1 Introduction

As a significant strategic support for economic development, high-speed railways bring immense changes in transportation mode of humans all over the world. Compared with the other currently existing transportation tools, the high-speed railway has the outstanding advantages such as high-speed, heavy-load, and being punctual, comfortable, environment-friendly and safe, and therefore has become one of the actively developed infrastructures in the world. In 1964, the world’s first high-speed railway was built in Japan with the design speed of 210 km/h. Since then, if the design velocity of a railway is higher than 200 km/h, this line can be defined as a high-speed railway. After that, many countries such as France, Germany and China started to build high-speed railways. The construction and development of high-speed railways in different countries are shown in Table 1. The rapid improvement of high-speed railway running speed leads the development of high-speed trains. For example, the trains with the maximum design speed within the range between 200 and 250 km/h, such as CRH1A and CRH2A, normally can provide a traction power about 5 MW. The trains with the maximum design speed within the range between 250 and 350 km/h, such as CRH2C and CRH3C, normally can provide a traction power more than 8.5 MW. The trains with the maximum design speed above 350 km/h, such as CRH380 and CR400, normally can provide a traction power about 10 MW. So far, the highest commercial travel speed of China’s high-speed railway has reached 350 km/h, ranking the first in the world. High-speed trains run at high speeds because they are powered by a constant supply of electrical energy, and the entrance to provide electrical energy for the trains is the pantograph–catenary system.

Table 1 High-speed railways of different countries
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As one of the three basic relationships of high-speed railway (i.e., pantograph–catenary relationship, wheel–rail relationship and fluid–solid relationship), pantograph–catenary system is the key to maintain the constant and reliable power supply for high-speed trains, as shown in Fig. 1a and b, which contains two main components—the catenary and the pantograph [1, 2]. The catenary is a special power supply line built along the track to provide electrical energy for high-speed trains, as it is mainly composed of contact wires. The pantograph is the electrical equipment for high-speed trains to obtain electrical energy from catenary, which is installed on the roof of the train. The interface that directly contacts with the contact wire is pantograph slide. As the key current collection component of the train, the pantograph slide is installed on the top of the pantograph. The pantograph–catenary system provides electric energy for trains through sliding electric contact, which is the only path of energy supply for high-speed trains.

Fig. 1
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a Three basic relationships of high-speed railway, b the pantograph–catenary system, and c the current collection process of electrical contact system

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The ideal electrical contact condition between pantograph and catenary is the guarantee of safe and stable current collection of trains. The current collection quality of the pantograph–catenary electrical contact system directly determines whether the train is able to operate safely and reliably. Figure 1c shows the electrical contact current collection process of the pantograph–catenary system. With the increase of train velocity, the mechanical–electrical coupling vibration between the pantograph and catenary is significantly intensified, resulting in the contact loss phenomena occurring between the contact wire and the pantograph slide, which might trigger arc. The arc ablation, friction and wear, vibration and mechanical impact tend to bring serious threats to the pantograph–catenary electrical contact interface, leading to the failure of the pantograph–catenary electrical contact system, which will seriously affect the safe running of trains and even cause the trains to stop running. The bottleneck restricting the safe and stable current collection of pantograph–catenary electrical contact systems consist mainly in three aspects: Firstly, there is a lack of pantograph slide materials with high performance in wear resistance, ablation resistance and impact resistance. Secondly, with the increase of speed, the matching of pantograph slide and contact wire deteriorates and the following performance of the pantograph–catenary system gets worse. Thirdly, there is a lack of effective equipment for online detection and early warning of pantograph–catenary system.

With the rapid development of the economy, the ultra-high-speed train is the development goal of high-speed railway. In pursuit of higher operational velocity and stronger carrying capacity, the matching condition of the pantograph–catenary electrical contact system must be improved to provide the outstanding transmission capability. Moreover, the high-speed railway tends to face more complex environments, along with the expansion of commercial railway. Some environmental uncertainty tends to bring more technique difficulties to the construction of railways, in terms of the geographical and climate elements. In addition, with the change of global climate, extremely harsh environments of different types and degrees may appear in different countries over the world. Take the climate and environmental factors in some typical countries with advanced high-speed train technology for example: earthquakes and typhoons sometimes occur in Japan [3,4,5], heavy rainfall occasionally occurs in Europe [6,7,8,9,10,11,12,13,14,15], and strong snowstorms usually occur in Russia [16, 17], as shown in Fig. 2. The operation and maintenance of existing high-speed railways and the development of future high-speed railways must face these complex environmental elements. China has a vast territory and different environments in different regions, such as high altitude and strong sandstorms in the west, rain and lightning in the south, typhoons in the southeast, and severe cold in the northeast [18,19,45, 46]. In fact, China is taking the lead in research and development of high-speed railways.

