How to prevent transformers from being struck by lightning?
Lightning damage to distribution transformers used to be simply thought to be caused by lightning waves entering the high-voltage windings. In fact, this understanding has a certain degree of one-sidedness. Theoretical analysis and practical tests show that the main cause of distribution transformer lightning accidents is caused by the overvoltage of “forward and reverse conversion” when the distribution system suffers lightning damage, and the reverse conversion overvoltage damage accident is especially serious. Now we will analyze the development process of forward and reverse conversion overvoltage and discuss the lightning protection of distribution transformers.
How to prevent a transformer from being struck by lightning
Methods/Steps
1 Positive transformation overvoltage When the low-voltage side line suffers a lightning strike, the lightning strike current invades the low-voltage winding and enters the ground through the neutral point grounding device. The ground current causes a voltage drop on the grounding resistor Rjd. This voltage drop causes the neutral point potential on the low voltage side to rise sharply. It superimposes overvoltage on the low-voltage winding, endangering the low-voltage winding. At the same time, this voltage rises to the high-voltage side through the electromagnetic induction of the high- and low-voltage windings according to the transformation ratio, and is superimposed with the phase voltage of the high-voltage winding, causing dangerous overvoltage in the high-voltage winding. This phenomenon in which the low-voltage winding suffers lightning overvoltage and is transformed to the high-voltage side through electromagnetic induction, causing overvoltage in the high-voltage winding, is called “positive transformation” overvoltage.
2 Reverse transformation 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 I produces a voltage drop on the ground resistor Rjd. This voltage drop acts on the neutral point of the low-voltage side, and the low-voltage side outlet is equivalent to being grounded through a resistor. Therefore, most of the voltage is added to the low-voltage winding. Through electromagnetic induction, this voltage drop rises to the high-voltage side with a variable ratio, and is superimposed on the phase voltage of the high-voltage winding, causing overvoltage in the high-voltage winding and resulting in breakdown accidents. This phenomenon in which the high-voltage side is struck by lightning, acts on the low-voltage side, and is converted to the high-voltage side through electromagnetic induction, causing overvoltage in the high-voltage winding, is called “reverse transformation overvoltage.”
3 Wiring methods. When a lightning strikes the low-voltage side line, the lightning current enters the two “half windings” on the low-voltage side, which are equal in size and opposite in direction. The magnetic fluxes on each core column cancel each other out, so there will be no generation in the high-voltage winding. Positive conversion overvoltage. When a lightning strike occurs on the high-voltage side line, the magnetic circuit is actually asymmetric due to reasons such as the transformer structure and magnetic flux leakage, so the magnetic flux cannot be completely offset. The forward and reverse conversion overvoltage still exists, but it is small, which can be considered to have better protection. Lightning effect
4 Installing a lightning arrester on the high-voltage side of the distribution transformer can effectively prevent lightning waves from invading and damaging the distribution transformer when the high-voltage side line is struck by lightning. Valve-type arresters are often installed on the high-voltage side of the distribution transformer in engineering projects.
5 The grounding wire of the high- and low-voltage side arrester, the distribution transformer shell and the low-voltage side neutral point should be connected together and grounded together (when the neutral point is not grounded, theInstall a breakdown fuse between the neutral point and the ground)
6 During a lightning strike, the lightning slope is too steep. In order to prevent this phenomenon, the first or second base pole of the distribution transformer can be installed At the same time, the insulator pins of high-voltage lines should be well grounded, so that in the event of a lightning strike, the lightning voltage breaks down the insulator and discharges to the ground, which can prevent the arrester and transformer from directly bearing the lightning current with too high a steepness, and ensure the safety of the distribution transformer.
Notes
The lightning arrester should be installed between the high-voltage fuse and the transformer.
Please do not operate outdoors on rainy days
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