulation strength and causing internal Flashover failure; poor connection of the lead wires and poor welding of the lead wire joints, resulting in disconnection of the lead wires after a short-circuit impact.
The situation of transformer core insulation failure: the insulating material between the transformer silicon steel sheets is damaged, causing the transformer core to be partially overheated and melted; the insulation of the core bolts clamping the transformer core is damaged, making the transformer core silicon steel sheets and core bolts damaged. A short circuit is formed between them; the residual welding slag during production forms the grounding of the transformer core at two points, causing a short circuit: the casing flange of the transformer tank, between the casing and the barrel, and the winding clamps are heated due to the local magnetic field, causing damage to the insulating material .
Automatic trip fault analysis
When the transformer automatically trips, if there is a backup transformer, the backup transformer should be put into operation, and then the working transformer should be stopped to check the reason for the automatic tripping of the transformer. If the inspection result proves that the tripping of the transformer is not caused by internal fault, but by overload, external short circuit or secondary circuit fault of protection device. Then the transformer can be put into operation again without external inspection. If there are signs of internal failure during inspection, internal inspection shall be carried out. If the trip occurs due to the action of the differential protection, the electrical equipment within the differential protection range, such as bushings, cable heads, oil temperature, oil color, etc., should be checked. After the fault is eliminated, power on again.