Technology Research on Lightning Strike Risk Evaluation of a Cable Car

Yong HU, He FENG

Hebei Lightning Protection Center, Shijiazhuang 050021, China

Lightning strike is one of ten most serious natural disasters in nature.A disaster is a kind of risk, and control of a disaster means managing,reducing or eliminating risk,so it is needed to recognize and assess risk correctly.Lightning strike risk evaluation means lightning strike risk is analyzed and calculated according to the function and structural features of a project and local meteorological,geographical, geological and environmental characteristics,and it is an effective way to evaluate and reduce risk.In recent years,scholars at home and abroad have paid more attention to technology research on lightning strike risk evaluation and have established related technical specifications for different evaluation objects. For instance,the standard IEC62305-2 Risk Management proposed by the International Electrotechnical Commission has been widely applied in lightning strike risk evaluation[1].

A cable car, a big modern recreation facility in a mountain scenic spot,is distributed along mountains and can be struck by lightning frequently.Moreover, the lightning strike can threaten visitors’ safety in the cable car seriously.Evaluating lightning strike risk of a cable car to seek scientific,effective,economic and reasonable protective measures is an effective way of preventing and reducing lightning damage of the cable car. However, there are perfect evaluation methods for lightning strike risk of buildings and electronic information systems in current evaluation standards, and these methods are not appropriate for lightning strike risk evaluation of a cable car. Here, according to the specific characteristics of a cable car, an appropriate method of lightning strike risk evaluation for a cable car would be put forward based on the evaluation model and evaluation method in the standard IEC62305-2.

Lightning Strike Danger of a Cable Car

A cable car is mainly set up in a mountain scenic spot,but by comparison with surrounding areas, lightning is very strong and frequent in the mountain scenic spot because of geographical factors. A cable car projects over mountains and their surface objects, and cableways and cable cars made of metal. According to the choice of lightning strike, towering and projecting metals can be struck by lightning very easily, so in comparison with mountains, cable cars can abstract cloud-to-ground lightning discharge. Frequency of lightning can affect lightning strike frequency of an evaluation object in the study area,and complex lightning environment in mountain scenic spots increases lightning strike loss and risk of a cable car[2].

Lightning Strike Risk of a Cable Car

According to the analysis above,cable cars has high lightning strike risk, and lots of tourists ride in a cable car in peak tourist season, and most tourists are exposed in field, so lightning strike can threaten tourists’ life safety. Running of a cable car depends on electric energy that will be transformed into kinetic energy. Once lightning electromagnetic pulse(LEMP) results in the cable car’s system fault and interruption of power supply, it will influence public service, so that large quantities of tourists are detained in danger zones. Therefore,people’s life loss risk and public service loss risk are main types of lightning strike risk of a cable car.

Different from common buildings,a cable car is a scattered close space,and lightning damages mainly include the direct damage to tourists caused by contact voltage, side flashover and heat effect as well as the damage to electronic information systems caused by lightning electromagnetic pulse. In addition, stopping running of cable cars due to system fault will affect tourists’safety in cable cars and waiting areas.Lightning strike damage and risk of various damage sources are shown in Table 1[3].

Table 1 Risk types of lightning disaster

Method of Lightning Strike Risk Evaluation for a Cable Car

According to the evaluation model in the standard IEC62305-2, lightning strike risk R is defined as the ratio of annual average possible loss caused by lightning strike to value of a protected object. It can be calculated according to the formula R=NPL=ΣRX=NXPXLX, where N is annual average frequency of lightning strike, which is related to lightning environment in the study area, characters and size of an evaluation object, surrounding environment and soil features; P is damage probability of buildings caused by lightning strike, which is related to characters and size of the evaluation object and protective measures;L is indirect loss, which is related to use of the evaluation object,involved peoples and measures of reducing loss. Lightning strike risk evaluation include calculation of relevant parameters and risk components, and the calculated values of risk are compared with allowable values[4].Based on the evaluation model above, lightning strike risk components of a cable car in a mountain scenic spot at a height of 50 m are studied from aspects of damage probability and loss. As shown in Fig.1, a cable car in the mountain scenic spot is 10 m in height h and 1 m in width w,and an overhead ground wire is set up over the cableway to preventing the cable car from being struck by lightning.

