Fire risk assessment method combining operation parameters and health conditions of high-voltage cable

Abstract

The invention provides a fire risk assessment method combining operation parameters and health conditions of a high-voltage cable. The fire risk assessment method comprises the following steps: constructing a fire risk hidden danger assessment set: dividing fire risk grades of a power cable into four grades; building a fire risk hidden danger evaluation index set, wherein the evaluation indexes influencing the fire risk hidden danger of the high-voltage cable are divided into three first-level evaluation indexes and eight second-level evaluation indexes of cable operation parameters, cable aging conditions and cable laying; determining the deterioration degree of the fire risk hidden danger evaluation index; comprehensive evaluation: the deterioration degree is calculated by combining various indexes of the high-voltage cable, and the membership degree corresponding to various fire risk hidden danger grades in the evaluation set is obtained according to the corresponding membership function; determining the weight of the index by adopting an analytic hierarchy process; and obtaining a judgment scale according to the calculated weight of each secondary evaluation index and the membership matrix. Compared with fire hazard risk evaluation performed by singly using operating parameters or aging conditions, the method is more comprehensive, and the evaluation result is more accurate.

Description

Technical Field

[0001] The invention relates to the field of cable fire risk assessment, and in particular to a fire risk assessment method combining operating parameters and health status of a high-voltage cable. Background Art

[0002] With the rapid development of economy and urban construction, power transmission has gradually developed from overhead lines to underground cables, which has led to an increasing use of cross-linked polyethylene high-voltage cables in cities. The sheath layer in the cable, cross-linked polyethylene, and epoxy resin in the cable joints are all combustible materials. In addition, the cable channel has the characteristics of closed space, hidden fire points that are difficult to be found in the early stage, and fire fighting is difficult. When there is an arc fault in the power cable, overheating of the core, or external heat source, it is easy to induce a fire, which will seriously threaten the safety of urban power supply. Therefore, the early warning of high-voltage cable fires is an important part of the fire prevention and control system and an important part of the safe and reliable power supply of the urban network system.

[0003] The causes of fires in cables and their passages can be divided into two aspects: internal and external: internal causes are mainly due to long-term full or overload operation of cables, high resistance of cable joints, short circuit faults in lines, etc.; external causes are mainly caused by external mechanical damage causing faults or ignition of external fire sources, etc. Therefore, in order to reduce the risk of cable fires, it is necessary to build an early fire warning system. At present, most of the research on high-voltage cable tunnel fires is based on monitoring smoke, fire characteristic gases or fire images to achieve the purpose of early warning. However, the existing technology uses gas sensors, image sensors, etc. to warn of cable fires, but only considers the single use of operating parameters or aging conditions for fire hazard risk assessment, and the factors considered are not comprehensive enough, resulting in inaccurate evaluation results. Summary of the invention

[0004] In view of this, the present invention provides a fire risk assessment method that combines the operating parameters and health status of high-voltage cables. Multiple aspects are taken into consideration during the evaluation, and an index system for fire hazard risk assessment is constructed from three aspects: cable operating parameters, cable aging status and cable laying. This is more comprehensive than using only operating parameters or aging status for fire hazard risk assessment, and the evaluation results are more accurate.

[0005] A fire risk assessment method combining high-voltage cable operating parameters and health conditions, comprising the following steps:

[0006] Step 1: Construct a fire risk hazard assessment set

[0007] The power cable fire risk level is divided into four levels, from small to large, namely green level, yellow level, orange level, and red level, so as to obtain the power cable fire risk hazard level assessment set S, that is,

[0008] S={s1,s2,s3,s4}={green,yellow,orange,red} (1)

[0009] Step 2: Construct a fire risk hazard evaluation index set

[0010] The assessment indicators that affect the fire risk of high-voltage cables are divided into three primary assessment indicators: cable operating parameters, cable aging status and cable laying. Cable operating parameters include five secondary assessment indicators: conductor temperature, load current, maximum and minimum ratio of single-phase grounding current, ambient temperature and ambient humidity. Cable aging status is evaluated by health index, which reflects the historical aging status of the cable. Cable laying includes two secondary assessment indicators: laying method and fixing method, which includes a total of 8 secondary assessment indicators.

[0011] Step 3: Determine the deterioration degree of fire risk hazard evaluation indicators

[0012] The degradation degree is used to indicate the degree to which the cable index deviates from the normal state. Its value range is [0,1]. When the degradation degree is 1, it indicates that the cable is in a faulty state. When the degradation degree is 0, it indicates that the cable is in an optimal state.