Fig. 15
figure 15

Development process of catenary and pantograph in China

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2.3.5 Other typical pantograph–catenary systems

In addition to the above-mentioned, the pantograph–catenary system of Sweden's X2000 high-speed pendulum train is also a typical one [47], as shown in Fig. 16. The catenary of the pendulum train pantograph–catenary system adopts simple chain suspension, and the pantograph adopts a WBL88-X2 type pantograph. The WBL88-X2 pantograph can be controlled laterally on a curved track to realize reverse inclination relative to the vehicle body, which can reduce pantograph–catenary system vibration and improve current collection quality. The adoption of high-speed pendulum trains is a significant feature of Swedish high-speed railways, which can increase the passing speed of trains by 30% to 40% in the curved section. At the same time, the tilt of the car body offsets 70% of the centrifugal force felt by the passengers, making the passenger experience more comfortable.

Fig. 16
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Sweden’s X2000 high-speed pendulum train with the WBL88-X2 pantograph

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3 The component evolution of pantograph–catenary system

3.1 Contact wire

Contact wire is an important part of catenary, which transmits electrical energy to the electric locomotive through pantograph slide. Contact wire is generally built in zig-zag to reduce the wear on pantograph slide. When the train is running at a high speed, the contact wire has to face the extreme working environment such as vibration shock, temperature difference, environmental corrosion, mechanical friction, and arc ablation [48]. Its material properties directly affect the current collection quality and operation safety of the train. There are many types of electrified railway contact wires, which are mainly classified into pure copper contact wires and copper alloy contact wires according to the material [49].

3.1.1 Pure copper contact wire

Pure copper has excellent electrical conductivity, thermal conductivity and corrosion resistance, but it has low strength, low softening temperature and poor heat resistance. The pure copper contact wire is easily broken, causing the pantograph–catenary system to fail. Therefore, pure copper contact wire cannot meet the requirements of high-speed and heavy-haul railways, it is generally only suitable for ordinary railway with speeds lower than 200 km/h [50]. At present, it is gradually replaced by copper alloy contact wire.

3.1.2 Copper alloy contact wire

The metal elements doped with copper alloy mainly include Ag, Sn, Cr, Mg, and Zr. Due to the addition of metal elements, the mechanical properties, softening resistance and wear resistance of copper alloy contact wire are greatly improved compared with pure copper.

Copper–silver alloy contact wire usually refers to copper as the main material, containing about 0.1% silver element. Ag and Cu are homologous elements with similar electronic structures, so copper–silver alloys can achieve an electrical conductivity of 96% IACS or more. Copper–silver alloy contact wire is used in Re250 catenary in Germany, and the train speed can reach 250 km/h. However, since Ag and Cu are cognate elements, the solid solution strengthening effect is not prominent, and the improvement of mechanical properties of copper–silver alloy at room temperature is limited, which is about 370 MPa. At present, the copper–silver alloy contact wire produced in China has reached a high level and can be used for high-speed railways below 300 km/h.

Copper–tin alloy contact wire is made by adding 0.3% tin element to copper. Compared with the copper–silver alloy wire, the copper–tin alloy contact wire has advantages of simple manufacturing process, high yield and low price. In 1997, Japan achieved the goal of 300 km/h by using the copper–tin alloy contact wire [51]. The copper–tin-120 contact wire developed in France and tested in a catenary with a speed of 300–350 km/h has a tensile strength of 537.5 MPa and an electrical conductivity of 70% IACS. The copper–tin alloy contact wire has high strength, high softening temperature and appropriate electrical conductivity, and its comprehensive performance is better than that of the copper–silver alloy contact wire [52].

