Analysis and preventive measures of lightning protection on UHVDC transmission lines

The causes of flashover on UHVDC transmission lines due to lightning strikes are analyzed. Then, after the introduction of the characteristics of flashover caused by lightning strikes, the types of lightning strikes and discrimination method, the calculation method of lightning performance, and the methods of identification of faults, the preventive measures are proposed.


Introduction
Electric energy as the support energy of the national economy, its role is quite prominent.It is not only the power source of modern industry, science and technology, agriculture, national defense, but also closely related to the production and life of the vast majority of people, electricity is the premise and foundation to ensure the development of the national economy, without the power industry, the rapid development of the national economy is impossible [1].The Durong transmission line of 1000 k V had lightning trip faults in 2015 and 2017 respectively.Besides, the Binjin, Fufeng and Jinsu transmission lines of ±800kV had lightning trip faults more than 10 times since 2010 [2].The statistics on classification of faults of overhead transmission lines in China in recent years show that among the tripping times caused by lightning strikes account for a large proportion, and that fault rate was higher especially in areas with frequent lightning, high soil resistivity and complex terrain.Therefore, adequate attentions should be paid to lightning protection in UHVDC transmission.

Analysis of the causes and characteristics of fault due to lightning strikes
The overhead transmission line is long and the transmission corridor environment is complex.There are some factors affecting the normal operation of the line, such as pollution flashover, lightning strike, ice-covered line breaking and crane line collision.According to the operation experience, the tripping rate caused by lightning strikes accounts for 40%~70% of the various factors that cause the tripping of HV and EHV transmission lines [3].Lightning fault can be roughly divided into three categories: direct lightning strike line fault, line fault caused by lightning induced overvoltage and reverse current lightning phenomenon.

Direct lightning strike line
Direct lightning strike is the most common cause of power system failure, including direct lightning strike, winding strike on conductor, lightning strike on tower, lightning wire, etc.The tower height, surrounding buildings and vegetation, terrain and landform are the main factors affecting direct lightning strike High towers have a high probability of being hit by direct lightning.buildings and vegetation have some shielding effect on the direct lightning strike of the line.Lightning flashover is more likely to occur in mountainous areas than in plains.When the tower is obviously prominent, its probability of being struck by lightning will greatly increase.

Lightning induced overvoltage
The high-frequency space electromagnetic field induced by the main lightning discharge process will generate induced overvoltage on the surrounding lines.Through the analysis of lightning monitoring data, it is believed that the main lightning discharge process is the main cause of the induced overvoltage fault in the power system, and the greater the induced overvoltage is the closer to the lightning.

Countercurrent lightning phenomenon
Countercurrent lightning occurs when lightning strikes a tall building, and part of the lightning current flows into the relevant line through the connection system between the building and the line, resulting in the line flashover or the corresponding electrical equipment damage.Since there is no direct lightning line, the damage caused by this phenomenon is different from that caused by direct lightning, but the actual line does bear the impact of direct lightning current [4].Countercurrent lightning often occurs in the distribution network lines, resulting in the burning of the lightning arrester at the end of the distribution network lines.

Discrimination of lightning strike flashover and calculation of lightning performance
The calculation of the lightning performance of lines covers the lightning shielding failure and lightning back flashover.

Lightning shielding failure
For UHVDC transmission lines, due to the requirements on stain resistance, their insulators are longer with high discharge voltage.Lightning accidents are mainly caused by the shielding failure.The current common calculation methods for evaluating the lightning withstand level of lines at the time of shielding failure include regulation method, electrical geometric model method, and the pilot development model method.The regulation method model is suitable for places where the accuracy requirement is not very high, but for UHV transmission lines, the tower height is higher and the terrain traversed is more complex, so there is a great error between the actual value and the calculated value [5].Due to the lack of test and operation data verification of LPM, there are different views on the criteria of LPM, and many calculation parameters are not the same, which will lead to large differences in calculation results [6].In the Literature [7], the electrical geometric model (EGM) is a concept of striking distance first proposed by Golde.This model has been widely used to evaluate the winding performance of conventional ultra-high voltage and below transmission lines in many countries.The basic principle of EGM is as follows: Before the thundercloud develops toward the ground and reaches the critical striking distance from the struck object, the striking point is uncertain.It discharges to the object whose striking distance is first reached.The striking distance is related to the amplitude of the lightning current.In the Literature [8], the formula for calculating the striking distance is as follows: , Where, I is the lightning current; s r is the striking distance of lightning from the lightning conductor; g r is the striking distance of lightning from the earth; c h is the average height of the lightning conductors; ph U is the instantaneous value of the working voltage on the line; c r is the striking distance of lightning from the line.
The electrical geometric model (EGM) of lightning shielding failure is shown in Figure 1.Where, AA is the center line of the tower; S is the shield wire; C is the conductor;  is the protection angle of the between the shield wire and the conductor of a line.If the lightning first reaches the arc AB , it will hit the shield wire, and the conductor is protected.If the lightning reaches the arc BD , it will hit the conductor.Shielding failure occurs due to failed protection of the shield wire.If the lightning reached the plane DE , it will hit the ground.As the amplitude of the lightning current increases to max