[0013] Step 4: Comprehensive Evaluation

[0014] The degradation degree is calculated by combining various indicators of high-voltage cables, and the membership degree corresponding to various fire risk hazard levels in the evaluation set is obtained according to the corresponding membership function;

[0015] The analytic hierarchy process was used to determine the weights of the indicators;

[0016] According to the calculated weights of each secondary evaluation index and the membership matrix, the evaluation scale S, that is, the comprehensive evaluation matrix, is obtained:

[0017]

[0018] Where W is the weight vector of the secondary evaluation index to the target layer; R is the membership matrix of the secondary evaluation index; is the fuzzy synthesis operator.

[0019] Furthermore, step three determines the deterioration degree of fire risk hazard evaluation indicators, including:

[0020] The five indicators of cable operation parameters are all positive degradation indicators, and their degradation degree is calculated using the following formula:

[0021]

[0022] Where: x m is the indicator status parameter threshold; x i is the actual measured value of the indicator;

[0023] The conductor temperature, load current, maximum-minimum ratio of single-phase grounding current, and degradation degree of ambient temperature and humidity in the indicators are calculated using equations (3)-(7) respectively.

[0024]

[0025] Where: x i is the actual measured value of the conductor temperature;

[0026]

[0027] Where: x m is the rated current carrying capacity of the line, x i is the actual measured value of the indicator;

[0028]

[0029] Where: x i is the actual measured value of the indicator; when x i ≤3, the phase k of the maximum value of the single item in phases A, B, and C is 1; the phase k of the intermediate value is 0.8; the phase k of the minimum value is 0.6; when x i >3, the phase k to which the single maximum value belongs is 1; the phase k to which the intermediate value belongs is 0.7; the phase k to which the minimum value belongs is 0.5;

[0030]

[0031] Where: x i is the actual measured value of the ambient temperature;

[0032]

[0033] Where: x i is the actual measured value of humidity;

[0034] Health level index calculation formula:

[0035]

[0036] Where: HI0——Cable initial health level index;

[0037] HI – Final health level index of the cable;

[0038] B——cable aging coefficient;

[0039] T1——the year when the new cable was put into operation, corresponding to HI0;

[0040] T2——current year, corresponding to HI;

[0041] For the health index, the health index is calculated according to formula (8), and its level is analyzed according to Table 2, and then its degradation degree is determined according to the following:

[0042]

[0043] Furthermore, in step 4, the degradation degree is calculated by combining various indicators of the high-voltage cable, and the corresponding

[0044] The function is obtained

[0045] The membership degree corresponding to various fire risk hazard levels in the evaluation set includes:

[0046] For the quantifiable indicators such as conductor temperature, load current, maximum-minimum ratio of single-phase ground current, ambient temperature, ambient humidity and health index, the membership functions are shown in equations (10)-(13):

[0047]

[0048] For the non-quantified indicators such as laying method and fixing method in the secondary indicators, a qualitative method is used to describe their degree. Combined with their laying and fixing methods, their membership degrees are determined according to the following formula:

[0049]

[0050]

[0051] According to the degradation degree obtained by the index value and the analysis of equations (10)-(15), the membership matrix R of the indicators corresponding to the evaluation set S is obtained, that is:

[0052]

[0053] Where: R is the membership matrix of the secondary evaluation index; n is the number of indicator sets; m is the number of evaluation sets. The R matrix is ​​composed of the membership of the degradation degree of n indicators in the fire risk hazard evaluation index set to the m state space of the fire evaluation set.

[0054] The present invention conducts fire hazard research from the perspective of cable operation and maintenance. On the basis of analyzing the ignition mechanism of internal fire in the cable, the aging condition of the power cable is analyzed in combination with the health index, and the cable fire risk hazard evaluation index and index degradation function considering the operating parameters, health index and cable laying method are constructed. The ridge distribution function is used to analyze the membership of the fire level. At the same time, the hierarchical analysis and comprehensive evaluation methods are used to establish a high-voltage cable fire risk hazard evaluation model, and the model is verified by combining examples. The results show that the method is effective, and its evaluation results can provide a scientific basis for the analysis and early warning of cable fire risk hazards. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 Schematic diagram of evaluation indexes for fire risk level of power cables according to an embodiment of the present invention;

