Design and development of high voltage cable for t

The design and development of high-voltage cables for electric vehicles

I. overview

the continuous rise of international raw oil prices, the increasing attention of the whole society to environmental degradation and global warming, as well as the tax preference and policy support of governments all over the world, promote the continuous growth of the market share of alternative energy, especially electric vehicles, all over the world

electric vehicles mainly include three categories, namely, and. Pure electric vehicles and vehicles are completely driven by an electric motor, while hybrid vehicles are combined with electric motors, which are supported by electric motors when the efficiency of the internal combustion engine is not high under the conditions of acceleration and low speed. Their common feature is that they use a driving voltage of up to 600V or higher. When it comes to wiring, they all have the same basic requirements, that is, to safely transmit high current and voltage under EMI (electromagnetic interference) protection system. As a high-voltage cable, it is used to connect high-voltage batteries, inverters, air conditioning compressors, three-phase generators and motors to realize the transmission of power and electric energy

the basic principle of electric vehicles seems simple. However, through in-depth analysis, system manufacturers are facing a series of challenges that need to be overcome. New technical requirements are put forward for the flexibility, shielding, safety, size and other items of high-voltage cables of electric vehicles, because these will affect the wiring of high current and high voltage components. In the face of different technical points of each possible powertrain, different special requirements are put forward for the required components

it should be noted that the high-voltage system of electric vehicles is not a typical high-voltage system, and the relevant terms such as "high voltage" and "high current" must be limited to the automotive field, as opposed to the low-voltage system of conventional vehicles. In other fields, the reference system adopts completely different standards. For example, the definition of high voltage in the power field starts at the level of thousands of volts

II. Requirements for high-voltage cables of electric vehicles

innovative electric vehicle design poses new challenges to high-voltage cables and system components, and these requirements cannot completely adopt existing solutions. The specific requirements are analyzed as follows

1. it is still a work of art. The basic difference between voltage

and conventional automotive cables is that the structure needs to be designed according to the rated voltage of 600 v. if it is used in commercial vehicles and buses, the rated voltage can be as high as 1000 V. By contrast, it is even higher. At present, the cables used in cars driven by internal combustion engines are designed to have a rated voltage of 60 v

power generated by the system (P = u × i) Without change, due to the use of lower current, high voltage can reduce the power loss of the comprehensive planning and management mechanism that fails to form system 1 in the transmission system (Ploss = I2 × r）。

2. Current

as cables connect batteries, inverters and motors, high-voltage cables need to transmit high current. According to the power requirements of system components, the current can reach 250A to 450A. Such a high current is difficult to find on a vehicle with conventional drive

3. Temperature

high current transmission results in high power consumption and component heating. Therefore, the high-voltage cable is designed to withstand high temperature. At present, it can be seen that there is a trend of further increase in temperature requirements

in contrast, the current vehicle usually uses the rated temperature of the cable to 105 ℃, as long as the cable is not used in the engine compartment or other areas resistant to higher temperatures. High voltage cables of electric vehicles are usually higher than this temperature, such as 125 ℃ or 150 ℃

if the route in the electric vehicle is unfavorable, the main engine manufacturer will even put forward higher requirements for high temperature resistance. Such as near the exhaust pipe, in front of the motor, on the back of the battery, etc

4. Working life

steam is mainly used on the surface of precision instruments, meters, precision measuring tools, and the friction surface of extremely important parts. In the car industry, the design service life of cables at the specified temperature level is 3000 H. In recognized cable standards (such as ISO 6722, ISO 14572), this value is usually used for long-term aging tests. The special requirements of customers in high-voltage applications may exceed 3000 h, and the cumulative operation time at the specified temperature may even reach 12000 H

5. shielding effect

the high-voltage cable itself does not need shielding, because it does not transmit data like coaxial cable, but it needs to prevent or reduce the high-frequency radiation generated by the switching power supply in the system from being induced to the surrounding components through the cable

unlike fuel driven vehicles, it is necessary to control the three-phase alternating current of the motor of electric vehicles. The sinusoidal voltage carrying energy is equivalent to square wave pulse signals with different frequencies. Because the high-frequency pulse has a steep edge, it will produce harmonics with strong energy to be emitted to the surrounding area

EMI problems can be completely solved by using appropriate shielding methods. In some cases, the combination of different shielding types is required to meet the different requirements of shielding effect

6. flexibility

the challenge faced by the development of hybrid electric vehicles in many cases is that the existing series platform was originally designed to only load the space of gasoline engine and its components, and more electrical components were included. Even if wiring is not considered, space constraints can be expected. In addition, cables and connectors also need space through routing. The usual consequence is a tight bending radius

due to the inherent design of conventional cables, high bending force is difficult to overcome. In order to solve this problem, the high flexibility and toughness of high-voltage cable is very important. Only a relatively flexible design can easily realize the routing through vehicles

7. bending resistance

if the motor is located near the moving part of the vehicle, and then causes the continuous vibration of the connected high-voltage cable, it is required to be designed to withstand high cyclic bending to ensure good bending resistance

