To meet the driving requirements of electric passenger vehicles and the requirements for wiring harness connection reliability and safety under various driving conditions, this paper designs a high-voltage, high-current (high-power) high-voltage wiring harness for electric passenger vehicles.
Electric passenger vehicles, as the name suggests, are powered by electricity. Therefore, they must use high-power motors and large-capacity batteries. To reduce charging time, high-voltage, high-current charging technology is employed, which necessitates a high-voltage electrical system. The high-voltage electrical system of an electric passenger vehicle connects all components, including the motor, battery, and power electronic components. The high-voltage wiring harness is the electrical path connecting the energy source (fuel cell) to the power unit. To meet the driving requirements of electric passenger vehicles and the reliability and safety requirements of the wiring harness connection under various driving conditions, this paper designs a high-voltage, high-current (high-power) high-voltage wiring harness for electric passenger vehicles.
Currently, the new energy vehicle industry is in the exploratory and small-scale trial production stage, and no industrial scale has been formed domestically or internationally. Therefore, related components are also in the trial production stage. However, compared with the domestic automotive wiring harness technology level, which is seriously lagging behind and mainly focuses on wiring harness assembly, foreign countries have a solid foundation in automotive wiring harness technology and already have high-voltage wiring harness solutions. For example, Amphenol, an industry leader that was the first to enter the field of charging connectors for electric and hybrid vehicles, has developed electric vehicle high-voltage wiring harnesses with simple structure, excellent performance, and high user acceptance. They can work reliably in ultra-high temperature, vibration, confined space and other harsh environments, and are now widely used by various domestic and foreign automakers. Other foreign companies such as TYCO, Delphi, and LS have followed suit and launched their own high-voltage wiring harness solutions and related products.
To fill the research gap in high-voltage wiring harnesses for electric passenger vehicles in my country and to break away from the current situation where my country primarily purchases foreign products for high-voltage wiring harnesses needed for electric passenger vehicles, this paper presents an independent research and development project for a high-voltage, high-current wiring harness for electric passenger vehicles. Based on the usage requirements of high-voltage wiring harnesses in the high-voltage electrical system of electric passenger vehicles, the designed high-voltage wiring harness should meet the following requirements: a) Usability requirements for high voltage and high current; b) Safety and reliability requirements such as electromagnetic interference resistance, waterproofing, vibration resistance, wear resistance, flame retardancy, and reliable contact.
1. Design of high-voltage cables
Traditional automobiles are powered by gasoline engines, and their cables primarily transmit control signals, handling relatively small currents and voltages. Therefore, the cables are small in diameter and structurally simple, consisting only of conductors and insulation. However, electric passenger vehicles (EVs) require high-voltage cables that primarily transmit energy, transferring battery power to various subsystems. Consequently, the designed high-voltage wiring harnesses must meet the demands of high-voltage, high-current transmission. EV high-voltage cables withstand higher voltages (up to 600V) and larger currents (up to 600A), resulting in stronger electromagnetic radiation. Therefore, the cable diameter is significantly increased. Furthermore, to prevent strong electromagnetic interference from surrounding electronic equipment and ensure its proper operation, the cables incorporate an anti-electromagnetic interference shielding structure. This coaxial structure utilizes the combined action of the inner and outer conductors (shielding) to create a concentric magnetic field within the cable, while the electric field extends from the inner conductor and terminates at the outer conductor. This effectively eliminates the external electromagnetic field around the cable, shielding it from electromagnetic radiation and ensuring the normal operation of the electric vehicle.
Early automotive cables primarily used PVC (polyvinyl chloride) as insulation material. However, PVC contains lead, which is harmful to human health. In recent years, it has been gradually replaced by materials such as LSZH (low smoke halogen-free material), TPE (thermoplastic elastomer), XLPE (cross-linked polyethylene), and silicone rubber. Since high-voltage cables for electric passenger vehicles must meet requirements for high voltage and high current, electromagnetic interference resistance, abrasion resistance, and flame retardancy, the properties of these materials were compared.
a. LSZH can be divided into two main categories: PO (polyolefin) and EPR (ethylene propylene rubber), with PO type cable material being the mainstream. The formulation of PO type LSZH flame-retardant cable material contains a large amount of Al(OH)3 and Mg(OH)2 inorganic flame retardants, giving it good flame retardant, low smoke, halogen-free, and low toxicity properties. However, this also makes it different from other non-flame-retardant and halogen-containing flame-retardant materials in terms of physical and mechanical properties, electrical properties, and extrusion process performance.
b. TPE is a polymer material that combines the properties of rubber and thermoplastic. It exhibits the high elasticity of rubber at room temperature and can be plasticized and molded at high temperature. However, this material is not wear-resistant and cannot meet the requirements for high-voltage wiring harnesses in electric passenger vehicles.
c. XLPE is made by irradiating and cross-linking ordinary PE (polyethylene) material with a temperature resistance of 75℃. Its temperature resistance can reach 150℃, and it has excellent physical and mechanical properties, overload resistance and long service life, but it is not flame retardant.
d. Silicone rubber has a high breakdown voltage, thus exhibiting resistance to electric arcing, tracking, and ozone. It also possesses good high and low temperature resistance, withstanding temperatures up to 200℃. It has excellent insulation properties, remains stable under high temperature and humidity conditions, and is flame-retardant. After comparing the properties of the above materials, silicone rubber has become the preferred insulation material for high-voltage cables in electric passenger vehicles due to its excellent physical and mechanical properties, long service life, and low cost. The final structure of the high-voltage cable for electric passenger vehicles is shown in the figure below.
