Analysis of Wire Breakage Fault Caused by Galloping on 66kV Debao A and B Line in November 2023

In early November 2023, affected by extreme weather conditions such as rain, snow, and freezing, some transmission lines in northeast region experienced icing and galloping, which brought lots of tripping and disconnection faults. Taking the wire breakage faults of the 66kV Debao A and B line as examples, the causes, processes, and characteristics were all detailed analysed. It can be basically identified that after the first-order galloping of icing the wire, due to the lack of or limited effectiveness of anti-galloping measures, interphase flashover occurred. The flashover melted some of the stranded of galloping wire, resulting in dynamic tension caused by galloping concentrated towards the broken position, the remaining cross-section of the stranded. Due to repeated bending and stretching, the remaining stranded wires exceeded the limit of fracture stress, leading to fracture and overall wire breakage and landing. So the wire breakage fault in this case is strongly related to the interphase flashover caused by galloping. This article conducted fault analysis work by collecting line design, on-site meteorological conditions, and fault characteristics, providing technical support for subsequent prevention and control work.


Preface
Due to the large-scale grid connection of new energy and the implementation of the policy of "carbon neutralization and carbon peak", China has the most complex power grid structure with lines widely distributed.As of 2020, China had a total of over 35,000 kilometers or 2,076.413 billion kWh of Ultra-High-Voltage (UHV) lines in operation and under construction, making it a key link in China for the sustainable development of energy and power and a pivotal role in the modern energy supply system.However, the abnormal climate changes in recent years have profound impact on China's power grid, causing extensive and long-running galloping that has led to frequent mechanical and electrical faults.Specifically, since the winter of 2007, the State Grid Corporation's transmission lines of 500kV and above have been shut down various times for failures, with galloping as a primary cause.Cold wave is one of the main reasons for the impact of rain, snow, and freezing disasters on power grid equipment [1][2][3][4].Since 2007, extreme weather conditions that may induce galloping frequently, and statistics show that the galloping of iced transmission lines has been the primary cause of line failure and outage.For most traditional anti-galloping techniques, the induction process of galloping is seen as the main target for prevention.Further analysis of extreme weather conditions in recent years shows that, the weather process of raining, snowing and icing disasters often poses great threats to the operation of the power grid and has been on the trend of frequent and extensive occurrence, with 1-2 times every winter, as well as the trend of its impact spreading to non-traditional heavy-iced and galloping-prone areas.Specifically, a typical case was the February ice disaster around the Bohai Sea, where a power line for the supply of liquefied natural gas (LNG) located in a Level-0 galloping area started galloping and caused a long power outage.Another typical case was the November ice disaster where a 66kV power line for water supply galloped in a Level-0 galloping area and caused the failure of the water supply pump station and some areas being out of heating and gas supply for a long time as water was also necessary for heating.In these cases, no measures or methods were available on-site as responses to the sporadic long-time galloping to effectively prevent the long-term failure and outage caused by galloping.The adverse effects of galloping on the power grid are becoming increasingly frequent.
From November 4th to 7th, 2023, the first cold wave weather in this winter affected most parts of northern China, with severe cooling in the north and extreme precipitation, mainly manifested as large-scale blizzards and freezing rain, causing icing [5,6] and galloping [7][8][9] on the 66kV Debao A and B line, and leading to tripping faults [10,11].
The fault point of the 66kV Debao A and B line was put into operation in 2009.Based on the design data collected in the early stage and compared with the actual observation on site, the 66 kV Debao A and B lines are 10mm as designed ice thickness, 30m/s as the design wind speed, and 16m/s as the measured wind speed.

Fault Site Situation
After the malfunction occurred (as shown in figure 1 and figure 2), it was discovered during the inspection that the average span of the section where the fault occurred was 300m.The wires were arranged vertically on the same tower and in a conventional triangle shape, with a designed ice thickness of 10mm and a designed wind speed of 30m/s.Both were in the level 0 galloping zone.Before this winter, bolt tightening was carried out on the entire line.In terms of anti-galloping devices, two lines have adopted differentiated anti-galloping measures.Line A has installed phase interval bars throughout the line, while Line B has not installed them.
When the fault occurred, the weather conditions in the section were rainy first and snowy later, with northeast winds of 6-7 levels, and the angle between the line and the wind direction was about 80°.An eccentric icing of 3mm was observed on the wire at the observation point, as shown in figure 3, with an equivalent standard thickness of 2mm.A special inspection of the fault found that the wire was galloping with an amplitude of 6 meters.The ice thickness and wind load on the conductors and towers caused by this round of rain, snow, and freezing disasters have not yet met the design standards.

