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How does dry-type transformer protect against lightning?

2019/7/12 16:34:49 Click:

How does dry-type transformer protect against lightning? Lightning damage damage transformers were previously thought to be caused by lightning waves entering high-voltage windings. In fact, this understanding has a degree of one-sidedness. Theoretical analysis and practical experiments show that the main cause of distribution transformer lightning accidents is caused by the "forward and reverse transformation" overvoltage when the distribution system is subjected to lightning, and the reverse transformation overvoltage damage accident is particularly serious. The development process of forward and reverse conversion overvoltage is analyzed, and the lightning protection of the distribution transformer is discussed.

1, positive and negative conversion overvoltage

1.1 Positive conversion overvoltage When the low-voltage side line is subjected to a lightning strike, the lightning strike current invades the low-voltage winding and enters the ground through the neutral grounding device. The ground current Ijd generates a voltage drop on the ground resistance Rjd. This voltage drop causes a sharp rise in the neutral point potential on the low side. It is superimposed on the low-voltage windings, causing an overvoltage, endangering the low-voltage windings. At the same time, this voltage is increased to the high voltage side by the electromagnetic induction of the high and low voltage windings according to the transformation ratio, and it is superimposed with the phase voltage of the high voltage winding, causing a dangerous overvoltage in the high voltage winding. The phenomenon that the low voltage winding is subjected to lightning overvoltage is transformed to the high voltage side by electromagnetic induction, which causes the overvoltage of the high voltage winding to be called a "positive conversion" overvoltage.

1.2 Reverse overvoltage When the high-voltage side line is subjected to a lightning strike, the lightning current is discharged into the ground through the high-voltage side arrester, and the ground current Ijd generates a voltage drop on the ground resistance Rjd. This voltage drop acts on the neutral point of the low-voltage side, and the low-voltage side outgoing line at this time is equivalent to grounding through a resistor, so the voltage is mostly applied to the low-voltage winding. After electromagnetic induction, this voltage drop is increased to the high-voltage side with a transformation ratio and superimposed on the phase voltage of the high-voltage winding, causing an overvoltage in the high-voltage winding and causing a breakdown accident. This phenomenon of high-voltage side being struck by lightning, acting on the low-voltage side, and then transforming to the high-voltage side through electromagnetic induction, causing the overvoltage of the high-voltage winding is called "inverse conversion overvoltage".

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