CN105785243A - Evaluation method for insulator icing flashover risk of ultrahigh voltage alternating-current transmission line - Google Patents

Abstract

The invention discloses an evaluation method for an insulator icing flashover risk of an ultrahigh voltage alternating-current transmission line. The method comprises: S1, carrying out meteorological and line basic data collection; S2, carrying out determination of an insulator icing flashover initial condition; S3, carrying out insulator icing flashover voltage calculation; and S4, carrying out insulator icing flashover risk evaluation. According to the invention, on the basis of the real-time meteorological data, an insulator icing flashover risk is evaluated by combining basic data of the ultrahigh voltage alternating-current transmission line and the powerful guidance can be provided for disaster prevention and reduction of the power grid operation management department. The method can be realized simply with high feasibility; and the prediction result has the high reference value.

Description

A kind of ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment

Technical field

The invention belongs to electrical network to prevent and reduce natural disasters field, particularly relate to a kind of ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment.

Background technology

UHV transmission has the advantage that transmission capacity is big, power transmission distance is remote, power transmission efficiency is high, it has also become the main electric energy mode of movement of the problems such as Energy Base improves day by day and line corridor is nervous all the more is alleviated with load center skewness weighing apparatus, transmission line capability in China." 12 " period, State Grid Corporation of China invests 500,000,000,000 yuan, built and at the UHV transmission line built 40,000 kilometers, transmission line capability 4.3 hundred million kilovolt-amperes, gradually forms the energy source configuration general layout in " transferring electricity from the west to the east, north and south mutually for, on national network ".In UHV transmission line process of construction, due to line corridor growing tension, increasing track section needs through locations with a varied topography such as high mountain high hill, valley, rivers, inevitably it is subject to the impact of the atmospheric environments such as ice and snow, low pressure, High aititude, filth, there is the risk of icing flashover in insulator, power grid operation constitutes direct threat.

Southeast Shanxi-Nanyang-Jingmen UHVAC demonstration project, as First extra high voltage line, there occurs 3 icing flashover faults altogether since on January 6th, 2009 formally puts into operation.By accident conditions are analyzed, show that failure cause is as follows: the 1st fault (2010-02-1612:41) is that typical covering ice for insulator flashover, wire and the icing situation on insulator are all very serious；The edge flashing fault that 2nd fault (2012-03-0109:47) occurs when ice dissolution and low temperature high humidity for contaminated insulator；3rd time fault (2012-03-0220:25) belongs to glaze icing flashover fault, and covering ice for insulator bridge joint degree is 60%～70%, has reached serious icing form.The insulator chain length of 3 icing flashover accidents is 10.5m, and height above sea level is 664.5～831m, and site pollution degree is c～d level.

It can be seen that the factor such as covering ice for insulator flashover and the icing degree of insulator, dunghill concentration, air pressure is closely related from UHVAC demonstration project ice sudden strain of a muscle accident.Mountain area at height above sea level about 500～1000m, particularly in c level and above gradation for surface pollution area, the probability of extra high voltage line insulator generation icing flashover is relatively big, needs to consider the weather environment residing for insulator contamination situation, icing situation and insulator in carrying out powerline ice-covering flashover risk assessment processes.

In real work, history ice sudden strain of a muscle data are mainly based upon for powerline ice-covering flashover risk assessment or icing on-line monitoring information realizes.But, the historical data dodged about UHV transmission line ice is few, and the risk evaluation model representativeness thus set up is not strong, and the error of assessment result is also bigger；In addition, for the on-Line Monitor Device scarcity of icing on UHV transmission line, and carry the extra high voltage line of bulk transmission grid not have a power failure as grid power and carry out the installation of icing on-Line Monitor Device, and Electromagnetic Environment of UHV AC Transmission Lines is severe, the properly functioning of on-Line Monitor Device and data communication requirements is significantly high, so causing being difficult to based on the icing flashover methods of risk assessment of on-line monitoring information.Therefore, it is necessary to set up one for ultrahigh voltage alternating current transmission lines practical, icing flashover methods of risk assessment accurately and reliably, dodge the real-time management and control of risk for realizing ice and ice damage emergence treatment scheme offer technological means is provided in advance, and can according to assessment result shaft tower that flashover risk class is higher, tune can be taked in advance to climb, change serial type, use coating, winter cleans the Flash Decontamination Measurements such as filthy before icing, contribute to fundamentally reducing the probability of happening of ice disaster weather ultrahigh voltage alternating current transmission lines icing flashover fault, improve electric power netting safe running stability.