2. Design of high-voltage connectors
2.1 Design of High Current Contacts
Connectors (primarily referring to their contacts) typically have operating temperature limits. If the operating temperature exceeds these limits, the connector will overheat, reducing its safety and potentially causing it to fail or be damaged. There are two main reasons for increased connector operating temperature:
a. The car itself. The hottest part of a car is around the engine; for example, the temperature around a traditional car engine can reach over 125°C.
b. The connector itself. Connectors generate heat during use. The mating contacts within the connector exhibit contact resistance; higher contact resistance leads to greater power loss, higher contact temperatures, and lower reliability. This is particularly important to consider when designing high-voltage, high-current connectors for electric passenger vehicles. To prevent excessively high operating temperatures from damaging the connector's insulation material, reducing its insulation performance, or even causing it to burn out, and to avoid decreased elasticity of the contacts after heating, or the formation of an insulating film in the contact area, reducing contact reliability, increasing contact resistance, and further exacerbating the temperature rise—a vicious cycle that ultimately leads to connection failure—it is essential to rationally design the high-current contacts in high-voltage, high-current connectors for electric passenger vehicles.
When designing high-current contacts, the choice of contact type directly determines the quality and cost of the connector. Generally, there are three main contact types: plate type, leaf spring type, and wire spring type, as shown in Figure 2.
The socket of the plate contact is a grooved cylindrical tube with a closed end. The socket is made of beryllium bronze wire (rod), which is expensive. Furthermore, the subsequent closing process is difficult to control, making it difficult to ensure product quality consistency and resulting in higher costs.
The insert of the leaf spring type contact is a crown spring hole, with 1-2 leaf spring coils placed inside. Each leaf spring coil consists of multiple spring plates, all of which arch inward to form an elastic spring coil. When the insert and the pin mate, each spring plate contacts the pin and generates a compressive force, ensuring stable multi-point contact. The leaf spring type insert is composed of machined brass parts and stamped crown spring parts, resulting in good product consistency and low cost. Amphenol's patented RADSOK insert structure (Figure 3) uses hyperbolic crown spring technology, which can increase the contact area by 65%, and its surface is a highly wear-resistant silver-plated layer.
The socket of the wire-spring type contact is a wire-spring hole, and its structure is similar to that of the leaf spring type socket, except that the wire-spring type socket is composed of spring wire. Although the wire-spring type socket has excellent performance, its manufacturing process is complex and its cost is high. After comparing the contact types mentioned above, the high-voltage, high-current connector for this electric passenger vehicle adopts a high-current leaf spring type contact. Furthermore, to improve contact reliability and current carrying capacity, and to meet other performance requirements of high-current contacts, this high-current leaf spring type contact uses a two-stage leaf spring socket with double springs. Finally, through calculation of the contact resistance, structural design, and prototype design correction, the high-current contact was successfully designed.
2.2 High-pressure resistance design
To meet the design requirements of high-voltage connectors for electric passenger vehicles, it is essential to ensure sufficient dielectric strength in all parts of the connector through structural design and material selection to guarantee its high-voltage resistance. The high-voltage resistance design of high-voltage connectors for electric passenger vehicles mainly includes creepage distance, interface air gap, and insulation materials.
Creepage distance refers to the distance between components when the operating voltage is too high. A momentary overvoltage can cause current to surge outwards along the gap between insulation layers, potentially damaging devices or even personnel. The creepage distance is determined by the operating voltage at which the arc continues. In the design of high-voltage connectors, the creepage distance should be maximized. Considering that the connector dielectric withstand voltage is above 400V, after careful calculation and verification, a creepage distance of 24mm or more is designed to fully meet the 600V operating requirements of high-voltage connectors.
To improve the high-voltage withstand performance of connectors, the interface areas should be perfectly sealed without air gaps during connector mating. The connector interface mainly includes the mating interface between the plug and socket connectors, and the connection points between the connector contacts and the wires. These areas require complete dielectric filling to reliably prevent connector breakdown. To eliminate the presence of air gaps at the interface, the following measures are taken in the design of high-voltage connectors:
a. Soft insulating material is used at the mating interface to ensure that the air gap is filled when the mating is in place.
b. The insulation outside the socket contact is made of molded material to fill the gap outside the contact.
c. The mating surfaces of the plug and socket adopt a conical structure.
d. The cable insulation of the rear part of the contact cable extends into the connector housing insulation.
To improve the high-voltage resistance of the connector, the high-voltage connector for electric passenger vehicles uses PPA (polyphthalamide) plastic, which has good insulation performance, high breakdown voltage, high insulation strength, good stability under high temperature and high pressure, arc resistance, leakage tracking resistance, and low moisture absorption.