I
, BD shrinks to zero, and no shielding failure will occur.According to the amplitude of the lightning current provided by the lightning positioning system, an electrical geometric model is established to determine whether shielding failure will occur.In the Literature [9], the amplitude of the lightning current max I of various tower structures is obtained when the arc BD is zero.When the actual lightning current amplitude is greater than max I , no shielding failure will occur.Taking the ±800kV DC transmission line as an example, the lightning performance was calculated and analyzed.The electric geometric model was used to calculate the skirting flashover rate, and the traveling wave method was used to calculate the counterattack flashover rate.Table 1 shows the lightning protection calculation results of Fufeng, Jinsu, Hazheng, Xizhe and Lingshao UHVDC transmission lines from 2010 to 2012.  1 imply that the stronger the lightning activity of the line, the greater the lightning flashover rate, and the shielding failure flashover accounts for the main part of the lightning flashover.However, the average lightning flashover rate of Fufeng line from 2010 to 2012 was 0.05 times per 100km per year.The calculated value of lightning flashover rate in Table 1 was greater than the operating value of Fufeng line, mainly because the mountainous terrain was complex and this factor  In Table 2, when the ground dip and protection angle remain unchanged, with the increase of wind speed, the shielding failure flashover rate increases, and the lightning performance weakens gradually.When the wind speed reaches 20m/s, the shielding failure flashover rate increases by more than 3 times.In Table 3, when the protection angle and wind speed remain unchanged, the shielding failure flashover rate increases with the increase of the ground dip, and the lightning performance decreases gradually.As the ground dip reaches 30 o , the shielding failure flashover rate increases almost 3 times.As can be seen from Table 4, when the ground dip and wind speed remain unchanged, the shielding failure flashover rate decreases with the reduction of protection angle, and the lightning performance increases gradually.When the protection Angle is -15 o , the flashover rate is only 0.0045 times per 100km per year, and almost no flashover accidents will occur.
The Literature [12] carries out statistical analysis on the transmission lines with serious lightning tripping in various aspects, and obtains the laws related to lightning fault distribution such as tower type, landform, span length, number of conductors on the same tower, and lightning protection configuration, and obtains the lightning risk assessment results, and determines the differential lightning protection transformation scheme with high technical and economic efficiency according to the input and output.Therefore, on the basis of the classical EGM calculation model, the electrical geometry model is improved by adding factors such as landform, wind deviation and lightning incidence angle so that the calculation results can be more accurate and better meet the needs of different actual situations [13].

Lightning back flashover
Back flashover refers to the circumstance in which the lightning current flows through the tower body and the grounding body when lightning strikes the top of the tower or the shield wire and causes induced overvoltage on the phase conductor in addition to increased electric potential of the tower.If , flashover will occur between the conductor and the tower.The lightning back flashover rate of lines can be calculated using the traveling wave method.Insulator U 50% is obtained by standard lightning shock discharge test, but the flashover voltage under the impact of short tail wave is greater than that under standard lightning wave, which also causes the deviation of calculation results [14].When the lightning strikes the top of the tower, the voltage c u on the conductor consists of the following three voltage components: . Where: R u is the induced overvoltage component formed on the conductor at the top of the struck tower; 1 U is the voltage at the striking point; DC U is the operating voltage on the pole line; 0 k is the geometric coupling coefficient between the shield wire and the conductor; 0 c k is the coupling coefficient between the shied wire and the conductor after taking into account the impact of impulse corona on the shield wire.

Causes
(1) The lightning is generally intensified in the area where the line is located, with increased flash density; (2) The topography and geological characteristics of some areas result in inadequate burial depth and grounding resistance of the tower's grounding device, which reduces the lightning protection effect.The design length of the grounding down lead does not consistent to the specification.Because the grounding down lead has not been checked or replaced in long-term use, it is subject to underground corrosion and damage in the process of use, which affects its normal drainage effect.(3) The designed line protection angle is too large; (4) With increased operating years of lines, their insulation level declines; (5) After the composite insulators of old and obsolete lines are replaced, the dry arc distance is relatively reduced, leading to lowered overvoltage level; (6) The operation and maintenance of some lines are improper.

