Introduction to high-voltage harness

In recent years, with the strong support and promotion of the country, the development of new energy vehicles in China has made considerable progress, and various new energy vehicles have gradually entered our daily life. In short, new energy vehicles mainly refer to vehicles with advanced technical principles, new technologies and new structures, which use unconventional vehicle fuels as the power source (or use conventional vehicle fuels, use new on-board power devices) and integrate advanced technologies in power control and drive. It mainly includes pure electric vehicles, incremental electric vehicles, hybrid electric vehicles, fuel cell electric vehicles, hydrogen engine vehicles and other new energy vehicles.

However, regardless of the form of new energy vehicles, their common feature is the use of voltage platforms up to 300V~600V or higher, which involves wiring, and they all have the same basic requirements, namely, the safe transmission of large current and large voltage under the electromagnetic interference protection system. High-voltage cables are used to connect high-voltage batteries, inverters, air-conditioning compressors, three-phase generators and motors to realize the transmission of power energy. However, it should be noted that the high-voltage harness of electric vehicles is not equivalent to the high-voltage transmission cable in our daily life, but is only relative to the low-voltage system of conventional vehicles.

Requirements for high-voltage harness of electric vehicle

1. Voltage

The basic difference between this cable and conventional automobile cable is that its structure needs to be designed according to the rated voltage of 600V. If it is used on commercial vehicles and buses, the rated voltage can be up to 1000V. In contrast, it is even higher. At present, the cables used in internal combustion engine driven vehicles are designed to have a rated voltage of 60V.

When the power generation remains unchanged (P=U × 1) In this case, the high voltage can reduce the transmission system (PLOSS=I2) due to the use of low current × R) Power loss.

2. Current

Because the cable connects the battery, inverter and motor, the high-voltage cable needs to transmit large current. According to the power demand of system components, the current can reach 250A to 450A. It is difficult to find such a high current on a traditional drive vehicle.

3. Temperature

High current transmission results in high power consumption and component heating. Therefore, high-voltage cables are designed to withstand higher temperatures. At present, it can be seen that the temperature requirement has a trend of further improvement.

In contrast, in current vehicles, as long as cables are not used in the engine compartment or other high-temperature resistant areas, it is usually enough to use cables with a rated temperature of 105 ℃. The high voltage cable of electric vehicle is usually higher than this temperature, such as 125 ℃ or 150 ℃.

If the wiring in the electric vehicle is unfavorable, the main engine factory will even put forward higher requirements for high temperature resistance. Such as near the exhaust pipe, front of the motor, rear of the battery, etc.

4. Working life

In the automotive industry, the design service life of the cable at the specified temperature level is usually 3000 h. In recognized cable standards (such as ISO 6722 and ISO 14572), this value is usually used for long-term aging test. The customer's special requirements in the field of high-voltage application may exceed 3000 hours, and the accumulated operation time under the specified temperature may even reach 12000 hours.

5. Shielding effect

The high-voltage cable itself does not need to be shielded, because it does not transmit data like the 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 peripheral components through the cable.

Unlike fuel-driven vehicles, the motor of electric vehicles needs to control three-phase alternating current. Sinusoidal voltage with energy is equivalent to square wave pulse signal with different frequencies. Because of the steep edge of the high-frequency pulse, the harmonic with strong energy will be generated and transmitted to the surrounding area.

The electromagnetic interference problem can be completely solved by adopting appropriate shielding methods. In some cases, in order to meet different requirements of shielding effect, different combinations of shielding types are required.

6. Flexibility

In many cases, the development of hybrid electric vehicles is faced with such challenges, that is, the existing series of platforms originally only designed the space for loading gasoline engines and their components, but incorporated more electrical components. Even if wiring is not considered, space limitations can be expected. In addition, cabling cables and connectors require space. The usual result is a tight bend radius.

Due to the inherent design of traditional cables, it is difficult to overcome high bending force. In order to solve this problem, the high flexibility of high-voltage cables is very important. Only flexible design can easily realize the path selection between vehicles.

7. Bending strength

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

8. Identification

Due to the increased application risk caused by high voltage, various standards stipulate that high voltage cables must be visually different from ordinary automobile cables, and the designated surface must be bright orange.

At the same time, you can also print warning contents and special marks, such as "Caution! High voltage 600V", high voltage lightning signs, etc.

Status of standardization of electric vehicle cables

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

This work was carried out by the Working Group on Vehicle Cables of the Electrical and Electronic Subcommittee of the Technical Committee on Road Vehicles of the International Organization for Standardization (ISO/TC 22/SC 3/WG4).

It can be seen from ISO 6722 that it is revised on the basis of ordinary 600V cable standards to meet the requirements of 600V cables. Because most of its requirements are universal, the special design of high-voltage cables is often not considered. A similar revision has been made to ISO 14572.

At present, the standardization of high-voltage cables with voltage higher than 600 V is the subject of various 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 V to 1000 V. The newly created standard SAE J2840 has not been published, and will be defined as a shielded type of cable.

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

China is developing a national automotive industry standard for high-voltage shielded cables, whose rated voltage will reach 1000V.

Structural Design of High Voltage Cables for Electric Vehicles

Standard products and very specific requirements are difficult to define. The purpose of this paper is to apply advanced high-voltage cable structure principles to solve the basic design ideas and overcome the above challenges.

1. Conductor design

The flexibility of high-voltage cables is mainly determined by the design of conductors. This is why high-voltage cables use special conductors with a large number of small diameter monofilament. First, a certain number of monofilament bundles are twisted together, and then twisted concentrically to form flexible conductors required for high-voltage cables.

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

2. Insulating material

The selection of insulation materials mainly considers the requirements of heat resistance and mechanical strength. Compared with standard battery cables, soft materials can be reasonably selected to maintain the flexibility of specially designed stranded conductors.

3. Cabling

When the cable is multi-core cable, it is usually necessary to lay the core. In order to compensate the deformation caused by the stranding of high-voltage cable core, a special equipment called untwisting is required. During this process, the paying-off reel of the special stranding machine rotates reversely relative to the direction of the stranding. It is necessary to prevent cable deformation and tension.

According to the structure of the cable, filling is usually used to ensure the high concentricity of the shielded cable and finally obtain a satisfactory high-voltage cable. The flexibility of the cable can be maintained by using wrapping tape in the stranded cable core.

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 oxidation and other environmental effects. Thin copper wire can maintain the flexibility of design.

In order to overcome the above electromagnetic interference problems, the shielding coverage must exceed 90%.

For different shielding effects, woven shielding can be combined with other shielding, such as aluminum-plastic film. A layer of non-woven fabric can be wrapped around the shield to ensure that the protective sleeve can be easily stripped during assembly.

5. Sheath

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

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

The selection of softer materials benefits from flexibility, which may reduce the wear resistance of high-voltage cables.

According to relevant specifications, the extruded sheath shall be bright orange, and special high-voltage warning signs can be added as required.

summary

As the main carrier of electric vehicle power output, high-voltage wire harness is one of the key components for vehicle performance and safety. The R&D and design of high-voltage wire harness should be considered not only from the perspective of the whole vehicle, but also from the perspective of raw material suppliers, connectors, components and other links. In the case of less standardized industry standards, we should work together to develop a unified standard that is not only in line with the current actual use environment, but also forward-looking in the industry. This can further improve the safety of electric vehicles and reduce the design and manufacturing costs of high-voltage harnesses, thus making due contributions to the development of electric vehicles.