Copper–magnesium alloy contact wire is a milestone wire after copper–tin alloy contact wire. It has the characteristics of high tensile strength, high wear resistance, high temperature softening resistance, moderate electrical conductivity, etc. It is mainly used in high-speed railway catenary with a speed of more than 300 km/h [53]. Copper–magnesium alloy contact wire was first developed by German Railway Company (DBAG), with a tensile strength of 490 MPa and an electrical conductivity of 62% IACS. It was applied in Re330 catenary, and the speed could reach 330 km/h. China has begun to develop copper–magnesium alloy contact wire since 1990s. In 2002, copper–magnesium alloy contact wires were successfully applied to Qinhuangdao–Shenyang high-speed railway. Since then, copper–magnesium alloy contact wires have been mostly used in lines with a design speed of 300 km/h and above in China.

Copper–chromium–zirconium alloy contact wire is a copper alloy product strengthened by heat treatment. It has high tensile strength and significantly improved electrical conductivity compared with contact wires made of copper–tin alloy and copper–magnesium alloy. The Chinese TB/T2809-2017 standard requires that the tensile strength of copper–chromium–zirconium contact wire shall not be less than 560 MPa and the electrical conductivity shall not be less than 75% IACS. In 2010, the 220 km pilot test section of Bei**g–Shanghai high-speed railway set a world record speed of 486.1 km/h with copper–chromium–zirconium contact wire for high-speed railroad operation test. Copper–chromium–zirconium alloy contact wire material has high strength, good electrical conductivity and high temperature softening resistance. It can be applied to high-speed railway above 350 km/h and has broad application prospects. However, its production process is totally different from that of ordinary copper–silver, copper–tin and copper–magnesium alloy contact wires. It requires multi-pass and long-time heat treatment, and due to its complex process, the production cost is higher than that of other contact wires [54].

Table 3 shows the key performance parameters of different types of contact wires. Facing the development direction of high-speed railways to higher speed, contact wire materials are required to have better comprehensive properties. For the existing types of contact wires, it is necessary to optimize the alloy composition design, optimize the process flow of copper alloy wire blank, improve the reliability of products, and reduce the cost. Meanwhile, it is necessary to strengthen the research and development of advanced production technology and processing equipment.

Table 3 The key performance parameters of different types of contact wires [49, 50, 55, 56]
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3.2 Pantograph slide

As the key current collection part of the high-speed train, the pantograph slide is installed on the top of the train. It directly contacts with the contact wire and gets electric energy to provide traction power for the stable operation of the locomotive [57, 58]. The current collection quality of the pantograph–catenary system determines whether the train can obtain sufficient traction power to run safely and reliably [59]. The performance of the slide plate material used in the pantograph directly determines the quality of the current collection. In addition, the pantograph slide is exposed to the ambient environment, so it would face mechanical impacts, arc erosion, and friction continuously during the running of trains [60,61,62]. Therefore, the design of the pantograph slide has very strict requirements which include sufficient impact resistance, high electrical conductivity, good abrasion and ablation resistance, excellent self-lubrication, and low wear property against the contact wire. The general requirements for the pantograph slide material are shown in Fig. 17.

Fig. 17
figure 17

Requirements for the pantograph slide material

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With the development of high-speed and heavy-haul electrified railway, a higher requirement for the performance of the pantograph slide materials is needed. Research on the pantograph slide, especially on its materials, has been studied extensively all over the world. Since the 1920s, countries like Japan, France and Germany have done a lot of studies on the pantograph, laying a solid foundation for the development of pantograph slide materials.

The development of pantograph slide materials has experienced different stages, including metal slide plate, pure carbon slide plate, powder metallurgy (P/M) slide plate, metal impregnated carbon slide plate, and composite slide plate [63,64,65]. The development of the pantograph slide corresponding to trains with different speed levels is shown in Fig. 18. Since the 1960s, China has made remarkable achievements in pantograph slide materials. In 1961, China’s the first electrified railway, namely Baoji–Fengzhou railway was put into operation with a maximum running speed of 70 km/h, where the mild steel pantograph slide was mainly used. This kind of pantograph slide has the advantages of high mechanical strength, low resistivity, low cost, and long service life. However, the lubricity of this pantograph is poor, which causes serious wear of the contact wire. To solve this problem, the pure carbon slide plate was developed by the Institute of Railway Sciences of China in 1967, and the mechanical wear of the contact wire was greatly mitigated.

