Research on Testing Method of Shielding Effectiveness of Shielded Cables for Electric Vehicles

The energy generation mode of new energy vehicles is the charging and discharging of batteries, so how to ensure the reliability of the internal electrical system of electric vehicles is the focus of electric vehicle design and quality control. As the artery of power transmission, power cable plays a key role. The performance and service life of the cable shielding structure are analyzed emphatically, and the traditional process and material design are changed. Through actual verification, experimental data are obtained, and various tests of the improved cable are compared with those of the cables that are widely used at present. Finally, it is concluded that the overall service life of the improved cable is obviously improved, and other test indexes can meet the requirements of relevant standards.

I introduction

With the development of the times, China's environmental protection is increasing, and the general trend of the automobile industry is that the traditional fuel vehicles will be gradually replaced by clean new energy vehicles. As the internal transmission power of new energy vehicles, special cables have a very broad development prospect and market environment. This cable should meet the technical requirements as follows: the voltage is 300 ~ 1000 V; The current is 80-500 A; Low temperature resistance-40℃; The high temperature resistance is 125 ~ 150℃, and the high temperature resistance of individual heating areas needs to reach 175 ~ 200℃. At the same time, the cable needs to meet the requirements of long life, resistance to various climatic environments and EMI (electromagnetic interference).

In this paper, aiming at the problems that the shielding layer of traditional high-voltage cables in cars has a short life and the shielding performance will drop sharply after aging and cracking, the service life of the cables can be increased as much as possible by improving the technological characteristics, changing the structure and manufacturing method of the shielding layer, and ensuring that the cables achieve the expected shielding effect and excellent softness, so that at least they can keep the same as the life of the cars, and the performance can't drop during the normal service life of the cars.

ⅡThe design concept

The warranty period of traditional cars is 3 ~ 5 years. Traditional car cables are mainly suitable for low voltage and low current, so the cable structure is simple (mostly simple structure of conductor and insulation). Because of its small size, low transmission voltage and current, and weak interference signal to the outside world, shielding design is generally unnecessary.

In recent years, the rapid development of new energy electric vehicles has driven the rapid development of intelligence and automation in the automobile industry. When designing many automobiles, details are often omitted and ignored, such as the matching between cable life and automobile life. The previous automobiles were highly mechanized, but the electronic degree was low, so the interference to signals can be basically ignored. Therefore, there is basically no need to consider the problem of shielding for automobile cables. However, nowadays, the electronic degree of automobiles is getting higher and higher, and the signal acquisition, transmission, reception, etc. will cause signal distortion due to the interference of internal and external signals of automobiles, and even cause safety accidents. So nowadays, many important electronic components of automobiles will seriously consider the anti-interference performance design. On the one hand, the anti-interference treatment is to shield the interference from the outside, and on the other hand, to shield the electromagnetic waves that may be generated by the components themselves from interfering with adjacent components.

Three-phase motors of new energy vehicles have been widely used in the industry. Sinusoidal voltage carrying energy is equivalent to square-wave pulse signals with different frequencies. Because high-frequency pulses have steep edges, they will generate powerful harmonics and emit them to the surrounding areas, which will cause strong electromagnetic interference to the surrounding electronic equipment. Although high-voltage cables for new energy vehicles do not need to transmit data like coaxial cables, they do not need to shield external interference. However, shielding is needed to prevent or reduce the high-frequency radiation generated during current transmission from affecting the peripheral components and the normal operation of other electronic devices through cables. To prevent serious consequences, appropriate shielding methods must be used to shield the possible interference of cables to corresponding components during power transmission.

In cable design, the shielding combination usually adopts the composite shielding structure of aluminum-plastic composite tape and copper wire weaving, but the aluminum-plastic composite tape is easy to be oxidized and cracked, and its life is short, and the shielding performance will be drastically reduced after cracking. In order to improve this situation, the newly designed cable of Kable-X adopts the way that the braided shielding layer is coated on the semiconductor shielding layer, and the wrapped aluminum-plastic composite tape is changed into the extruded semi-conductive shielding layer. On the basis of ensuring good shielding performance, the service life of each part of the cable is consistent, which fully meets the service life requirements of all parts of new energy vehicles.

III. Analysis of various shielding structures

3.1 Separate copper wire shielding

Although the single copper wire shield is not affected by the service life of aluminum-plastic composite belt, and its overall service life can meet the service life requirements of new energy vehicles, the structure of the braided shield is in the form of cross-weaving. When the cross-weaving, there will always be diamond-shaped gaps, resulting in the area coverage rate of copper wire weaving not reaching 100%, and the area and shape of the gaps will be different due to different weaving specifications and parameters. The area and shape of the gap will have a direct impact on the shielding effect, resulting in the shielding effect not reaching the best state. According to the calculation and actual test, it can be found that with the change of knitting angle (the shape of rhombic gap will also change accordingly), the transfer impedance will also change. Nowadays, the increasingly intelligent and automatic automobiles make the requirements for anti-interference ability of various components more stringent. It is difficult to meet the requirements of shielding with copper wire weaving alone, and this scheme is basically no longer used.

3.2 Copper tape shielding

Among the conventional shielding methods, the shielding effect of copper-clad tape is excellent. However, although copper-clad tape can achieve good shielding effect and service life requirements, the disadvantage of copper-clad tape is that the whole cable is hard after being wrapped with copper tape, resulting in too large bending radius. It is not suitable for wiring in a narrow space of automobiles, and cannot meet the most basic requirements of automobile design, so this scheme can also be directly eliminated.

