Feasibility analysis of dynamic improvement of current carrying capacity of overhead lines

The demand for electricity in China has been increasing year by year. However, the construction of transmission lines is costly and has significant composite short-term peak characteristics. This article aims to study the feasibility of improving the conductor’s current capacity, analyze data characteristics of typical conductor states and micrometeorological data, figure out the real-time conductor’s current capacity based on the calculation model, analyze the margin for improving the CCC, and lift the power supply potential.


Introduction
Some transmission lines have experienced heavy overload.Considering the huge cost and timeconsuming construction of new lines, adopting new technologies to enlarge the short-term power supply of the power grid is a significant research topic.The supply capacity of overhead lines is designed based on typical environmental conditions.To ensure the safety of line operation, it is chosen to design the the current carrying capacity (CCC) of the line in relatively harsh meteorological conditions.However, in actual line operation, considering that the overhead line often passes through mountainous areas and the environmental temperature is low, the CCC of the wire can be much greater than the calculated rated value.Domestic grid researchers have conducted certain research based on the channel characteristics, and the research on wire CCC monitoring technology has become a major research topic [1][2][3][4][5][6].However, the current monitoring technology for CCC mainly focuses on the research of monitoring technology, and the improvement of CCC is still in reliminary stage.There are few practical applications.The space for improving CCC still needs further analysis.
This article focuses on the calculation of CCC, analyzes the characteristics of the influencing factors of CCC, and analyzes the variation characteristics of wire CCC for a certain 220 kV overhead line using a micrometeorological monitoring device.It explores the margin for improving CCC of the line at different time periods, and discusses the improvement of CCC.It also analyzes the characteristics of deepening the improvement of line CCC.

Analysis of Factors Influencing CCC
The calculation method for the CCC of overhead lines is based on the requirements of the "GB 50545-2010", and the main influencing factors include wire material, environmental temperature, environmental wind speed, and lighting conditions.When the environmental temperature, environmental speed, lighting, and line current remain stable, the maximum CCC that the conductor can withstand remains stable.The calculation is based on the heat balance: where I is the allowable CCC, A; Ws is the solar heat absorption power, W/m; WR is the radiation heat dissipation power, W/m; WF is the convective heat dissipation power, W/m; ' T R is the AC resistance of the conductor at allowable temperature, Ω/m.
WR and WS are shown in Equations ( 2) and (3): where θ is the current carrying temperature rise of the wire, ℃; D is the outer diameter of the conductor, m; E1 is the radiation coefficient of the wire surface; Stephen Boltzmann constant is 5.67×10 -8 W/m 2 ; T is the environmental temperature, ℃; α is the wire heat absorption coefficient; Is is the intensity of sunlight, W/m 2 .WF is shown in Equations (4): 2.42 10 7( + 2) 10 where λf is the heat transfer coefficient of the air layer on the surface of the wire, W/m•℃; Re is Reynolds number; V is the speed of the wind, perpendicular to the conductor, m/s; v is the kinematic viscosity of the air layer on the surface of the wire, m 2 /s.The CCC of typical conductors is calculated and typical parameters are specified.Among them, the sunshine coefficient is 1000 watts/square meter, the wind speed is 0.5 meters/second, the allowable temperature of the conductor is 70℃, and the ambient temperature is 40℃.The radiation coefficient and absorption coefficient are 0.9.
The environmental temperature and speed have a significant impact on the calculation results.LGJ-300/40 wire is selected to calculate the CCC of the wire under different environmental temperatures and wind speed conditions.Other parameters refer to typical design values.The results are shown in Figure 1.

Line selection
An analysis was conducted on 16 lines with micrometeorological monitoring devices installed in a certain area, considering that dynamic capacity increase calculation requires temperature monitoring, wind speed monitoring, and solar radiation monitoring.At the same time, the number of monitoring devices should be as rich as possible, and Line 1 was ultimately selected.
8 sets of micrometeorological monitoring devices and 6 sets of wire temperature monitoring devices have been installed along Line 1.At the same time, each section of Line 1 is located in a plain area with good terrain consistency, and Line 1 is located in a new energy transmission area, which has the actual demand for line capacity expansion and dynamic analysis of wire CCC.Those device collect physical data, such as environmental temperature, light intensity, equivalent wind speed, wire current, and wire temperature.Through implementation, the line data is obtained, and based on the CCC algorithm shown in Chapter 1, the dynamic monitoring of wire CCC is achieved, and the status of the line is calculated.The calculation process of CCC is shown in Figure 2.

