Design and Development of High-Voltage Wiring Harnesses for Electric Passenger Vehicles

Design of High-Voltage Cables

Gasoline engines power traditional automobiles. The role of cables in traditional automobiles is to transmit control signals, with the current and voltage they bear being very small. Hence, the cable diameter is small, and the structure is simply a conductor with insulation added, which is quite simple. However, according to the usage requirements of high-voltage cables for electric passenger vehicles, the high-voltage cables for electric passenger vehicles mainly play the role of transmitting energy and need to communicate the energy from the battery to various subsystems. Therefore, the designed high-voltage wiring harnesses for electric passenger vehicles must meet the requirements of high-voltage and high-current transmission. The high-voltage cables for electric passenger vehicles bear relatively high voltages (the rated voltage is up to 600 V) and large currents (the rated current is up to 600 A), and the electromagnetic radiation is relatively strong. As a result, the diameter of the cables is significantly increased. Meanwhile, to avoid the strong electromagnetic interference caused by electromagnetic radiation on surrounding electronic equipment and affect the normal operation of other electronic equipment, the cables are also designed with an anti-electromagnetic interference shielding structure, that is, a coaxial structure is adopted. Using the combined action of the inner conductor and the outer conductor (shielding), the magnetic field inside the cable is distributed in concentric circles, and the electric field points from the inner conductor to and terminates at the outer conductor, making the electromagnetic field outside the cable zero, that is, shielding the electromagnetic radiation, thereby ensuring the normal operation of electric vehicles.

In the early days, the main insulating material for automobile cables was PVC (polyvinyl chloride). 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 and halogen-free materials), TPE (thermoplastic elastomer), XLPE (cross-linked polyethylene), and silicone rubber. Since the high-voltage cables for electric passenger vehicles need to meet the requirements of high voltage, large current, anti-electromagnetic interference, as well as wear resistance and flame retardancy, the performance of these materials has been compared as follows:

a. LSZH can be divided into two major categories: PO (polyolefin) and EPR (ethylene-propylene rubber). Among them, PO-based cable materials are the mainstream. The formulation of PO-based LSZH flame-retardant cable materials contains a large amount of AI(OH)3 and Mg(OH)2 inorganic flame retardants, which endow the cable materials with good flame retardancy, low smoke, halogen-free, and low toxicity characteristics. However, at the same time, it also makes them different from other non-flame-retardant materials and halogen-containing flame-retardant materials in terms of physical and mechanical properties, electrical properties, and extrusion process properties.
b. TPE is a polymer material with rubber and thermoplastic plastic characteristics. It exhibits high elasticity of rubber at room temperature and can be plasticized and molded at high temperatures. However, this material is not wear-resistant and cannot meet the usage requirements of high-voltage wiring harnesses for electric passenger vehicles.
c. XLPE is obtained by irradiating and cross-linking ordinary PE (polyethylene) materials with a temperature resistance grade of 75 °C. Its temperature resistance grade can reach 150 °C, and it has excellent physical and mechanical properties, overload resistance, and long service life characteristics, but it is not flame-retardant.
d. Silicone rubber has a high breakdown voltage, so it has arc resistance, tracking resistance, and ozone resistance. It also has good high and low-temperature resistance, can withstand temperatures up to 200 °C, has good insulation performance, is stable under high temperature and high-humidity conditions, and is flame-retardant. After comparing the performance of the above materials, silicone rubber has become the first choice for the insulating material of high-voltage cables for electric passenger vehicles due to its good physical and mechanical properties, long service life, and low price. The structure of the finally designed high-voltage cable for electric passenger vehicles is shown in Figure 1.

Figure 1 Structure of the high-voltage cable for electric passenger vehicles

Design of High-Voltage Connectors

Usually, connectors (mainly referring to the contacts therein) have usage temperature limitations. Once the usage temperature exceeds the specified limit, the connectors will reduce their safety due to heat generation and may even fail or be damaged. There are mainly two reasons for the increase in the usage temperature of connectors:

a. The automobile itself. The area with the highest temperature in an automobile is around the engine. For example, the temperature around the engine of a traditional automobile can reach above 125 °C.
b. The connector itself. The connector will generate heat during use. There is contact resistance between the mated contacts in the connector. The larger the contact resistance, the greater the power loss, the higher the temperature of the contacts, and the lower the reliability. In this regard, special attention should be paid when designing high-voltage and high-current connectors for electric passenger vehicles. To avoid damage to the insulating materials in the connectors due to excessive usage temperature, reduce their insulation performance, or even cause burnout and failure, as well as prevent the contacts from experiencing a decrease in elasticity after being heated or forming an insulating film in the contact area, reducing contact reliability, increasing contact resistance, and further exacerbating the increase in usage temperature, which will eventually lead to connection contact failure in a vicious cycle, it is necessary to reasonably design the high-current contacts in the high-voltage and high-current connectors for electric passenger vehicles.