Lightning strike risk components of a cable car

Components of people’s life loss risk R1are shown as follows:RGis risk component of biological damage in a cable car caused by high voltage and heat effect when the cable car is struck by lightning, RG=NEPGLG; RHis risk component related to failure of electronic information systems caused by LEMP as power lines are struck by lightning,RH=NLPHLH;RKis risk component related to failure of electronic information systems caused by LEMP as areas near power lines are struck by lightning,PK=NIPKLK;RDis risk component related to biological damage caused by contact voltage and step voltage when waiting areas are struck by lightning,RD=NFPDLD;RNis risk component related to biological damage caused by direct lightning strike when waiting areas are struck by lightning,RN=NFPNLN.

Components of public service loss risk are shown as follows: RWis risk component related to failure of electronic information systems caused by LEMP as power lines are struck by lightning, RW=NLPWLW; RZis risk component related to failure of electronic information systems caused by LEMP as areas near power lines are struck by lightning,RZ=NIPZLZ.

Modern big cable cars may be equipped with safety system, fire protection system and automatic control system and involve signal network lines and other service facilities, and system fault will affect normal running of the cable cars, thereby increasing risk components related to failure of electronic information systems caused by LEMP on power lines and in areas near power lines struck by lightning[5].

Expected annual average frequency of lightning strike

Expected annual average frequency of lightning strike involved in lightning strike risk evaluation of a ca-ble car include annual average number of dangerous events when a cable car is struck by lightning NE, annual average number of dangerous events as waiting areas are struck by lightning NF, and annual average number of dangerous events when service facilities and areas near the service facilities are struck by lightning NLand NI.Here, annual average numbers of dangerous events when cable cars and waiting areas are struck by lightning that are not involved in the standard are calculated.A mountain scenic spot is large and open, so the effects of surrounding environment on expected annual average frequency of lightning strike are considered separately. Annual average numbers of dangerous events in a cable car and a waiting area can be calculated according to the formulas as follows: NE=NgAe, NF=NgAf, where Ngis ground flash density; Aeand Afare equivalent collection areas of a cable car and a waiting area.

Ground flash density Ground flash density Ngis the frequency of lightning strike per square kilometer every year,and its formula based on annual average thunderstorm days Tdare shown in the standard, namely Ng=0.1Td.However, a cable car, different from common buildings, runs from the foot to the top of mountains, and its length ranges from thousands of meters to decade thousands of kilometers. Due to the effects of mountain terrain and geology, there is a big difference in rules of lightning activity along a cableway, but ground flash density calculated based on annual average thunderstorm days can not reflect the difference[6].In regions with mature lightning location system, ground flash density can be calculated based on lightning location data, which can be applied in lightning strike risk evaluation of a cable car. When lightning strike risk of a waiting area is calculated,the difference between ground flash density at the top and foot of mountains. However, ground flash density in a direction can be calculated based on lightning location data, but lightning location system has a certain range error due to terrain, location method and instrument itself, and the range of waiting areas is far smaller than the range error,so collecting data from waiting areas may affect the accuracy of calculation results. To reduce the effects of inaccuracy of lightning location data on ground flash density as much as possible, a large range with lightning activity rules similar to the evaluated zone is chosen to analyze and calculate ground flash density according to choice of lightning strike and local actual characteristics.

Equivalent collection area Equivalent collection area of a cable car or a waiting area is regarded as a straight line during the process of calculation.In the method recommended in IEC62305-2,equivalent collection area refers the ground area projected by a line with slope of 1/3 and contacting with the evaluated object. The surface of mountains is tilted, so expand width 3h is taken in each vertical plane to draw equivalent collection interval.Ground flash density is the frequency of lightning strike per square kilometer every year, so it is needed to project equivalent areas in tilted mountains on the ground (Fig.2), namely Ae=200×60.27×10-6=1.21×10-2km2[7].

