Accident analysis of 220, 500 kV overhead lines of the main power systems network

. The statistical analysis of overhead lines (OHL) of the unified national power grid (UNPG) of the Central European part of the country for the previous 15 years is subjected to statistical analysis. The structure of the failure flow parameter (failure frequency) of 220 and 500 kV overhead lines with a total length of 17.6 and 8.5 thousand kilometers, respectively, is compared. It is shown that the last decade is characterized by the most favorable reliability characteristics. At the same time, a tendency was detected, when the parameter of overhead line failure flow became insignificantly dependent on the nominal voltage of the network. This fact makes us doubt the existing belief of specialists, according to which the specific (per 100 km) failure rate of overhead transmission lines decreases with the increase of the nominal voltage of the network. The above trend once again demonstrates the objective possibility of purposeful management of electric networks reliability.


Introduction
The problem of ensuring the reliability of power systems has been and remains relevant.One of its aspects is the study of reliability indicators of power grid elements.The most important of them are the failure flow parameter (more precisely -the average failure flow parameter or failure rate), the average restoration time, as well as the frequency and duration of scheduled repairs.According to its functions, the power grid is subdivided into the main grid and the distribution grid [1].The main grid includes the Unified National Power Grid (UNPG), which forms the backbone of the country's energy systems.It includes overhead power lines with voltage of 220 kV and higher.Reliability of overhead power lines attracts close attention because of their relatively high accident rate due to natural, climatic and socio-economic phenomena and events.As is known, these are the least reliable elements of power grids.
Failures of overhead transmission lines were analyzed in a large number of publications [2-7, etc.]see Table 1, where references to literature sources are given in descending order of the chronological period of statistical sampling.In Table 1, the values (per 100 km of overhead line length) of the failure flow parameter are highlighted: unrecoverable ɷ н ; the sum of unrecoverable and recoverable by automatic reclosure ɷ ∑ .Characteristics are given per circuit or line.If the literature source differentiates the corresponding values depending on the type of supports (reinforced concrete or metal), the data on metal supports, as the most frequently used ones, were taken as a basis.

Accident analysis
Let us turn to the parameters characterizing the accident rate of overhead power lines.Among the data from [2][3][4][5][6][7] stands out the work [4], which, in fact, is the desktop book of the country's design engineers.It refers to the use of statistical data [6].But in fact it reflects the values of ɷ ∑ .Thereby, the failure rate of overhead lines used to assess the reliability of electrical connection schemes is inappropriately overestimated.In [8] the data of more than half a century ago are presented.At the same time, for a number of voltage classes (220 and 330 kV) the values of ɷ ∑ have not changed much since then.As for 500 kV overhead power lines, it was directly stated in [8] that ɷ ∑ =2.5 1/(year 100 km) cannot be regarded as a natural reflection of their peculiarities: the data covered only the first experience of operation of overhead power lines from Volga HPPs to Moscow, when the «dancing» of wires was the main cause of emergency outages of these lines.The parameters of the flow of unrecoverable failures in [2,3,5,6] are generally comparable, despite the fact that, say, the statistical sample [3] covered the time period from 2002-2007, and [5,6] -the period up to the mid-1980s of the last century.The reason lies in the fact that failures in the core networks are subjected to thorough investigations with subsequent development of organizational and technical measures to prevent them.
Table 1 shows that the specific (per 100 km) failure rate of overhead lines steadily decreases with increasing nominal voltage of the network.This is due, as many believe, to the increasing overall dimensions of lines (phase-to-phase distances, distances relative to the ground, etc.) -see Table 2.As is known, in the early 2000s, the organizational structure of management of the domestic electric power industry was changed.The 220 kV power grids referred to the UNPG were separated from regional power systems and transferred to the newly established Federal Grid Company.This ensured uniform rules for repair and maintenance of 220-750 kV grids.It turned out that these transformations had a noticeable impact on the reliability of these grids.The relevant arguments are given below.
The structure of unrecoverable failures of 220 and 500 kV overhead power lines with the total length of 17.6 and 8.5 thousand km in the central European part of the country for the period 2006-2022 was analyzed (Table 3).It follows from Table 3 that the predominant influence on overhead power line failures is caused by natural and climatic phenomena and events (items 2, 3 and 5 -more than half of cases for 220 and 500 kV overhead power lines), as well as by conditionally social reasons (items 1, 4 and 6 -about 40% of failures for both voltage classes).As can be seen from Table 3, the structure of failures of 220 and 500 kV overhead lines is generally comparable.This peculiarity, as well as the unified since 2006 principles of repair and operational maintenance of UNPG, prompted the authors to compare the accident rate of 220 and 500 kV overhead lines.Fig. 1 shows the time series of the unrecoverable failure flow parameter of the above mentioned 220 and 500 kV overhead lines.As can be seen from Fig. 1, at the initial stage (2006) ɷ n of 220 and 500 kV overhead lines was approximately 0.5 and 0.15 1/(year 100 km), respectively, which confirmed the data of Table 1.However, since 2012 the failure rate of overhead power lines has practically equalized, and in some cases it became more favorable for 220 kV overhead power lines compared to 500 kV: in 2020 the failure flow parameter of 220 kV overhead power lines was 0.08 1/(year 100 km), and that of 500 kV -0.10 1/(year 100 km).Thus, the uniform natural and climatic conditions and repair and maintenance resulted in the fact that the parameter of overhead line failure flow became little dependent on the nominal voltage of the network.Consequently, the identified trend once again demonstrates the objective possibility of managing the reliability of power grid elements.
As follows from Fig. 1, the failure rate of overhead transmission lines is subject to a certain variability from year to year.Therefore, it is not surprising that depending on the time period of the statistical sample, the overhead line reliability parameters, other things being equal, differ (Table 1).The maximum values ɷ н 0.54 and 0.67 1/(year 100 km) at 220 and 500 kV respectively were observed in 2010 -the year of abnormal natural fires.At the same time, the last fiveseven years have revealed a historical minimum of the accident rate of the considered overhead lines (0.1-0.15 1/(year 100 km)), despite the fact that about 40% of the lines have been in operation for more than 50 years (exceeding their standard service life).
At present, operating organizations are equipped with modern mechanisms and means for inspection and elimination of overhead power line failures.Analytical work is underway to collect and process statistical data on their accident rate in order to determine the causes and develop measures to prevent failures.
For example, starting from 2010, a program for the expansion of clearings has been developed and implemented.Trees in the overhead line protection zone that threaten to fall (leaning and dead trees), as well as trees and shrubs under the wires, are cut down annually.This significantly reduced the accident rate due to the fall of trees on wires and overlaps on tree and shrub vegetation (Fig. 2).For more visual dynamics of accident rate reduction in Fig. 2 and subsequent figures, both non-recoverable and recoverable by reclosure action failures are taken into account.As can be seen from Fig. 2, there was a multiple reduction in the accident rate of 220 and 500 kV overhead lines due to the considered cause.

