Control strategy for large-scale ﬁres near power transmission lines and its application

To prevent ﬁres causing the power transmission lines trip, a control strategy including the methods of ﬁre forecasting, ﬁre monitoring and ﬁre extinguishing for large-scale ﬁres under power transmission lines is proposed. A water mist ﬁre extinguishing method for transmission lines with high-voltage was presented to ensure the ﬁre extinguishers safe. Compared with conventional measures of ﬁre risk evaluation and ﬁre detection, the proposed control strategy can proactively control the ﬁres to ensure the transmission lines run continuously, rather than passively stop running to avoid the ﬁre. The timeliness of ﬁres rescue for power grid is analysed statistically, and the control strategy for ﬁres near transmission lines is presented and discussed. The proposed strategy had been widely used in Hunan province power grid of State Grid in China. After the applications, the average ﬁre tripping number of transmission lines with the voltage levels of 220


INTRODUCTION
The power transmission lines usually pass through the highfire area with dense vegetation. The gas insulation of power transmission lines will be greatly reduced in the case of fires, and phase-to-ground or phase-to-phase short circuit event may occur [1][2][3][4]. Most of the transmission line trips caused by fires are difficult to be re-closed successfully due to the continuous burning, so the adverse effects of fire faults on transmission lines operating are more serious than that of other transient faults such as lightning strikes. 5.2% of large blackouts were caused by fires in the United States [5]. According to the field statistics, 63.7% of the transmission lines failed to re-close after the tripping caused by fire in Hunan province power company of China. With increasing of power demand in China, the voltage level of transmission lines becomes higher and higher (such as ultra-high voltage power (UHV) AC 1000 kV, DC ±800 kV) to enhance power supply ability of the lines, and the UHV lines operation safety is very important. However, the UHV transmission lines have broken down several times due to fires and brought huge impact on the power grid. The ±800 kV Yunguang and Jin-su lines were suspended due to fires in 2013 and 2014, respectively. The 1000 kV Chang-nan line suffered fires to outage in Shanxi province in 2013. A lot of transmission lines with voltage levels from 110 to 1000 kV were tripped due to fires in China [6]. On the other hand, fires have the characteristics of high frequency in a period of time in a large-scale area, and it causes a large number of transmission lines tripping, which poses a serious threat to the safe operation of power grid. For example, 9 transmission lines with a voltage level of 220 kV and above were tripped caused by fires on January 31, 2014 in Hunan power grid, almost causing the power grid to collapse. Therefore, it is significant to study a control strategy for large-scale fires near transmission lines to improve the safe operation of transmission lines and ensure the reliable power supply.
The issue of transmission lines fires trip has attracted the attentions of many researchers. Some researchers have done a lot of work on this subject, including fire trip mechanism and analysis, fire monitoring, and so on. In the aspect of fire trip mechanism, A. Robledo-Martinez et al. studied dielectric characteristics of a model transmission line in the presence of fire [7]. Sukhnandan. A et al. discussed various models of flashover under varying fire conditions [8], and Li Peng et al. studied the dielectric characteristics of transmission line gap under fire conditions [1,9]. In the aspect of fire trip possibility evaluation method, some researchers presented some calculation methods [10][11][12][13][14][15][16]. The tripping risk warning methods for transmission lines in the case of fire were discussed in literatures [10] to [14]. Such as, literature [10] presented a method to evaluate the impact of forest fire on line ageing degree based on dynamic heat balance equation and Weibull distribution. The fire occurrence risk prediction methods were proposed in [15,16]. In the aspect of fire monitoring and spread analysis, satellite-based fires warnings method and fire spread trend nearby transmission line were studied in literatures [17][18][19][20][21]. In the above researches, the research only focus on the fire warning for transmission lines, but there was no overall strategy to control fires near transmission lines with high-voltage. IEEE Std 979-2012 provides fire protection and safety guidelines for substations for electric power engineers and fire protection professionals [22], however there was no standard or effective measure in application to control the fires near transmission lines. The difficulty and challenge of fire control for transmission lines are that: 1. Transmission lines operate with high voltages, fire extinguishers are at a great risk of electric shock. There was not effective method of fire extinguishing near high-voltage transmission line to ensure the fire extinguishers safe in the past. 2. The fires near transmission lines occur in a large-scale geographical area, and the timeless requirement for fire rescue is very high. However, the trip possibility evaluation methods presented in the literatures aim at transmission line when fire was occurred, which cannot satisfy the application requirement for large-scale fires.
The main contributions of this paper are summarized as the following. This paper presented a control strategy for large-scale fires near transmission lines to ensure the continuous operation of the transmission lines. A fire extinguishing method for transmission line with high-voltage was presented to ensure the safety of firefighters. A large-scale fire risk forecasting method for transmission lines was proposed to early prepare fire extinguishing equipments in the high-risk fire area, and also a fire monitoring system was developed to provide real-time fire information.
This paper was organized as follows. Section II analysed the timeliness of fire rescue for transmission lines, and presented a control strategy for large-scale fires near transmission lines. A fire risk forecasting method for transmission lines and a real-time large-scale monitoring technology for