Fig. 18
figure 18

Development of pantograph slide with the increase of train speed in China

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Subsequently, as the train speed increased to 100 km/h, the pure carbon slide plate could no longer be used because of its low mechanical strength, poor impact toughness, and low service life at higher speeds. In order to solve this problem, a P/M slide plate with good impact toughness was developed, which was based on the metal (iron and copper) powder. This approach greatly reduced the wear of contact wire, and the slide plate had a better performance in wear resistance as well. It was therefore used as an ideal solution for trains on 100 km/h trunk lines.

Later on, the further increase of the train speed resulted in an increase of the contact loss rate of the pantograph–catenary and more frequent arcing phenomena during train service. Due to the loose structure and poor arc resistance, the existing P/M slide plate could no longer meet the safety requirements at 160 km/h. In the 1990s, the emerge of the metal impregnated carbon slide plate with dense structure in high-speed electric locomotive offered a promising approach to compensate for the material defects at high-speed. The metal impregnated carbon slide plate had many excellent performances such as high mechanical strength, low resistivity, low wear, easy to form a lubricating film, and strong arc extinguishing ability. It was recognized as a novel slide plate with the best adaptability to the contact wire, and got widely applied in high-speed trains running with a speed above 200 km/h. However, the existing metal impregnated carbon slide plate may not meet the requirements of ultra-high-speed trains in the future, and it is necessary to develop novel composite pantograph slides with high comprehensive performance.

3.2.1 Pure carbon slide plate

The pure copper and soft steel are mainly used as pantograph slide materials for electrified trains running at a speed below 70 km/h. However, due to the serious wear on the contact wire, the pure metal slide plate could not meet the needs of electrified railway development. Therefore, the pure carbon slide plate with self-lubricating performance replaced the pure metal slide plate and became the best choice of the pantograph slide in the low-speed stage of electrified railway.

The pure carbon slide plate is mainly composed of carbon black, pitch coke, and petroleum coke. The production process of the pure carbon slide plate is shown in Fig. 19 (green line). First, the components are made into aggregate powder, and pitch is added as the binder for the mixing process. After hot pressing and roasting, a blank is formed, and this blank is impregnated with pitch and is roasted before the preparation is complete [66].

Fig. 19
figure 19

The methodology of pantograph slide design and preparation

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The pure carbon slide plate has good self-lubrication, which can form a layer of lubricating carbon film on the wire when it rubs against the copper contact wire. This design effectively reduces the wire wear and prolongs the service life of the wire. Moreover, the pure carbon slide plate has advantages of low electromagnetic noise and high temperature resistance when sliding, and there is no welding problem between the carbon slide plate and the contact wire. Therefore, in the early days, the pure carbon slide plate is adopted by many countries.

Japan has been devoted to the research and development of pantograph slide materials and it began to develop and use the pure carbon slide plate in the 1920s. The pantograph slide materials were widely used in private railways in the 1940s [67]. Netherlands began to use pure carbon slide plate in 1934. In 1991, Germany ICE was founded and started to use the pure carbon slide plate with a wear rate in the range of 1.7 to 2.2 mm 10,000 km [68]. In the 1960s, the pantograph slide of electric locomotive in China adopted mild steel and pure carbon slide plate, which were gradually replaced by the pure carbon slide plate due to its excellent performances in self-lubrication and wear reduction [87]. The main preparation method of metal impregnated slide plate is based on melt infiltration method which can be achieved in a container with high pressure, atmospheric pressure, and negative pressure at high temperature. It can be divided into pressure impregnation [88,89,90], pressureless melt infiltration [91, 92], and negative pressure infiltration [93, 94].

As mentioned above, the pressure impregnation method was initially used for the preparation of the metal impregnated carbon slide plate. The platform of pressure impregnation is shown in Fig. 20. The high density of the prepared composite and high utilization of copper alloy can be achieved by pressure impregnation. However, the obtained wettability of copper and carbon is very poor. Therefore, if the vacuum pressure impregnation is not used, it is difficult to impregnate the copper melt into the carbon matrix. Even if the impregnation is successful, the performance of the prepared pantograph slider would not be stable. In addition, the preparation of slide plate materials using the pressure impregnation method is very complex with low operational safety. These drawbacks prevented the industrialization application of this technique, and to develop pantograph slide materials with simple process and stable performance is required.