[0056] Figure 2 It is a membership function diagram of fire risk of evaluation index according to an embodiment of the present invention. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] The embodiment of the present invention provides a fire risk assessment method combining high-voltage cable operation parameters and health conditions, comprising the following steps:

[0059] Step 1: Construct a fire risk hazard assessment set

[0060] According to the severity of the fire, the embodiment of the present invention divides the power cable fire risk level into four levels, namely green level, yellow level, orange level, and red level from small to large, thereby obtaining a power cable fire risk hazard level assessment set S, namely:

[0061] S={s1,s2,s3,s4}={green,yellow,orange,red} (1)

[0062] The higher the fire risk hazard level, the greater the possibility of fire and the scope of impact. For the sake of clarity, Table 1 describes the size of fire risk hazards and countermeasures accordingly. It is a specific representation of the evaluation set of high-voltage cable fire risk hazard levels. According to different levels, different measures can be taken to timely reduce the probability of high-voltage cable fires.

[0063] Table 1 Power cable fire risk hazard level assessment set

[0064]

[0065] Step 2: Construct a fire risk hazard evaluation index set

[0066] In the process of constructing the fire risk hazard evaluation index set, not only the current operation data of the cable should be considered, but also the historical condition of the cable. The evaluation indicators that affect the fire risk hazards of high-voltage cables can be divided into three first-level evaluation indicators: cable operation parameters, cable aging conditions and cable laying. Among them, cable operation parameters include five second-level evaluation indicators: conductor temperature, load current, maximum and minimum ratio of single-phase grounding current, ambient temperature and ambient humidity. The cable aging condition is evaluated by the health index, which reflects the historical aging condition of the cable. Cable laying includes two second-level evaluation indicators: laying method and fixing method, which contains a total of 8 second-level evaluation indicators. Evaluation indicators such as Figure 1 shown.

[0067] Step 3: Determine the deterioration degree of fire risk hazard evaluation indicators

[0068] The fire risk hazard of high-voltage cables corresponds to their ability to maintain their specific performance in the current use environment. During the operation of the cable, the monitored parameters are characteristic parameters that characterize its specific performance of power transmission capacity, and these parameters are functions of time. Under normal power transmission conditions, the set of these measured parameters can determine the normal state of the cable, while the fault state is when one or part of these monitoring parameters exceeds the normal range. According to reliability theory, the degree of deviation of cable indicators from the normal state can be expressed by the degree of degradation, and its value range is [0,1]. When the degradation degree is 1, it indicates that the cable is in a faulty state, and when the degradation degree is 0, it indicates that the cable is in the optimal state.

[0069] For situations where condition monitoring parameters (online and offline) can be used to reflect the cable status, Figure 2 The five indicators in the cable operation parameters are all positive degradation indicators, and their degradation degree can be calculated using the following formula:

[0070]

[0071] Where: x m is the indicator status parameter threshold; x i is the actual measured value of the indicator.

[0072] The following is a detailed analysis of the calculation of cable operating parameters and health index degradation degree.

[0073] (1) Cable operating parameters

[0074] Specifically, according to the cable operation specifications such as "Q / GDW 11223-2014 Technical Specifications for High Voltage Cable Status Detection", and considering that air humidity has a very close relationship with the incidence of fire, the conductor temperature, load current, maximum-minimum ratio of single-phase grounding current, and degradation degree of ambient temperature and humidity are calculated using formulas (3)-(7) respectively.

[0075]

[0076] Where: x i is the actual measured value of the conductor temperature.

[0077]

[0078] Where: x m is the rated current carrying capacity of the line, x i is the actual measured value of the indicator.

[0079]

[0080] Where: x i is the actual measured value of the indicator. i ≤3, the phase k of the maximum value of the single item among the three phases A, B, and C is 1; the phase k of the intermediate value is 0.8; and the phase k of the minimum value is 0.6. i When >3, the phase k to which the single maximum value belongs is 1; the phase k to which the intermediate value belongs is 0.7; and the phase k to which the minimum value belongs is 0.5.

[0081]

[0082] Where: x i is the actual measured value of the ambient temperature.

[0083]

[0084] Where: x i is the actual measured value of humidity.