The overview of fires near transmission lines in world region
Most countries in the world suffered from forest fires. The transmission lines were inevitably affected by fires. The most serious countries on fires near transmission lines are mainly China, the United States, Canada, Australia and so on, as shown in Table 1.
In China, the south provinces are the serious area where the transmission lines are affected by fires, such as Hunan, Hubei, Sichuan, Guizhou and Hainan. For China Southern power grid, during the period of 2012-2017, the power transmission line trips caused by fire was more than 10% of the total trips, and the most serious year reached 18.60% [23]. In Hunan power grid of State Grid of China, the proportion of fire caused faults was 16.04% of the total faults of 220 kV and above power transmission lines from 2005 to 2014 [24], and the proportion was 6.02% from 2007 to 2018 in Southern Coastal Provincial power grid of China [25]. In the United States, there were about 100,000 wildfires each year [26]. There have been many huge fires in the United States in recent years, such as Washington big fire in 2014, California huge fire in 2016 and so on. And 5.2% of large blackouts were caused by fires in United States between 2003 and 2012 [5]. In Canada, a wildfire impacted the northern  [27]. The fire burn an estimated 590,000 ha, and burned down a lot of power facilities and caused widespread blackouts. Australia is a country with many fires, the states of New South Wales, Queensland and Victoria are the fire serious area in Australia. Between 2019 and 2020, a huge fire has burned for months in the several states including New South Wales, Victoria and South Australia [28]. The power transmission lines were seriously damaged in these areas, leading to a large-scale power outage.

Timeliness of fire rescue for transmission lines
Fire event near power transmission lines is an emergency. There is a certain period of time from the fire occurrence to transmission line trip. If no response measure is taken during this time period, the transmission line fire might be tripped. Therefore, the time from the fire occurrence to the transmission line trip should be known, and this is an important information for the rescue of transmission line fires.
It is assumed that the distance between fire and transmission line is d , and the average burning velocity of vegetation in the direction of the vertical transmission line is The time from the fire beginning to fire spreading below the transmission lines is According to the field statistics, the fires that affects the transmission lines are generally closed to the line (within 500 m), and the times of more than 78.6% of the power line trip events caused by fire in Hunan province power company are no more than 1 h from fire beginning to tripping. However, the transmission lines fire field is usually very far from the rescue devices and rescue workers, thus it often costs a long time for workers and fire-fighting equipments to reach the fire field, and it finally fails to rescue the fire. Therefore, the timeliness requirement of fires rescue of transmission line is very high. It is very significant to obtain the future fires risk and deploy fire precautions in advance, which needs an integrated control strategy to suppress fires.

Control strategy for fires near transmission lines
In order to prevent the transmission lines trips caused by fires, a control strategy for fires near transmission lines is proposed, as shown in Figure 2. The strategy includes three key steps, which are fire forecasting, fire monitoring and fire extinguishing, and their time sequence of transmission line fires rescue is shown in Figure 3. First, the fire risk near the transmission lines in the next few days can be obtained by the method of fire forecasting, and then fire extinguishing equipments can be arranged in the high-risk fire area in advance. Second, when fire is occurring, they can be monitored and located in real time by fire monitoring, so that the fire-fighting equipments can reach the fire field in time. Last, the fire extinguishing equipments can ensure the power workers put out the fires safely and quickly under high voltage lines to prevent lines trip with no electric shock risk. The above three steps are an organic whole and have internal relations with each other. The fires monitoring The schematic diagram of geographical area unit for fires counts data provides historical data for fire forecasting, and fire forecasting can provide high-risk area to arrange fire extinguishing equipments in advance to shorten the rescue distance, and fire monitoring provides the real-time fire location information for fire extinguishing to quickly rescue. Because of the preparation, the time period from fire occurring to fire extinguishing is not more than 1 h, and the rescue is effective.