Fig. 20
figure 20

Pressure impregnation experimental platform: a overall design scheme of experimental equipment; b experimental equipment

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An improved technique has been developed by using the pressure impregnation method to make the alloy melt easily to impregnate or even impregnate the carbon matrix under ambient pressure [95]. Zuo et al. [96] proposed a strategy based on copper–boron alloying to improve the C/Cu interfacial bonding force, and the Cu–B/sintered-carbon composites were successfully prepared by pressure impregnation. The results showed that the C/Cu contact angle decreased from 123.6° to 21.3° in the case of 2.5% (by mass) B do** (see Fig. 21), and the compressive and flexural strengths of the modified composites were increased by 39% and 54%, respectively. Rambo et al. [97] used the pressureless melt infiltration method to form dense C/Cu composites by infiltrating Cu–Ti alloy into carbon preforms prepared by 3D printing under an argon gas protected pressureless condition. Negative pressure infiltration, on the other hand, is a way in which infiltration occurs under vacuum. To infiltrate the molten metal into the porous body, Yang et al. [98] prepared C/C–Cu composites with density of 2.7–4.9 g/cm3 by placing Cu–Ti alloy melt infiltrated and C/C blanks in a sealed container at 1300 °C under 10–2 Pa.

Fig. 21
figure 21

Variation of contact angle with B content. As the mass fraction of B increase to 2.5%, the C/Cu contact angle decreases from 123.6 to 21.3 (modified after [96])

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This method has low cost and is easy to be applied industrially. In addition, the prepared composites have better interfacial bonding and excellent overall performance, and the preparation of C/Cu composites by negative pressure infiltration technology has been widely used at present.

In recent years, in addition to the use of copper alloy impregnated carbon matrix, pure aluminum or aluminum alloy have been found to have potential advantages in preparing high electrical conductivity and light weight metal impregnated carbon slide plate. For example, aluminum/sintered carbon composites with good mechanical properties were prepared by using aluminum alloy to infiltrate the sintered carbon matrix under high pressure [99].

Table 6 shows a comparison of mechanical properties for the metal impregnated carbon slide plates developed in different countries. The hardness, electrical conductivity, flexural strength, and impact strength of the metal impregnated carbon slide plate produced in China have reached or even exceeded those of Germany, Britain and other advanced countries.

Table 6 Some key parameters of metal impregnated carbon slide plates in different countries [73, 81, 86,87,88]
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3.2.3.3 Advantages and limitations

The metal impregnated carbon slide plate has many advantages compared to the P/M slide plate, as it is more suitable for the pantograph of high-speed electric locomotive [139, 140]. The MCC slide plates are characterized by good electrical conductivity, high impact resistance, high wear resistance, high arc ablation resistance, and low wear on the contact wire, which will effectively solve the problems of fast wear, easy breakage, and large damage occurring to the contact wires of carbon matrix slide plates.

However, due to the nature of ceramic material retained by MCC slide plate, its stability needs to be improved. In addition, the material density is higher than that of carbon slide plate, which increases the load loss of pantograph. Due to the limitation of purity in the preparation process of Ti3SiC2 powder, it is still in the experimental stage and has not been produced on a large scale. Therefore, after further improvement of its stability, the MCC slide plate has a high possibility to be used as an ideal choice for the pantograph slide of ultra-high speed train in the future.

To sum up, with the increase of the train operation speed, the materials of pantograph slide have mainly experienced the process of metal, carbon, powder metallurgy, metal impregnated carbon and composite. The application field, advantage and disadvantage of different types of pantograph slide plates are shown in Table 7.

Table 7 The application field, advantage and disadvantage of different types of pantograph slide plates
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4 Detection and evaluation of pantograph–catenary system

In the electrified railway system, due to the long-term contact between pantograph slide and contact wire, the surface of electrical contact material is worn under the joint action of sliding wear, arc erosion and temperature stress, gradually leading to internal cracks. This causes irreversible damage to the slide plate.

If the defects cannot be detected in time, they could gradually expand and endanger the safety and stable running of the train [141,142,143]. The detection methods of the pantograph slide include manual detection, laser detection, ultrasonic detection, and image detection. Manual detection has been gradually replaced by other advanced detection methods. In the following content, we focus more on laser detection, ultrasonic detection, and image detection.

4.1 Laser detection

The detection system installed above the train emits a laser to the pantograph slide to realize the three-dimensional (3D) reconstruction figure of the slide plate. By this approach, the wear or crack of the slide plate can be obtained based on the processor online calculation. Its working principle and overall structure are shown in Fig. 25. The existing pantograph detection systems are developed based on the laser displacement sensor with anti-interference and accurate measurement capabilities [144, 145].

