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. Positive and negative 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 overvoltageâ€.
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 normal to maintain the voltage of each phase unchanged, while providing two different voltages of 380/220V to meet user requirements; 2 when single-phase grounding short circuit occurs, can avoid the rise of the other two phase voltage; 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.
3.2 Install a lightning arrester on the low voltage side to limit the positive overvoltage.
For the Yyn0 distribution, even if the arrester is mounted on the high voltage side, it is inevitable that the reverse conversion of the wave from the high voltage side or the positive conversion overvoltage of the wave from the low voltage side is inevitable. When a set of arresters is installed on the low-voltage side, the forward and reverse conversion overvoltages can be limited.
The forward and reverse transform overvoltage theory analysis. The positive and negative transitions are generated due to overvoltages in the low voltage windings. Therefore, as long as trying to limit the magnitude of the overvoltage of the low voltage winding, the forward and reverse conversion overvoltage can be limited. The arrester 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 forward and reverse conversion overvoltages are effectively suppressed, thereby protecting the high-voltage winding.
4. Specific requirements for installing lightning arresters
4.1 The correct lightning protection wiring is shown in Figure 1.
4.2 The transformer should be installed between the high voltage fuse and the transformer.
4.3 Lightning arrester lightning protection grounding down the line using the "three in one" grounding method. That is, the lightning arrester grounding down conductor, the distribution transformer metal casing and the low voltage side neutral point are connected together, and then connected to the grounding device.
4.4 A set of low-voltage arresters shall be installed in the multi-mine area at the outlet of the low-voltage side of the transformer.
5, the installation of the grounding device
The quality of the grounding device installation determines whether the lightning protection device of the distribution transformer plays a good role in protecting the grounding. Therefore, the grounding is reliable and conforms to the technical specifications, so that the shunting function can be well performed to protect the transformer.
5.1 The grounding wire of the high and low voltage side arrester, the transformer housing and the neutral point of the low voltage side should be connected together to be grounded together (when the neutral point is not grounded, the breakdown fuse is installed at the neutral point to the ground)
5.2 The grounding resistance should meet the requirements of the regulations. For the distribution of 100kVA or above, Rjd≤4Ω; repeat grounding not less than three places, each Rjd≤10Ω. 2 For 100kVA and below, Rjd≤10Ω; repeat grounding not less than three places, each Rjd≤30Ω.
5.3 The arrester grounding down conductor (ie the connection to the transformer housing) should be as short as possible. Because even if the grounding wire is 0.6m long, its inductance L is about 1mH. When the lightning wave steepness di/dt=10kA/μs is small, the voltage drop on the grounding wire is not so small as Ldi/dt≈10kV. The value. It overlaps with the residual voltage of the arrester on the distribution transformer and will also greatly increase the destructiveness. For this reason, for the high voltage side, the arrester should be installed at the lower end of the high voltage drop fuse. This can not only reduce the length of the grounding lead, but also bring convenience to the pre-test of the arrester (take the drop-out fuse, do the safety measures can be carried out, will not affect the high-voltage line operation); secondly, when the arrester is of poor quality, the discharge cannot be When the arc is extinguished, the power frequency continues to flow, so that the high-voltage drop fuse is blown, and the melt pipe is automatically dropped, thereby avoiding the influence on the power supply of the high-voltage line and reducing the trip rate of the line.
6 Conclusion
It can be seen from the above analysis that it is necessary to install a lightning arrester on the low-voltage side of the transformer, which is also a shortcoming of our previous understanding. The installation of lightning arresters on the low-voltage side of the distribution transformer is of great significance for reducing the accident trip rate and improving the reliability of power supply. Therefore, doing a good job of lightning protection in rural distribution has not only direct economic benefits, but also great social benefits.
1. Positive and negative 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 overvoltageâ€.
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 normal to maintain the voltage of each phase unchanged, while providing two different voltages of 380/220V to meet user requirements; 2 when single-phase grounding short circuit occurs, can avoid the rise of the other two phase voltage; 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.
3.2 Install a lightning arrester on the low voltage side to limit the positive overvoltage.
For the Yyn0 distribution, even if the arrester is mounted on the high voltage side, it is inevitable that the reverse conversion of the wave from the high voltage side or the positive conversion overvoltage of the wave from the low voltage side is inevitable. When a set of arresters is installed on the low-voltage side, the forward and reverse conversion overvoltages can be limited.
The forward and reverse transform overvoltage theory analysis. The positive and negative transitions are generated due to overvoltages in the low voltage windings. Therefore, as long as trying to limit the magnitude of the overvoltage of the low voltage winding, the forward and reverse conversion overvoltage can be limited. The arrester 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 forward and reverse conversion overvoltages are effectively suppressed, thereby protecting the high-voltage winding.
4. Specific requirements for installing lightning arresters
4.1 The correct lightning protection wiring is shown in Figure 1.
4.2 The transformer should be installed between the high voltage fuse and the transformer.
4.3 Lightning arrester lightning protection grounding down the line using the "three in one" grounding method. That is, the lightning arrester grounding down conductor, the distribution transformer metal casing and the low voltage side neutral point are connected together, and then connected to the grounding device.
4.4 A set of low-voltage arresters shall be installed in the multi-mine area at the outlet of the low-voltage side of the transformer.
5, the installation of the grounding device
The quality of the grounding device installation determines whether the lightning protection device of the distribution transformer plays a good role in protecting the grounding. Therefore, the grounding is reliable and conforms to the technical specifications, so that the shunting function can be well performed to protect the transformer.
5.1 The grounding wire of the high and low voltage side arrester, the transformer housing and the neutral point of the low voltage side should be connected together to be grounded together (when the neutral point is not grounded, the breakdown fuse is installed at the neutral point to the ground)
5.2 The grounding resistance should meet the requirements of the regulations. For the distribution of 100kVA or above, Rjd≤4Ω; repeat grounding not less than three places, each Rjd≤10Ω. 2 For 100kVA and below, Rjd≤10Ω; repeat grounding not less than three places, each Rjd≤30Ω.
5.3 The arrester grounding down conductor (ie the connection to the transformer housing) should be as short as possible. Because even if the grounding wire is 0.6m long, its inductance L is about 1mH. When the lightning wave steepness di/dt=10kA/μs is small, the voltage drop on the grounding wire is not so small as Ldi/dt≈10kV. The value. It overlaps with the residual voltage of the arrester on the distribution transformer and will also greatly increase the destructiveness. For this reason, for the high voltage side, the arrester should be installed at the lower end of the high voltage drop fuse. This can not only reduce the length of the grounding lead, but also bring convenience to the pre-test of the arrester (take the drop-out fuse, do the safety measures can be carried out, will not affect the high-voltage line operation); secondly, when the arrester is of poor quality, the discharge cannot be When the arc is extinguished, the power frequency continues to flow, so that the high-voltage drop fuse is blown, and the melt pipe is automatically dropped, thereby avoiding the influence on the power supply of the high-voltage line and reducing the trip rate of the line.
6 Conclusion
It can be seen from the above analysis that it is necessary to install a lightning arrester on the low-voltage side of the transformer, which is also a shortcoming of our previous understanding. The installation of lightning arresters on the low-voltage side of the distribution transformer is of great significance for reducing the accident trip rate and improving the reliability of power supply. Therefore, doing a good job of lightning protection in rural distribution has not only direct economic benefits, but also great social benefits.
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