Fault Cause Analysis
In order to conduct a detailed analysis of the galloping characteristics and fault situation of the Debao A and B lines, and based on relevant information, the design parameters of their wires and segments are clearly defined, as shown in the table 1 below: It should be noted that due to the adoption of differentiated anti-galloping measures, Line A has installed interphase spacers (as shown in figure 4), while Line B has not installed them.Meanwhile, referring to the wire model, detailed parameters such as cross-sectional area, operating tension, and elastic stiffness could be obtained, as shown in table 2. There was thin eccentric icing on the wire, and the cross-section is shown in the following figure 5.    Where: considering the span setting of the line, take the first to third-order modes common in line galloping for analysis, so the mode order marks J = 1, 2 and 3; M. K and C are the mass, stiffness and damping coefficient matrices of the system obtained by discrete finishing; FNL, J (x, y, ) are the nonlinear terms under different modes of each degree of freedom.
Therefore, there is a significant difference in the response amplitude of different wind speeds between the two under 3.23mm eccentric icing, as shown in figure 7.As shown in figure 8, when the fault occurred, although the interphase spacing rod was installed on Line A, it also experienced galloping.Due to freezing and rain condensation, as well as the variation of wind speed with height, the amplitude and synchronization characteristics of upper, middle, and lower phase conductors will significantly differ with slight differences in ice and wind speed.According to research, the circuits that triggered the tripping fault during this galloping were all arranged in a triangular or vertical pattern, and the fault point was also the middle and lower phases on the same side.This is because the near ground wind speed in this round of rain, snow, and freezing weather was relatively stable, and the combined effect of ice and wind made the upper and middle phase conductors galloping synchronously, resulting in asynchronous galloping of the middle and lower phase conductors.When galloping occurs, the curve of the dynamic tension of the wire over time is shown in figure 9.The normal operating tension of the Debao A-B line is about 1.35kN.According to the above theoretical calculations, the peak tension of the galloping wire is about 1.6 times that of the static state, which can reach 38.8% of the standard value of the calculated tensile force of the wire.Considering the design standards of steel core aluminum stranded wire, even if the calculated tensile strength reaches 100%, the wire will not easily break.
However, according to the wire breakage diagram 10 taken during on-site investigation, the wire breakage occurred near the center of the span (galloping belly), and the cross-section was all inclined.The strand breakage on one side showed burning and melting marks, with obvious burnt black, accounting for about 35%; The other half of the stranded wire exhibits necking characteristics, which

Conclusion
Through on-site research and theoretical analysis, the wire breakage fault in this round of ice disaster can be basically identified as: (1) After the first-order icing galloping of the wire, due to the lack of galloping measures or limited effectiveness of anti-galloping measures, interphase flashover occurred.
(2) The discharge caused by flashover melted some of the stranded wires, resulting in dynamic tension caused by galloping.The remaining cross-section of the stranded wires concentrated towards the broken position.Due to repeated bending and stretching, the remaining stranded wires exceeded the limit of fracture stress, leading to fracture and overall wire breakage and landing.
So the wire breakage fault in this case is strongly related to the interphase flashover caused by galloping.

Figure 1 .
Figure 1.Breakage of Debao Line A and B.Figure 2. Wire breakage on the ground.

Figure 2 .
Figure 1.Breakage of Debao Line A and B.Figure 2. Wire breakage on the ground.

Figure 4 .
Figure 4. Diagram of Installation of Interphase Spacer for Debao A Line.

Figure 5 .
Figure 5. Eccentric icing section.The aerodynamic lift, drag, and torque coefficients acting on the wire under the action of air flow vary with the angle of attack, as shown in the following figure 6:

Figure 6 .
Figure 6.Curve of coefficient variation with angle of attack.

Figure 7 .
Figure 7. Wind speed -Response amplitude curve.Using nonlinear dynamics methods and numerical simulation modeling methods, analyze the galloping response characteristics of the Debao A-B line.

Figure 8 .
Figure 8. Galloping trajectory curve under wind speeds of 6-7 level.(The third lines on the left are line A, and the third lines on the right are line B).

Figure 9 .
Figure 9.The variation curve of dynamic tension of wires over time.
fault caused by the tensile force exceeding the fracture stress limit of the aluminum stranded wire.