Design and development of high voltage cables for electric vehicles
I. Overview
The rising international crude oil price, the whole society's concern about the global warming of environmental degradation, coupled with the tilt of government taxation and policy support, have led to a growing market share of alternative energy sources, especially electric vehicles, worldwide.
Electric vehicles mainly include three types, namely pure electric vehicles, hybrid vehicles and fuel cell vehicles. Pure electric vehicles and fuel cell vehicles are driven entirely by an electric motor, which is combined with an internal combustion engine and an electric motor, and is supported by an electric motor when the efficiency of the internal combustion engine is not high under acceleration and low speed conditions. Their common feature is the use of drive voltages of up to 600V or higher, involving wiring, they all have the same basic requirements, both safely transmitting high currents and voltages under EMI (electromagnetic interference) protection systems. As a high-voltage cable, it is used to connect high-voltage batteries, inverters, air-conditioning compressors, three-phase generators and electric motors to realize the transmission of power and power.
The basic principle of electric vehicles seems to be simple. But in-depth analysis, system manufacturers are facing a series of challenges that need to be overcome. For electric vehicle high-voltage cables, new requirements for flexibility, shielding, safety, and size have been put forward because they affect the wiring of high-current and high-voltage components. In the face of the different technical points of each possible power system, different special requirements are also imposed on the required components.
It should be noted that the high voltage system of the electric vehicle is not a typical high voltage system, and related terms such as "high voltage" and "high current" must be limited only in the automotive field, as opposed to the low voltage system of a conventional automobile. Reference systems in other areas use completely different standards, such as the high voltage definition in the power sector, which starts at a few kilovolts.
Second, the requirements for high-voltage cable for electric vehicles
Innovative electric vehicle designs present new challenges for high voltage cables and system components that do not fully utilize existing solutions. The specific requirements are analyzed as follows.
Voltage
The basic difference from conventional automotive cables is that the structure needs to be designed for a rated voltage of 600 V, and if used on commercial vehicles and buses, the rated voltage can be as high as 1000 V. In comparison, it is even higher. Cables currently used in automobiles driven by internal combustion engines are designed to have a rated voltage of 60 V.
In the case where the generated power (P = U × I) is constant, the high voltage can reduce the power loss in the transmission system (PLOSS = I2 × R) due to the use of a lower current.
2. Current
Since the cable connects the battery, the inverter and the motor, the high voltage cable needs to transmit a high current. The current can reach 250A to 450A depending on the power requirements of the system components. Such high currents are difficult to find on conventionally driven vehicles.
3. Temperature
The result of high current transmission results in high power consumption and heating of the components. High voltage cables are therefore designed to withstand higher temperatures. It can be seen that there is a tendency for further increase in temperature requirements.
In contrast, current vehicles typically use a cable rated at 105 ° C, as long as the cable is not used in the engine compartment or other areas that are resistant to higher temperatures. Electric vehicle high voltage cables are usually higher than this temperature, such as 125 ° C or 150 ° C.
If the route passed through the electric car is unfavorable, the OEM will even propose higher temperature resistance requirements. Such as near the exhaust pipe, the front of the motor, the back of the battery, etc.
4. Working life
The automotive industry typically has a designed service life of 3000 h at a specified temperature grade. In recognized cable standards (eg ISO 6722, ISO 14572), this value is typically used for long-term aging tests. The special requirements of customers in high-voltage applications may exceed 3000 h, and the cumulative operating time at specified temperatures may even reach 12,000 h.
5. Shielding effect
The high voltage cable itself does not need to be shielded because it does not transmit data like a coaxial cable, but it is necessary 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, three-phase alternating current that controls the electric motor's motors becomes a must. The sinusoidal voltage carrying energy is equivalent to a square wave pulse signal of different frequencies. Since the high frequency pulse has a steep edge, it generates a very strong harmonic emission to the surrounding area.
The EMI problem can be completely solved by using an appropriate shielding method. In some cases, a combination of different shielding types is required to meet the different requirements of the shielding effect.
6. Flexibility
The challenge in the development of hybrid vehicles in many cases is that the existing series of platforms originally only designed the space for loading the gasoline engine and its components into more electrical components. Even if wiring is not considered, space limitations can be expected. In addition, cables and connectors also require space for routing. The usual consequence is the bending radius that leads to tension.
