1 lightning intrusion channel
It is unlikely that lightning will directly hit the substation equipment, because the design and construction will consider installing independent lightning rods, lightning protection belts and lightning protection networks.
Lightning may invade along the power line, and lightning waves may invade the substation along the line. For example, when the lightning arrester operates, the residual voltage of the lightning arrester is coupled to the low-voltage network through the electromagnetic induction of the transformer used, which damages the power supply module of microcomputer protection and integrated automation. At this time, the amplitude of overvoltage in low-voltage power grid is mainly related to the residual voltage of arrester, the distance between arrester and transformer and the length of grounding down lead of arrester.
Lightning may invade along the communication line, and the overvoltage caused by lightning has a certain potential difference between the communication line and the equipment, which directly acts on the serial communication port (RS232/422/485, etc. ). The fundamental reason is that the 400V low-voltage power supply side lacks the necessary lightning protection measures, especially the lightning arrester protection of the corresponding voltage level, which makes the lightning overvoltage in the low-voltage network unable to be effectively limited. At the same time, the power supply of microcomputer monitoring system, dispatching automation system and communication system is not separated from other power supplies, or special measures are taken to prevent lightning accidents caused by lightning interference.
Lightning induction often occurs, and it is induced to 400V low voltage lines through 35kV or 10kV high voltage. If the low-voltage network is large, or there are low-voltage overhead lines, when lightning moves near it, it will induce high overvoltage on the 400V low-voltage network and damage the power supply part of microcomputer protection, integrated automation system, dispatching system or communication system connected to the low-voltage network.
In addition, lightning also acts on the equipment through counterattack, wave interception and backward flow, as shown in figure 1.
2 Problems existing in lightning protection of secondary equipment at present
2. The residual pressure of1mov is not reasonably matched with the withstand voltage of secondary equipment.
Due to the limitation of the current manufacturing process, the residual voltage of lightning arrester is 6 times higher than the rated voltage. For example, the residual voltage of the low-voltage arrester used on the 220V line is 1.3kV, while the withstand voltage of some sensitive chips is only 6 ~ 10V, which greatly exceeds the safe voltage of the chips.
2.2 MOV action caused by chopper overvoltage.
When the lightning arrester is running, the voltage decreases (cut-off), and the conductor inductance between the lightning arrester installation point and the secondary equipment end and the capacitance of the secondary equipment input end to the ground form a resonant circuit. The intercepted wave passing through the resonant circuit will produce high overvoltage (chopping overvoltage).
3 Countermeasures to solve the problem
3. 1 Equipotential technology of power supply system
C 1024 stipulates that in order to achieve lightning protection potential balance, equipotential conductors or overvoltage protectors should be used to connect external lightning protection devices, building steel frames, installation equipment, various conductors, power supplies and communication equipment in the protected space. When lightning strikes, the potential of the grounding grid rises φ = IRCH = 100 Ka× 2ω = 200 kV, and the horizontal potential drops at the speed of1kV/m. As the electricity for the secondary equipment is provided by the transformer in the substation, the secondary equipment in the substation is distributed in different positions, and the equipment shell is grounded nearby, and the potential difference between the neutral point of the power supply and the equipment shell causes a counterattack, as shown in Figure 2. The huge potential difference caused by the power supply system leads to counterattack and "backward flow".
Calculation of potential difference. The shell potentials of the three devices are: φ (a, b, c) = IR-L (1, 2,3) ε; The potential of power supply is: φ d = IR-L4ε; The potential difference between the equipment shell and the power supply is Δ φ = φ (a, b, c)-φ d, where ε is the voltage drop constant1kv/m. ..
See table 1 for the potential difference data table of each secondary equipment and power supply system.
Solution: The power supply of the secondary equipment supplies power to the secondary equipment through the isolation transformer of 1: 1, so that all parts of the protected object can form equipotential as much as possible, thus preventing the potential difference from damaging the electronic equipment. As shown in fig. 3
Function of isolating transformer: potential fluctuation. The secondary equipment is powered by the power supply through the 1: 1 isolation transformer. By using the principle of "a rising tide lifts a boat", the potential of the local grounding network of the secondary equipment is "floating" and the counterattack is eliminated.
Lightning wave isolation, through the principle of isolating the primary and secondary open circuit of the transformer, isolates the lightning wave invading along the power supply, and the isolated lightning energy enters the earth as a primary and secondary lightning arrester through isolation.
3.2 Residual Voltage Attenuation Technology of Lightning Arrester
In view of the fact that the residual voltage of arrester is far greater than the lightning withstand level of secondary equipment chip, a new type of neutralization transformer can be used for attenuation.
Neutralization transformer consists of annular iron core and coils wound on the iron core. As shown in Figure 4, the working principle of the new neutralization transformer: In general, the neutralization transformer is in the state of differential mode input, and the induced potentials in opposite directions cancel each other, which has no influence on the secondary system.
When lightning invades, the transformer is in the input state of * * * mode, and the lightning flows through the arrester to discharge, which will induce a high potential in the coil, and this part of the changing voltage will offset part of the residual voltage, thus achieving the purpose of reducing the residual voltage.
U output =U input residual pressure -δU,
Δ u = l (di/dt), and the current directions in the two coils are the same, then
Φ=Φ 1+Φ2,
Therefore, the total inductance value is:
L=L 1+L2+2M .
The mutual inductance m of the two coils is determined by magnetic flux and lightning current.
The lightning current is steep. According to the above formula, it can be clearly seen that the voltage output from the neutralization transformer to the secondary system is greatly reduced.
In addition, the use of neutralization transformer can also eliminate the overvoltage caused by direct use of lightning arrester.
4 conclusion
With the large-scale use of computer communication equipment in power dispatching automation system, the harm caused by lightning is becoming more and more serious, and the previous protection system can no longer meet the requirements of computer communication network security. We should systematically and comprehensively consider the prevention of direct lightning, induction lightning wave invasion, lightning protection electromagnetic induction, and ground potential counterattack. Strictly following the lightning protection and grounding regulations, applying new technologies and equipment, adopting equipotential technology of power supply system and residual voltage attenuation technology of lightning arrester are important means to ensure that the lightning damage of power dispatching automation system is greatly reduced.
References:
Tang xingzuo High voltage technology [M]. Chongqing university press, 199 1.
Wang Jian, Zhang,. Study on lightning protection improvement measures for transmission lines [J]. North China Electric Power Science and Technology,1998 (10):1~ 5.
Zhou zecun. High voltage technology [M]. China Electric Power Press, 2004.
[4] Wang Jufeng. Modern lightning protection new technology [M], July 2007.
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