Lightning Protection For Transmission Lines

1. Chapter 9 Lightning Protection of Transmission Lines Requirements of this chapter: Induced overvoltage of transmission lines: Calculation of induced overvoltage on conductors when lightning strikes the ground and towers Direct lightning overvoltage and lightning withstand level on transmission lines Arc creation rate and lightning strike Calculation of tripping rate. Analysis of Lightning Protection Measures and Functions of Transmission Lines Because transmission lines are long, widely distributed and located in the wilderness, they are vulnerable to lightning strikes. There are two types of atmospheric overvoltages that appear on transmission lines: one is caused by lightning strikes on transmission lines, which is called direct lightning overvoltage; (1) direct lightning strikes on conductors, which are most likely to occur on lines without lightning protection wires, but even if there are lightning protection wires , lightning may still bypass the protection range of the lightning protection line and hit the conductor (shield strike). (2) When a lightning strike tower or lightning protection wire, a strong lightning current passes through the tower and grounding resistance, causing the potential of the tower and lightning protection wire to suddenly increase. When the potential difference between the tower and the wire exceeds the flashover voltage of the line insulator, a flashover occurs on the insulator, and a flashover occurs on the wire. Very high voltage.This kind of power tower

2. The bit rises and in turn discharges the wire, which is called counterattack. The other is caused by lightning striking the ground near the line. It is caused by electromagnetic induction and is called induced lightning overvoltage. (3) When lightning strikes the ground near the transmission line and strikes the ground 65m away from the conductor horizontally, the overvoltage induced on the conductor due to the rapid change of the space electromagnetic field is called induced lightning overvoltage.Hazards of induced lightning overvoltage: (3-1) Cause line tripping, affecting normal power supply. Overvoltage causes insulator flashover, short-circuiting wires to ground, lightning overvoltage lasts for a short time (tens of seconds), and the relay protection device has no time to act. , but the power frequency freewheeling current continues to discharge along the discharge channel. After forming a stable burning arc, the relay protection device will trip the circuit breaker, affecting normal power transmission (3-2) Lightning waves intrude into the substation wires to form lightning overpasses. The voltage wave will eventually invade the substation and appear on the electrical equipment after complex refraction.

3. Very high overvoltage endangers equipment insulation and causes accidents. The lightning protection performance of transmission lines is mainly measured by the lightning resistance level and lightning tripping rate. Lightning withstand level: The maximum lightning current amplitude that does not cause impact flashover of the line insulation when a lightning strikes the line, the unit is KA. The higher the lightning resistance level of the line, the smaller the chance of impact flashover of the line insulation. Lightning tripping rate: the number of trips caused by lightning strikes per 100km of line per year. It is a comprehensive index to measure the lightning protection performance of the line. Line lightning protection is a comprehensive technical and economic issue. When determining specific lightning protection measures for lines, it should be based on the line's voltage level, load properties, system operation mode, intensity of lightning activity, topography characteristics and soil resistivity. The conditions such as high and low should be determined through technical and economic comparison, especially in combination with the operating experience of the original local lines.Section 1 Induced lightning overvoltage of transmission lines 1. Lightning strikes the ground near the line, induced overvoltage on the line

4. Sources of pressure-induced overvoltage: electrostatic component: bound charges are instantly released to form induced lightning overvoltage. Electromagnetic component: The main discharge current produces changes in the magnetic field to form induced overvoltage. When S > 65m, the induced overvoltage on a single conductor is: the lightning current amplitude KA; the average height of the conductor hanging m; S the distance between the lightning strike point and the conductor.Characteristics of induced overvoltage: (1) Due to lightning strikes to the earth, the natural grounding resistance of the earth is large, and the amplitude of the lightning current is average.

5. Two or three phases flashover to the ground at the same time, causing an interphase flashover accident. (4) The waveform is relatively gentle, with a wave head of tens of seconds and a wavelength of several hundred seconds. When a lightning protection wire is hung above the conductor: due to its shielding effect, the induced charge on the conductor will be reduced, and the induced overvoltage on the conductor will be reduced. . The shielding effect of the lightning protection wire can be obtained by the following method. Assume that the average height of the conductor and the lightning protection wire to the ground is respectively. If the lightning protection wire is not grounded, the induced overvoltage on the lightning protection wire and the conductor can be obtained respectively. So but the lightning protection The wire is actually grounded through each base tower, so it can be imagined that there is a potential on the lightning protection wire to keep the lightning protection wire at zero potential. Due to the coupling effect of the lightning protection wire and the wire, this will generate a coupling voltage on the wire. Among them, K is the coupling coefficient of lightning protection wire and conductor. In this way, the potential on the conductor will be. The above formula can show that the existence of the ground lightning protection line can reduce the induced overvoltage on the conductor to .coupling