Due to the inherent design of conventional cables, high bending forces are difficult to overcome. To solve this problem, the high flexibility of high voltage cables is critical. Only a more flexible design can be easily implemented by routing the vehicle.
7. Resistance to bending
If the motor is located close to the moving part of the vehicle and then causes the connected high-voltage cable to continuously vibrate, it is required to be designed to withstand high cyclic bending to ensure good bending endurance.
8. Identification
Because of the increased application risk due to high voltages, various standards define that high-voltage cables must be visually distinguished from ordinary automotive cables, and the designated surface must be bright orange.
At the same time, it can also print warning content and special marks, such as "Caution! High voltage 600V", high voltage lightning logo.
Third, the standardization status of electric vehicle cables
In response to the above challenges and requirements for high voltage cables for electric vehicle applications, it is necessary to establish new cable standards to meet the needs of suppliers, wire harness plants and OEMs.
This work is being carried out by the Automotive Cable Division of the Electrical and Electronic Subcommittee of the International Organization for Standardization Road Vehicle Technical Committee (ISO/TC 22/SC 3/WG4).
As seen on ISO 6722, it has been revised based on the common 60 V cable standard to meet the requirements of 600V cable. Because most of its requirements are still very versatile, the special design required for high voltage cables is often not considered. A similar revision was made to ISO 14572.
The standardization of high voltage cables with voltages higher than 600V is currently a topic for each working group. The standard number is ISO 17195.
SAE will adjust the current high voltage (600 V rating) specification SAE J1654 for high voltage cables and cover voltage ratings from 600 to 1000 V. The newly created standard SAE J2840 will be defined as a shield type cable.
LV is the common procurement standard of Germany's five major auto companies, and currently introduces the standard LV 216 for high-voltage cables for electric vehicles with a rated voltage of 600 V. It covers single-core and multi-core shielded cables.
China's national automotive industry standards for high-voltage shielded cables are under development and will be rated at 1000 V.
Fourth, electric vehicle high-voltage cable structure design
Standard products and very specific requirements are difficult to define. The purpose of this paper is to solve the basic design ideas and overcome the challenges described above by applying advanced high voltage cable construction principles.
Conductor design
The flexibility of high-voltage cables is mostly determined by the design of the conductors. This is why high voltage cables use special conductors with a large number of very small diameter monofilaments. A certain number of monofilaments are first bundled and then concentrically twisted to form the soft conductors required for the high voltage cable.
Another benefit of multiple root numbers is better resistance to bending. The twisted pitch is shortened, which also improves the bending life of the high voltage cable.
2. Insulation material
The choice of insulating material is mainly considering heat resistance requirements and mechanical strength. Compared to standard battery cables, a softer material can be chosen to keep the specially designed stranded conductor flexible.
3. Cable
When the cable is multi-core, it is usually necessary to twist the core. In order to compensate for the deformation caused by the twisted high-voltage cable core, it is necessary to use a special device called back-twist. In this process, the dedicated stranding machine is equipped with a pay-distributing disc that rotates in the opposite direction with respect to the twisting direction. This is necessary to prevent the deformation tension of the cable.
Depending on the structure of the cable, padding is usually used to ensure a high degree of concentricity of the shielded cable, resulting in a satisfactory high voltage cable. The use of a strap in the stranded core maintains the flexibility of the cable.
4. Shielding
Due to EMC (electromagnetic compatibility) requirements, multiple copper wires are used to form a braided shield. Tinned copper wire can make it more resistant to environmental influences such as oxidation. Use a thin copper wire to maintain design flexibility
Shielding requires a coverage of over 90% to overcome the EMI problem described earlier.
The shielding effect can be combined with other shieldings, such as aluminum-plastic film. A layer of non-woven fabric can be wrapped around the shield to ensure easy peeling of the jacket during assembly.
5. Sheath
As with the insulation of the core, the jacket material is selected according to thermal and mechanical requirements. Due to direct contact, environmental properties such as resistance to liquids and abrasion are also of particular importance to the sheath. These characteristics are primarily dependent on the type of jacket material selected and, to some extent, by the jacket design.
