Internet of Things Technology Helps Intelligent Power Supply System of Dual-source Pure Electric Bus

In recent years, with the popularity of cars in large and medium-sized cities, the proportion of exhaust emissions in exhaust emissions is getting higher and higher, causing serious environmental problems and traffic jams in cities. Among them, urban air pollution is particularly serious, smog has swept almost half of China, and many large and medium-sized cities across the country have been heavily "occupied". Under the current severe environmental protection situation, the state vigorously promotes the development of new energy vehicles to alleviate the increasingly serious air pollution problem.

Considering that pure electric drive is the main strategic orientation for the development of new energy vehicles, pure electric drive vehicles have been widely used in public service fields such as public transportation, rental, official business, sanitation and postal services. As a "zero-emission" pure electric vehicle, dual-source pure electric bus plays an increasingly important role in new energy public transportation. Taking Beijing Public Transport Group as an example, there are currently more than 1200 dual-source pure electric buses in operation, with a power supply network of more than 200 kilometers and 24 bus lines. Among them, BRT1, BRT2 and BRT3 lines have all been electrified and 18m long double-source pure electric buses are used for operation. In early 2017, Shanghai opened 71 trolley buses with medium capacity on Yan 'an Road, and also adopted 18m long electric buses for operation, effectively relieving the operation pressure of public transportation.

The power supply line network erected above the road is mainly used to provide power for the dual-source pure electric bus. The power supply station converts AC 10KV electric energy into DC electric energy, which is transmitted to the feeder isolation switch through the feeder cable, and the electric energy is transmitted to the power supply line network by the isolation switch. The power supply line network can provide electric energy for the dual-source pure electric bus to directly drive the vehicle, and can also provide a continuous and stable charging current for the on-board energy storage device. Therefore, the power supply station, feeder cable, feeder isolation switch and power supply line network constitute the key elements of the dual-source pure electric bus power supply system.

Fig. 1 Schematic diagram of dual-source pure electric bus power supply system

dual-source pure electric bus power supply system not only provides power load and charging load for the vehicle, but also provides electric energy for the air conditioning (heating), air pump and other equipment installed on the vehicle. With the continuous increase in the number of trams, the load pressure of the power supply system has increased significantly, which is easy to cause problems such as overheating and insulation aging of power station power supply equipment and underground cable joints, and cannot provide electric energy to the line network during maintenance. At the same time, the underground construction operations in the city are also easy to cause the power supply cable to be cut off, damage to the grounding and other problems, which also adversely affect the normal power supply of the two vehicles. When the above problems occur, it is necessary to disconnect the feeder isolation switch, isolate the corresponding line network from the cable, and then use the adjacent line network for emergency power supply to ensure that the line network supplies normal power to the vehicle.

power supply line network will also affect the normal operation of the vehicle motor. When vehicles travel intensively under the long-distance power supply network, the voltage at the end of the network is generally low. Generally speaking, for a power supply system with a nominal voltage of 600V, the minimum driving voltage for normal driving of the vehicle is about 400V, and the 750V power supply system requires 500V. At the same time, the line network is also used as the charging network of the tram, and its voltage fluctuation will also affect the normal charging of the on-board energy storage device. Taking the 750V power supply system as an example, the on-board lithium battery needs at least 600V voltage to charge normally. Therefore, the measurement and monitoring of the network voltage, especially the terminal voltage, is particularly important. When the power supply voltage is found to be low, the terminal voltage equalization and other operations can be carried out to improve the power supply voltage at the end of the network.

Fig. 2 Dual-source pure electric bus running under online network

, however, the management of the current power supply network, such as isolation, contact and terminal voltage measurement, remains in the field manual mode. Since the feeder isolation switch is set near the power supply line network, the contact emergency power supply and terminal voltage measurement must also be completed at the end of the line network. The operator must rush to the site to operate as soon as possible to isolate the line network and restore power supply. For a city, the power supply network is distributed in a wide range, and it takes a long time for personnel and overhead vehicles to arrive at the scene. When the city meets the morning and evening peak, the time to restore power supply is longer. Taking Beijing as an example, the time for personnel and vehicles to arrive at the scene during peak hours sometimes exceeds 2 hours, which seriously affects the normal driving power supply and charging of vehicles.