Fig. 25
figure 25

Schematic diagram of laser detection: a overall structure and equipment installation; b working principle

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Dwarakanath et al. [146] developed a laser sensor-based non-contact pantograph slide online detection system. When the pantograph passes below the detection system, the laser transmitter emits three laser lines to the upper surface of the slide plate. The laser lines on the slide plate are photographed by the high-speed camera, and the 3D shape of the slide plate is reconstructed through the three-dimensional point cloud. On this basis, the wear of the slide plate can be detected.

The laser detection method can realize the 3D reconstruction of the pantograph slide, which is a future research direction. However, the laser sensor is expensive and easily influenced by illumination, which limits the further application of the laser method. Meanwhile, the 3D reconstruction is related to the structure shape of the pantograph, and it is difficult to realize the complete 3D reconstruction if the thickness of pantograph slide is small.

4.2 Ultrasonic detection

Ultrasonic detection is a non-destructive detection method using ultrasonic wave to detect the internal defects of the workpiece. The basic principle of the ultrasonic detection method is that the ultrasonic sensor installed above the pantograph slide sends ultrasonic waves to the slide plate, and the sound wave will be reflected when it meets the interface between two media with discontinuous medium or large acoustic impedance difference. The damage of the slide plate can be obtained according to the time between ultrasonic emission and reception combined with the propagation speed of ultrasonic in the air [147,148,149]. Its working principle and overall structure are shown in Fig. 26.

Fig. 26
figure 26

Schematic diagram of ultrasonic detection: a overall structure and equipment installation; b working principle of surface wear; c working principle of inner damage

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For the detection of pantograph slide surface wear, Aydin [150] proposed a novel type of the pantograph slide fault detection method to automatically locate the pantograph fault area, which can effectively eliminate the influence of weather and other factors on image quality. Ostlund et al. [151] proposed an online automatic detection method of pantograph by using pattern recognition technology. The system analyzes the collected pantograph image and uses template matching to obtain the wear value of the pantograph slide.

Based on the Rayleigh integral theory, Wei et al. [152] established a Rayleigh integral model of ultrasonic propagation in pantograph slide. The optimum range of the probe parameters was determined by studying the distribution law of the ultrasonic field inside the pantograph slide under different probe frequencies and diameters. The differences of probe parameters between single and multiple cracks were compared. This method provides a theoretical basis for the evaluation of the internal damage degree of pantograph slide. The ultrasonic field distribution using different ultrasonic frequencies and different probe diameters are shown in Fig. 27.

Fig. 27
figure 27

Ultrasonic field distribution using a different ultrasonic frequencies and b different probe diameter

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The ultrasonic detection method [153] has the advantages of convenience, low cost, high detection efficiency, and high precision. But this method could be affected by environmental factors such as temperature, humidity, wind speed, and ambient noise.

4.3 Image detection

The image detection method mainly uses two sets of CCD (charge coupled device) cameras to capture pantograph images, and uses image processing technology to perform image filtering, image enhancement, edge detection, and information extraction on pantograph images. Its detection principle and overall structure are shown in Fig. 28. The use of image processing technology can improve the image quality, and has advantages in many aspects such as detection accuracy, detection efficiency and adaptation to train speed [154, 155].

Fig. 28
figure 28

Schematic diagram of image detection: a overall structure and equipment installation; b detection principle

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The key of image detection method is image processing. In [156], region-based-convolutional neural network (R-CNN) was used to extract the ROI (Regions of Interest) of the slide plate, as the wear size of the slide plate was detected based on the least square method and laser line. Aydin et al. [157] provide a method based on image processing and pattern recognition for online monitoring of the pantograph–catenary system. The images and data are provided to a D-Markov based state machine, so that faults such as overheating and pantograph arc can be identified. In [158], Gaussian homomorphic filtering and a variety of edge extraction algorithms were used to detect the wear of pantograph slide, and wavelet operator was used to detect the edge of pantograph slide more effectively. Although different image processing methods proposed in the above studies produced promising detection results, they all are in the experimental development stage.

