Distribution transformer lightning protection technology

Lightning damage damage distribution is simply thought to be caused by lightning waves entering the high-voltage winding. In fact, this understanding has a one-sidedness. Theoretical analysis and actual tests show that the main cause of the lightning damage 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 especially 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 forward and reverse conversion overvoltage

1.1 Positive transformation overvoltage When the low voltage side line is struck by lightning, the lightning current invades the low voltage winding through the neutral point grounding device and the grounding current Ijd produces a voltage drop across 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.

1.2 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 on 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 Influence of different wiring of transformer on forward and reverse conversion overvoltage

2.1 Yzn11 wiring. When the low-voltage side line falls, the lightning current enters the two "half windings" on the low-voltage side, the same size, the opposite direction, the magnetic flux on each core column just cancels each other, and thus does not generate in the high-voltage winding. Positive overvoltage. When the high-voltage side line falls, the magnetic circuit is asymmetrical due to the structure of the transformer and magnetic flux leakage. Therefore, the magnetic flux cannot be completely cancelled. The over-voltage of the forward and reverse conversion still exists, but it is small, and it can be considered that it has better defense. Thunder effect.

2.2 Yyn0 wiring

This type of transformer is a standard wiring in China. It has many advantages:

1 Normally, the voltage of each phase can be kept constant, and two different voltages of 380/220V can be provided to meet the user's requirements;

2 When a single-phase grounding short circuit occurs, the voltage rise of the other two phases can be avoided.

3 to avoid the danger of high pressure entering the low pressure side. Therefore, almost all distributions in the distribution network use this connection.

3 Yyn0 wiring distribution transformer lightning protection

3.1 Install a lightning arrester on the high voltage side to prevent lightning overvoltage.

3.1.1 Installed lightning arrester on the high-voltage side of the distribution transformer, which can effectively prevent the lightning wave from entering the high-voltage side line and damage the distribution transformer. The FS-10 valve type arrester is often installed on the high-voltage side of the transformer.

3.1.2 After installing the arrester on the high voltage side. The grounding wire of the arrester should be grounded together with the transformer casing and the neutral point of the low-voltage side to fully exert the voltage limiting function of the arrester and prevent reverse flashover.

(2) There is enough arcing distance at the upper end of the circuit breaker, refer to the instruction manual of each type and specification product.