Shielding Failure Rate Research Articles

OVERVIEW OF FACTORS AFFECTING THE ESTIMATION OF LIGHTNING SHIELDING PERFORMANCE OF OVERHEAD TRANSMISSION LINES

This paper examines the risk of lightning stroke to overhead transmission line. The estimation procedure for the lightning performance of overhead power lines is based on the selected lightning attachment model, available lightning parameter statistics, the transmission tower design and voltage levels, type of overvoltage and other characteristics. In this paper the overview of factors affecting the estimation of lightning shielding performance of overhead transmission lines was performed. Among the factors that can affect the estimation accuracy, one can list insufficiently accurate data on the ground flash density in the area of interest and lack of complete data on statistical distribution of lightning current magnitudes. The paper shows that the influence of wind on the increase in the horizontal exposed distance of the phase conductor is not also taken into account. In this research traditional electro-geometric model was used for estimation of lightning performance of 220 kV overhead power line. Results obtained suggest that swing of suspension insulator strings caused by strong winds may lead to increased risk of lightning shielding failure during thunderstorm. Calculation performed for 3 kA lightning current magnitude shows that at swing angle equal to –1 degree, the horizontal unprotected distance of phase conductor increases by 3.1%, that corresponds to 5.240 m uncovered width. When the swing angle is increased to –5 degree, the uncovered width is increased by 15.8% that corresponds to 5.885 m uncovered width. It is proposed that an increase in the risk of lightning shielding failure as a result of wind load can be accounted by applying an appropriate correction factor in expressions for calculation of shielding failure rate, shielding failure flashover rate, etc. Proposed correction factor should account frequency and strength of wind in the area of transmission line route and depend on transmission line voltage level and tower design. Further efforts should be focused on obtaining and justifying the numerical values ​​of this correction factor.

Study on the Lightning Protection Performance for a 110 kV Non-Shield-Wired Overhead Line with Anti-Thunder and Anti-Icing Composite Insulators

Due to micro landforms and climate, the 110 kV transmission lines crossing the mountain areas are exposed to severe icing conditions for both their high voltage (HV) conductors and shield wires during the winter. Ice accumulation on the shield wire causes excessive sag, which leads to a reduced clearance between earth and HV wires, and could eventually result in tripping of the line due to phase-to-ground flashover. Due to the lack of effective de-icing techniques for the shield wires, removing them completely from the existing overhead line (OHL) structure becomes a reasonable solution to prevent icing accidents. Nevertheless, the risk of exposure to lightning strikes increased significantly after the shield wires were removed. In order to cope with this, the anti-thunder and anti-icing composite insulator (AACI) is installed on the OHLs. In this article, the 110 kV transmission line without shield wire is considered. The shielding failure after installation of the AACIs is studied using the lightning strike simulation models established in the ATP software. The lightning stroke flashover tests are carried out to examine the shielding failures on various designs for the AACIs. Assuming the tower’s earth resistance is 30 Ω, the LWL of back flashover and direct flashover are 630.88 kA and 261.33 kA, respectively, after the installation of AACIs on an unearthed OHL. Due to the unique mechanism of the AACI, the operational voltage level and the height of the pylon have a neglectable influence on its lightning withstand level (LWL). When the length of the parallel protective gap increases from 450 mm to 550 mm, the lightning trip-out rate decreases from 0.104 times/100 km·a to 0.014 times/100 km·a, and the drop rate reaches 86.5%. Therefore, increasing the gap distance for the AACI to provide additional clearance is proven to be an effective method to reduce the shielding failure rates for non-shield-wired OHLs.

