In this section, we first review existing transmission line inspection methods over the last 20 years, focusing on comparing their advantages and disadvantages. Among the existing inspection methods of transmission lines, the UAV inspection technology is mainly discussed. The UAV inspection technology has relatively low cost, flexible use and relatively high efficiency. Because of these advantages, UAV inspection be-comes a very ideal inspection method.
2.2 Airspace patrol
Airspace checks are usually performed by climbing systems or air systems. The former uses mobile robots to cross obstacles found along power lines and inspect com-ponents passing along them. The climbing robot can get high quality images because of its proximity to the wire. However, the disadvantages of the climbing system include damage to the line, low efficiency, incomplete detection, obstacles and so on, which limit its application. Another aerial system checks power lines based on helicopters, drones and other aircraft. The aircraft flies along electric lines, manned or automatic. During this process, multiple onboard sensors are used for visual observation and data collection.
Leena Matikaine et al.[17] reviewed the possibilities offered by modern remote sensing sensors in power line corridor measurements and discussed the potential and limitations of different methods. Includes monitoring of power line components and surrounding vegetation. Remote sensing data sources discussed in the evaluation include synthetic Aperture radar (SAR) images, optical satellite and aerial images, thermal images, airborne laser scanner (ALS) data, land-based mobile mapping data, and UAV data. The review shows that most of the previous studies focus on the mapping and analysis of network components. Especially, automatic extraction of power line conductors has attracted wide attention and gratifying results have been reported.
In order to solve the problems of manual inspection, airspace inspection is pro-posed. The methods of airspace inspection are as follows: helicopter-assisted inspection, line patrol robot inspection and UAV inspection, which are briefly introduced below.
In helicopter-assisted inspections, teams of two or three people, usually including a pilot, an inspector and a videographer, are flown by helicopter to conduct online inspections. Photographer photograph power lines, poles, insulators, transformers and objects around pylons. Inspectors look for obvious failures, such as vegetation too close to transmission lines, snow or hail piling up on power lines, and downed poles. After the flight, a team of inspectors usually goes through the collected data manually, focusing on smaller failures, such as broken insulators or missing splints. Although this method improves the detection speed, it also has disadvantages such as high cost, low accuracy, reliance on the visual skills of inspectors, and always has safety problems.
Wang Liming et al.[18] studied the safe distance for helicopter inspection of 500KV AC transmission lines. The determination of electromagnetic safety distance for helicopter power line inspection is the guarantee for operators. The results show that when there is no helicopter, the electric field intensity around the transmission line is inversely proportional to the distance of the transmission line. When a helicopter is present, its maximum electric field strength is inversely proportional to its distance from the transmission. The maximum strength on a helicopter is usually found in the helicopter wing, which is 6 to 10 times greater than without a helicopter. Moreover, if the helicopter is close to the ground wire, the field strength of the helicopter will be lower due to the shielding effect of the ground wire, so as not to affect the helicopter inspection. Based on the relationship between the maximum strength and the distance from the helicopter to the line, the safe distance under different limits of the electric field strength can be determined, for example, about 2.2m and 500kV/m.
In the inspection of the line patrol robot, the designed line patrol robot moves along the transmission line. In this process, the navigation and detection of the trans-mission line are completed, and the data results are analyzed in real time to find out the fault of the line, improve the safety and reliability of the transmission line, and then ensure the safety of the transmission line. Compared with the artificial ground detection method, this method has a higher level of automation, and can detect various power facilities in a close distance. However, there are still some technical difficulties when the robot crosses the barrier on the line, and the climbing and moving of the line patrol robot on the line will cause certain irreversible wear and tear on the trans-mission line. Because of these drawbacks, this solution can only be used on certain lines.[19]
Compared with helicopter assisted inspection of line patrol robot inspection, UAV inspection is an ideal and promising method. With the development of UAV technology, UAV inspection technology has relatively low cost, flexible use, relatively high efficiency and stronger safety. It is these points that make UAV have a high starting point, and there are newer and better options for transmission line detection. Therefore, UAV inspection technology for transmission lines has been gaining importance and devel-oping rapidly. UAV used for power line inspection can be divided into four categories: fixed-wing UAVs, rotor wing UAVs, unmanned helicopters, and vertical take-off and landing fixed-wing UAVs. The representative of fixed wing UAV is shown in Fig. 3a, whose flight speed can reach more than 100km/h. Compared with the other three types of UAVs, fixed-wing UAVs consume the least power during flight and have the longest cruising distance. Therefore, fixed-wing UAVs are generally used to perform long-range line detection. However, its disadvantage is that it is impossible to analyze each part of the line in detail, so it can only take the form of individual shooting for rough pattern detection. In addition, because fixed-wing aircraft need a certain initial velocity to take off, it is greatly affected by terrain. As shown in Fig. 3b, the rotor wing UAV has the following advantages: easy operation, simple structure and low cost.[20–23] It usually uses wireless signals for real-time control, and can be suspended in the air. But the disadvantage is that such drones generally fly slowly, lack endurance and can only be used for inspection of specific parts. Unmanned helicopter As shown in Fig. 3c, unmanned helicopter has better air maneuverability, can take off and land in a short distance, hover at key detection locations for shooting. However, the disadvantage is that the unmanned helicopter has poor endurance capability, and the speed is slightly slower than that of the fixed-wing UAV. It is only suitable for short-distance inspection and the cost is more expensive. Fixed wing UAV of vertical take-off and landing type is shown in Fig. 3d. It takes into account the advantages of convenient takeoff and landing of rotorcraft and long flight time of fixed wing UAV, so it is more and more favored by dynamic detection. vertical take-off and landing fixed-wing UAVs have two flight states that can be switched arbitrarily, namely rotor and fixed wing state. The fixed-wing status realizes patrol, and the rotor status realizes takeoff and landing.[24–26]
In UAV inspection, the UAV is equipped with multiple sensors and cameras, navigates along the power line, conducts online inspection, detects obvious faults, and collects data. UAV detection provides a great possibility to solve the problems of other detection methods such as high cost, slow speed and safety. Compared with manual inspection and helicopter-assisted inspection, the operation cost of UAV inspection is relatively low. In addition, drones can get relatively close to power lines and take detailed images of conductors, towers and power components, which can significantly improve detection accuracy without damaging transmission lines. The inspection method based on UAV has strong scalability.[27, 28]