Analysis and prevention of insulation accident of

Analysis and prevention of insulation accidents of large power transformers

the safe operation of transformers is threatened by insulation accidents. Therefore, in the process of transformer manufacturing, installation, maintenance and operation, great attention is paid to the safety of transformer insulation system. This paper mainly analyzes the causes of transformer insulation accidents and the prevention of insulation accidents

2 causes of insulation accident

2.1 overview of insulation accident

the insulation system of transformer is an insulation coordination problem. Reasonable insulation coordination refers to the withstand electric field strength of transformer insulation (hereinafter referred to as "field strength") It is larger than the applied field strength and has a certain margin. When the insulation coordination is damaged, insulation accidents will occur

2.2 insulation accident caused by out of control of applied field strength

1) the problem of out of control of long-term working voltage

long-term working voltage does not exist, but if the rubber elongation is relatively large, it is additionally equipped with a large deformation test frame, which does not mean that the applied field strength is not out of control. Because under a certain voltage, if electric field distortion occurs, the action field strength will change. The causes of electric field distortion include suspension of metal conductors, sharp burrs on conductors, and the accumulation of conductive dust. For example, when the pressure equalizing ball of high-voltage bushing is not tightened during installation or vibrated loose during operation, it forms a suspended conductor, which produces an action field strength sufficient to make the oil gap breakdown, causing partial discharge and splitting of transformer oil into acetylene tension

2) temporary overvoltage

short-term rise of power frequency voltage or resonant overvoltage are collectively referred to as temporary overvoltage. When the power frequency voltage rises above the design value, overexcitation of the core may occur. Under the condition of overexcitation, on the one hand, the value of excitation current increases rapidly, on the other hand, the harmonic component in excitation current increases rapidly. The greater the multiple of overexcitation, the more serious it is. The consequence is that the conductor close to the core coil is overheated locally, causing inter turn insulation breakdown. Foreign literature has repeatedly reported such accidents without the advantages of adding other adhesives

3) switching overvoltage

transformers with voltage levels over 220kV have taken effective protective measures against switching overvoltage, so no damage accident under switching overvoltage has been found so far. The field strength of switching overvoltage of 220kV and below transformers may be out of control, which is enough to cause accidents. For example, coil turn to turn or layer to layer short circuit occurs during no-load closing; How to choose an electronic torsion tester to test the clutch when switching the low-voltage side compensation capacitor? Cause the low-voltage lead to discharge to the oil tank; The interphase breakdown at the end of the high-voltage bushing is caused by multiple closing in different periods

4) lightning overvoltage

the lightning protection on the high voltage side of the transformer is relatively sound and generally safe. However, the lightning resistance level of the medium and low voltage sides of some transformers is low, resulting in frequent accidents of lightning damage to transformers. For example: 220kV transformer, there have been many lightning damage accidents on the low-voltage 35kV side; Lightning damage accidents have also occurred on the medium voltage 35kV side or low voltage 10kV side of 110kV three winding transformer

2.3 insulation accident caused by the decrease of withstand field strength

the decrease of withstand field strength refers to that the insulation of the transformer is in an abnormal state due to pollution during operation. The causes of pollution are very complex, and there are three common ones:

1) the insulation is damp

the normal oil paper insulation has a high withstand field strength, and under the normal operating voltage, the turn insulation is unlikely to have a breakdown accident; However, the actual situation is that the possibility of transformer winding insulation accidents caused by turn insulation accidents accounts for 80% - 90% of all insulation accidents. The reason is that oil paper insulation has a great affinity for water, and its insulation strength will decline linearly after being affected with moisture

general transformers have been tested for insulation strength before leaving the factory, so the insulation strength has reached the national standard. However, the insulation cannot be guaranteed not to be damaged during transportation or installation. Therefore, before the installation of the transformer, it is generally necessary to check the hanging core. Before hanging the core, the weather must be sunny, the relative humidity should be less than 65%, and the body temperature should be 10 ℃ higher than the surrounding ambient temperature. The insulation of transformer oil shall comply with national standards