In addition, some researchers [159,160,161] introduced a Pancam locomotive pantograph online monitoring system to detect the condition of the slide plate and the state of the ram’s horn, and extracted the image target using the designed image target extraction algorithm. However, the system required the train to return to the depot to collect pantograph images in static conditions and its image processing method was more complex. Hu et al. [162] located the slide plate based on the line projection transformation principle, and used the adaptive Canny edge detection algorithm and the sub-pixel edge detection algorithm based on cubic polynomial fitting to realize the wear detection of pantograph slide.

In addition to the surface wear detection of pantograph slide, the image detection technology can also realize crack detection. The curve coefficient directional map** algorithm and the second-generation curve detection algorithm based on moving parallel windows can effectively distinguish the line singularity features of slide crack images from the point singularity features of other interference images, and accurately detect and locate the crack damage of slide plate with an accuracy rate of 94.1% [163,164,165,166].

The pantograph image detection technology is now develo** rapidly, but it still cannot realize comprehensive online detection. More efforts should be made on new algorithms. Apart from that, the study of image acquisition equipment with higher resolution, efficiency, and precision is still needed.

4.4 Evaluation of pantograph slide

The pantograph–catenary electrical contact system is designed to cater to trains running conditions, i.e., high-speed sliding, high current transmission, and complex operational environment for a long time. Once the pantograph–catenary system fails, the current collection quality of the train will be affected, which may cause the train to stop running and has a significant impact on the safety of passengers and transportation. Therefore, in addition to the damage detection of pantograph slide, it is also important to evaluate the current collection condition of pantograph slide. The high voltage team of Southwest Jiaotong University developed an electrical contact current collection condition detection system based on the acoustic-photic-electric multi-information fusion [167], which is a comprehensive evaluation system covering ‘characteristic parameters-detection technology-condition evaluation’ (see Fig. 29). The system can effectively evaluate the service performance of the pantograph–catenary electrical contact current collection system of high-speed train.

Fig. 29
figure 29

Condition detection and evaluation system of the pantograph–catenary electrical contact system

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At present, there are still some deficiencies in the existing evaluation methods of pantograph slide. Accurate in-situ detection and online early warning cannot be achieved. What’s more, it is still unknown if the detection accuracy of these methods could hold for ultra-high-speed trains in the future.

5 Challenges and outlooks

5.1 Challenges

As the development of high-speed railways move toward higher speeds and more complex operational environments, the design of pantograph–catenary system will face more technique challenges. Since the traction power required increases exponentially with the running speed of the train, the pantograph–catenary electric contact system must meet the transmission capacity of extra high power. With the increase of the train operation speed, the difference between the train speed and the catenary fluctuation speed becomes larger; the deterioration of the pantograph–catenary matching condition will aggravate the mechanical, electrical, and material coupling damage. Meanwhile, high power makes the intensity of arc burning greater, leading to more serious ablation of the pantograph electrical contact system more serious. When the train is running, the pantograph–catenary electrical contact system is normally in a vibration state. Under high-speed conditions, the vibration and impact between pantograph and catenary will intensify, thus causing more serious cracking of the pantograph slide and even leading to the eccentric wear phenomenon of the pantograph slide. All these will seriously affect the service life of the pantograph–catenary electrical contact system. In addition, the current-carrying friction and wear of the pantograph–catenary electrical contact system are significantly affected by the current. When the train runs at a higher speed, the traction current increases greatly, and the wear of the pantograph slide and the contact wire becomes more serious, which will further shorten the service life of the pantograph–catenary contact system. Therefore, when the high-speed train runs at a higher speed in the future, higher requirements are placed on the pantograph–catenary electrical contact system.

The construction, operation and maintenance of high-speed railway pantograph–catenary system are facing severe challenges in the complex environments of various countries, such as the earthquakes in Japan, the floods in Germany and Britain, the heavy rainfalls in France, the heavy sandstorm, lightning, severe cold and high altitude environment in China. In earthquake and flood environment, it is necessary to conduct seismic design and flood-prevention design of catenary to prevent the collapse of catenary equipment caused by the earthquake and flood. In heavy rainfall and sandstorm environment, it is necessary to develop pantograph–catenary electrical contact materials with better comprehensive performance to reduce the severe wear of electrical contact materials caused by rainwater and sand grains. In thunder and lightning environments, the catenary needs to be designed with lightning protection to avoid serious damage to the pantograph–catenary system when the high-speed train is struck by lightning. In the severe cold environment, it is necessary to conduct a deicing design for the pantograph–catenary system to reduce the damage of the pantograph–catenary system caused by frequent arcing occurrence.