Improved Electrogeometric Model of UHV Transmission Line Based on Long Gap Discharge and Its Application

Based on long air gap discharge test data and lightning return stroke observation data, an improved electrogeometric model (EGM) considering terrain conditions is established and verified to analyze the lightning shielding performance of UHV transmission lines. The striking distance formula is modified as rs = 0.13 (I2 + 40I)0.814. In this paper, the lightning shielding failure rate of three-phase conductors of EHV and UHV transmission lines calculated by the improved EGM model is consistent with the lightning observation data of actual transmission lines in Japan and the scaled lightning discharge simulation experimental results of UHV transmission lines in plains and mountainous areas of China, which verifies the applicability of the improved EGM model to evaluate the lightning shielding performance of large-scale UHV transmission lines. The improved EGM model is applied to evaluate the influence of tower type and slope steepness on the shielding failure tripping rate of UHV transmission lines. The shielding failure tripping rate of the SZ322 tower is higher than that of the SZT1 tower, and the shielding failure tripping rate of UHV transmission lines is greatly affected by slope gradient and increases with the increase of slope gradient.

Study on the lightning shielding characteristics of the crossover area of UHV/HV transmission lines

Due to space limitations of transmission corridors, ultrahigh voltage (UHV) lines inevitably cross over other lines. UHV lines in a crossover section have a lightning shielding effect similar to that of a lightning conductor of lower lines. While, whether a 500-kV line below a crossover section meets the lightning protection requirements is unknown. Therefore, a calculation method for the shielding failure trip rate of transmission lines in a crossover section in a three-dimensional space is proposed. The calculation of the shielding failure trip rate of the 500-kV line without a ground wire below the UHV line was performed. The results revealed that the shielding failure rate of the lower line was positively correlated with the length of the lower line and was negatively correlated with the crossing angle and length of the upper line. The maximum traversal lengths for a 500-kV line without a ground wire below a UHV AC or DC line were 200 m and 150 m, respectively. These findings provide a scientific basis for lightning protection design of ultra-UHV line crossing areas.

The Calculation Method of Shielding Failure Trip Rate of UHV Transmission Lines in Mountainous Areas

Since the second industrial revolution, electric energy has always been the most important and indispensable clean energy in the international community. As a kind of transmission technology with small loss ratio and long transmission distance, UHV transmission is widely used and vigorously promoted in the world. However, UHV transmission lines are often assumed to be vulnerable to thunderstorm discharge in some harsh geographical environment. In order to improve the calculation accuracy of line shielding failure rate, an improved electrical geometric model is proposed to study the shielding failure characteristics of transmission lines under complex mountainous areas, and various typical characteristics including mountain tops, valleys, slopes, climbing, and crossing trenches are summarized.

Estimation of HVDC transmission lines shielding failure using LPM method and an adapted SLIM model

In this study, the shielding system of HVDC transmission lines against direct lightning strokes is analysed and the shielding failure rate is investigated. The lightning downward leader is modelled using 3D leader progression model (LPM) and the striking point is distinguished employing the self-consistent upward leader inception and propagation model (SLIM). For determination of striking point more accurately, the required parameters of SLIM model are extracted from the recent experimental results. Different effective objects in the space of study including cloud, lightning channel, towers, wires, and ground are modelled using fictitious charges. Moreover, the charge simulation method is used for electric field calculation. Using the foregoing model, the shielding failure rate of an HVDC transmission line is calculated. Also, the percentages of lightning strokes to each part of the structure are obtained in order to investigate the differences between the positive and negative poles in attracting the lightning. In addition, the effect of height and shielding angles of positive and negative poles are studied. All the results are compared with those of electrogeometric model models.

A fractal-based stepped downward leader model including branched channel charge distribution and branch fading

In this paper, a fractal-based model is proposed for lightning downward leader propagation through the air. The model simulates the zigzag movement and the branching nature of lightning close to reality in comparison to previous existing models. The probabilistic behavior of lightning is well modeled using the fractal approach. A new model is also introduced for charge distribution along different branches of the lightning downward leader. The model calculates the charge density of each branch based on the concept of electric field through the branches. Moreover, the probability of branch fading is considered in the simulations. The proposed model is evaluated for striking distance calculation of a transmission line structure and the results are compared to those of Leader Progression Model (LPM), and different models of Electrogeometric Model (EGM). Moreover, Shielding Failure Rate (SFR) of some structures is obtained by the proposed method and the results are compared to those of field observations. In addition, the electrical field on the ground near the striking point of lightning is calculated and compared to the measurements. The branch fading and the fractal dimension of simulated lightning are also discussed. The simulations show how the proposed model simulates the lightning in an acceptable manner.