FIRE RISK FORECASTING METHOD FOR TRANSMISSION LINES
Forest fires are mainly caused by dry weather conditions. However, besides weather conditions, fires near transmission lines are mainly caused by the customs of human using fires, such as burning crops, and burning paper money to ancestor worship in China etc. In the past, fire-fighting equipments could not be arranged in the high-fire area in advance due to no effective and practical fire forecasting method for transmission lines. Even though the fires near transmission lines were found in time, the fires cannot been extinguished in time. This paper proposes a fire forecasting method for transmission lines, which can be used to know the risk of fires near transmission lines in future days, and relevant measures can be taken in advance to prevent transmission line from tripping due to fires.
15 years historical satellite monitoring fire hots of China were collected by the authors. Through the big data association analysis, two key factors are closely related to the fires occurrence near transmission lines are found to be historical fires density and amount of precipitation. The historical fire density denotes the average number of fires per year in a given geographical area. In this paper, the geographical area unit is regarded as a nearly quadrangle, the geographical coordinates of four vertex are respectively (x, y), (x ′ , y), (x, y ′ ) and (x ′ , y ′ ), where x and x ′ is the longitude values of fires, and y and y ′ is the latitude value of fires, as shown in Figure 4.
The difference in edge length caused by the change in latitude is ignored in this paper, then, the quadrangle is a rectangle. It is denoted that the two side lengths of the rectangle are l x and l y , respectively. The acreage of the rectangle unit is S = l x ⋅ l y (2) In this paper, the longitude difference and latitude difference for every unit can be chosen as equal, so where is the longitude and latitude difference value, in degrees ( • ), in this paper = 0.5 • . Then The acreage of the rectangle can be calculated by above equation. So the history fires density is where d f is the historical fires density (unit/10,000 km 2 ), N refers to the average historical fires number within a certain period of nearly 5 years for an geographical area unit. The period refers to the days before and after the forecasting date (according to the date of the Gregorian calendar). The calculation formula is where i denotes the recent year, j denotes the number of days, n i j denotes the fire number in the i-th year j-th day, d 1 and d 2 are the days before and after the forecasting day, usually d 1 = d 2 = 3. p is defined to be the amount of precipitation for geographical area unit, which can be obtained from the common weather forecasting system.
The fire risk grade for transmission lines can be classified to four levels, which are grade 1 (no colour), grade 2 (yellow), grade 3 (orange) and grade 4 (red) from low to high. Those are explained as below.
• Grade 1 (no colour): There will be no fire near power lines or the possibility of fire is very small, and there is no fire risk for transmission lines. The power workers take no care to it.  workers should strengthen the monitoring of fires near power lines, especially in the key areas arranging special personnel to inspect them and prepare fire-fighting equipments in advance. • Grade 4 (red): Large-scale area of fires will occur, and the fire risk level of transmission lines is very high. The power workers should comprehensively monitor the fires near power lines, arrange fire extinguishing devices in advance and make preparations for fire-fighting at any time.
The fire risk grades can be obtained by the rules based on the values of history fires density (d f ) and amount of precipitation (p), as shown in Table 2.
Based on the above methods, a transmission lines fires risk warning system was developed, which can automatically produce the transmission line fire risk forecasting map for next 3 days, as shown in Figure 5, which can provide message guiding the power workers to take fire prevention measures in advance.