8. Identification

due to the increased application risk caused by high voltage, various standards define that high-voltage cables must be visually distinguished from ordinary automobile cables, and the designated surface must be bright orange

at the same time, warning contents and special marks can also be printed, such as "caution! High voltage 600V", high voltage lightning logo, etc

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III. standardization status of electric vehicle cables

in view of the above challenges and requirements of high-voltage cables for electric vehicle applications, it is necessary to establish new cable standards to meet the needs of suppliers, harness manufacturers and host manufacturers

the vehicle cable working group of the electrical and electronic Subcommittee of the road vehicle technical committee of the international organization for Standardization (ISO/TC 22/SC 3/wg4) is carrying out this work

it can be seen from ISO 6722 that the common 60 V cable standard has been revised to meet the requirements of 600V cables. Because most of its requirements are very general, but the special design of high-voltage cables is often not considered. ISO 14572 also made a similar revision

at present, the standardization of high-voltage cables with voltage higher than 600V is a topic of all working groups. The standard number is ISO 17195

sae will adjust the requirements of the current high voltage (Rated 600 V) specification SAE j1654 for high voltage cables, and cover the rated voltage from 600 to 1000 V. The newly created unpublished standard SAE j2840 will define cables as shielded types

lv is a common procurement specification of five major German automobile companies. At present, LV 216, a high-voltage cable standard for electric vehicles with a rated voltage of 600 V, has been introduced. It covers single core and multi-core shielded cables

the national automotive industry standard of China's high-voltage shielded cable is being formulated, and its rated voltage will reach 1000 V

IV. it is difficult to define the standard products and very specific requirements for the structural design of high-voltage cables for electric vehicles. The purpose of this paper is to solve the basic design ideas and overcome the challenges mentioned above by applying advanced high-voltage cable structure principles

1. conductor design

the flexibility of high-voltage cables is mostly determined by the design of conductors. This is why high voltage cables use special conductors with a large number of monofilaments with very small diameters. A certain number of monofilaments are twisted first, and then concentrically re twisted to form the soft conductor required by the high-voltage cable

another advantage of more roots is better bending resistance. Shortening the strand pitch can also improve the bending life of high-voltage cables

2. Insulating material

the selection of insulating material mainly considers heat resistance requirements and mechanical strength. Compared with standard battery cables, soft materials can be reasonably selected to keep the specially designed stranded conductors flexible

3. cabling

when the cable is multi-core, the core usually needs to be twisted. In order to compensate for the deformation caused by the stranding of high-voltage cable cores, special equipment called untwisting is needed. In this process, the pay off reel equipped with the special strander rotates in the opposite direction to the stranding direction. This is necessary to prevent cable deformation and tension

according to the structure of the cable, filling is usually used to ensure a high concentricity of the shielded cable and finally achieve a satisfactory high-voltage cable. The use of wrapping tape in the stranded cable core can maintain the flexibility of the cable

4. Shielding

due to the requirements of EMC (electromagnetic compatibility), multiple copper wires are used to form a braided shield. Tinned copper wire can make it more resistant to environmental effects such as oxidation. Using thin copper wire can maintain the flexibility of the design

shielding requires a coverage rate of more than 90% to overcome the EMI problem described above

according to the different needs of shielding effect, braided shielding can be combined with other kinds of shielding, such as aluminum-plastic film. A layer of non-woven fabric can be wrapped around the shield to ensure that the sheath can be easily peeled off during assembly

5. The sheath

is the same as the insulation of the core, and the sheath material is selected according to the requirements of heat and mechanics. Due to direct contact, environmental properties such as liquid resistance and wear resistance are also particularly important for the sheath. These characteristics mainly depend on the type of sheath material selected, and are also affected by the sheath structure design to a certain extent

if special requirements, such as overcoming the wear of the installation vehicle environment, require to increase the wear resistance, which needs to be considered when selecting materials. Test equipment is usually used to simulate real situations to verify these characteristics

choosing soft materials benefits from flexibility, which may lead to low wear resistance of high-voltage cables

according to the relevant specifications, the extruded jacket should be a bright orange, and special signs warning high voltage can also be added according to the regulations

v. characteristics and optimization of electric vehicle high voltage cable

perfect and complex design and use of high-quality materials will result in expensive cable costs. Experience has shown that specific high-voltage cables can often be tailored through the optimization of cross-section, temperature requirements, flexibility and shielding effect. Weight and cost savings can be highlighted, and excessive size and excessive components can be avoided

1. optimization of cross-sectional area and temperature grade

cable selection is mostly based on the ambient temperature and transmission current indicators. In this regard, the most important characteristics are "cable section" and "heat resistance grade of materials used in cables"

the voltage drop of the conductor is converted into the conductor of the high-voltage cable heated by thermal energy. This heat can be partially transferred to the environment, reducing the operating temperature of the conductor. A lower temperature gradient can transfer less heat. Cables with continuous load current can withstand the highest rated temperature. This temperature can cause the material used