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One, positive and negative transformation over voltage
When the low-voltage side line is subjected to a lightning strike, the lightning current enters the low-voltage winding through the neutral point grounding device and the ground current Ijd generates a voltage drop across the ground resistance Rjd. This pressure drop makes the potential of the neutral point on the low voltage rise sharply. It superimposes overvoltage on the low voltage windings, 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, and it overlaps with the phase voltage of the high voltage winding, causing a dangerous overvoltage in the high voltage winding. This kind of low-voltage winding is subject to lightning overvoltage, and it is transformed to the high-voltage side through electromagnetic induction, causing the overvoltage of the high-voltage winding to be called “transformation†overvoltage.
Reverse 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 ground resistance Rjd. This voltage drop acts on the neutral side of the low voltage side, and the low voltage side outlet is equivalent to being grounded via a resistor. Therefore, most of the voltage is applied to the low voltage winding. Through electromagnetic induction, this pressure drop is increased to the high voltage side with a transformation ratio, and is superimposed on the phase voltage of the high voltage winding, resulting in a high voltage winding overvoltage and breakdown accidents. This kind of lightning strike on the high-voltage side acts on the low-voltage side, and then changes to the high-voltage side through electromagnetic induction, causing the overvoltage of the high-voltage winding to be called "inverse conversion overvoltage." If you need to know more, please contact Zhengzhou Leidi Electronic Engineering Co., Ltd. staff directly!
Second, the impact of transformer different wiring on the positive and negative conversion over-voltage
Yzn11 wiring. When lightning strikes on the low-voltage side, the lightning current enters the two “half-windings†on the low-voltage side. The two are equal in magnitude and opposite in direction. The flux on each core column exactly cancels each other and thus does not generate in the high-voltage winding. Overvoltage is changing. In the event of lightning strikes on the high-voltage side line, the magnetic circuit is actually asymmetric due to the transformer structure and magnetic flux leakage. Therefore, the magnetic flux cannot be completely canceled, and the over-voltage of the forward and reverse transformation still exists, but it is considered to be better when the overvoltage is small. Thunder effect.
Yyn0 wiring. This connected transformer is a standard wiring in our country. It has many advantages: 1 It can keep the voltage of each phase unchanged during normal operation, and can provide two different voltages of 380/220V to meet the user's requirements; 2 When single-phase grounding short-circuit occurs, the voltage of the other two phases can be prevented from increasing; 3 The danger of high pressure pumping into the low pressure side can be avoided. Therefore, almost all distribution transformers in distribution networks use this method.
Third, Yyn0 wiring with lightning protection
Arresters are installed on the high-voltage side to prevent lightning overvoltages.
The lightning arrester installed on the transformer high voltage side can effectively prevent the lightning wave from entering and damaging the distribution transformer when lightning strikes the high-voltage side line. The FS-10 valve arrester is often installed on the high-voltage side of the transformer.
After the arrester is installed on the high-voltage side. The arrester grounding wire should be grounded together with the transformer housing and the low-voltage side neutral point to fully utilize the arrester's pressure limiting function and prevent reverse flashover.
An arrester is installed on the low-voltage side to limit the overvoltage.
For Yyn0 strain distribution, even if the surge arrester is installed on the high-voltage side, it is still unavoidable to perform reverse conversion of the wave from the high-voltage side or a positive conversion over-voltage from the low-pressure side. When a group of surge arresters is installed on the low-voltage side, the over-voltage in the forward and reverse directions can be limited.
Use positive and negative transformation overvoltage theory analysis. The positive and negative conversion overvoltages are caused by the overvoltage of the low voltage winding. Therefore, as long as it tries to limit the amplitude of the overvoltage of the low voltage winding, the overvoltage can be limited by changing the forward voltage and the negative voltage. The arrester installed on the low-voltage side is used to limit the amplitude of the overvoltage of the low-voltage winding. With the low-voltage arrester, the overvoltage is effectively suppressed in the positive and negative directions, and thus the high-voltage winding can be protected.
From the above analysis, it can be seen that it is necessary to install a surge arrester on the low-voltage side of distribution transformers. This is also a deficiency of our previous understanding. The installation of lightning arresters on the low voltage side of distribution transformers is of great significance for reducing the tripping rate of accidents and improving the reliability of power supply. Therefore, improving the lightning protection technology of distribution transformers not only has direct economic benefits, but also has great social benefits.
Lightning damage distribution in the past simply thought that the lightning wave into the high voltage winding caused, in fact, this understanding with a degree of one-sidedness. Theoretical analysis and practical tests show that the main reason for lightning accidents with distribution transformers is due to the overvoltage of “forward and reverse transformation†when the distribution system is exposed to lightning, and the accidental overvoltage damage is particularly serious. Now Zhengzhou Leidi Electronic Engineering Co., Ltd. technical personnel on the positive and negative transformation overvoltage development process analysis, discuss the distribution of lightning protection, to share with you the distribution transformer lightning protection technology.