[0085] (2) Health Index

[0086] The cable health index (HI) is a comprehensive measure of the cable's operating status, health level and service life from the beginning of operation to the present. The index provides a new perspective and dimension. The health index quantifies the cable status under the influence of many factors that cause long-term aging. The accumulation of these factors will lead to the end of the cable's life. The health level index calculation formula is:

[0087]

[0088] Where: HI0——Cable initial health level index;

[0089] HI – Final health level index of the cable;

[0090] B——cable aging coefficient;

[0091] T1——the year when the new cable was put into operation, corresponding to HI0;

[0092] T2——Current year, corresponding to HI.

[0093] Generally speaking, HI∈[0,10], the lower the value, the better the condition of the cable body. If HI∈[0,2.8] is between, it indicates that the cable body is in good condition. At this time, the performance of the cable body is stable, the probability of failure is low and does not change much over a period of time; if HI∈(2.8,6.5] is between, it indicates that the cable body has shown obvious aging phenomenon, and the aging process begins to increase significantly. At this time, although the probability of failure of the cable body is at a relatively low level, it has begun to enter the rising stage, and the aging rate has also begun to gradually increase; HI∈(6.5,10] is between, it indicates that the cable body has shown serious aging phenomenon, during which the probability of failure increases significantly, and the corresponding aging rate shows a gradual upward trend. The relationship between the health index and the aging state of the cable body is shown in Table 2.

[0094] Table 2 Correspondence between health index and cable aging status

[0095]

[0096]

[0097] For the health index, the health index is calculated according to formula (8), and its level is analyzed according to Table 2, and then its degradation degree is determined according to the following:

[0098]

[0099] Step 4: Comprehensive Evaluation

[0100] Before comprehensive evaluation, it is necessary to calculate the degree of degradation in combination with various indicators of high-voltage cables, and the corresponding membership degree of various fire risk hazard levels in the evaluation set in Table 1 can be obtained based on the corresponding membership function. The ridge distribution function in the membership function has the characteristics of wide main value interval and smooth transition zone, which can better reflect the fuzzy membership relationship between various cable indicators and the degree of fire hazard risk. Therefore, the embodiment of the present invention selects the ridge distribution function to calculate the membership degree of each indicator, and its membership function curve is as follows: Figure 2 shown.

[0101] For the quantifiable indicators such as conductor temperature, load current, maximum-minimum ratio of single-phase ground current, ambient temperature, ambient humidity and health index, the membership functions are as shown in equations (10)-(13).

[0102]

[0103]

[0104] For the non-quantified indicators such as laying method and fixing method in the secondary indicators, a qualitative method is used to describe their degree. Combined with their laying and fixing methods, their membership degrees are determined according to the following formula.

[0105]

[0106] According to the degradation degree obtained by the index value and the analysis of equations (10)-(15), the membership matrix R of the indicators corresponding to the evaluation set S can be obtained, that is:

[0107]

[0108] Where: R is the membership matrix of the secondary evaluation indicators; n is the number of indicator sets; m is the number of evaluation sets.

[0109] The R matrix is ​​composed of the membership of the degradation degrees of n indicators in the fire risk hazard evaluation index set to the m state spaces in the fire evaluation set.

[0110] In the evaluation process, the weight distribution of fire risk hazard evaluation indicators has a great influence on the evaluation results. Therefore, how to reasonably determine the weight is also critical. The present invention adopts a hierarchical analysis method to determine the weight of the indicator. The hierarchical analysis method has been introduced in detail in many literatures and will not be repeated here.

[0111] According to the weights of each secondary evaluation index and the membership matrix calculated above, the evaluation scale S, that is, the comprehensive evaluation matrix, can be obtained:

[0112]

[0113] Where W is the weight vector of the secondary evaluation index to the target layer; R is the membership matrix of the secondary evaluation index; is a fuzzy synthesis operator. The present invention adopts the M(·,∨) operator, which can reflect the role of non-main indicators and can better reflect the importance of a single indicator to the evaluation result. After calculating the comprehensive evaluation matrix, according to the maximum membership principle and referring to Table 1, the fire risk hazard level of the evaluation scale S can be obtained.

[0114] Example of comprehensive assessment of fire hazard risk

[0115] (1) Case monitoring data

[0116] The second 220kV line in a certain city is a cable line with a total length of 9.422km. The cable model is YJLW03--220-1*1000. The cable and accessories are produced by a certain company. The second line was put into operation on June 30, 2008. There are 16 sets of intermediate joints (15 insulated, 1 straight-through), 1 set of cable GIS terminals, and 1 set of cable outdoor terminals. Before the accident at joint No. 3, the operating environment of the fault section was good. On December 19, 2019, the city cable operation and inspection room conducted routine tests on the joint, including temperature measurement, circulating current measurement, and partial discharge detection. The measurement results were all normal. The operating and environmental parameter detection values ​​are shown in Table 3 (because the partial discharge index is not considered in this invention, this data is not included in Table 3).