When designing high-current contacts, the choice of contact form will directly determine the quality and cost of the connector. Usually, the contact forms of contacts mainly include three types: plate type, leaf spring type, and wire spring type, as shown in Figure 2.

Figure 2 Structures of the three types of contacts

The socket of the plate-type contact is a cylindrical barrel with slots and a constricted opening. The socket is processed using beryllium bronze wire (rod). The price of the raw material is relatively high, and the subsequent constricting process is difficult to control. It is difficult to ensure the consistency of product quality, and the cost is high.
The socket of the leaf spring-type contact is a crown spring hole. One or two leaf spring coils are placed in the socket. Each leaf spring coil is composed of multiple spring leaves, and all the spring leaves arch inward to form an elastic spring coil. When the socket and the pin are mated, each spring leaf contacts the pin and generates a squeezing force to ensure stable multi-point contact. The leaf spring-type socket is composed of a brass turning part and a crown spring stamping part, with good product consistency and low cost. The patented RADSOK socket structure (as shown in Figure 3) of Amphenol Corporation adopts the hyperbolic crown spring technology, which can increase the contact area by 65%. Its surface has a high-wear-resistant silver-plated layer.

Figure 3 Structure of the RADSOK socket of Amphenol Corporation

The socket of the wire spring-type contact is a wire spring hole. The structure of the wire spring hole is similar to that of the leaf spring-type socket, except that the wire spring-type socket is composed of spring wires. Although the wire spring-type socket has excellent performance, its process is complex and the cost is also high.
After comparing the contacts of the above various contact forms, the high-voltage and high-current connectors for electric passenger vehicles adopt the high-current leaf spring-type contact. Meanwhile, to improve contact reliability and current-carrying capacity and meet other index requirements of high-current contacts, the high-current leaf spring-type contact adopts a two-stage leaf spring-type socket with double reeds. Finally, through the calculation of the contact resistance of the high-current contact, the design of the structure, and the design correction of the sample, the high-current contact has been successfully designed.

Design of High-Voltage Resistance Performance

In order to meet the design requirements of high-voltage connectors for electric passenger vehicles, it is necessary to ensure that each part of the high-voltage connector has sufficient dielectric strength through structural design and material selection to ensure its high-voltage resistance performance. The design of the high-voltage resistance performance of high-voltage connectors for electric passenger vehicles mainly includes aspects such as creepage distance, interface air gap, and insulating materials.
The creepage distance refers to the situation where when the working voltage is too high, the instantaneous overvoltage will cause the current to release an arc along the gap between the insulations, damaging the devices or even the operators. This insulation gap is the creepage distance, and the working voltage at which the arc lasts determines the creepage distance. When designing the structure of the high-voltage connector, the creepage distance should be increased as much as possible. Considering that the dielectric withstand voltage of the connector is above 400 V, after careful calculation and verification, the creepage distance of the connector is designed to be above 24 mm, which can fully meet the usage requirements of the high-voltage connector at 600 V.
To improve the high-voltage resistance performance of the connector, when the connector is mated, its interface should be closely fitted without air gaps. The interface of the connector mainly includes the mating interface of the plug connector the socket connector, and the connection parts between the connector contacts and the wires. These parts need to be filled with a medium without air to reliably ensure that the connector is not broken down. To eliminate the existence of interface air gaps, the following measures have been taken in the design of high-voltage connectors:

a. Soft insulating materials are used at the mating interface to ensure that the air gaps are filled while the mating is in place.
b. The insulation outside the socket contact is in the form of molding to fill the gaps outside the contact.
c. The mating surfaces of the plug and the socket adopt a tapered structure.
d. After the contact is connected to the cable, part of the cable insulation extends into the insulation of the connector housing.

In order to improve the high-voltage resistance performance of the connector, PPA (polyphthalamide) plastic with good insulation performance, high breakdown voltage, high insulation strength, good stability under high temperature and high pressure, arc resistance, tracking resistance, and low hygroscopicity is selected for the high-voltage connector of the electric passenger vehicle.

Overall Structural Design

The structure of the finally designed high-voltage connector for electric passenger vehicles is shown in Figure 4. The structure of the high-voltage connector from the inside to the outside is the inner conductor, the insulation layer, the shielding layer, and the outer shell in sequence.

