Lightning Strike Analysis And Lightning Protection Countermeasures For 10kV Overhead Insulated Lines

This article analyzes the causes of lightning overvoltage of overhead insulated lines and proposes how to solve lightning protection countermeasures for insulated lines based on the mechanism of lightning damage.

The power supply reliability of distribution network is a basic concept and important indicator in production, application and life. It not only affects the production and consumption of electricity by industrial enterprises and residents' lives, but also affects the quality and social benefits of electricity. With the rapid development of the distribution network, the power supply area is covered by trees, buildings under the lines, typhoons and many other factors have caused the reliability of the distribution network to face new difficulties.

Due to various threats posed by nature to the distribution network, overhead insulated lines are produced. Zhaoqing Power Supply Bureau began to use overhead insulated wires in suburbs, counties and towns during the transformation of the distribution network in 2002. However, lightning strikes and broken wires frequently occur during the thunderstorm season every year. Therefore, lightning protection of overhead insulated lines is a very important task.

Characteristics of overhead insulated lines

Good insulation properties. Due to the addition of an insulating layer, overhead insulated conductors have superior insulation performance than bare conductors, which can reduce the distance between lines, reduce the insulation requirements for line supports, and increase the number of loops of lines erected on the same pole. Good moisture-proof and anti-corrosion properties. Since overhead insulated wires have an insulating layer on the outside, they are less susceptible to oxidation and corrosion than bare wires, and have strong moisture-proof and corrosion resistance, which can extend the service life of the lines. Prevent external damage, reduce the impact of external factors such as trees, floating objects, metal oxide films and dust, and reduce phase short circuits and ground faults. The strength meets the requirements. The insulated wire is tough, and the mechanical strength of the entire wire meets the stress design requirements. However, some new problems also arise during the application of insulated wires. Among them, the most prominent problem is that disconnection failure occurs easily when struck by lightning.

Lightning strike fault condition

Guangdong Province is an area prone to thunderstorms. According to the reference materials provided by the meteorological department, the average thunder and lightning days in our city in the past three years is 75 to 80 days, which is an area with high thunder and lightning days. In 2009, there were a total of 1,514 line fault trips in the city's distribution network, of which 751 were lightning strikes, accounting for 49.6% of the failure rate. It can be seen that the main cause of line failures is lightning strikes.

There are 145km of 10kV overhead insulated lines in the city, accounting for 9% of the city's distribution lines. According to incomplete statistics, the city's overhead insulated lines were disconnected 25 times from 2007 to 2009, including 10 times in 2007, 9 times in 2008, and 7 times in 2009. Second-rate.

Analysis of lightning strike disconnection mechanism

Lightning overvoltage flashover in distribution network is a large current discharge at atmospheric pressure or higher than atmospheric pressure, which is a form of arc discharge. The cause of disconnection of insulated wires caused by lightning is the same as that of bare wires. The lightning overvoltage destroys the insulation of the line, causing a short circuit to the ground. Since the 10kV distribution network of our office is a grounding system through arc suppression coils, the current flowing through the ground is compensated when single-phase grounding occurs, and single-phase grounding faults generally do not cause disconnection. When a flashover occurs between two or three phases, a metallic short-circuit channel is formed, causing thousands of amperes of power frequency freewheeling, and the arc energy will increase sharply.

When bare conductors are used in distribution lines, line flashover will occur when struck by lightning (including direct lightning and induced lightning). At this time, the arc caused by the continuous power frequency short-circuit current causes the arc to spread rapidly to both sides of the lightning strike point of the conductor under the action of electromagnetic force. The lightning current quickly flows into the earth through the lightning arresters of lines, switches, transformers and other equipment, or at work. Frequency current burns out the wire before causing it to trip, so disconnection accidents rarely occur.

However, when an insulated conductor is struck by lightning, the situation is different. The lightning overvoltage causes the insulator to flashover and breakdown the insulation layer of the insulated conductor. The insulation near the breakdown point prevents the arc from moving rapidly to both sides along the surface of the conductor. Therefore, the arc can only burn at the breakdown point. The heat generated by power-frequency arc currents (kA/ms level) of up to thousands of amps is concentrated at the insulation breakdown point and fuses the wires quickly before the circuit breaker trips. The flashover fuse point is usually within 10 to 30cm at both ends of the insulator of the conductor, because the electric field distribution in this range is the weakest.

