In the past, distribution transformers were damaged by lightning strikes, which were thought to be caused by lightning waves entering the high-voltage windings. In fact, this perception is not comprehensive.
Theoretical analysis and actual tests show that the main cause of the dry-type transformer lightning accident is caused by the over-voltage of the “positive and negative transformation” when the distribution system is exposed to lightning, and the reverse-transformed over-voltage damage accident is particularly serious. Now we will analyze the development process of the over-voltage and over-voltage, and discuss the lightning protection of the distribution transformer.
1.Positive and negative conversion overvoltage positive overvoltage When the low voltage side line is struck by lightning, the lightning strike current invades the low voltage winding through the neutral point grounding device and the grounding current Ijd produces a voltage drop on the grounding resistance Rjd. This pressure drop causes the neutral point potential on the low voltage side to rise sharply. It superimposes an overvoltage in the low voltage winding, jeopardizing the low voltage winding. At the same time, this voltage is raised to the high voltage side by the electromagnetic induction of the high and low voltage windings, superimposed with the phase voltage of the high voltage winding, causing a dangerous overvoltage in the high voltage winding. This phenomenon is caused by a lightning strike overvoltage caused by a low voltage winding, and is converted to a high voltage side by electromagnetic induction, causing a phenomenon of overvoltage of the high voltage winding, which is called a “positive change” overvoltage. Reverse conversion overvoltage When the high voltage side line is struck by lightning, the lightning current is discharged into the ground through the high voltage side arrester, and the ground current Ijd generates a voltage drop across the grounding resistance Rjd. This pressure drop acts on the low-voltage side neutral point, while the low-voltage side outlet line is equivalent to grounding through the resistor. Therefore, most of the voltage is applied to the low-voltage winding. Further, by electromagnetic induction, this voltage drop rises to the high voltage side in a variable ratio and is superimposed on the phase voltage of the high voltage winding, causing an overvoltage in the high voltage winding to cause a breakdown accident. This phenomenon is caused by lightning strike on the high voltage side, acting on the low voltage side, and changing to the high voltage side through electromagnetic induction, causing the phenomenon of overvoltage of the high voltage winding is called “inverse conversion overvoltage”.
2.The wiring of the oil-immersed distribution transformer wi