At present, the Sichuan–Tibet Railway under construction in China is extremely difficult and is known as the most challenging railway project. The Sichuan–Tibet Railway has remarkable characteristics such as high altitude, crossing tunnel groups (over 80%), and a large number of unmanned areas; the line slope is as high as 30‰ (the highest in the world), the bridge span is large, and the operational environment is extremely harsh. As the most challenging railway project in human history, once a fault occurs and the power supply of the line is interrupted, rescue will be extremely difficult. Therefore, the Sichuan–Tibet Railway intends to use a rigid catenary power supply. For a high-speed rigid pantograph–catenary electrical contact system, there is no relevant design and operation experience all over the word. As the core equipment of energy transmission of high-speed train, the pantograph–catenary electrical contact system faces great technique challenges: Firstly, the rigid pantograph–catenary system has poor following performance, so the coupling vibration is severe during high-speed running, and the frequency of arcing occurrence increases significantly. Secondly, the low pressure caused by the high altitude significantly reduces the breakdown voltage of the air gap, resulting in a decrease in the threshold value of the train pantograph–catenary arc and its intensity. Thirdly, under the low pressure and strong airflow conditions, the maintenance time and extinguishing distance of the pantograph–catenary arc increase nonlinearly, and the distance between the support insulators of the Sichuan–Tibet railway catenary is small (the interval is about 6 m). These two factors cause the arcing to stretch and drift under the action of airflow, which is easy to develop rapidly on the catenary busbar, and even cause insulator string flash. Finally, the traction current transmission density is high when the train is climbing a long ramp, which leads to more serious corrosion of the electrical contact material by the pantograph arc. The above factors may accelerate the service performance degradation of the high-speed rigid pantograph–catenary system, reduce the current collection quality, and cause power supply interruption in severe cases. Therefore, the pantograph–catenary electrical contact system of the Sichuan–Tibet Railway need to overcome the above severe challenges from the extremely complex environments.

5.2 Outlooks

The improvement of the stability, reliability and service life of high-speed railway pantograph–catenary system is a key research direction in the future. To this end, the future work need to be conducted from the following aspects.

Firstly, for the catenary:

  • Explore the type of catenary that is suitable for higher speed and increase the fluctuation speed of the catenary;

  • Develop contact wire materials with better comprehensive performance using advanced large-scale heat treatment and processing equipment;

  • Optimize production process, improve product reliability and reduce costs.

Secondly, for the pantograph:

  • Explore the pantograph frame structure and pantograph head material with higher strength and lighter weight;

  • Develop novel pantographs with better stability and following performance;

  • Improve the matching performance between pantograph and catenary.

Thirdly, for the pantograph slide:

  • Explore novel fabrication method for the metal impregnated carbon slide plate and improve its comprehensive mechanical properties;

  • Explore advanced method to enhance the interfacial bonding strength between reinforcement and matrix;

  • Develop brand-new composite slide plate with excellent comprehensive performance to improve the service life of pantograph slide.

Finally, for the detection:

  • Develop high-precision online detection equipment for pantograph–catenary system;

  • Improve the accuracy of online detection;

  • Realize the in-situ detection and online early warning of the service condition of the pantograph–catenary system.

Last but not the least, increasing the loading capacity of high-speed railway is an important development direction in the future. At present, the high-speed train with the largest passenger capacity is China’s CR400AF-S, with a passenger capacity of 1584. In 2020, China developed the world’s first freight high-speed train with a speed of 350 km/h, with a cargo capacity nearly 110 t. Although the running of high-speed railway has reached a higher speed, its carrying capacity is small. For example, when a major disaster such as an earthquake or flood occurs in a country, a large number of disaster supplies and relief teams need to be sent to the disaster area at the first time. When a country is at war, it needs to send a large number of heavy weapons and troops to the battlefield quickly. However, the carrying capacity of high-speed railways cannot meet these demands by far at present. Therefore, develo** large loading capacity high-speed railways and improving their carrying capacity can not only promote the rapid development of the national economy, but also ensure the social stability and people’s safety. The traction power of high-speed train with large carrying capacity is greatly improved, as its pantograph–catenary system must have greater current collection capacity and superior service performance.