When a lightning strike acts on an insulator, the flashover of the insulator depends on the overvoltage value and the line insulation level. The probability of arc generation usually depends on multiple parameters: rated line voltage U2, flashover path L, the time when the lightning strike occurs, and the lightning current size and line parameters, etc. Among these parameters, it is mainly determined by the average gradient of the operating voltage along the flashover path. According to the calculation formula, the heat generated by the power frequency freewheeling current will be 10,000 times greater than the heat generated by the lightning current. It can be seen that induced overvoltage is the inducement of lightning strike disconnection, and power frequency freewheeling is the decisive factor causing disconnection of insulated wires.

Look at the relationship between the arc current value and fusing time of bare conductors and insulated conductors of the same cross-section. The fusing time of insulated wires is extremely short. For insulated wires and bare wires of the same thickness, the fusing time of insulated wires is only 1/5 to 1/10 of the fusing time of bare wires. This means that before the substation protection operates and interrupts the fault current, the possibility of wire disconnection becomes greater. Since insulated wire breakage usually occurs when a two-phase short circuit or a three-phase short circuit fault occurs, there are many cases of two-phase breakage and three-phase breakage.

Protection measures against lightning strike disconnection

Generally speaking, the methods to prevent overhead insulated conductors from being disconnected by lightning strikes are mainly summarized into two methods: "blocking" and "drainage".

"Blocking" means to improve the insulation level of the line, reduce the probability of lightning flashover of the insulation, and prevent the power frequency from continuing to flow and arc building after lightning flashover; "draining" means to allow the line to have a certain probability of lightning flashover, but to prevent lightning flashover. The arc root of the subsequent power frequency freewheeling current is "groomed", that is, the insulated wire is partially stripped, the insulated wire near the insulator is partially exposed, and an anti-arc clamp is installed to transfer the arc root of the power frequency arc, and the arc root is transferred during the stripping process. Partially slides instead of being fixed at one point to ablate, thus protecting the wires from burns. The specific methods and measures are as follows.

⑴.Install overhead ground wire

It mainly converts the lightning overvoltage with a large amplitude into current and discharges it through the grounding resistance of the tower, thereby greatly reducing the lightning overvoltage and protecting the conductors. The insulation level of the 10kV distribution network is low, and lightning strikes on overhead ground wires can easily cause counterattack flashovers. Failures such as power frequency freewheeling and burning of insulated wires can still occur.

According to statistics, the probability of distribution lines suffering from direct lightning strikes or bypass lightning strikes is relatively small, accounting for only about 20% of lightning accidents. 80% of lightning overvoltage faults on distribution lines are induced overvoltages. The maximum induced voltage of a 10kV overhead line without an overhead ground wire can reach 550kV. After installing an overhead ground wire, the maximum induced overvoltage drops by as much as 40%. Therefore, installing overhead ground wires on distribution lines in areas with frequent lightning strikes is an important measure to limit induced overvoltage.

⑵. Install gapless line composite jacket zinc oxide arrester

Zinc oxide arresters can limit the amplitude of induced overvoltage, absorb discharge energy after lightning flashover, and prevent power frequency freewheeling arcing, thereby achieving the purpose of protecting insulated wires. The protection range, lightning characteristics and arrester parameters of zinc oxide arresters are related to the ground resistance value of the arrester grounding network and the line insulation level. It is currently difficult to collect field data on the lightning characteristics and can only be based on simulated field test data provided by research institutions. , Use line arresters to protect against lightning and suppress lightning damage. The key is the grounding impedance of the grounding wire.

Four double-circuit lines on the same pole, including the 10kV Ruihua line, which were put into operation in 2003 at the foot of Beiling Mountain in the suburbs of Zhaoqing City, were designed according to the induced overvoltage amplitude of 200-300kV peak value and the wave head of 2μS; the line adopts FP-1-0/ 2.5 or FZS-0/5T pin type, pillar composite insulator, lightning protection grounding resistance is 10~25Ω, a set of lightning arresters is installed every 2~3 gear poles combined with the planned down switch, and only 2 lightning strikes occur in 8 years of operation Disconnected.