Fig. 2. Number of violations related to tree and slab falls on woody and shrub vegetation
Programs to improve lightning resistance of the most accident-prone overhead power lines have also been implemented: -inspection and, if necessary, replacement of the grounding circuit of supports; -strengthening of lightning rods grounding by increasing the number of additional electrodes, changing the grounding scheme; -increasing the length of the insulating suspension by adding insulators.If there was no effect from increasing lightning resistance, surge arresters were additionally installed on the supports.As a result, the number of lightningrelated accidents was significantly reduced (Fig. 3).
The situation is more complicated with accidents related to the impact of unauthorized persons and organizations not involved in the technological process.First of all, work was carried out to replace outdated and exhausted operational life of supports with new ones with improved characteristics (replacement of reinforced concrete supports with metal ones with increased dimensions).A program was implemented to reconstruct overhead line sections with wire-to-ground dimensions that do not meet modern requirements (Electrical installation rules, edition 7) due to the growth of settlements.A comprehensive system of notification and informing the population (by authorities, mass media, information and warning posters, signs on poles, day and night markings, etc.) about the inadmissibility and danger of violation of overhead line protection zones has been developed.4), there was a «surge» of overhead line accidents.This is due to the rapid development of agricultural lands, increase in the number of industrial and social facilities, intensification of construction of large road interchanges, highways, as well as individual housing near overhead line protection zones in the region.
At that time, the existing regulations on interaction (during work) between electric grid and third-party organizations were not efficient enough; as a result, overhead line accidents increased.After the introduction of property and monetary agreements between these organizations, which provide for compensation of potentially caused damage to power grids, as well as the implementation of measures to fence territories, installation of information signs, inspection of overhead line protection zones by field crews, regulations for work in overhead line protection zones, etc., the number of accidents decreased (Fig. 4).Thus, if in 2006, 12 such failures were registered on 220 kV overhead lines (see Fig. 4), in 2022 -only four.
Table 1 shows the ratio ɷ /ɷ н∑ , i.e. the number of unsuccessful failures divided by their total number.As can be seen from Table 1, as the nominal voltage of the network increases, the share of unsuccessful failures increases approximately 1.5-2 times and more: 0.19-0.28and 0.36-0.48 at 220 and 500 kV, respectively.Fig. 5 shows the time series of the share of unrecoverable failures of 220 and 500 kV overhead power lines for the period under consideration 2006-2022.As can be seen, the ratio ɷ n /ɷ ∑ varies in wide ranges.Thus, in 2006 its value was about 0.45 for both voltage classes, and in 2022 -0.18 and 0.34 for 220 and 500 kV, respectively.Thus, this indicator is subject to noticeable temporal variability.However, there is an increase in the efficiency of successful reclosure on 220 kV OHL.It should be assumed that this is due to the abovementioned list of measures to improve their reliability.

Conclusion
For many decades there has been a stable opinion of specialists that with increasing nominal voltage of the network the specific (per 100 km) frequency of overhead line failures decreases.This fact is reflected in many literature sources.
The changes in the organizational structure of management of the domestic electric power industry that took place in the early 2000s provided uniform rules for repair and maintenance of 220-750 kV networks.This led to the fact that the parameter of overhead line failure flow became little dependent on the nominal voltage of the network.
Analysis of the organizational structure of failures of 220 and 500 kV overhead power lines in the vast region for the period 2006-2022 revealed the main factors influencing reliability, including their weight in the total accident rate: natural, climatic and social.Targeted impact of operation on these factors led to the historical in this time interval minimum accident rate of 220 and 500 kV overhead lines at the level of 0.1-0.151/(year 100 km).This is two to four times less than previously published statistical data.
The efficiency of reclosing on 220 and 500 kV overhead power lines is subject to significant temporal variability and is within the previously known limits.Never the less, the efficiency of reclosing on 220 kV overhead lines in the period 2006-2022 increased approximately twice, which, it should be assumed, is associated with the implementation of organizational and technical measures to improve reliability.

Fig. 1 .
Fig. 1.Flow parameter of unrecoverable failures of 220 and 500 kV overhead lines

Fig. 5 .
Fig. 5. Ratio of the number of unrecoverable failures to their total number to their total number

Table 1 .
Failure flow parameter of overhead line

Table 2 .
Parameters of 220 and 500 kV overhead lines

Table 3 .
Organizational causes of overhead line failures for the period 2006-2022