Compared with fuel buses, pure electric buses have a particularly significant difference in the use of large-capacity, high-voltage power battery packs and high-voltage electric drive systems, as well as high-voltage electrical components such as high-voltage electric auxiliary systems. The high-voltage electrical system is a general term for the power circuit composed of high-voltage electrical components on the entire vehicle. It usually consists of a rechargeable energy storage system, a high-voltage power distribution system, an electric drive system, an electric auxiliary system and a charging system. The circuit connection and control interaction between the systems ensure the reliable and safe operation of the whole vehicle.
Due to the particularity of high-voltage electrical safety, the system should meet the corresponding regulations or standards. The main contents are summarized . According to the different forms of the combined structure of the power control conversion module and the electrical load, the high-voltage electrical system is mainly divided into two configurations: centralized and distributed.
1. In the centralized configuration , the high-voltage control conversion modules are integrated together to form an integrated controller, and the power battery pack is directly connected to the integrated controller (including the drive motor controller, the steering motor controller, and the air compressor motor). Controller, etc.), the integrated controller outputs and connects to various electrical loads (drive motor, steering motor, air compressor motor, etc.). The control module of this configuration is easy to integrate design, the environmental adaptability of the control module is low, and the design of electric load is simple. However, there are more AC circuits and the problem of electromagnetic compatibility is more prominent.
2. In the distributed configuration , each high-voltage control conversion module is integrated with the applied electric load, and the power battery pack output is directly connected to the electric load after power distribution. The power distribution lines of this configuration are simple and all are DC circuits, but the integrated design of the electrical load is difficult, and the environmental adaptability of the control module is high.
In the actual application process, the two configurations are combined according to the needs of the vehicle and the development status of the components to carry out a comprehensive design. The principle of cost-saving of the high-voltage electrical system of the vehicle is shown.
① The rechargeable energy storage system is the electric energy storage system of the whole vehicle and the energy source for the whole vehicle drive. The system is mainly composed of a power battery pack and a power battery management system (BMS). BMs mainly collect and analyze power battery voltage, temperature, current and other information to achieve high-voltage safety management, battery status analysis, energy management, fault diagnosis management, battery information management and other functions, so as to realize the safe and effective management of the power battery system and avoid The battery is overcharged and over-discharged to extend the life of the power battery.
②The function of the high-voltage power distribution system is to realize the electric energy management and distribution of the various high-voltage components of the vehicle, and it can be connected and disconnected reliably, so as to realize the safe and efficient operation of the high-voltage electrical system of the vehicle. The system is mainly composed of contactors, fuses, power resistors, conductive copper bars, and high-voltage wiring harness connection components.
③The electric drive system, as the driving power source of the vehicle, controls the rotation of the drive motor by converting the direct current of the vehicle energy into alternating current to provide the driving force of the vehicle. The system is mainly composed of a power motor and its controller.
④The electric auxiliary system is an accessory system that provides auxiliary functions to ensure the normal operation and comfort of the whole vehicle. The system is mainly composed of electric power steering motor and motor controller, electric air compressor and controller, electric air conditioner/electric defroster, DC/DC converter, etc.
⑤The charging system is the energy supplement channel and interface of the vehicle energy system, which realizes the conversion and supplement of external electric energy and ensures the continuous operation of the whole vehicle. The system is mainly composed of charging interface, on-board charger (AC charging configuration) and corresponding connection and control protection circuit.
2. Key points of high-voltage electrical system design.
The high-voltage electrical system plan needs to be designed according to the high-voltage electrical load determined by the vehicle's power, comfort and other indicators, such as the voltage, power, maximum charge and discharge power of the on-board energy storage system, and the power of the electric drive system and other electrical equipment. . The design and development process is basically the same as the design of passenger car electrical system, the main steps are as follows:
① Demand analysis: what functions the whole vehicle needs to achieve, what are the installation requirements, working environment conditions (temperature, humidity, vibration, etc.), requirements for external power grid equipment, frequency of use, etc.
②Function design: Allocate the functions that meet the needs to form a list of functions, such as different setting requirements for cooling and heating of the air-conditioning system.
③System scheme design: Determine the index of the electrical system, specify the topology structure of the electrical system, specify the electrical system schematic diagram according to the electrical load, and output the selection of main components. The schematic diagram needs to reflect the connection relationship and main specifications of each component ( Wire diameter, interface, fuse, relay current, etc.)