Influence of Placement of Overhead Ground Wires on Steady State Induction in AC-DC Hybrid Transmission Corridor

To meet the growing power demand of the future, utilities and researchers have considered the addition of dc circuits in existing transmission corridors. The previous research has focused on analyzing the field environment, steady state induction and the negative effects of induction in the hybrid transmission lines. The overhead ground wires (OHGWs) have been used only for improving the lightning performance of overhead lines (OHL). In this paper, the spatial position of OHGWs has been studied with the objective of minimizing the steady state induction between power conductors as well as poorly grounded metallic objects like railroads and fences present near the ac-dc OHL. Candidate space for placement of OHGWs having shielding failure rate of less than four is identified using the conventional electro-geometric model. The steady state induction in the power conductors and nearby metallic objects is studied as a function of position of OHGWs using electro-magnetic transient simulation. The results indicate a strong correlation between the height of OHGWs and magnitude of induced voltage in power conductors, and metallic objects underneath the transmission line. The conscious effort in placement of OHGWs can reduce the magnitude of induced voltages while ensuring adequate lightning performance.

Fractal‐based lightning model for shielding failure rate calculation of transmission lines

In this study, performance of lightning shielding system of transmission lines is investigated by means of a new fractal-based model of simulated lightning. A new lightning downward stepped leader based on fractal approach is introduced in this study. The zigzag movement of lightning downward leader as well as branching nature are simulated using the proposed model. Moreover, a new charge distribution is proposed for branched lightning channel. In addition, the branch fading during lightning downward movement is also simulated in order to simulate the lightning closer to reality. The proposed method is applied for calculating the striking distance for a transmission line and the results are compared with those of leader progression model and different models of electrogeometric model. Furthermore, the shielding failure rate of some practical structures obtained by the proposed method is compared with the field observations. The results are discussed and superiority of the proposed simulated lightning model is concluded.

Improved electro‐geometric model for shielding failure analysis of transmission lines

An improved electro-geometric model (EGM) based on upward corona leader system consideration is introduced. The proposed method is a modified version of EGM and its high accurate results are independent of the height, voltage level, and circuit type of structure. The equations of the proposed method are modified by means of genetic algorithm based on the leader progression models (LPM) results which are consistent with the experimental results. To evaluate the performance of the proposed method, the shielding failure rates (SFR) of a single-circuit transmission line with different protection angles are calculated and compared to those of LPM and a revised EGM. The comparisons show that the results of the improved method roughly agree with the results of LPM. To ensure about the performance of the proposed method, it is applied on ten different practical structures with different characteristics. The results of IEGM are compared to the field data and the results of the revised EGM. The comparisons demonstrate that the proposed IEGM is an appropriate method for SFR calculation irregardless of the height, voltage level, and circuit type. In other words, the proposed IEGM is as simple as conventional EGM and as accurate as LPM.