6. The larger the coefficient K, the lower the induced overvoltage on the conductor. 2. When lightning strikes a line tower, the above formula for the induced overvoltage on the conductor is more suitable when S>65m. The closer lightning will hit the line due to the lightning-inducing effect of the line. When lightning strikes a line tower, due to the rapid change of the electromagnetic field generated by the lightning channel, an overvoltage with the opposite polarity to the lightning current will be induced on the conductor. When there is no lightning protection wire, the induced overvoltage coefficient () is equal to the average steepness of the lightning current (). Section 2: Direct lightning overvoltage and lightning resistance level of transmission lines. We use the neutral point to directly ground the line with lightning protection wire in the system. Taking this as an example for analysis, the analysis principles for other lines are the same. The situation of lightning protection lines can be divided into three types, namely, lightning strike on the top of the tower, lightning strike in the center of the lightning protection line, and lightning bypassing the lightning protection line and wire (shielding).1. Overvoltage and lightning resistance level when lightning strikes the top of the tower. When lightning strikes the top of the tower, the lightning channel

7. The negative charge in the tower and the positive charge on the lightning protection line quickly neutralize to form a lightning current. At the moment of lightning strike, a lightning current wave moves downward along the tower from the lightning strike point (i.e., the top of the tower); another two identical negative current waves move from the top of the tower along the lightning protection lines on both sides to adjacent towers, and at the same time, from the top of the tower There is a positive lightning current wave moving upward along the lightning channel, and the value of this positive lightning current wave is equal to the sum of the three negative current wave values. The overvoltage on the line insulation is caused by these currents. The induced overvoltage caused by the movement of positive current waves in lightning channels on conductors has been explained in the previous section. Here we mainly analyze the overvoltage caused by lightning current flowing through towers and ground wires. 1. Tower top potential/lightning protection line potential. For towers with a height of less than 40 meters, in engineering, the tower and lightning protection wires are often replaced by concentrated parameter inductance. The equivalent inductance of different towers can be found in 9-2-2.The equivalent inductance of a single lightning protection wire is approximately (is the length of the span

8. Degree, m), the two lightning protection wires are approximately. It is the tower impact grounding resistance. Considering that the impedance of the lightning strike point is low, the influence of lightning channel wave impedance can be omitted in the calculation. Due to the shunting effect of lightning protection wires, the current flowing through the tower will be less than the lightning current. : The shunt coefficient of the tower can be found from 9-2-3. The potential of the tower top cross arm to the ground can be expressed as Substituting the following two equations into the above equation: Then the amplitude of the cross arm potential to the ground is where is the amplitude of the lightning current; The inductance of the tower body below the cross arm can be expressed as. Bringing the above equation into the above equation, we can get the potential of the tower top cross arm to the ground: where: is the total inductance of the tower; is the height of the tower; is the height of the tower cross arm.The potential at the top of the tower can be expressed as: The potential amplitude at the top of the tower can be expressed as: 2. The potential of the conductor and the voltage borne by the line insulation: (1) The lightning polarity of the coupled wave component is the same (coupling coefficient) (2) Lightning strike on the tower The induced overvoltage on the conductor is the same as the lightning polarity.

9. Inverse (geometric coupling coefficient), so the potential of the conductor can be expressed as the potential difference between the two ends of the insulator: it represents the difference between the cross arm potential and the potential of the conductor, so the voltage amplitude on the line insulation can be expressed as: The above three The following equations can be obtained as a simplified calculation. In the above calculation, it is assumed that the amplitude of each voltage component appears at the same time, and the operating voltage component with uncertain polarity is not considered. For lines of 220kV and below, the proportion of working voltage is not large and can generally be omitted; but for ultra-high and ultra-high voltage lines, it cannot be ignored, and the instantaneous value and polarity of the working voltage on the conductor during lightning strike should be used as a random To consider variables, for strict requirements, the case of different polarity with the cross arm potential should be taken in the calculation. 3. Lightning resistance level When the voltage does not exceed the insulation level of the line, there will be no flashover between the conductor and the tower. From this, it can be concluded that the lightning withstand level of the line when lightning strikes the tower is: It should be noted that the positive polarity in the insulator string should be 50