Monitoring data accuracy
Tables 1 and 2 show the comparison of the monitoring data of 2 towers on Line 1 with local meteorological station data.From the comparison of monitoring data between monitoring devices and meteorological stations, it can be seen that some time periods of the observation point line section are relatively close, and the monitoring data of the monitoring device can be effectively used to analyze the CCC of the line.

Monitoring data analysis
Due to the relatively small changes in environmental temperature during noon and night, it is considered to conduct CCC analysis based on typical period monitoring environmental temperature data.Two observation points were selected to analyze the meteorological monitoring data and to analyze the characteristics of ambient temperature changes in typical time periods.The average ambient temperature at each time point in July 2023 for observation points 1 and 2 is shown in Figures 3 and 4.
In 11:40-2:40 pm and 0:00-5:30, the environmental temperature is relatively stable, and CCC change is slow.Observation point 2 shows a significant change in the CCC in the noon, where the solar radiation situation changes fastly.
Meanwhile, through data analysis, it can be concluded that the CCC of different observation points on the same absorption road will exhibit different changes due to environmental meteorological and topographical factors.Further analysis of the changes in environmental temperature, environmental speed, and other information on the same line channel is still needed to obtain the minimum CCC of the line and to ensure the safety of the line CCC adjustment process.

Capacity Analysis
Considering that observation point 2 is the location with the minimum CCC of the line channel, the dynamic improvement of the line's CCC is analyzed based on the results of observation point 2.
Considering the ambient temperature of 40℃, the CCC of this line is 215 megawatts.The minimum value of the maximum steady-state current within 30 minutes is used as the CCC of the line during this period.The dynamic monitoring CCC is lower than the specified 40℃ ambient temperature CCC during the multi-day peak hours of July 6-8, 10-12, 15-16, 20, 22-23, 25-26, 28, and 30 (10:30-14:00).
During peak hours (10:30-2:00pm), the CCC of the line changes within the range of 169 to 311 MW.Dynamic monitoring is used during the evening peak (18:00~21:30), and the CCC of the line varies within the range of 275 MW and 333 MW.
The maximum supply power of the double circuit line is 259 megawatts (3:45 on July 8th), and there is no situation of exceeding the control power operation.The demand for improving CCC during is relatively small in the summer.
Referring to the dynamic monitoring data of the CCC of the line in the early morning of July (0:00~7:30), the CCC of the line varies within the range of 285 MW to 337 MW, corresponding to the requirements for the dual circuit control of the system varying within the range of 341 MW to 403 MW.Therefore, there is great room for CCC improvement in the early morning.
For this line, there is a small demand for improving its CCC during summer peak periods.There is a CCC demandin the double circuit transmission, and clean energy transmission mainly occurs in the early morning.Dynamic monitoring of CCC can effectively improve the transmission capacity of the double circuit line and solve the problem of clean energy transmission checkpoints in this channel.

Conclusion
This article conducts a dynamic adjustment analysis of the CCC of a certain 220 kV line.Based on existing monitoring data, the feasibility of a dynamic capacity increase of the line in summer is analyzed.The main work is as follows: Analyzed the factors affecting the CCC of overhead lines, and identified environmental temperature and wind speed as important influencing factors for the CCC of conductors.
We selected a certain 220 kV line for conducting wire load adjustment analysis and sorted out the existing monitoring devices of the line.
Two observation points on the line were selected and the characteristics of the CCC monitoring data for July were analyzed.It was found that the composite characteristics at night were relatively stable and had the feasibility of capacity adjustment.
Based on the calculation of real-time CCC, an analysis was conducted on the improvement of CCC.It was found that the 220 kV line has significant room for improvement in CCC during periods of high nighttime and new energy generation.

Figure 1 .
Figure 1.CCC under different environmental temperatures and wind speed conditions.

Figure 2 .
Figure 2. Calculation Process for CCC of Line 1.

Figure 3 .
Figure 3. Average Environmental Temperature and CCC of Observation Point 1 in July.

Figure 4 .
Figure 4. Average Environmental Temperature and CCC of Observation Point 1 in

Table 1 .
Comparison of Monitoring Device and Meteorological Station Data at Observation Point 1.

Table 2 .
Comparison of Monitoring Device and Meteorological Station Data at Observation Point 2.