# The Trouble to Watch out for the Transformer

A short-circuit suddenly occurred when we use a transformer, the high- and low-voltage windings may pass through a short-circuit current of 10 times the rated value, so that it generates a large amount of heat and the transformer is highly heated. The capacity of a transformer to withstand short circuit current is short and the thermal stability is very poor, the insulation material inside the transformer will be damaged in a large area, resulting in accidents such as transformer penetration and damage.

When the transformer is shorted to the outlet, the absolute value of the short-circuit current is: Id(n)=m(n)Id1(n)(1) where (n) is a short-circuit type of the corner; m(n) is a ratio coefficient. Its value is related to the type of short circuit; Id(n) is the absolute value of the positive sequence current of the short circuit type.

The absolute value of the positive sequence current of different types of short circuit is: Id1(n)=E/(Xl+X1(n)(2) where E is the pre-fault phase voltage; Xl is the equivalent positive sequence impedance; X1(n ) an additional impedance.

The short-circuit of the outlet of the transformer mainly includes three cases three-phase short circuit, two-phase short circuit, single-phase ground short circuit and two-phase ground short circuit. According to the statistical data, in the neutral grounding system, the single-phase grounding short circuit accounts for about 65 percent of all short-circuit faults, and the two-phase short-circuit accounts for about 10 percent to 15 percent. The ground short circuit accounts for about 15% to 20%, and the three-phase short circuit accounts for about 5%. Among them, the short-circuit current value when the three-phase short circuit is the largest.

Ignore the effect of system impedance on short-circuit current. Then the three-phase short-circuit expression is: (3) where: I(3)dt-three-phase short-circuit current; U-voltage transformer connected system rated voltage; Zt-transformer short-circuit impedance; IN-transformer rated current; UN-transformer The percentage of short circuit voltage.

For a 220kV three-winding transformer, the short-circuit impedance of the high-voltage centering and low-voltage is between 10% and 30%. The medium-voltage low-voltage short-circuit impedance is mostly less than 10%, so when the transformer has a short-circuit fault, the strong short-circuit current will cause the transformer insulation material to withstand high heat and damage.

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