[0117] Table 33 Joint test data (2019.12.19)

[0118]

[0119] (2) Health Index Analysis

[0120] The design life of cross-linked polyethylene insulated cables is designed by the manufacturer according to relevant standards. Generally speaking, its design life can reach more than 30 years. In this invention, 30 years is selected for analysis. Assuming that after 30 years of operation, the health status of the cable is obviously aged and the cable failure rate is high, it can be considered that HI = 6.5. The health level index of the cable when it is put into operation is generally taken as 0.5. According to formula (7), at the same time, considering the load and environmental factors, the cable aging coefficient can be calculated as:

[0121]

[0122] Substituting the aging coefficient into formula (8), the health index in 2019 can be calculated:

[0123] HI=1.7304 (19)

[0124] From Table 2, we can see that the insulation condition is in good condition. According to formula (9), the degradation degree can be calculated to be 0.2400.

[0125] (3) Membership matrix calculation

[0126] Combined with the data of joint No. 3 in Table 3, the corresponding degradation degree is calculated according to formulas (3)-(7) and (9). According to the degradation degree, the corresponding membership degree can be obtained according to formulas (10)-(13). The index weight is calculated using the hierarchical analysis method. The corresponding data are shown in Table 4.

[0127] Table 4 Index parameter weights, degradation degree and membership degree list

[0128]

[0129] (4) Comprehensive evaluation

[0130] Finally, a comprehensive evaluation is performed according to formula (17), and the comprehensive evaluation result sets of phases A, B, and C are calculated respectively. The results are as follows.

[0131]

[0132] At this time, according to the maximum subordination principle, the fire risk hazard level of phases A and B is green, while the fire risk hazard level of phase C is red. The risk of fire is very high, and further inspection should be carried out on phase C to find out the cause.

[0133] The real situation is that the data of the routine cable inspection conducted by the municipal company on December 19, 2019 were normal according to the technical specifications for high-voltage cable status inspection. However, at 23:46 on July 11, 2020 (when the weather conditions and operating environment are good, and lightning strikes, external damage, or moisture on the main body can be ruled out), the second C phase of the line tripped due to a fault. The fault point was connector No. 3, and the cause of the fault was poor installation process of the accessories. After the fault, the grounding arc ignited the waterproof casting agent (AB glue) in the glass fiber reinforced plastic shell, and the open fire lasted for hours. The patrol personnel then used a fire extinguisher to put out the fire.

[0134] It can be seen that according to the method proposed by the present invention, a fire warning can be issued nearly half a year before the fire occurs, which can provide effective reference for patrol personnel and should be taken seriously. Therefore, the fire risk hazard warning technology proposed by the present invention is effective. The reason is that the technology takes into account the cable operation parameters and historical health conditions at the same time, and combines the current conditions with the historical conditions to analyze the cable fire risk hazards. It has a good warning effect for fires caused by the cable body, but it has no effect on sudden fires caused by external factors such as ignition of combustibles and fire spread. However, this type of fire can be warned by other early fire warning devices.

[0135] The present invention proposes a cable fire hazard risk assessment method that considers cable operating parameters and combines historical health conditions. This method constructs an index system for fire hazard risk assessment from three aspects: cable operating parameters, cable aging conditions, and cable laying. It is more comprehensive than using only operating parameters or aging conditions to conduct fire hazard risk assessment.

[0136] In addition, the proposed method combines different types of indicators and designs different degradation functions to calculate the degradation degree reflected by the value. At the same time, the corresponding membership function is constructed based on the fire risk hazard evaluation level and combined with the ridge distribution function for the analysis and calculation of the indicator membership. According to the weight and membership matrix, it can be used for the comprehensive evaluation of cable fire risk hazards. This method can provide a scientific basis for the analysis and decision-making of cable fire risk hazards, and can also provide a reference for the discovery of cable faults.