In fact, the power supply station of the dual-source pure electric bus power supply system has completed technological innovation. The intelligent box-type power station, remote background system, brand-new fault detection technology and information interaction technology have enabled the power supply station to be unattended. Intelligent box-type and room-type power stations have realized digital remote monitoring of all kinds of electricity and non-electricity, automatic fault identification, automatic power supply recovery, and remote operation of equipment. Under the premise of ensuring the safety of the power station, the work efficiency of the power station has been greatly improved. In contrast, the management of the power supply network still stays in the technology of the 1990 s, mainly relying on manual work to complete various operations on site, which not only has low operation efficiency, but also causes the network to be unable to supply power normally for a long time.

Fig. 4 Box-type Power Supply Station of Beijing Bus Field Operation

Figure 5 Beijing bus remote background system

In order to reduce the power outage time of the network and improve the operating efficiency of the isolating switch, it is the most convenient and feasible way to realize the remote operation of the isolating switch and the remote real-time monitoring of the network voltage value by means of communication. Since the power supply network is widely dispersed in the streets of the city, the use of wired communication will inevitably lead to re laying of communication cables, which is difficult for various cities to do. In the face of this problem, we can use the rapid development of the Internet of things technology based on wireless communication. The Internet of Things, the English name IoT(Internet of things), is simply "the Internet of things connected to things". The Internet of Things is widely used in the integration of networks through communication perception technologies such as intelligent perception, identification technology and pervasive computing. There are many technical directions for the Internet of Things, such as 3G/4G,GPRS,NB-IoT,Lora,Sigfox, etc. Considering the wide coverage of the power supply network in the city, and the small amount of data interaction between switch operation and voltage measurement, GPRS technology with wide coverage and mature application can be selected.

For the remote operation of the isolating switch and the measurement of the terminal voltage, the relevant industry standard for trolleybuses "CJT_1-1999 Urban Trolley and Tram Power Supply System" has already mentioned:

8.3.2 Feeder and contact network telecontrol device

a. Monitoring of terminal voltage of contact network in low voltage area

B. Remote control operation of feeder box knife switch

, from the content of the standard description, the operation of the isolating switch and the monitoring of the terminal voltage can be completely completed by the remote background, which also solves the problem of electrical management of the power supply network widely distributed in the city, and lays the foundation for the intelligence of the dual-source pure electric bus power supply system.

figure 6 new generation of isolating switch and remote background communication

This new generation of isolation switch based on Internet of Things technology should have the following functional characteristics:

>> with the help of the operator's mature network and remote background connection, the switch status, voltage on both sides of the switch, temperature in the switch box and other data can be updated within tens of seconds, and the switch can be opened and closed by instructions issued by the background.

>> can support on-site WIFI connection. considering that the isolation switch is often installed in a position that is not easy for personnel to operate, data interaction through WIFI can greatly reduce the workload of on-site operation and maintenance.

>> The isolating switch can automatically realize voltage interlock during operation to prevent the switch from operating with voltage and load.

>> Have comprehensive diagnostic functions, such as: switch state diagnosis, energy storage equipment working parameter diagnosis, ambient temperature diagnosis, etc.

Fig. 7 Feeding disconnector and contact disconnector for field operation

isolation switch can complete a variety of applications for the work requirements of the dual-source pure electric bus power supply system. By understanding the voltage distribution of the line network, especially in the morning and evening peak traffic congestion, we can evaluate whether the catenary can provide sufficient power and charging voltage for vehicles. The background system can also analyze the collected voltage data and provide a big data basis for the layout and configuration of the line network.

Fig. 8 Measurement of network voltage distribution of power supply line

, when the isolating switch is combined with the power supply station, the insulation detection of the underground cable can be completed efficiently. Close the circuit breaker of a DC feeder in the power supply station through the background system, disconnect the other DC feeder circuits, then disconnect the isolation switch corresponding to the DC feeder, and then evaluate the insulation leakage of the feeder cable to the ground through the insulation detection equipment in the power supply station. Due to the high efficiency of the background system to the isolation switch operation, all DC feeder cables can be evaluated regularly and relevant data can be recorded, by contrast to understand the changes in the leakage of feeder cables to the ground.

Figure 9 Cable insulation detection principle

dual-source pure electric bus vehicles have undergone rapid changes in recent years. At the same time, the power supply stations of dual-source pure electric buses in Beijing, Jinan, Qingdao and other cities have completed comprehensive automation and unattended operation. While greatly improving the efficiency of power stations and the accuracy of various judgments, it has also significantly reduced Labor costs. As the "last kilometer" of the power supply system service vehicle, the power supply quality and continuous power supply time and other data have become the key factors affecting the normal operation of the vehicle. With the rapid development of the Internet of things technology, the new generation of isolation switch can significantly improve the efficiency of the power supply network, and lay a solid foundation for the intelligence of the whole power supply system.