10. % impulse discharge voltage, because the current flowing into the tower is mostly negative polarity. At this time, the conductor is at a positive potential relative to the tower, and the insulator string is lower when the conductor is of positive polarity. If the lightning current amplitude exceeds the lightning resistance level of the line when lightning strikes the tower, it will cause a flashover of the line, which is called "counterattack". The concept of "counterattack" is very important, because the tower that was originally considered to be grounded is now carrying a high potential, which in turn discharges the transmission line, applying lightning overvoltage to the line, and then invading the substation. In order to reduce counterattacks we must improve the level of mine resistance. The analysis shows that the main measures to improve the lightning resistance level are: (1) Reduce the ground resistance of the tower at ordinary heights. The voltage drop at the top of the tower is the main component of the potential at the top of the tower. (2) Increasing the coupling coefficient can reduce the voltage and induced overvoltage on the insulator string. Use double lightning protection wires and install coupling ground wires. (3) Reduce the shunt coefficient (4) Strengthen line insulation and add insulators

11. Number of pieces, use better insulating materials. 2. Overvoltage when the lightning strike lightning protection wire is in the center. The schematic diagram of the lightning protection wire spacing in the center is as shown in the figure above; from the perspective of breakdown of the air gap between the conductor and the ground wire caused by lightning strike. You see, the most serious situation of a lightning strike on a lightning protection line is when the lightning strike point is in the center of the span, because at this time, the voltage wave with a different sign reflected from the tower ground point takes the longest time to reach the lightning strike point, and the overvoltage amplitude at the lightning strike point is the largest. The highest voltage at lightning strike point A is: This voltage wave moves from the lightning strike point along the lightning protection wires on both sides to the adjacent towers. Since the ground resistance of the tower is much smaller than the wave impedance of the tower and lightning protection wires, it can be approximately considered to be equal to zero. In this way, negative voltage total reflection will occur at the ground point. The voltage wave starts from point A, reaches the ground point of the tower, is reflected, and then returns to point A. The elapsed time can be expressed as: (us) is the length of the span m, is the height of the tower m, and is the wave speed m/us. The impact of corona is 0.75

12 times the speed of light in vacuum. If the lightning current is a flat-top oblique wave, and the expression of its wavefront is (is the lightning current wavefront steepness), then the expression of the lightning strike point voltage can be obtained. Please note the relationships in the above formula, and their expressions; It can be seen from the above formula that when the lightning voltage reaches the maximum value, it is the voltage that will be coupled on the conductor due to the coupling effect of the lightning protection wire and the conductor. Therefore, the highest voltage endured by the air gap S between the lightning protection wire and the conductor is: It can be seen from the above formula that when lightning strikes the center of the lightning protection wire span, the voltage on the air gap S between the lightning protection wire and the conductor at the lightning strike is related to the coupling coefficient k, the lightning current steepness a, the tower height, and the span; when When this voltage exceeds the power generation voltage of the air gap, the gap will be broken down and cause a short circuit accident. Based on theoretical analysis and operating experience, the regulations stipulate that the air distance S between the conductor and the lightning protection wire at the center of the span satisfies (m) when there is no wind. The formula represents the length of the span, m.when

13. When the span length is long (such as a long span span), if S calculated according to the above formula is greater than the value specified in the table below, it can be determined according to the values ​​in the table below, but the conductors of the long span span and lightning protection wires The air distance between them shall not be less than the value specified in the table below. The system voltage distance m367..5 between the large spanning gear line and the lightning protection wire required to prevent counterattacks. As long as the above distance requirements are met, many years of operating experience have shown that when the lightning strike lightning protection wire is at the center of the span, flashover generally does not occur between the conductor and the lightning protection wire. network. Therefore, this situation will not be taken into account when calculating the lightning tripping rate below.3. Overvoltage and lightning resistance levels during shielding. Install lightning protection wires so that all three-phase conductors are within its protection range. There is still the possibility of lightning bypassing the lightning protection wires and directly hitting the conductors. This happens. The probability of bypass is called bypass rate. Once this happens, it will often cause line failure.

14. The flashover of fate string. The shielding rate is related to factors such as the protection angle, the height of the tower, and the topography of the area where the line passes. It can be obtained by the following formula: For plain areas and mountainous areas, the protection angle refers to the connection between the lightning protection line and the side phase conductor and the line passing through the lightning protection line. The angle between plumb lines. The smaller the protection angle, the better the shielding and protection effect of the lightning protection wire on the conductor; is the height of the tower. The amplitude of the voltage on the conductor when shielding the conductor can be expressed as: From the above formula, it can be seen that the voltage amplitude on the conductor increases with the increase of the current amplitude during shielding. If it exceeds the impact flashover voltage of the line insulator string, then The insulator string will flashover, and the lightning withstand level during shielding can be calculated by making it equal to 50% of the flashover voltage of the insulator string. Because the regulations are considered, the calculation method is based on the regulations. The shielding withstand voltage levels of 35, 110, 220 and 330kV lines are about 3.5, 7, 12 and 16kA respectively, which are much smaller than the lightning withstand level when a lightning tower is struck.