Summary of the invention

The technical problem to be solved in the present invention is in that feature few for ultrahigh voltage alternating current transmission lines history ice sudden strain of a muscle data, about icing on-Line Monitor Device scarcity, thering is provided the method that the real time meteorological data of a kind of basic data according to ultrahigh voltage alternating current transmission lines and location realizes covering ice for insulator flashover risk assessment, the operational reliability and fine-grained management level for improving ultrahigh voltage alternating current transmission lines provides practical, technological means accurately and reliably.

The technical solution adopted for the present invention to solve the technical problems is:

The present invention provides a kind of ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment, comprises the following steps:

S1, meteorology and line tower foundation data collection: investigate the basic data of ultrahigh voltage alternating current transmission lines to be assessed, and obtained the real time meteorological data in transmission line of electricity corridor along the line by the automatic meteorological observing station of meteorological department or the microclimate on-Line Monitor Device of grid company；

S2, covering ice for insulator flashover initial conditions judge: judging whether to meet icing condition according to the meteorological data of insulator place to be assessed section, the insulator for meeting icing condition further determines whether the risk that there is icing flashover；

S3, Flashing Voltage in Ice Coated Insulator calculate: when insulator exists the risk of icing flashover, the icing weight of estimation insulator, and the correction formula substituting into the flashover voltage of the insulator calculates insulator flashover voltage value under ice coating state；

S4, covering ice for insulator flashover risk assessment: contrasted with when the actual motion voltage at front insulator two ends by Flashing Voltage in Ice Coated Insulator value of calculation, calculate covering ice for insulator probability of flashover, and carry out the division of flashover risk class according to the size of probability of flashover.

Further, in the step S1 of the present invention, the basic data of transmission line of electricity includes shaft tower information, line information, insulation configuration information, insulator contamination information and special handling information；Real time meteorological data includes the median radius of wind speed, wind direction, temperature, humidity, air pressure, rainfall, raindrop.

Further, temperature when transmission line of electricity starts in the step S2 of the present invention icing, humidity and air speed value are as the condition judging insulator whether icing.

Further, the step S2 of the present invention judges whether the risk of icing flashover according to the filthy situation before the type of insulator, special handling measure and icing.

Further, in the step S3 of the present invention, extra-high voltage insulator icing weight estimation formula is:

$m=N\[BIn\left(\underset{i=1}{\overset{H}{\Σ}}\frac{\mathrm{\ζ}\·\mathrm{\β}\·\mathrm{\γ}\·LWC\·v_i}{\mathrm{\ρ}}\mathrm{\Δ}T\right)-C\]$

In formula, N is the sheet number of insulator；ζ is water droplet capture coefficient；β water droplet collision coefficient；γ is freezing fraction；LWC is Liquid water content；v_iIt is the wind speed in the i-th time period, m/s；Δ T is meteorological data collection interval, h；H is the hourage that insulator location meteorological data reaches icing condition；ρ is the iced insulator of insulator, g/cm³；B and C is the constant relevant with insulator structure pattern, can be obtained by trial curve matching.

Further, the step S3 of the present invention considers affect the principal element of covering ice for insulator flashover, air pressure when being specially the equivalent salt density before covering ice for insulator, equivalent grey close and icing, Liquid water content, temperature, icing weight.

Further, in the step S3 of the present invention, Flashing Voltage in Ice Coated Insulator computing formula is:

$U_f=A{\left(ESDD\right)}^{-a}\×{\left(NSDD\right)}^{-b}\×{(1-\frac{H}{45.1})}^{5.36n}\×{\[1+0.02(t-20)\]}^{-w_t}\×{(1-\frac{m}{45.1})}^{5.36\mathrm{\α}}$

Wherein, A is the constant relevant with insulator pattern, material, air pressure, Liquid water content, voltage type etc.；A, b respectively salt close effect characteristics index close, grey；Equivalent salt density and the equivalence ash of ESDD, NSDD respectively insulator surface are close, mg/cm²；H is the height above sea level of shaft tower；N is air pressure, Liquid water content effect characteristics index, takes 0.53 in engineer applied；T is ambient temperature, DEG C；W_tFor temperature Intrusion Index, engineering exchanges and generally takes 0.2, it is possible to obtained by result of the test；M is covering ice for insulator weight, kg；α is the characteristic index value that icing ghost image rings.