3.3 Wrapped aluminum-plastic composite tape+copper wire braided composite shield

At present, the shielding methods of high-voltage cables in cars on the market are the composite shielding method of wrapped aluminum-plastic composite tape and copper wire weaving. This scheme has excellent shielding effect, and will not affect the overall flexibility and bending radius of the cables. It is easy to be wired and installed in the narrow space of cars. After actual testing and verification, it is found that the shielding effect and bending radius can meet the corresponding requirements of car companies and related standards.

However, through the use test of simulating the special environment inside the automobile, it is found that the aluminum-plastic composite tape will accelerate the oxidation of the aluminum film, make it brittle and easy to crack under the long-term hot and humid or acid, alkali, salt and other special conditions, resulting in some unpredictable risks. In order to solve this problem, we need to find other suitable alternatives.

3.4 Extruded semiconductive layer+copper wire braided shield

Combined with many years' production experience of high-voltage power cables, we found that the insulation and shielding methods used in high-voltage cables can be fully applied to the shielding design of high-voltage cables in new energy vehicles. After many years' use verification of high-voltage power cables, the service life of extruded semi-conductive shielding layer will not have a big performance degradation problem in just a few years or even more than ten years. Moreover, on the basis of the mature three-layer co-extrusion production process, a slight change is made to use double-layer co-extrusion to extrude the insulation and semi-conductive layer at the same time, which can not only solve the bonding problem between the insulation and shielding layers, but also avoid the problems such as air in the interlayer, bulging of the cable after being heated, and other impurities in the two-layer structure will not lead to quality problems. The thickness of the semi-conductive shielding layer can also be controlled at about 0.3mm, which will not increase the cable outer diameter and the cable weight, just in line with the requirements of the vehicle.

After sample preparation test, it is found that the structure combination of extruded semi-conductive shielding layer and tinned copper wire braided composite shielding can fully meet the requirements of life and shielding rate.

In the production process, when the insulation layer is extruded, the double-layer co-extrusion method is adopted, and the production efficiency is much higher than that of wrapping the aluminum-plastic composite tape after extrusion insulation, and the cost will be more advantageous. After many times of production and test verification, it is found that this structural design can completely replace the current traditional composite shielding mode of wrapping aluminum-plastic composite tape and copper wire weaving, which perfectly solves the influence of the service life of aluminum-plastic composite tape on the overall service life of cables.

IV. Cable shielding effect

At present, the national standard GB/T25087-2010 Round, Shielded and Unshielded 60V and 600V Multicore Insulated Cables for Road Vehicles, clause 6.3 of shielding efficiency, and the automobile industry standard QC/T1037-2016 High-voltage Cables for Road Vehicles, clause 4.10.5 of shielding efficiency, have clear requirements for the shielding effect test of such cables, mainly including transfer impedance test and shielding attenuation test, among which At present, this test data is usually used to measure the shielding effect in the industry.

V. Testing of cable bending performance

In many cases, the challenge faced by the development of new energy vehicles is the lack of internal space. The existing series of platforms only design the space for loading gasoline engines and its components. Now, to incorporate more electrical components, the rational use of space must be considered.

Because of the limited space inside the car, the wiring space is very narrow, the corner radius is also small, and the cable with better flexibility is easier to construct when assembling in the whole car factory, which is convenient for customers to produce, and the capacity of customers will be higher. The "double-layer co-extrusion" method mentioned in this paper can control concentricity together without adding one more process, which reduces the outer diameter of the finished product and improves the quality of the product. The overall flexibility of the cable is better than that of the cable produced by the traditional method of wrapping aluminum-plastic composite tape, and the overall performance and customer convenience are stronger. The dual requirements of productivity and quality control are taken into account.

VI. Life Test of Cable

The internal high-voltage cable structure of the traditional new energy vehicle is conductor+insulation+aluminum-plastic composite tape shielding+copper wire braided shielding+sheath, and the aluminum-plastic composite tape with auxiliary shielding has a short life because of its own material. In a good environment, the life can not reach the minimum warranty period (3 years) of the existing vehicle, and even reach 3 months in extreme (damp, acid and alkali, salt fog, etc.) environments.

On the basis of ensuring good shielding effect, the new designed cable hopes to make the service life of all parts of the cable consistent, and the warranty period of high-voltage cable for new energy vehicles should at least reach that of traditional vehicles.

Through the comparison of test results, it is found that the aging speed of aluminum-plastic composite belt is accelerated under the conditions of humidity, high temperature, acid and alkali, salt fog, etc. The material becomes extremely fragile, and will be broken under the action of bending or external force, making it in a state of failure and reducing the overall shielding effect. Thereby seriously affecting the whole service life of the cable.

The extruded semi-conductive layer is designed to replace the wrapped aluminum-plastic composite tape. The life of the extruded semi-conductive layer is consistent with the whole life of the cable, and it can be used for 5 ~ 10 years without external force or foreign object damage.

VII. Concluding remarks

In this paper, through the analysis and comparison of various shielding structures of new energy high-voltage cables, the extruded semi-conductive layer+copper wire braided composite shielding structure, compared with the commonly used wrapped aluminum-plastic composite tape+copper wire braided shielding structure, improves the shielding effect and service life of cables without increasing the manufacturing cost. The extruded semi-conductive layer+copper wire braided composite shielding structure has superior performance and is worth popularizing.

Under the environment of protecting the environment and building ecological civilization in China, fossil energy will be gradually replaced by clean energy. In the near future, new energy vehicles will definitely become the mainstream of automobiles, the sales volume will continue to rise, and the market demand for high-voltage cables will also continue to increase. Therefore, the quality of high-voltage cables must be continuously improved to ensure the use requirements of new energy vehicles, and the development prospect of high-voltage cables will be very broad.