The design principles of the high-voltage electrical system are: stable power supply for working parts, no power when not working, branch fault protection by branch circuit, main circuit must have a fuse and maintenance switch, and the external interface is simplified. According to the configuration characteristics of the power battery pack of the passenger car, taking into account the power, current and efficiency, the voltage level of the high-voltage electrical system is generally above 500V, and the maximum working voltage is no more than 750V. The design process is mainly carried out from the following aspects: circuit power matching design, characteristic parameter matching design, connection line and interface design, electromagnetic compatibility design.
(1) Power matching design
For the current-carrying components of the electrical system, the power matching is determined according to the working requirements of the load. The power electrical circuit mainly includes cable wires, fuses, high-voltage relays and other main components. The matching is mainly determined according to the following parameters
Working environment temperature: the maximum ambient temperature range for the normal working of electrical components.
Working voltage: The maximum voltage range of the circuit. The rated voltage of cables, fuses and relays should not be less than this value.
Working current: The rated continuous current and peak current of the electrical components work. For the matching of the fuse, the fusing current of the cable should also be considered.
Working life: Mainly the contact life of the high-voltage relay should meet the requirements of the design life of the vehicle electrical system.
1) Selection of fuse. The rated voltage of the fuse should not be less than the maximum operating voltage of the system, usually DC 750V or DC 800V. The current must be initially selected for the rated current according to the stable working current.
2) Selection of cable conductors
① Orange for high-voltage cables and wiring harness sheaths: high-voltage wiring harnesses with shielding structure; high-voltage wiring harnesses are selected with a weaving density of not less than 85% high-voltage cables; high-voltage wiring harnesses use 360° surround shielding locking heads or 360° surround shielding connectors and electrical equipment connect. According to the voltage level of the car cable, the high voltage DC 1500V cable is usually selected because of the higher voltage platform of pure electric passenger cars.
②Cable diameter selection: Determine the high-voltage cable load current according to the working characteristics of the high-voltage wiring harness connected to the electrical equipment, select the appropriate conductor cross-sectional area according to the load current and the working environment, and perform the verification according to the working temperature rise and voltage loss of the high-voltage wire.
When pure electric passenger cars are operating in different working conditions, the electrical load of electrical equipment changes greatly. The design of high-voltage wiring harness needs to consider the rated current, peak current and working time of the electrical load and other parameters.
3) Matching of fuse and cable.
The circuit overload or short-circuit current must be matched in the fuse and cable selection. When there is an overcurrent in the circuit, the fusing time of the fuse must be less than the smoking time of the cable wire. The fusing time of the 30A fuse and the smoking time of the wire are shown.
4) Check the matching of fuses, relays and cables.
Resistive load and motor load can be selected and designed according to the principle of power matching, but the high-voltage power distribution circuit of power conversion module components has large-capacity support capacitors on the internal interface circuit, which causes excessive impulse current, and special design is required. Charging circuit (Figure 2-80). The matching of the resistance in the pre-charging circuit must be determined according to the power-on time requirements and the load supporting capacitance, and the calculation method is the same as that of the passenger car.
The selection of the relay should fully consider the inrush current and time relationship of the typical load
The state of its contacts, and the adhesion detection related circuit is set according to the monitoring requirements of the relay state, which can be realized by its own auxiliary contacts or circuit voltage detection.
The selection of relays should start from the aspects of rated control capacity, contact resistance, and electrical life. The focus is on:
① Fully consider the load size, type, polarity, inrush current, switching frequency, etc. of the contact.
②Consider the position and connection of the contactor in the circuit
③Whether the life of the contactor is balanced with the life of the equipment used.
④ Consider the ambient temperature in actual use.
⑤ Confirm under actual usage conditions (actual circuit, actual load, etc.).
(3) Connection circuit and interface design
High-voltage circuit interfaces are mainly divided into plug-in type and bolt-fastened type according to different connection forms. According to the different structures of its different parts, there are mainly the following three types of application products:
①Plug-in connection interface
②Terminal bolt interface
③Terminal via connector interface
The matching design should be selected according to the current-carrying capacity, operational advantages, and cost reliability. High, but easy to operate.
(4) MSD matching design
Manual Service Disconnect (MSD) is used to inspect and repair electric vehicles. In order to ensure the safety of people and vehicles, the power supply of the high-voltage system is manually disconnected, so that the high-voltage circuit has a visible breakpoint.
(5) Electromagnetic compatibility design
① Shield
②Ground
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