Methods for risk assessment of old overhead transmission lines

In view of the problem that the transmission line body elements are not considered comprehensively in the current evaluation methods of old transmission lines, this paper proposes a comprehensive risk assessment method for old overhead transmission lines based on practical operation experience. Firstly, the risk factors affecting the safe operation of the old overhead transmission lines were summarized. The pylons in the line segment were investigated item by item. All the hidden trouble points in the line segment were sorted out, and the risk value was calculated by danger source identification and risk assessment method. Secondly, according to the possibility of fault caused by technical factors on the old line, the weight coefficient is given to calculate the deduction value to judge the risk grade state. Finally, when the total deduction is greater than 60 points, or the single deduction is greater than 24 points, it is necessary to formulate a corresponding management plan to guide the orderly management of line hidden dangers.


Introduction
With the gradual advancement of power grid construction, the contradiction between the rapid growth of line scale and the high-quality demand for power supply and the difficulty in accurately grasping the risk state of the line has become increasingly prominent.In order to effectively master the health status of the old overhead transmission lines and carry out the line operation, maintenance, and overhaul in an orderly way, a practical and effective risk assessment method for overhead transmission lines has attracted the attention of many scientific and technological workers and equipment operation managers at home and abroad [1][2][3].
In [4], the toughness assessment of electrical systems under earthquake disasters was studied.In [5], a transmission line risk assessment method was proposed considering the impact of extreme natural disasters, aiming at frequent lightning activities and lightning damage of transmission lines in the rainy season in southern China.In [6], a transmission line risk assessment method considering the impact of extreme natural disasters, aiming at the phenomenon of frequent lightning activities and lightning damage of transmission lines in the rainy season in southern China was proposed.In [7], an overhead transmission line operation risk assessment system was established considering various natural disasters.
The above risk assessment methods for overhead transmission lines take into account the impact of lightning, typhoons, ice and snow, earthquake, mountain fire, and other natural disasters, but the factors considered are relatively simple and not comprehensive enough.
In [8], the principal component analysis method was used to extract three main factors affecting the operation and maintenance quality of overhead power lines in mountainous areas.An intelligent evaluation model for the operation and maintenance quality of overhead power lines was built.In this method, only the main factors are selected, and the factors are not considered comprehensively.
Therefore, a comprehensive risk assessment method for old overhead transmission lines is proposed in this paper, considering all the technical factors affecting the transmission line body safety.First of all, technical elements based on actual operation experience are combined and screened; Secondly, the risk status of each section of the line is summarized, and the evaluation conclusion is drawn.Then, the risk level of technical identification is considered.Finally, a reasonable governance strategy is proposed according to the transmission line status.

Risk assessment process of old overhead transmission lines
Based on the actual operation experience, this paper summarizes six technical elements of the transmission line body and puts forward the risk assessment process of the old overhead transmission line.The workflow of risk assessment and management of old transmission lines is shown in Figure 1.

Collect technical elements and deduct points
Segment risk status summary

Evaluation conclusion
Medium-high risk level

Technical appraisal
Medium-low risk level Governance strategy  Each technical element checked out shall be evaluated as a line section based on the continuous tower section with the same elements.According to the results of hidden danger screening, the risk value is calculated by the hazard source identification and Risk assessment (HIRA) method, namely: where R stands for risk value; C stands for consequence, defined as the most likely outcome of an accident due to a hazard, including injury and equipment failure; E represents exposure time and is defined as the frequency of hazardous events.L stands for possibility, defined as the chance that a hazardous event will lead to consequences once it occurs.
According to the risk value R, the risk degree of each technical element is divided into four levels from light to heavy, which are Ⅰ, Ⅱ, Ⅲ, and Ⅳ, and the corresponding basic deduction points are 2, 4, 8, and 10, as shown in Table 1.According to the relevant national and industrial standards, combined with the actual operation experience, and according to the investigation focus and technical route, we carry out a hidden danger investigation on the six technical elements, including channel environment, foundation, tower, guide ground, fixtures, and insulators, and carry out a risk assessment of the old transmission lines [11-14] .According to the possibility of technical factors causing tower toppling, line breaking, and series breaking of old transmission lines, technical factors are divided into four levels from light to heavy, with weights corresponding to 1, 2, 3, and 4 and corresponding scores 1, 2, 3 and 4, as shown in Table 2.
Table 2 For the continuous tower section, when the hidden danger of the same technical element is the same, it can be evaluated as a line section, and the deduction value of the technical elements of the line section can be calculated, which is equal to the weight score × the corresponding basic deduction value.
c) Determine the risk status of the section The deduction values of all technical elements in the line segment are superimposed.The old transmission line segment is divided according to the principle that the total deduction values of all technical elements are the same and continuous.The risk state of the segment is determined according to the single maximum and total deduction values.The risk status of the line section is set to four levels: normal state, low-risk state, medium-risk state, and high-risk state.As shown in Table 3, when any technology element in the section is evaluated as a low-risk state, medium-risk state, or high-risk state, the overall evaluation of the section should be the most serious state.

Hidden danger investigation focus points and technical route
According to the actual situation, six technical factors (channel environment, foundation, tower, guide ground, fixtures, and insulators) should be assessed for the risk of old transmission lines.The investigation focus and technical route should be determined to carry out on-site base-by-base and item-by-item investigations.If necessary, the relevant parts can be sampled for test analysis.

