Shielded High-voltage Connection Technology for E-mobility Applications

The demands placed on shielded cable glands for the high-voltage systems of electrically powered commercial vehicles, construction plant and agricultural machinery are far higher than those placed on customary electrical connection technology. Conventional EMC cable glands are not capable of durably and reliably withstanding the harsh operating conditions. Agro has developed a pluggable cable gland system with high current-carrying capacity and a 360° shield connection.

As safety-relevant components, shielded cable glands for use in the high-voltage vehicle electrical systems of construction plant, agricultural machinery and commercial vehicles must be able to withstand heavy mechanical loading and demanding thermal effects. However, the commonly available cable glands from the field of automation technology cannot guarantee this over the long term. The use of such cable glands thus risks consequences such as inadequate Electromagnetic Compatibility (EMC) shielding, loss of sealing performance, short circuits and even vehicle fires.

For these reasons, Agro has expanded its range of EMC-shielded cable glands to include a completely new type for e-mobility applications. Based on already well-established EMC cable glands and the testing of their performance in high-voltage vehicle electrical systems, the manufacturer defined an application-specific requirements profile. This involved identifying the potential disadvantages of the use in electric vehicles of connection technology designed for automation applications and tailoring the entire product design to the specific requirements of vehicle manufacturers, component fabricators and cable assembly companies.

Evolution EMC

The new EMC-shielded cable glands of the type Evolution EMC, Figure 1, are designed for the protected feedthrough of high-voltage cables with braided copper shields and cross- sections of 16 to 120 mm², and multi-core cables with smaller cross-sections. These cable glands are available with short or long connection threads in sizes M20 to M32 and designed to meet the demands of ingress protection class IP68 or IP6K9K. Depending on the thread size, a current-carrying capacity of up to 200 A is achieved with only minimal temperature increase, Figure 2. The shielding effectiveness in the frequency range from 100 kHz to 300 MHz is at least 86 dB, and the solution also offers reliable protection against interference induced by the high switching frequencies of DC/DC converters and AC transformers.

Figure 1

The EMC-shielded cable gland ensures high current-carrying capacity and high shielding effectiveness (© Agro)

Figure 2

Temperature rise of shield current of Evolution EMC M25 type 184, cable 70 mm2 (© Agro)

High Low-impedance Current-carrying Capacity

The range of applications for the new cable gland extends to permanently installed electrical equipment in commercial vehicles, Figure 3, construction plant and agricultural machinery, the connections of which are rarely covered by routine maintenance work - which makes a long service life particularly important. This applies, for example, to AC inverters, traction motors, PDUs and DC/DC converters, as well as auxiliary drives, ancillary units such as air and air-conditioning compressors, coolers, steering pumps, fuel cells and, if appropriate, onboard charging equipment. The demands placed on shielded cable glands are very high, for example, on cables between inverters and traction motors. In such cases, in addition to good EMC shielding, high low-impedance current-carrying capacity and current dissipation with only minimal heating effect are also important.

Figure 3

Mounting position of a permanently installed electrical equipment in a commercial vehicle (© Agro)

The quality of the EMC shielding is verified by measuring the transfer impedance. The lower the delta of the transfer impedance, the higher the EMC shielding effect. The standard requirement is < 2.5 mΩ/m at 2 MHz or < 5 mΩ/m at 30 MHz. Evolution EMC has significantly lower impedance values, Figure 4, and ensures this protective effect even at extreme operating temperatures down to -40 and up to +140 °C.

Figure 4

Transfer impedance of Evolution EMC M25 type 184, cable H&S 70 mm2 (© Agro)

Durable All-around Contact

A second special challenge for such connection technology concerns the connection of DC/DC converters to auxiliary devices. Due to the high switching frequencies, EMC performance of the highest standard is required, with a shield connection that ensures lifelong reliability. In principle, however, a high level of protection is necessary for the entire high-voltage vehicle electrical system, as it is often subjected to parasitic EMC signals and therefore the shielding of all power and signal lines should be EMC-tight throughout. With increasing length of service, shielding defects often occur at interfaces such as plug-type connections, direct cable entries or EMC cable glands. This can be prevented by using a material with optimally supplementing surface coating, by ensuring continuous 360° contact between the cable shield and the high-voltage component at all transmission levels, as well as by designing the contact points with a frictional connection and the largest possible contact area. The shielding effect should also be preserved, over the entire service life of the vehicle, in accordance with the applicable standards - even under very rough operating conditions and at extreme temperatures.

The choice of materials strongly affects the cable gland's functionality and durability. The most important criteria include high electrical conductivity, good protection against electrochemical corrosion, high mechanical strength, good workability, high temperature resistance and suitability for break-proof compression connections. Of course, sustainability and environmental safety must also be considered. The metal components are made of a lead-free alloy of RoHS-compliant brass which is equivalent to conventional brass compounds in terms of mechanical properties, corrosion resistance and electromagnetic compatibility, and meets the requirements of IEC EN 62444 and UL 514B.