Figure 4 Structure of the high-voltage connector for electric passenger vehicles

Overall Design of High-Voltage Wiring Harnesses

1 Design of Shielding Performance

To make the designed high-voltage wiring harnesses have excellent electromagnetic shielding performance in addition to meeting the basic requirements of reliable electrical connection, the shielding performance design of high-voltage wiring harnesses has been carried out. The shielding performance design of high-voltage wiring harnesses mainly includes the shielding performance design of high-voltage cables themselves, the shielding performance design at the joint between high-voltage cables and high-voltage connectors, the shielding performance design of high-voltage connectors themselves, and the shielding performance design at the mating interface of high-voltage connectors. To improve the shielding performance of high-voltage cables themselves, high-voltage cables adopt a shielding structure. This should be paid more attention to when the cable is composed of a combination of signal lines and power lines. To improve the shielding performance at the joint between high-voltage cables and high-voltage connectors, on the premise of ensuring the reliability of the contact between the two, especially ensuring that the connection will not become loose under strong vibration conditions after the inner conductors of the high-voltage cable and the high-voltage connector are connected, the cable braid contacts the shielding layer, and a separate shielding metal braid is added at the joint between the cable braid and the connector to strengthen the shielding effect. To improve the shielding performance of high-voltage connectors themselves, the connectors adopt a metal housing design. To improve the shielding performance at the mating interface of high-voltage connectors, a shielding spring structure is adopted in the design to ensure reliable contact between the plug and socket housings. The inner conductor at the head of the connector is lower than the interface of the outer shell to prevent the inner conductor from touching fingers or other metals, playing a certain protective role and increasing safety. After mating, the shielding layers of the socket connector and the plug connector are in reliable contact, making the mating surface shielded from the outside.

2 Mechanical Protection and Dust and Waterproof Design

Since the diameter of the high-voltage cable for electric passenger vehicles is relatively large and requires a special wiring route, that is, the high-voltage wiring harness for electric passenger vehicles is laid outside the vehicle, it is necessary to carry out mechanical protection and dust and waterproof design for the high-voltage wiring harness for electric passenger vehicles. To improve the mechanical protection and dust and waterproof performance of the high-voltage wiring harness, protective measures such as sealing rings are adopted between the plugged connectors and at the positions where the connectors are connected to the cables to prevent water vapor and dust from entering, thereby ensuring the sealed environment of the connectors, avoiding the risk of short circuits between the contacts, and preventing moisture from entering to avoid safety problems such as spark generation.

3 Design of Service Life

Electric passenger vehicles travel on the road and will be affected by factors such as uneven road surfaces and vehicle speeds, resulting in high vibrations, which will cause friction and wear between the high-voltage wiring harness and the contacted parts and other wiring harnesses, as well as the fatigue wear of the high-voltage wiring harness itself. To improve the service life and quality of the high-voltage wiring harness, the connection between the high-voltage cable and the high-voltage connector should be strengthened, a locking structure should be adopted for the connection between high-voltage connectors, the wiring scheme should be optimized, wear-resistant materials should be selected for the high-voltage wiring harness, and anti-fatigue copper stranded wires should be used for the conductors. In addition, the connection link between high-voltage connectors is a weak point of the high-voltage wiring harness itself. To improve the service life of the high-voltage wiring harness and meet the usage requirements of the high-voltage electrical system at the same time, it is necessary to ensure the insertion and extraction times and connection quality of high-voltage connectors.

4 Overall Structural Design

The structure of the finally designed high-voltage wiring harness for electric passenger vehicles is shown in Figure 5.

Figure 5 Structure of the high-voltage wiring harness for electric passenger vehicles

This article briefly introduces the functions and applications of high-voltage wiring harnesses for electric passenger vehicles and the research and development situations at home and abroad. Starting from the usage characteristics, requirements, and environment of electric passenger vehicles, it analyzes the performance requirements and design key points (high voltage resistance, high current resistance, environmental resistance, shielding performance, safety, and reliability, etc.) of high-voltage wiring harnesses for electric passenger vehicles, and elaborates on the main design of cables, the main design schemes of connectors and their contacts respectively, and gives the overall scheme of the wiring harness. Finally, it introduces the test situation of the developed samples. From the usage requirements and test results, it can be concluded that the developed high-voltage wiring harness can meet the usage requirements of electric passenger vehicles. With the development of the electric vehicle industry, high-voltage wiring harnesses will surely develop further, be able to withstand higher voltages and larger currents, and will be used in various vehicle models. Meanwhile, in terms of functions, they will also be more perfect, for example, having their testability, that is, being able to monitor changes in the current, temperature, etc. of the wiring harness in real-time.