The inspection found that the disconnection points all occurred on the tower between the two sets of arresters. As a line overvoltage protection, the composite jacketed zinc oxide arrester can reduce lightning strike disconnection faults. However, due to the limitation of the protection range, lightning strike disconnection accidents cannot be completely eliminated. Moreover, long-term exposure to operating voltage accelerates the deterioration of the resistance valve piece and causes damage or the grounding down lead is stolen, which will lose the protective effect of the line.

⑶.Install overhead line overvoltage protectors

It is composed of a non-linear resistor current limiting element (zinc oxide valve plate) and a series discharge gap (stainless steel drainage ring). When lightning overvoltage or other fault causes a flashover to the ground to form a metallic arc discharge short circuit, the line protector The specially designed stainless steel drainage ring can directly guide the thousand-ampere power frequency freewheeling current to the nonlinear current limiting element, and convert the sinusoidal power frequency freewheeling current into a peak wave with the help of the nonlinear characteristics of the resistor current limiting element.

The peak wave current has a small current amplitude for a period of time before crossing zero. At the same time, the residual voltage of the current limiting component reduces the discharge voltage, causing the arc to instantly extinguish and quickly cut off the power frequency freewheeling, effectively preventing overhead insulated wires from being damaged due to work. The purpose of continuous flow of high temperature and fusing (breakage due to lightning strike). This method is installed in parallel with the insulator without destroying the insulation layer of the insulated conductor. It improves the impact withstand voltage level of the line and ensures that flashover occurs only under the action of particularly high lightning-induced overvoltage. Power-frequency aftercurrent will be caused by the synthetic insulator. The discharge creepage distance is too large to establish arc and extinguish.

Advantages of overhead line overvoltage protectors: First, maintenance and management are simple; second, when the protector operates, the power frequency freewheeling current is immediately interrupted, the circuit breaker will not trip, and the power supply will not be interrupted; third, there is no need to strip the insulated wires. The insulation layer will not expose the live parts and can be installed live. Disadvantages: First, the investment is large; second, the number of equipment on the pole increases; third, grounding devices need to be installed.

Our bureau began to install overvoltage protectors on overhead insulated lines in 2008. After more than a year of operation, no disconnection fault occurred on the lines where the overvoltage protectors were installed.

⑷.Install arc-proof fittings on overhead insulated lines

The hardware strips off the insulation layer of a short section of the insulated wire within a range of 150 to 200mm from the center of the insulator (load side), and installs the arc-proof hardware, so that the lightning overvoltage can flashover at a fixed point between the arc-proof hardware and the insulator steel feet. The arc root of the continuous power-frequency short-circuit current arc is fixed on the arc-proof fittings and burns, thereby protecting the wires from burns.

This method is easy to operate, has low investment, does not require a grounding device, and can prevent disconnection due to lightning strikes. However, it requires stripping off the insulation layer, leaving some exposed conductors, sealing and insulation defects, and burnt arc-proof fittings must be replaced after a lightning strike.

In 2009, a lightning strike occurred on a 10kV overhead insulated line in the urban area. The short-circuit current measured on the substation side was as high as 18kA. The column-mounted vacuum circuit breaker on the insulated line burned out. The arc-proof gold installed on the line had obvious traces of discharge. Analysis of the overhead insulated line It was attacked by direct lightning. The lightning current at the lightning strike point exceeded 30kA. The arc-proof fittings and arresters on the line failed to effectively "block" and "drain", causing the insulated wires to break.

Conclusion

Lightning is a complex natural phenomenon. To sum up, it is difficult to solve the lightning protection problem of insulated lines simply by relying on a certain protective measure. Comprehensive lightning protection countermeasures must be taken to effectively prevent lightning accidents, using "drainage" and "blocking" A combination of lightning protection measures, that is, installing overvoltage protectors every 3 to 4 steps on overhead insulated lines and installing arc-proof fittings on the remaining towers, can effectively reduce the probability of lightning flashover and avoid lightning disconnection faults. , to ensure the safe operation of the distribution network.

(Compiled from "Electrical Technology", the authors are Lu Jun and Ding Jianyong.)