Ground flash density can reflect difference in the frequency of lightning strike at the top and foot of mountains,so it is not needed to consider the vertical height of a waiting area,and there is no building or a layer of common building in a waiting area,so the equivalent collection area of a waiting area can be regarded as the area of the waiting area, or it is calculated based on the method recommended in the standard.

Probability of lightning damage

Probability of biological damage caused by high voltage and heat effect PGLightning is an electric discharge phenomena that release strong energy and has hundreds of thousands and tens of millions volt of impulse voltage, and the peak current ranges from several tens of thousands to hundreds of thousands ampere. A cable car is made of metal,and once it is struck by lightning, the current can be discharged into the ground throng a support tower. The lightning strike point is far from the ground, and ultrahigh voltage will produce at the light-ning strike point because of ground impedance,electric resistance of a cable car and metal perceptual impedance. Meanwhile, lightning energy will be transformed into large amounts of thermal energy, so that temperature is extremely high in a part. Only a cable car is close can the damage be evaded,so it is impossible to damage a close cable car, while as for open cable cars, only protective measures against direct lightning are adopted can the lightning strike frequency of open cable cars reduce.Protection efficiency of direct lightning can affect the probability of biological damage caused by high voltage and heat effect[8]. The empirical value of physical damage probability of buildings struck by lightning PBis given in the standard,the probability of biological damage caused by high voltage and heat effect when an open cable car is struck by lightning is calculated using the method. An overhead ground wire is set up over the cableway to prevent a cable car from being struck by direct lightning. As shown in Fig.3, when the radius of a rolling sphere R is 13.82 m, an overhead ground wire can protect the cable car effectively; when the radius is smaller than 20 m (LPLI grade),PGis 0.01 according to PB.

Probability of casualty caused by direct lightning strike in a waiting area PNWhen there is a thunderstorm,large quantities of tourists gather in a waiting area, and people’s safety depends on protective capability of direct lightning. When there are no protective measures against direct lightning in a waiting area, the probability of casualty caused by direct lightning strike in a waiting area PNis 1; as there are protective measures against direct lightning, but some regions are not protected,PNis the ratio of protection area to total area; when there are complete protective measures against direct lightning in a waiting area, it is 10-4.

Probabilities of casualty caused by failure of electronic information systems PHand PKWhen power lines of a cable car and areas near power lines are stuck by lightning, due to failure of electronic information systems caused by LEMP, lots of tourists are detained in a cable car and a waiting area, which increases the probability of casualty. Failure of electronic information systems, detention of tourists in dangerous areas and lightning strike in dangerous areas are conditions of people’s life loss risk related to failure of electronic information systems.As power lines of a cable car and areas near power lines are stuck by lightning, probabilities of casualty caused by failure of electronic information systems PHand PKcan be calculated according to the formulas as follows: PH=PU(PG+PN), PK=PV(PG+PN),where PUand PVare probabilities of system failure when service facilities and areas near the service facilities are struck by lightning, and they are given a value according to the standard IEC62305-2.

Loss L

In the standard IEC62305-2, indirect loss L of lightning strike risk components can be calculated according to casualties, total number of people and damaging time. Lightning strike risk evaluation of a cable car involves people’s life loss risk and public service loss risk, so the loss can be calculated according to the method in the standard.

Conclusion

According to the lightning strike characteristics of a cable car, an appropriate method of lightning strike risk evaluation for a cable car was put forward based on the evaluation model in the standard IEC62305-2.The method can be applied in lightning strike risk evaluation of a cable car and has certain guiding significance to the establishment of a scientific, effective, economic and reasonable lightning protection scheme. However, the method depends on current evaluating regulation and is suitable for simple cable cars, but the environment is complex in regions with cable cars, so it is needed to analyze local lightning environment and layout structure of a cable car to put forward a scientific and reasonable quantitative evaluation method.

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