Investigation on Shielding Failure of a Novel 400-k$\text{V}$ Double-Circuit Composite Tower

In this paper, the lightning shielding performance of a newly designed 400-k $\text{V}$ double-circuit composite tower is investigated. Based on a revised electrogeometric model (EGM) method, traditional shielding failure regions located at both sides of a traditional tower are no longer a big issue for the fully composite tower, due to its unusual $^{\prime }Y^{\prime }$ configuration. Instead, a new unprotected region exists in the tower center. The maximum lightning current that can lead to shielding failure and the shielding failure rate of the new tower are calculated. To verify results from the revised EGM method, a scale model test is conducted. Spatial shielding failure probability around the tower is calculated based on the ratio of discharge paths recorded in the test. Moreover, based on test results, the maximum shielding failure lightning currents are obtained. Analysis and results derived from the revised EGM method and scale model test have a small deviation, verifying the feasibility of these two methods in the lightning performance evaluation of the fully composite tower. Both methods confirm that the unusual negative shielding angle of $-\text{60}^{\circ }$ in the newly composite tower provides good lightning shielding performance. The feasibility of the novel composite tower is verified from the lightning protection point of view.

Shielding failure analysis of a hybrid transmission line with AC and DC systems on the same tower

Hybrid AC/DC transmission lines have different systems on the same tower and combine HVDC transmission over long distances with common HVAC transmission. Evaluation of the risk of lightning stroke outage of high power transmission lines is very important because lightning strokes cause mainly outages. The present paper illustrates how to combine efficiently different methods to support insulation coordination studies. Maximum lightning current amplitude that does not cause flashover across insulator string is estimated in response to first and subsequent lightning strokes with three different flashover models using several electrogeometric models and simulations in EMTP-ATP. Characteristic quantities like Shielding Failure Rate (SFR) and Shielding Failure Flashover Rate (SFFOR) are calculated for different shielding configurations.

LPM-Based Shielding Performance Analysis of High-Voltage Substations Against Direct Lightning Strokes

The leader progression model (LPM) is employed to investigate the shielding system performance of high-voltage substations against direct lightning strokes in the presence of substation instruments. The substation is simulated in detail for high accurate calculation of the shielding failure rate. Almost all of the apparatuses are modeled as fictitious charges in the stepwise simulations of LPM. The charge simulation method is used for finding the electrical field during the lightning movement. The probability of upward leader propagation is considered from all apparatuses. For more practical simulation, the environmental conditions are also included in simulations. The investigations are implemented on a test case and a practical high-voltage substation. The results are compared to those of the electrogeometric model. It is very likely that the results of this paper better simulate reality because lots of parameters have been taken into account. The results reveal that the presence of substation instruments increases the striking distance and the SF since they attract the lightning moving down near the substation.

Adaptive Strategies in the Leader Propagation Model for Lightning Shielding Failure Evaluation: Implementation and Applications

Lightning shielding failure accounts for the highest proportion of lightning accidents for transmission systems above 500 kV. The leader propagation model (LPM) was found to be effective in the analysis of lightning shielding failure of high-voltage transmission lines, especially for the ultrahigh voltage (UHV) transmission systems. However, it was generally inefficient to calculate the shielding failure rate by LPM, since the progress of lightning development must be simulated repeatedly, which is time-consuming, and no adaptive implementation of LPM has been reported. In this paper, adaptive strategies to accelerate the computation of shielding failure analysis by LPM were proposed, which include the adaptive time step control according to the electric field at the conductor surface, non-uniform lightning current sampling based on the lightning current amplitude probability distribution, and the adaptive lightning location sampling based on recursive adaptive Simpson’s method. The analysis results were validated by the field data of the UHV transmission line in Japan, and the efficiency of the adaptive strategies was shown by the shielding failure analysis of a typical 500 kV ac transmission line.

A hybrid approach to calculate the Shielding Failure-Caused Trip-out Rate

Lightning has become a big threat to the safe operation of the main transmission line. Reasonable and accurate calculation of shielding failure rate plays important role in transmission line and tower design. This paper proposes a hybrid approach to calculate the shielding failure-caused trip-out rate, based on the typical electro-geometric model and the regulation method. The case study prove the validity and correctness of this approach, by comparing with the actual operation shielding failure rate.