REAL-TIME LARGE-SCALE MONITORING TECHNOLOGY FOR FIRES NEAR TRANSMISSION LINES
Transmission lines are widely spread throughout the field, and the conventional ground point-to-point online monitoring method is difficult to fully cover all transmission lines. Satellite-based monitoring technique can realize the fire detecting requirement of large-scale area fires. A large-scale fire satellite monitoring system was developed by the research group of the authors. Synchronous and polar orbit satellite infrared remote sensing signal detection is used to identify fires and then calculate the distance between the fire point and the nearest transmission line and corresponding tower. It is very useful for guiding power workers to find the fires occurring near the power lines in real time.
According to the Planck's formula for absolute blackbodies, the formula for calculating brightness temperature can be derived where c is the speed of light in vacuum, k is Boltzmann's constant and k = 1.38 × 10 −23 J K −1 ; h is the Planck's constant, and h = 6.63 × 10 −34 J⋅S, denotes radiation emission. The fire burning is with abnormally high brightness temperature and high radiation capacity, and the bright temperature difference between the burning pixel and the surrounding pixel is obvious. The judgment model of the ignition point is where T c and T r represent the brightness values of main channel and reference channel respectively. ΔT is the bright temperature difference between the two values. According to the fire statistics have been detected, the bright temperature difference is greater than or equal to 8, so ΔT is taken as 8. T h is the fire judgment threshold, which can been chosen from 290 to 310, depending on the environment. The schematic diagram of fire monitoring and warning system is shown in Figure 6. The hardware of transmission line FIGURE 6 Schematic diagram of fire monitoring and warning system fires monitoring system consists of satellites receiving device, front-end receiving server, back-end application server and fire warning server. Satellites get ground temperature data through infrared remote sensing sensors and send it to the ground satellite receiver. The satellite receives device transmits data to the front-end receiving server for image pre-processing, and then transmits images to the back-end application server for fire judgement. Fire warming server calculates the distance between fire and the nearest transmission line tower, and outputs the fire warming message. Finally, the monitoring and warning information will be issued to the transmission line operation and maintenance units for the emergency management of fires.
Defining d is the distance between the fire and the nearest transmission line tower, the warning fires near transmission lines can be classified three warning levels as Equation (9). where d is the distance between the fire and the nearest transmission line. The polar-orbit satellite data receiving device and the synchronous satellite infrared data receiving device are developed, as shown in Figure 7. The average interval of the polar-orbit satellite data is about 2 h, and the interval of the synchronous satellite data is 10 min.

High-voltage insulation test of fire extinguishing
Fire extinguishing is an effective method to prevent transmission line fires tripping after the fires happening near transmission lines [29]. However, the ordinary water is conductive and the operating power lines carry high voltage. If the fire extinguishers use water to extinguish a fire under high voltage transmission lines, they may be electrocuted and the transmission lines might be tripped, as shown as Figure 8, this is because the high voltage may discharge to the people by fire water, so it is very dangerous [30]. A method of water mist fire extinguishing is proposed to improve the insulation performance of A high-voltage insulation test device for water mist was designed, which mainly included a booster power supply and a water mist generator, as shown in Figure 9, and the photos of the test device are shown in Figure 10. The voltage of the booster power supply can be adjusted from zero to 500 kV. The water mist droplet diameter can be adjusted from 20 to 5000 µm, and the water mist sprayer is conductive and connected to the ground. The water mist breakdown test and leakage current test were carried out.
In the high-voltage insulation test for water mist, the environment temperature was 8.4 • C, and the relative humidity was Diagram of high-voltage insulation test device for water mist 54.2%. The atmospheric pressure was 103.3 kPa. First, the gap between the sprinkler head and the grading ring was set to be 50 cm, then the water mist spray toward the gap. The mist equivalent diameter is measured to be 380 µm, and the water is common municipal water with conductivity 156 µs cm −1 . Then, the voltage of the test power supply was stepped up until the gap breakdown, and the breakdown voltage was recorded. Following, the air breakdown voltage under the same gap condition was tested, meanwhile the measured value of air breakdown voltage was recorded. The photos of water mist breakdown are shown in Figure 11, and that of air breakdown are shown in Figure 12. The test results are shown in Table 3.  It can be seen from Table 3 that the breakdown voltage of water mist is 5.8% to 10.2% higher than that of air, which means the insulating ability of water mist is higher than that of air, while the insulation of air is very good. This indicates the insulation of the tested water mist is very high.
In order to further verify the insulating property of water mist, a water mist leakage current test was carried out under the same experimental environment. The water mist was sprayed on the high pressure equalising ring, and the leakage current of the water mist insulation was measured by series ammeter in the loop.
The distance between the spray nozzle and the pressure equalising ring was set at 1.5 m. And the flow of spraying water mist is 15 L min −1 , and the droplet diameter of water mist was measured to be 380 µm. The power supply voltage was increased gradually, the circuit leakage current values under the different voltage condition were measured, as shown in Table 4. With the increase of voltage, the leakage currents of water mist increased, but the value is still small (<1 mA), which meets the safe requirement of fire extinguishing under high-voltage transmission lines.