If special requirements, such as overcoming the wear and tear of the installed vehicle environment, require increased wear resistance, this needs to be considered when selecting materials. Test equipment is often used to simulate real-world conditions to verify these characteristics.
Choosing a softer material benefits from flexibility, which can result in lower wear resistance of the high voltage cable.
According to the relevant regulations, the extruded jacket should be a bright orange, and special warning high pressure markings can be added according to regulations.
5. Characteristics and optimization of high-voltage cables for electric vehicles
The perfect complex design and the use of high quality materials result in costly cable costs. Experience has shown that specific high-voltage cables can often be tailored to the cross-section, temperature requirements, flexibility and shielding effectiveness. Weight and cost savings can be seen, over-sized and excessive components can be avoided.
1. Optimization of cross-sectional area and temperature class
Cable selection is mostly based on ambient temperature and current transfer. In this respect, the most important characteristics are the "cable section" and the "heat resistance rating of the material used for the cable".
The voltage drop of the conductor is converted into a conductor of a high voltage cable that is heated by heat. This heat can be partially transferred to the environment, causing the wire to run at a lower temperature. Lower temperature gradients can transfer less heat. Cables with continuous load current can be subjected to the highest rated temperature. This temperature can cause aging of the materials used.
Cable designers face the challenge of designing the most suitable cable for use: Excessive conductor specifications can result in increased cost and weight, and larger outside diameter. In the worst case, considering only the highest possible load current and ambient temperature will result in the use of large cross-section cables, high temperature resistant materials such as organic fluorine or silicon materials.
Determining the relationship between current and load ambient temperature makes sense from a technical and economic point of view. The true dynamic peak current of the driver should be considered, allowing a reasonable definition of the worst case load current and peak current.
A prerequisite for a good design is an understanding of the basic conditions, such as the need to first determine the ambient temperature and cable load. Generally, large-section high-voltage cables have large inertia in terms of temperature variation, so the peak current of the vehicle to accelerate or decelerate does not cause a large conductor temperature. Allowing for short-term temperature peaks to exceed the cable temperature levels defined above, the ability of high-voltage cables to handle these peaks is usually defined by thermal overload performance. Therefore, the cable does not need to be designed for a higher operating temperature class, and it is not necessary to use a cable that exceeds the specified operating temperature. The resident load current as well as the single pulse or series of pulses can be considered together with various parameters such as ambient temperature.
The combination of theoretical foundations and real-world experience allows for the initial determination, selection and optimization of high-voltage cables for applications.
Fig.1 Current carrying capacity and temperature rise of a 150° XLPO cable in AC state
2. Optimization of flexibility
The available space for the cable routing of the vehicle deserves careful consideration. A tight bend radius only in a specific area of ​​the vehicle requires an increase in the flexibility of the overall cable. If it is possible to make small changes in the overall design, it is very meaningful to avoid tight bending problems.
It is not necessary for the cable to have the highest flexibility. The exact definition of the bending force, combined with the structure and the corresponding test equipment, allows the cable designer to create the design for the most appropriate application. Especially for cables with larger cross-sections, replacing the highly flexible design with a more flexible or conventional structural design can significantly reduce costs.
3. Optimization of shielding effect
The shielding effect defined in a certain frequency range is very necessary for the development of the cable. There is no shielding effect on frequency information that is not very useful, which can lead to solutions that use excessive size and use expensive combination shielding, which is technically unnecessary.
Normally, the development and design phase of the electric vehicle cable can be theoretically calculated to give an expected effect value for consideration. Then, the physical shielding method is used to verify the shielding effect of the high-voltage cable.
Sixth, the conclusion
High-voltage harnesses for electric vehicles and traditional wiring systems still have a long way to go. Specific specification requirements are often not clearly defined, which can complicate the technology and result in expensive solutions.
All parties involved in the development phase must adopt a system-oriented approach to rationally optimize high voltage cable technology and cost. These parties may include cables, connector and component suppliers, wire harness factories, and OEMs. The knowledge accumulation of the entire system and the specification of a high-voltage cable for target requirements are the basis for optimal design. The R&D department of cable manufacturing uses theoretical calculations and appropriate measurement equipment to verify the ability to develop cables that are more suitable for the application.
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