Further, in the step S3 of the present invention covering ice for insulator flashover method for calculating probability particularly as follows:

The flashover voltage Normal Distribution rule of--icing insulator, i.e. U～N (μ, σ²), wherein μ represents flashover voltage U_50%, σ is standard deviation, and the unified mean standard deviation σ % that IEC/IC28 recommends is 10%.It is made under working voltage different--icing insulator U corresponding to probability of flashover accordingly_50%Flashover voltage curve, searches U_fCorresponding numerical value, can obtain the icing flashover probability of insulator.

Further, carrying out the risk stratification criteria for classifying according to covering ice for insulator probability of flashover in the step S4 of the present invention is:

P < 10%, for safe level；10% < P < 30%, for low-risk level；30% < P < 70%, for risk level；P > 70%, for excessive risk level.

The beneficial effect comprise that: the ultrahigh voltage alternating current transmission lines icing flashover real-time assessment of the present invention and method for early warning, based on the calculating to ultrahigh voltage alternating current transmission lines icing flashover probability of the current weather condition, it is achieved thereby that the real-time assessment to ultrahigh voltage alternating current transmission lines icing flashover；According to Weather Forecast Information, in advance the icing flashover risk class on certain period interior lines road following can be carried out early warning, for the guidance that the work offer of preventing and reducing natural disasters of relevant operating management department is strong；The present invention uses simply, and feasibility is strong, it was predicted that precision is high；It addition, the present invention can provide reference for the drafting of ultrahigh voltage alternating current transmission lines icing flashover risk prog chart.

Accompanying drawing explanation

Below in conjunction with drawings and Examples, the invention will be further described, in accompanying drawing:

Fig. 1 is the flow chart of ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment of the present invention；

Fig. 2 is the flow chart of the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment of the embodiment of the present invention；

Fig. 3 is the flow chart of the icing flashover probability calculation of the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment of the embodiment of the present invention；

Fig. 4 is the curve chart of the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment of the embodiment of the present invention.

Detailed description of the invention

In order to make the purpose of the present invention, technical scheme and advantage clearly understand, below in conjunction with drawings and Examples, the present invention is further elaborated.Should be appreciated that specific embodiment described herein is only in order to explain the present invention, is not intended to limit the present invention.

As it is shown in figure 1, the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment of the embodiment of the present invention, comprise the following steps:

S1, meteorology and line tower foundation data collection: investigate the basic data of ultrahigh voltage alternating current transmission lines to be assessed, and obtained the real time meteorological data in transmission line of electricity corridor along the line by the automatic meteorological observing station of meteorological department or the microclimate on-Line Monitor Device of grid company；

S2, covering ice for insulator flashover initial conditions judge: judging whether to meet icing condition according to the meteorological data of insulator place to be assessed section, the insulator for meeting icing condition further determines whether the risk that there is icing flashover；

S3, Flashing Voltage in Ice Coated Insulator calculate: when insulator exists the risk of icing flashover, the icing weight of estimation insulator, and the correction formula substituting into the flashover voltage of the insulator calculates insulator flashover voltage value under ice coating state；

S4, covering ice for insulator flashover risk assessment: contrasted with when the actual motion voltage at front insulator two ends by Flashing Voltage in Ice Coated Insulator value of calculation, calculate covering ice for insulator probability of flashover, and carry out the division of flashover risk class according to the size of probability of flashover.