Experimental research on lightning shielding performance of large scaled UHV transmission line

In actual operation, the shielding failure rate of UHV transmission line is much higher than that in simulation, so it is crucial to study the lightning shielding performance by experimental research. The testing UHV transmission line model is scaled down in the ratio of 12.5 which is relatively large comparing to other testing models. Two kinds of negative switching impulse waveform were applied to HV rod to simulate the lightning descending leader. The probability of transmission line being stroked was calculated statistically by recording more than two thousand discharges and the factors that affecting the discharge probability were studied through contrast tests. The results show that the grounding condition, DC bias, the protection angle, the switching impulse voltages have an influence on the discharge probability, breakdown voltage and breakdown time. The spatial boundary of shielding failure area was determined in the middle of the span. The upper and lower spatial boundary and the distance between them increased with the increasing of the lateral distance between HV rod tip and transmission line model simultaneously.

Estimation of the shielding performance of overhead transmission lines: The effects of lightning attachment model and lightning crest current distribution

Shielding failure of overhead transmission lines is a major cause of transmission system outages, affecting reliability of power supply and resulting in economic losses. In this study the shielding performance of overhead transmission lines is evaluated with the aid of general expressions, derived by implementing in shielding analysis different categories of lightning attachment models. Thus, the effects of lightning attachment model, transmission line parameters and lightning crest current distribution on shielding failure rate of overhead transmission lines have been quantified. Alternative approaches to that proposed by IEEE Std 1243 for assessing the shielding performance of transmission lines, including computer simulations of lightning attachment, are evaluated. Shielding failure rate results are discussed and compared with field data reported in literature. For typical overhead transmission lines, an upper limit of the estimated shielding failure rate is 0.4% of the rate of lightning strokes to the line. More and reliable field data is needed in order to evaluate lightning attachment models with respect to the lightning performance of overhead transmission lines.

Physical mechanism and numerical simulation of the inception of the lightning upward leader

The upward leader is a key physical process of the leader progression model of lightning shielding. The inception mechanism and criterion of the upward leader need further understanding and clarification. Based on leader discharge theory, this paper proposes the critical electric field intensity of the stable upward leader (CEFISUL) and characterizes it by the valve electric field intensity on the conductor surface, EL, which is the basis of a new inception criterion for the upward leader. Through numerical simulation under various physical conditions, we verified that EL is mainly related to the conductor radius, and data fitting yields the mathematical expression of EL. We further establish a computational model for lightning shielding performance of the transmission lines based on the proposed CEFISUL criterion, which reproduces the shielding failure rate of typical UHV transmission lines. The model-based calculation results agree well with the statistical data from on-site operations, which show the effectiveness and validity of the CEFISUL criterion.

Sensitivity of lightning shielding failure of double-circuit transmission line on tower geometric structures

A numerical calculation method based on the leader propagation model of lightning is proceeded to investigate the effect of overhead line tower geometric constructions to the lightning shielding failure. In this paper, the height and the spacing of the ground wire were changed to study the sensitivity of the shielding effectiveness on tower constructions. As the conclusion, an efficient layout scheme of ground wire and phase wire was proposed. Even if the protecting angle of the ground wire keeps unchanged, if the height of the ground wire is various, the total shielding failure rate is totally different, so we should take into account other factors except for the protection angle during the design of tower structure. This paper compared the total trip out rate of four types of double-circuit towers and concluded that the concave drum-type tower is the best design.

Comparison of numerical analysis models for shielding failure of transmission lines

The shielding failure is the occurrence of a lightning stroke that bypasses the overhead shielding wires and strikes on the phase conductors. The improvement of the shielding effect of transmission line to lightning is one of key problems in transmission line design. Some analysis models for shielding failure performance were proposed and discussed, mainly including the traditional electrogeometric model (EGM) and improved EGM, the leader progression model (LPM). The paper compares the results of shielding failure rate, lightning current range and location region for shielding failure by numerical calculation applying the three models. The influence of ground slope angle, voltage class of transmission line and protection angle of ground wire is also taken into account.