15. More! Section 3 Lightning Trip Rate of Transmission Lines When a transmission line is struck by lightning, if the lightning current exceeds the lightning withstand level of the line, an impact flashover will occur on the line insulation, and the lightning current will flow to the ground through the flashover channel. However, the impact flashover lasts only a few seconds. After ten seconds, the relay protection and circuit breaker have no time to operate. After the lightning current passes, the power frequency current (power frequency freewheeling) generated by the power frequency voltage flows through the flashover channel in the form of an arc. Only if the arc burns stably will the continuous Electrical protection operates and the circuit breaker trips. Therefore, when studying the lightning tripping rate of lines, the above factors must be considered! 1. Arc establishment rate When a line is struck by lightning, the probability of turning into a power frequency arc due to impact flashover is related to the average electric field strength in the arc, as well as to the instantaneous value of the power frequency voltage at the flashover moment and the deionization conditions. According to experiments and operating experience, the probability of an impact flashover turning into a stable power frequency arc is called the arc establishment rate. In the formula, E is the average operating voltage gradient of the insulator string kV (effective value)/

16. m. For a system with a directly grounded neutral point and a system with an ungrounded neutral point, for a system with a directly grounded neutral point and no iron cross arm, it is the effective value of the line rated voltage (kV); it is the insulator flashover distance (m). For systems with ungrounded neutral points, single-phase flashover will not cause tripping. Only when the second phase conductor flashes again will it cause phase-to-phase flashover and trip. Therefore, it should be the line voltage and the insulation length between phases. Practice has proved that when (effective value), it can be approximated. 2. Trip rate when lightning strikes the tower. Strike rate: The ratio of the number of times the tower is struck by lightning to the number of lightning strikes lines is called the strike rate. It is generally expressed by letters. It is related to the terrain characteristics and the number of lightning protection lines. The tripping rate is used to express, and its meaning refers to: assuming that there are 40 lightning days per year, the number of possible trips per 100 kilometers of lines due to lightning strikes.It can be used to measure the relative merits of different line design solutions, but does not represent the interruption situation during the actual operation of the line.

17. Indicates the probability that the lightning current amplitude exceeds, expressed as the arc building rate. 3. Shielding tripping rate. Let the shielding rate be 4. The lightning tripping rate of the line. The striking rate g in the above formulas is shown in the table below. Number of lightning protection lines. Terrain 012 Plain 1/21/41/6 Mountainous area 1/31/4 Section 4 Lightning protection measures for transmission lines 1. The development process of lightning accidents on lines and protective measures The development process of lightning accidents on transmission lines and corresponding protective measures are shown in the figure above Explanation: Here is a brief introduction to various lightning protection measures used on modern transmission lines; 1. Lightning protection wire (overhead ground wire) Function: 1. Reduce direct lightning strikes on wires 2. Diverting effect reduces tower top potential 3. Reduced shielding effect Induced overvoltage Installation of lightning protection lines along the entire line is still the most important and effective lightning protection measure for overhead transmission lines of 110KV and above. In addition to preventing lightning from directly hitting the conductor and generating extremely high lightning overvoltage,

18. It is also one of the effective measures to improve the lightning resistance level of lines. The regulations stipulate: a. Double lightning protection wires (protection angle) for the entire line of 220kV and above b. Lightning protection wires (protection angle) for 110kV except for small mine areas. 2. Reduce the ground resistance of the tower. In order to prevent counterattack, the ground resistance of the tower should be reduced to improve Lightning resistance level, very effective for normal height towers. 3. It is used when it is difficult to set up a coupling ground wire to reduce the ground resistance. It has shielding and shunt functions to reduce the induced overvoltage on the wire. 4. Adopt unbalanced insulation method for double-circuit lines on the same pole. 5. Install automatic reclosing device. In order to prevent the establishment of power frequency arc after lightning flashover, install automatic reclosing device. 6. The arc suppression coil grounding operation mode is used to form inductive current and the capacitive current of single-phase grounding (arc discharge) caused by lightning, which offset each other; so that the system returns to normal.7. Install tube-type arresters in weak lines. 8. Reinforce insulated high towers and increase the number of insulators. 9. Install line-type arresters.