As shown in Figures 2 and 3, in one particular embodiment of the present invention, according to the basic data of ultrahigh voltage alternating current transmission lines to be assessed and the real time meteorological data of insulator location, covering ice for insulator probability of flashover is calculated, and complete ultrahigh voltage alternating current transmission lines covering ice for insulator flashover risk assessment according to respective risk grading standard, specifically include following steps:

Step 1: meteorological and line tower foundation data collection: investigate the basic data of ultrahigh voltage alternating current transmission lines to be assessed, and obtained the real time meteorological data in transmission line of electricity corridor along the line by the automatic meteorological observing station of meteorological department or the microclimate on-Line Monitor Device of grid company.For being provided with the shaft tower of microclimate on-Line Monitor Device, it is possible to replace obtaining meteorological data from automatic meteorological observing station, for data such as the meteorology that cannot obtain, filths, it is possible to use close on the on-line monitoring information approximate substitution of low-voltage-grade circuit.

The data added up such as table 1:

The basic document of table 1 ultrahigh voltage alternating current transmission lines

Shaft tower information	Pole and tower design drawing, residing features of terrain, residing dirty district attribute
		Line information	Line account data, circuit geographic area
Insulation configuration information	Insulator model, insulator material, insulator serial type, insulator creep distance, structure height
		Special handling information	Whether have RTV coating, whether adjust climb, last time cleans the time

Step 2: covering ice for insulator flashover initial conditions judge: judging whether to meet icing condition according to the meteorological data of insulator place to be assessed section, the insulator for meeting icing condition further determines whether the risk that there is icing flashover.

The necessary meteorological condition of covering ice for insulator is: 1. have the congealable temperature of foot, namely less than 0 DEG C；2. having higher humidity, namely relative air humidity is typically in more than 85%；3. there is the wind speed that water in air can be made to drip motion, namely more than lm/s.When relative air humidity is little or calm and wind speed is only small, even if air themperature is below 0 DEG C, insulator does not occur icing phenomenon substantially.So for assessment time ambient temperature more than 0 DEG C or relative air humidity less than 85% or wind speed less than lm/s in the case of, it is believed that icing will not be there is.

Ultrahigh voltage alternating current transmission lines insulator chain is longer, and the electrical safety nargin left is high, and therefore icing flashover typically occurs in ice-melting phase.Within ambient temperature time period less than 0 DEG C, covering ice for insulator cannot melt and cause that the probability of flashover is less, only need to complete the estimation of icing weight；Composite insulator has good effect antifouling, anti-icing, it is believed that the probability that icing flashover occurs is relatively low, it is not necessary to its icing flashover risk is estimated；Filth is the key factor affecting covering ice for insulator flashover, according to UHVAC demonstration project operating experience, the probability of c level and above gradation for surface pollution area generation icing flashover is relatively big, and for the insulator in the following gradation for surface pollution area of c level, it is believed that its icing flashover risk is level of security.

Step 3: Flashing Voltage in Ice Coated Insulator calculates: when insulator exists the risk of icing flashover, the icing weight of estimation insulator, and the correction formula substituting into the flashover voltage of the insulator calculate insulator flashover voltage value under ice coating state.

The icing weight specific formula for calculation of insulator is as follows:

$m=N\&lsqb;BIn\left(\underset{i=1}{\overset{H}{\&Sigma;}}\frac{\mathrm{\&zeta;}\&CenterDot;\mathrm{\&beta;}\&CenterDot;\mathrm{\&gamma;}\&CenterDot;LWC\&CenterDot;v_i}{\mathrm{\&rho;}}\mathrm{\&Delta;}T\right)-C\&rsqb;$

In formula, N is the sheet number of insulator；ζ is water droplet capture coefficient；β is water droplet collision coefficient；γ is freezing fraction；LWC is Liquid water content；v_iIt is the wind speed in the i-th time period, m/s；Δ T is meteorological data collection interval, h；H is the hourage that insulator location meteorological data reaches icing condition；ρ is the iced insulator of insulator, g/cm³；B and C is the constant relevant with insulator structure pattern, can be obtained by trial curve matching.

Insulator water droplet capture coefficient ζ is relevant with factors such as wind speed and drop diameter d, and its mathematic(al) representation is:

$\mathrm{\&zeta;}={(1+\frac{{\mathrm{Cv}}_1}{vd})}^{-1}$

In formula: v₁For the kinematic viscosity of air, during glaze, take v₁=13.2 × 10^-6m²/s；C is constant, and glaze takes C=1.64 often.