The silicone split sealing rings ensure reliable sealing performance thanks to their form-fitting corrugated profile and the special design and effectiveness of the compression nut. This has been confirmed by ingress protection testing in accordance with ISO 20653 (IP6K9K) and EN 60529 (IP68) and by means of aging tests. The compression nut also protects the sealing insert from damage in normal service.

Reliable Assembly Concept

To achieve an optimal design with a simple and reliable assembly concept, it was necessary to consider not only the industry's particular requirements relating to shielding and screwed connections, but also the current standards for electric vehicles, which are based on the use of "plugged" connections. A strong focus was placed on developing a solution that can be pre-assembled in just a few steps, achieving firmly secured 360° shielding to ensure long-lasting EMC protection. Only five steps are required, Figure 5: Cutting the shield to length, fitting the support sleeve, folding over the shield braid, put on the contact sleeve and finally crimping it.

Figure 5

The reliable, cost-saving pre-assembly in five steps reduces the installation work on the vehicle (© Agro)

The secure gripping of the solution's supporting ring and contact sleeve is achieved by means of an innovative axial crimping technology. It ensures that the cable shield cannot be damaged following assembly, and that the EMC shield contact remains securely in place. The shield is firmly clamped onto the cable sheath over a very short distance. This results in a completely stiff and durable connection between the shield contact and the cable sheath, in accordance with the standards LV 214 and LV 215:2017 of the German automobile manufacturers. This has been verified by means of 50 insertion and removal cycles with simultaneous testing of the shield contact resistance. Thanks to the decoupling of the shield contact and the sealing function, the shielding effectiveness and current-carrying capacity are not affected by temperature changes or vibrations. The cable gland thus withstands the most demanding operating conditions over the long term.

The developed pre-assembled solution combines the advantages of a pluggable solution with the benefits of cable glands. The connection system facilitates significantly quicker installation in the series manufacturing of vehicles since the cable glands with plug-in connections can be reliably installed in just a few carefully defined steps. The cable plug is simply fed through the cable gland, the contact sleeve clicked into place, the sealing insert with anti-rotation design pushed in, and the compression nut easily tightened to the limit without torquing. This eliminates the need for tricky, error-prone cable shield work directly on the vehicle. Furthermore, repairs can be carried out quickly and without loss of fitting or performance because the cable shield is not affected by repeated removal and re-insertion of the shield contact and the specially split sealing insert can be replaced without disconnecting the inner conductor from the system.

Traceable Documentation of Installation Steps

The new solution makes both the upstream pre-assembly and the installation work on the vehicle simple, reliable and traceable. Since, with Evolution EMC, the cable shield is completely crimped between a supporting ring and a contact sleeve, shield strands are not able to protrude and potentially cause a short circuit. Furthermore, there is no longer any need to secure the end of the shield with shrink tubing or adhesive tape.

The crimping is done using Axi-Press, a handy cordless tool developed by Agro for crimping by means of patent-pending crimping technology, Figure 6. The crimping tool is supplied with appropriate tool inserts. In a matter of seconds, it provides the EMC shield contact sleeve with a secure and unchangeable all-around grip on the cable shield and cable sheath, and ensures a long- lasting, reliable shield connection. The crimped cable shield is thus protected from wear and damage under even the most demanding operating conditions. During crimping work, the compression force exerted by the Axi-Press tool can be precisely determined using its integrated force/pressure measurement technology and documented via an interface and recording log for each individual use. Together with the identification markings of the individual parts and the batch number, this ensures the reliability and traceability of the work on every cable shield.

Figure 6

The Axi-Press tool facilitates easy crimping of the shield sleeve for an enduringly reliable shield connection (© Agro)

Corrosion-protected Shielding

The shield contact offers considerably higher reliability than the spring shielding of conventional EMC cable glands. Under the tough, dynamic operating conditions that typically arise in commercial vehicles, the quality achieved by spring contacts varies. The springiness of such solutions causes rubbing against the tin-coated strands of the shield, and the spring pressure depends on the cable's diameter, strong banding radius and movements, resulting in reduced shielding effectiveness under thermal stress. Over time, such losses have a negative impact on the EMC protection of the entire vehicle and reduce its operational reliability. In addition, weak points in the EMC shielding make fault analysis during servicing more difficult, often resulting in drastically increased workshop costs. An increased risk of electrochemical corrosion also speaks against the use of conventional fittings with stainless steel springs. Depending on the type of contact and the high-voltage component, corrosion can lead to a massive deterioration in EMC shielding performance, or even its complete loss, within the shield contact. Due to the higher contact resistance, this results in greater heating of the cable gland which reduces the service life, functionality and reliability of the EMC connection.

Conclusion

With Evolution EMC, Agro has expanded its EMC range with a solution for high-voltage vehicle electrical systems. This new type of "pluggable" cable gland, with its all-around crimped shield contact, offers significantly higher reliability than conventional EMC cable glands. The connection system, which can be reliably pre-assembled in just a few steps, ensures long-lasting high current-carrying capacity and shielding effectiveness, even under very demanding operating conditions. It is therefore ideally suited for use in electrically powered trucks, buses and other mobile machines such as construction and agricultural plant, and in motorsport vehicles with electric or hybrid motors.