Identification methods
(1) More than 90% of metallic or near-metallic grounding is single-phase grounding, and the fault waveform is a sine wave.In this case, the fault location is more accurate, and traces of discharge can be found on the insulator.After occurrence of failure, the tripping time should be queried in the lightning location system at first.Then, the tower number is calculated based on the protection directions in the schedule or ranging of oscillography.
(2) When it is difficult to identify the defects in insulators during ground inspections, it is necessary to climb up the pole to find the fault under satisfactory grounding condition.
(3) After the tower is struck by lightning, the traveling wave will gradually spread to both sides along the lightning arrester at the location of the lightning strike.The longer the straight length of the two towers is, the larger the straight distance to the tower is.Of course, there is also a large relationship with the lightning resistance level [15].The continuous emission of these waves will generate high-frequency transient voltages and currents on the line.The instantaneous incremental relationship between voltage and current is as follows: Where, ) (t a is the forward wave; ) (t b is the backward wave;  is the --wave impedance.After the instantaneous voltage and current are sampled, the travelling wave value and the position of ground flashover can be calculated based on the known wave impedance of DC line.

Preventive measures
The design of lightning protection systems can be simplified by classifying structures according to the degree of hazard [16].Starting from the "four lines of defense" of lightning protection: (1) Avoid direct lightning; (2) No flashover due to lightning stroke; (3) Impact flashover does not form stable power frequency continuous current; (4) Prevent tripping and power failure.Viewed from the operating of ±800kV system in China, the lightning flashover is mainly caused by lightning shield failure.Therefore, the lightning protection of EHV DC transmission lines should focus on prevention against lightning shield failure, and the following measures can be adopted: (1) Strengthen the basic work of lightning protection of transmission equipment.It is necessary to carry out the work such as observation of lightning activities, statistics of lightning days, special investigations of lightning faults and collection of data for the existing and planned transmission lines.It mainly includes weather, season, time period, terrain, temperature, humidity, wind power, historical failure times, thunderstorm days, mean value of transition resistance, transition resistance, amplitudefrequency characteristics of voltage and current wave, etc. Attentions should be paid to the continuous accumulation of the basic data such as annual average lightning density, distribution of lightning current amplitude, lightning flashover rate, lightning shielding failure rate, thereby providing technical basis for the design and technical transformation of transformation of transmission lines [17].
(2) Strengthen the acceptance inspection of concealed projects of transmission equipment, and conduct effective supervision and inspection of their intermediate links with regard to the grounding devices of transmission equipment which is newly put into operation.It should be checked whether the burial depth of the grounding body should be checked meets the process requirement and whether the ray length reaches the designed length.Furthermore, a basic record of the grounding resistance value and direction should be kept for the transmission equipment.The corrosion of grounding materials is a difficult problem that affects the operation and maintenance of grounding facilities.Therefore, increasing the research on new grounding materials is a key measure to reduce the lightning counterattack trip rate of transmission lines.
(3) Improve the insulation of transmission equipment.Inspections should be timely carried out to replace low-resistance insulators, so as to ensure adequate insulation strength of transmission equipment.In areas with frequent lightning, the composite insulators of new lines should be appropriately extended to ensure the required lightning resistance level.For high towers, the number of insulators should be appropriately increased [18].
(4) Unbalanced insulation.Learning from the experience of unbalanced insulation operation in Japan, the insulation level of one or more circuits in the same tower is improved, and the insulation coordination is used to protect the lines with improved insulation level.
(5) Reduced protection angle.The protection angle is the included angle between the lightning wire and the conductor line and the vertical line.The smaller the protection angle is, the better the protection performance is.For areas with frequent lightning activities, even negative protection angle is used to protect the line.For areas with frequent lightning activities, even use negative protection angle to protect the line [19].
(6) Add lightning wire and coupling ground wire.The lightning wire has the function of reducing the shielding failure rate of the line, shunting and shielding the conductor, and is usually erected above the conductor.The coupling ground wire can strengthen the coupling effect of the lightning wire and also has the function of shunt.It is usually erected under the conductor when it is difficult to reduce the grounding resistance, which can effectively limit the overvoltage on the insulator and reduce the lightning trip rate.
(7) Use parallel gap.Parallel gap is a lightning protection measure that our country began to develop in the 1990s after absorbing the excellent operating experience of foreign countries, and it is one of the representatives of "scatter type" lightning protection measures.By connecting the two ends of the line insulators in parallel, the lightning flashover at one end of the gap and dredging the power frequency arc can be used to protect the insulator from being ablated by lightning arc.After the lightning flashover arc is extinguished along the gap, the power supply is restored by an automatic reclosing device at the expense of a certain trip rate.
(8) Reduce the grounding resistance of the tower.In addition to improving the testing cycle and management system of grounding resistance should be carried out, so as to ensure that the grounding resistance meets the design requirements.For powers in mountains or places with high soil resistivity and easy soil erosion, the resistance testing cycle should be shortened.Before the lightning and rainy season, the testing and individual excavation inspections should be carried out.Furthermore, grounding modules and conductive cement, etc. should be adopted to minimize the grounding resistance of the tower.The grounding resistance testing should be standardized to ensure the accuracy of measurement data [20].
(9) Strengthen the prevention of lightning shielding failure of transmission equipment.At present, line arrester is widely used for line lightning protection.Line arrester plays a significant role in line lightning protection [21].This is a very reliable lightning protection countermeasure obtained by experts and scholars through a lot of practice.In areas where the towers are prone to lightning strikes, the prevention measures such as installation of zinc oxide lightning arrester with gaps and erection of bypass shield wire and lightning rods can be adopted against lightning shielding failure.Transmission equipment also may be equipped with tower rod.Tower pin mainly includes tower lightning rod and side rod.The key point of using the tower rod is to estimate the critical length of the rod through the experiment.If the rod is too short, it will be shielded by corona, and if the rod is too long, it is not convenient for construction and utilization.

