Lightning: Transformer damaged by lightning strike, which is the mainly reason and occupied in 50% during the accident of damage .
Faulty Arc ：
Mechanism of faulty arc :
1 the insulated tie (oil and solid) punctured by over-voltage and faulty arc occurs.
2 energy transmission between oil and arc , which lead to temperature increasing surround arc At the same time , the oil in transformers vapour and became flammability gas in the form of plasma .
3 with the increasing of pressure of bubble surround arc, a very concentrated partial phase change and prevention of expansion of gas by oil in transformer , they make the balance of pressure between gas and fluid.
4 the wave of pressure created by pressure difference between gas and fluid ,transmits from arc to whole oil tank in speed of sound.
5 Wave of pressure makes the permanent deformation, fire accident and damage.
Harmonic:
Arrays with strong nonlinear characteristics as transformer, in that, the current of harmonic will increase at the deceasing of resistance.
Three phase unbalance :
The unbalance currents in load Caused by three phase unbalance.
Problem :
1 Increase the loss of distributing transformer by harmonic and three phase unbalance.
2 lower the capacity of distribution. ( winding designing according to the load in every phase, when harmonic and three phase unbalance happen , the capacity of distribution and over-load lowered.
3 zero-sequence current created by three phase unbalance. With three phrase unbalance increasing , zero-sequence current become more bigger. Hysteresis and eddy current by the zero-sequence creates high temperature on steel structure of the distribution transformer.