Fire extinguishing performance test of nozzles
To test of the fire extinguishing performance of different nozzles, a test was carried out according to the national standard of China GB 50898-2013 [31]. A square pan filled with diesel oil was used as the simulated fire, and the side length of the pan is 1 m, the thickness of the diesel oil is 10 mm. The fire extinguishing system was composed of water tank, water pump, water pipeline and nozzles, and the photos of fire extinguishing test device are shown in Figure 13. Four nozzles are installed right above of the oil pan in square, and the adjacent space between them is 2.5 m. The vertical distance from the oil pan to the nozzles is 3 m. The water pressure of the fire extinguishing system is 2.0 MPa, and the total flow rate of system is 60 L min −1 .
The experiment process was to ignite the fuel first. Subsequently, the water mist apparatus was started when the pre-burning duration reached 30 s until filling flame over the pan, and then the extinguishing time was recorded, as shown in Figure 14. The extinguishing time for four test conditions was listed in Table 5. It was observed that the extinguishing times were different with varying the droplet diameters, and the time was the shortest when the droplet diameter was selected as 400 µm. The time was grown with increasing of droplet diameter. The reason was that the surface area of the droplet was reduced owing to the increasing of the droplet size, leading to weakening of the evaporation to absorb the heat. As a result, the optimal cooling effect cannot be obtained, and the extinguishing time was longer than the droplet diameter of 400 µm. In addition, it was not that the smaller the droplet size, the shorter the The main reason was that those smaller droplets were easier to be evaporated so that they were difficult to move into the flame root in a larger fire. In such a case, the fire was not controlled by the suppression mechanism of the cooling effect. The above results showed that the droplet size existed in an optimal range using the water mist to extinguish fire. Hence, a reasonable droplet size range of

FIGURE 15
Movable water mist fire extinguishing device and its application. (a) Movable water mist fire extinguishing device, (b) field application 350-400 µm was recommend considering the fire extinguishing effect and insulation performance in this paper.

Water mist fire extinguishing equipment
The feasibility of the water mist fire extinguishing method under high voltage liens was verified. The movable water mist fire extinguishing equipment (see Figure 15) was developed and applied. The rated working pressure is 15 MPa, and the horizontal range of water mist gun is 25 m.

APPLICATION RESULTS
The strategy of fire forecasting, fire monitoring and fire extinguishing under high-voltage has been widely used in 27 provinces in China since the year of 2015, such as Anhui, Hubei, Hunan provinces etc. In Hunan power grid of State Grid in China, before the strategy was used, the average fire tripping number of transmission lines with the voltages of 220 kV and above was 19.8 times per year during the period of 2010 to 2014. After the presented strategy was used, the average fire tripping number was reduced to 7 times per year, reduced by 64.6% compared to previous 5 years. The corresponding data is listed in Table 6. In April 2015, it was predicted that a large-scale fires would occur in Hunan province, 42 fire extinguishing equipments were arranged to the high-risk fire regions in advance to shorten the rescue distance and save the rescue time. The field photos are shown in Figure 16. From April 3 to 5, 165 first-level alarm fire points within 1 km near the transmission lines were detected. By using of movable water mist fire extinguishing devices, 116 fires near the transmission lines such as ±800 kV Fu-feng Line and the 500 kV Jin-min Line were rapidly extinguished, some cases are listed in Table 7.
There was no transmission line tripping caused by fires during the Qing-ming high fires period for six consecutive years from 2015 to 2020 in Hunan power grid, while there were a lot of trips during Qing-ming periods of years before the presented method used.

CONCLUSIONS
In order to control the fires causing the power transmission lines trip, a control strategy for large-scale fires near power transmission lines was presented in this paper. The proposed strategy was composed of the methods of fire forecasting, fire monitoring and fire extinguishing under high-voltage lines. The fire risk grades for transmission lines can be calculated based on parameters of historical fires density and amount of precipitation. A transmission lines fires risk warning system was developed to provide message guiding the power workers to take fire prevention measures in advance. A large-scale fire satellite monitoring system based on synchronous and polar orbit satellite infrared remote sensing signal was developed, which can identify fires and then calculate the distance between the fire point and the nearest transmission line. A method of water mist fire extinguishing is proposed to improve the insulation performance of extinguishing medium. The breakdown voltage of water mist is 5.8% to 10.2% higher than that of air. A