Conclusions
Lightning activity is a complex natural phenomenon, which requires the cooperation of the power sector to reduce the occurrence of lightning damage and minimize the loss caused by lightning damage.
was not taken into account in calculation, leading to the deviation of the value [10].In the Literature[11], according to the principle of the improved electrical geometric model, the effects of wind speed, ground dip and protection angle on the performance of the UHVDC transmission line are analyzed and calculated respectively.
between the increased tower body and the induced overvoltage of the phase conductor exceeds the flashover voltage value of the insulation of the high-voltage transmission line just [1] Bose B K 2013 Global Energy Scenario and Impact of Power Electronics in 21st Century IEEE Transactions on Industrial Electronics [2] Xi Chongyu, Wang Haiyue, Duan Feifei, et al. 2016 Typical cases analysis of ±800 kV UHV DC transmission line Hunan Electric Power [3] Song Pan, Li Shipeng and Wang Daping 2016 Optimal and Economical Design of UHV Line Grounding for Lightning Journal of Shandong Electric Power College [4] Dai Qi 2018 Research on Lightning Flashing Arc Characteristics and Power System Fault Analysis Guangxi University [5] Power Industry Standard of the People's Republic of China DL/T 620-1997 Overvoltage protection and insulation coordination for AC electrical installations 1997 [6] Taniguchi S, Okabe S, Takahashi T, et al. 2008 Flashover Characteristics of Long Air Gaps with Negative Switching Impulses IEEE Transactions on Dielectrics and Electrical Insulation [7] Golde R H 1967 The lighting conductor Journal of the Franklin Institute [8] IEEE Standard 1243-1997 IEEE Guide for Improving the Lightning Performance of Transmission Lins 1997 [9] Ma Yutang, Wu Guangning, Zhang Xinghai, et al. 2010 Influence of topography on max shielding failure lighting current of transmission lines Insulators and Surge Arresters [10] Zhou Wei, Chen Xingfu and Ren Wei 2015 Research on lightning flashover of UHV and EHV DC transmission lines Sichuan Electric Power Technology [11] Tang Chunlin 2019 Research on influencing factors of the lightning protection performance of UHVDC transmission lines Xihua University [12] A Chusov, M Murmann, R Fuchs, et al. 2017 Investigation of impulse arc quenching in multichamber systems The 10th Asia Pacific International Conference on Lightning Krabi, Thailand [13] A J Erikssion 1987 The Incidence of Lighting Strikes to Power Lines IEEE Trans on Power Delivery [14] Hao Y, Han Y, Tang L, et al. 2015 Leader propagation models of ultrahigh-voltage insulator strings based on voltage/time curves under negative lightning impulses at high altitude IEEE

Table 1 .
±800kV UHVDC transmission lines of lightning protection for checking.

Table 2 .
Effect of wind speed on lightning performance of winding.

Table 3 .
Effect of ground dip on lightning protection performance.

Table 4 .
Effect of protection angle on lightning protection performance.