Obviously, wind speed is big, and when drop diameter is big, the inertia force of water droplet is big, and the capture coefficient of insulator is just big.Being typically in wind speed when glaze is formed, v < 10m/s, the median particle volume diameter of supercooling water droplet is between 20～30 μm, and therefore, ζ value is between 0.7～0.9.

The relational expression of Liquid water content LWC and rainfall P (mm) is:

LWC=0.067P⁰ _. ⁸⁴⁶

Insulator water droplet collision coefficient β is that in air under certain Liquid water content LWC and in certain time t, the quantity N that water droplet and insulator collide and initial position are in insulator at the projected area S coming on flow path direction_prInterior all water droplet quantity N_prRatio, can be obtained by emulation experiment.

$\mathrm{\&beta;}=\frac{N}{N_{S_{pr}}}$

The correction formula of Flashing Voltage in Ice Coated Insulator is:

$U_f=A{\left(ESDD\right)}^{-a}\&times;{\left(NSDD\right)}^{-b}\&times;{(1-\frac{H}{45.1})}^{5.36n}\&times;{\&lsqb;1+0.02(t-20)\&rsqb;}^{-w_t}\&times;{(1-\frac{m}{45.1})}^{5.36\mathrm{\&alpha;}}$

Wherein, A is the constant relevant with insulator pattern, material, air pressure, Liquid water content, voltage type etc.；A, b respectively salt close effect characteristics index close, grey；Equivalent salt density and the equivalence ash of ESDD, NSDD respectively insulator surface are close, mg/cm²；H is the height above sea level of shaft tower；N is air pressure, Liquid water content effect characteristics index, takes 0.53 in engineer applied；T is ambient temperature, DEG C；W_tFor temperature Intrusion Index, engineering exchanges and generally takes 0.2, it is possible to obtained by result of the test；M is covering ice for insulator weight, kg；α is the characteristic index value that icing ghost image rings, it is proposed that exchange takes 0.1.

Step 4: covering ice for insulator flashover risk assessment: Flashing Voltage in Ice Coated Insulator value of calculation is contrasted with when the actual motion voltage at front insulator two ends, calculate covering ice for insulator probability of flashover, and carry out the division of flashover risk class according to the size of probability of flashover.

As shown in Figure 4, as the pollution flashover of a kind of specific form, the flashover voltage Normal Distribution rule of--icing insulator, i.e. U～N (μ, σ²), wherein μ represents flashover voltage U_50%, σ is standard deviation, and the unified mean standard deviation σ % that IEC/IC28 recommends is 10%.It is made under working voltage different--icing insulator U corresponding to probability of flashover accordingly_50%Flashover voltage curve, searches U_fCorresponding numerical value, can obtain the icing flashover probability of insulator.

The probability of icing flashover is there is when current weather, by such as the risk class criteria for classifying of table 2 below, it is achieved the assessment to ultrahigh voltage alternating current transmission lines covering ice for insulator flashover risk according to insulator.

The table 2 ultrahigh voltage alternating current transmission lines covering ice for insulator flashover risk class criteria for classifying

It should be appreciated that for those of ordinary skills, it is possible to improved according to the above description or converted, and all these are improved and convert the protection domain that all should belong to claims of the present invention.

Claims (10)

1. a ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment, it is characterised in that comprise the following steps:

S1, meteorology and line tower foundation data collection: investigate the basic data of ultrahigh voltage alternating current transmission lines to be assessed, and obtained the real time meteorological data in transmission line of electricity corridor along the line by the automatic meteorological observing station of meteorological department or the microclimate on-Line Monitor Device of grid company；

S2, covering ice for insulator flashover initial conditions judge: judging whether to meet icing condition according to the meteorological data of insulator place to be assessed section, the insulator for meeting icing condition further determines whether the risk that there is icing flashover；

S3, Flashing Voltage in Ice Coated Insulator calculate: when insulator exists the risk of icing flashover, the icing weight of estimation insulator, and the correction formula substituting into the flashover voltage of the insulator calculates insulator flashover voltage value under ice coating state；

S4, covering ice for insulator flashover risk assessment: contrasted with when the actual motion voltage at front insulator two ends by Flashing Voltage in Ice Coated Insulator value of calculation, calculate covering ice for insulator probability of flashover, and carry out the division of flashover risk class according to the size of probability of flashover.

2. ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment according to claim 1, it is characterized in that, in step S1, the basic data of transmission line of electricity includes shaft tower information, line information, insulation configuration information, insulator contamination information and special handling information；Real time meteorological data includes the median radius of wind speed, wind direction, temperature, humidity, air pressure, rainfall, raindrop.

3. ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment according to claim 1, it is characterised in that temperature when transmission line of electricity starts in step S2 icing, humidity and air speed value are as the condition judging insulator whether icing.

4. ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment according to claim 1, it is characterized in that, step S2 judges whether according to the filthy situation before the model of insulator, special handling measure and icing the risk of icing flashover.

5. according to the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment described in claim 1, it is characterized in that, reaching in the time of icing condition at weather environment in step S3, the insulator for having icing flashover risk estimates its icing weight according to below equation:

$m=N[\mathrm{BIn}\left(\underset{i=1}{\overset{H}{\mathrm{\&Sigma;}}}\frac{\mathrm{\&zeta;}\&CenterDot;\mathrm{\&beta;}\&CenterDot;\mathrm{\&gamma;}\&CenterDot;\mathrm{LWC}\&CenterDot;v_i}{\mathrm{\&rho;}}\mathrm{\&Delta;T}\right)-C]$

In formula, N is the sheet number of insulator；ζ is water droplet capture coefficient；β water droplet collision coefficient；γ is freezing fraction；LWC is Liquid water content；v_iIt is the wind speed in the i-th time period, m/s；Δ T is meteorological data collection interval, h；H is the hourage that insulator location meteorological data reaches icing condition；ρ is the iced insulator of insulator, g/cm³；B and C is the constant relevant with insulator structure pattern, can be obtained by trial curve matching.

6. according to the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment described in claim 1, it is characterized in that, step S3 considers affect the principal element of covering ice for insulator flashover, air pressure when being specially the equivalent salt density before covering ice for insulator, equivalent grey close and icing, temperature, icing weight.

7. according to the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment described in claim 1, it is characterised in that in step S3, Flashing Voltage in Ice Coated Insulator computing formula is:

$U_f=A{\left(\mathrm{ESDD}\right)}^{-a}\&times;{\left(\mathrm{NSDD}\right)}^{-b}\&times;{(1-\frac{H}{45.1})}^{5.36n}\&times;{[1+0.02(t-20)]}^{-w_t}\&times;{(1-\frac{m}{45.1})}^{5.36\mathrm{\&alpha;}}$

Wherein, A is the constant relevant with insulator pattern, material, air pressure, voltage type etc.；A, b respectively salt close effect characteristics index close, grey；Equivalent salt density and the equivalence ash of ESDD, NSDD respectively insulator surface are close, mg/cm²；H is the height above sea level of shaft tower；N is effects of air pressure characteristic index, takes 0.53 in engineer applied；T is ambient temperature, DEG C；W_tFor temperature Intrusion Index, engineering exchanges and generally takes 0.2, it is possible to obtained by result of the test；M is covering ice for insulator weight, kg；α is the characteristic index value that icing ghost image rings.

8. according to the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment described in claim 1, it is characterised in that according to the rule of--icing insulator flashover voltage Normal Distribution, i.e. U～N (μ, σ in step S4²), wherein μ represents flashover voltage U_50%, σ is standard deviation, calculates different--icing insulator U corresponding to probability of flashover under working voltage_50%Flashover voltage.

9. according to the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment described in claim 1, it is characterised in that step S4 searches different--icing insulator U corresponding to probability of flashover under working voltage_50%Flashover voltage curve, obtains U_fCorresponding probability of flashover value.

10. according to the ultrahigh voltage alternating current transmission lines covering ice for insulator flashover methods of risk assessment described in claim 1, it is characterised in that step S4 carries out the risk stratification criteria for classifying according to covering ice for insulator probability of flashover and is:

P < 10%, for safe level；10% < P < 30%, for low-risk level；30% < P < 70%, for risk level；P > 70%, for excessive risk level.