[0137] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A fire risk assessment method combining high voltage cable operating parameters and health conditions, characterized in that: The steps include: Step 1: Construct a fire risk hazard assessment set The power cable fire risk level is divided into four levels, from small to large, namely green level, yellow level, orange level, and red level, so as to obtain the power cable fire risk hazard level assessment set S, that is, S={s1,s2,s3,s4}={green,yellow,orange,red} (1) Step 2: Construct a fire risk hazard evaluation index set The assessment indicators that affect the fire risk of high-voltage cables are divided into three primary assessment indicators: cable operating parameters, cable aging status and cable laying. Cable operating parameters include five secondary assessment indicators: conductor temperature, load current, maximum and minimum ratio of single-phase grounding current, ambient temperature and ambient humidity. Cable aging status is evaluated by health index, which reflects the historical aging status of the cable. Cable laying includes two secondary assessment indicators: laying method and fixing method, which includes a total of 8 secondary assessment indicators. Step 3: Determine the deterioration degree of fire risk hazard evaluation indicators The degradation degree is used to indicate the degree to which the cable index deviates from the normal state. Its value range is [0,1]. When the degradation degree is 1, it indicates that the cable is in a faulty state. When the degradation degree is 0, it indicates that the cable is in an optimal state. Step 4: Comprehensive Evaluation The degradation degree is calculated by combining various indicators of high-voltage cables, and the membership degree corresponding to various fire risk hazard levels in the evaluation set is obtained according to the corresponding membership function; The analytic hierarchy process was used to determine the weights of the indicators; According to the calculated weights of each secondary evaluation index and the membership matrix, the evaluation scale S, that is, the comprehensive evaluation matrix, is obtained: S=W o R=(s1,s2,L,s m ) (17) Where W is the weight vector of the secondary evaluation index to the target layer; R is the membership matrix of the secondary evaluation index; and "○" is the fuzzy synthesis operator.

2. The fire risk assessment method combining high voltage cable operation parameters and health conditions according to claim 1, characterized in that: Step 3: Determine the deterioration degree of fire risk hazard evaluation indicators, including: The five indicators of cable operation parameters are all positive degradation indicators, and their degradation degree is calculated using the following formula: Where: x m is the indicator status parameter threshold; x i is the actual measured value of the indicator; The conductor temperature, load current, maximum-minimum ratio of single-phase grounding current, and degradation degree of ambient temperature and humidity in the indicators are calculated using equations (3)-(7) respectively. Where: x i is the actual measured value of the conductor temperature; Where: x m is the rated current carrying capacity of the line, x i is the actual measured value of the indicator; Where: x i is the actual measured value of the indicator; when x i ≤3, the phase k of the maximum value of the single item in phases A, B, and C is 1; the phase k of the intermediate value is 0.8; the phase k of the minimum value is 0.6; when x i >3, the phase k to which the single maximum value belongs is 1; the phase k to which the intermediate value belongs is 0.7; the phase k to which the minimum value belongs is 0.5; Where: x i is the actual measured value of the ambient temperature; Where: x i is the actual measured value of humidity; Health level index calculation formula: Where: HI0——Cable initial health level index; HI – the final health level index of the cable; B——cable aging coefficient; T1——the year when the new cable was put into operation, corresponding to HI0; T2——current year, corresponding to HI; For the health index, the health index is calculated according to formula (8), and its level is analyzed according to Table 2, and then its degradation degree is determined according to the following:

3. The fire risk assessment method combining high voltage cable operation parameters and health conditions as claimed in claim 2, characterized in that: In step 4, the degradation degree is calculated by combining various indicators of high-voltage cables, and the membership degree corresponding to various fire risk hazard levels in the evaluation set is obtained according to the corresponding membership function, including: For the quantifiable indicators such as conductor temperature, load current, maximum-minimum ratio of single-phase ground current, ambient temperature, ambient humidity and health index, the membership functions are shown in equations (10)-(13): For the non-quantified indicators such as laying method and fixing method in the secondary indicators, a qualitative method is used to describe their degree. Combined with their laying and fixing methods, their membership degrees are determined according to the following formula: According to the degradation degree obtained by the index value and the analysis of equations (10)-(15), the membership matrix R of the indicators corresponding to the evaluation set S is obtained, that is: Where: R is the membership matrix of the secondary evaluation index; n is the number of indicator sets; m is the number of evaluation sets. The R matrix is ​​composed of the membership of the degradation degree of n indicators in the fire risk hazard evaluation index set to the m state space of the fire evaluation set.

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