Check the channel environment
First, we determine whether the distance of the line to the ground and the crossing distance of all kinds of poles, trees and roads, bridges, railways, and navigable rivers under construction meet the operation requirements.Then, we determine whether the design conditions of the line meet the latest version of the ice area and wind layout.

Check the tower
First, we determine whether the cross arm, tower material, bolts, or foot nails of the angle steel tower are corroded.We consider the bending condition of the tower material and determine whether it is located in an area prone to corrosion, whether the tower material cracks, whether there is corrosion and a loose strand of angle steel tala wire and check the skew of the cross arm and ground support.Second, we determine whether the steel pipe tower body, bolts, cross arm, foot nails, tower material, and flange are corroded.We check the bending condition of tower material and determine whether it is located in an area prone to corrosion.
Third, we determine whether the steel pipe bolt, cross arm, and cable are corroded.We check the bending of tower materials and determine whether it is located in the corrosion-prone area, whether the weld of the steel pipe rod is cracked, and whether the cross arm is skewed and twisted.
Last, we check whether there is a crack in the concrete rod, whether the bolts, hoop, fork beam, fork beam hoop, ground wire top frame, cable, and stable tensile tools are corroded, and whether they are located in corrosion-prone areas.

Check the guide ground
First, we check whether there are pollution sources near the corridor and whether it is an area prone to corrosion.Second, we check whether corrosion occurs, whether there are obvious rust spots, whether there are potholes, bulges, hulls, slag and other phenomena, whether there is a loose stock, broken stock, split stock situation, and whether the strength of guide and ground wire meets the current standard requirements.Third, we check whether it is in the second or third-level dancing zone.Last, we check whether it is in an area with severe wind vibration or secondary distance oscillation.

Check the foundation
First, we check whether it is in the mining-affected area, whether it is in an area prone to geological disasters such as landslides and debris flow, and whether there is settlement and uplift.Second, we check whether it is located in a flood control area and whether the tower foundation surface has obvious signs of damage.Finally, we check whether the buried depth of the cable foundation is insufficient and whether the rod is corroded.

Check the fixtures
First, we check whether it runs in corrosion-prone areas.Second, we check whether there are familial defects.Then, we check whether there is corrosion, crack, abnormal heating, and other phenomena.Finally, we check whether there is a burst wire clamp (connecting pipe), welding type tensile wire clamp steel anchor, and other fixtures, whether the closing pin of the bearing tool is corroded or missing, and whether there is any mismatch between the type of the instrument and the design (material, type, strength).

Check the insulator
First, we check whether the zinc layer of the steel foot of the composite insulator is lost and whether the neck is corroded, whether the end of the insulator gold plating zinc layer is seriously peeling, whether the zinc layer of steel foot is seriously corroded and precipitates appear in the neck, whether the diameter of the neck is significantly reduced, whether the steel foot head is deformed and the composite insulator hydrophobicity.
Second, we check whether the infrared imaging results of the composite insulator mandrake are abnormal.
Third, we check whether the glaze surface of porcelain and glass insulators is burned, whether the steel foot is deformed, whether it is corroded, and whether the locking pin is defective.
Then, we check whether it runs in corrosion-prone areas.Finally, we check whether there are familial defects.

Formulate management plans and measures
According to the risk status and importance of the old transmission lines, governance plans and measures are formulated from the implementation of countermeasures, operation and maintenance control, and other aspects to carry out targeted governance.
(1) For the old transmission lines in a high-risk state or the risk level of a single technical element reaching level Ⅳ, a near-term governance plan is formulated, and priority is given to governance.
(2) For the old transmission lines in the medium risk state or the risk level of a single technical element up to level Ⅲ, a near-term governance plan is formulated, and governance is gradually carried out according to the importance of the lines.
(3) For the old transmission lines in the state of low risk or the risk level of a single technical element reaches level Ⅱ, a long-term governance plan is formulated, and governance is carried out in stages.
(4) For the old transmission lines in the medium or high-risk states or the risk level of single technical elements such as channel environment, tower, and foundation up to level Ⅲ, relevant detection and verification should be carried out.If necessary, technical identification of the old transmission lines should be carried out, and timely rectification and treatment should be carried out.

Figure 1 .
Figure 1.Workflow of risk assessment and management of old transmission lines2.1.Check the technical elementsFirst, according to the relevant national and industrial standards, combined with the actual operation experience, according to the investigation focus and technical route, we determine the channel environment, foundation, tower, guide ground, gold tools, and insulators to carry out a hidden danger investigation.Then, we determine the investigation route of each technical element.The setting of the investigation route focuses on checking whether the design meets the requirements of the current distribution map of wind and ice regions and relevant countermeasures, as well as conformity with the actual situation of field operation.We consider the differential configuration factors of microtopographic and micro-meteorological regions.Finally, we conduct on-site base-by-base and item-by-

Figure 2 .
Figure 2. Defect map of old overhead transmission line 2.2.Risk assessment a) Value-at-risk calculation.Each technical element checked out shall be evaluated as a line section based on the continuous tower section with the same elements.According to the results of hidden danger screening, the risk value is calculated by the hazard source identification and Risk assessment (HIRA) method, namely:

Table 1 .
Technical element risk degree.
. Weight coefficients of technical factors.

Table 3 .
Table of evaluation criteria.