Development of high-voltage cable harnesses and their series testing

Transcription

1 Development of high-voltage cable harnesses and their series testing Summary The high-voltage wiring of electric and hybrid vehicles currently constitutes a particular challenge for vehicle manufacturers and their suppliers, as it does for producers of cable harnesses. High currents must be transferred at voltage levels that conform to safety standards. The degree of standardisation is still low, as is the number of fully developed and available components. This makes the task of HV interconnection a difficult one. High safety demands on the "HV cable harness" product also require special series testing. This article presents the requirements placed on the HV cable harness and its series testing as well as pointing out some of the most important interactions between the system components. 1. Introduction The cable harness in a car undoubtedly fulfils an indispensible function: it connects practically all electric components with each other. From a technical aspect, this structure is rather simple; in organisational terms, however, it is highly complex, as any changes to whatever electric component (and there are hundreds of them in a vehicle) in many cases lead to changes to the cable harness. Different models and platforms often contain the same electric components, whereas there are hundreds of thousands of varieties of cable harness. It is against this backdrop that the cable harness within the vehicle has developed its most important characteristic: it is hugely adaptable! It is able to respond to each new requirement with lightning speed because a huge number of tested components (cables, connectors, fixings ) are available as standard parts and can normally be combined without any problems. The parts can thus be quickly exchanged or supplemented if required. The cable harness itself can then be made to assume any form and length. The market has got used to this flexibility of wiring systems and has come to rely on the fact that changes in the later development process can be accommodated by the cable harness supplier without any difficulties. So, what does this have to do with the development of the HV cable harness? The case of the HV cable harness is at present a completely different one: requirements are high; the number of finished available parts is low. If the HV wiring and the interfaces are fixed late or if changes are made in a late phase of development, this can lead to big problems. In the best case, they result in additional costs deriving from special developments with low unit numbers. In the worst case, the comparatively simple component that is the HV cable harness can jeopardise the timetable of an entire project. This article seeks to explain the interrelated aspects involved in the development of HV cable harnesses. The testing of the safety-relevant HV products is also important. For instance, highvoltage resistance as a rule demands 100% series testing. Some special considerations must be taken into account in production with regard to voltage protection, where usually the quality of products is ensured exclusively on open test benches with minimum voltages.

2 2. Scope of the HV cable harness The main additional wiring requirement in an electric or hybrid vehicle relates to the connection between the HV battery and the power electronics and the connection between power electronics and the electric motor/generator. In many drive concepts there are several motors/generators, connection usually takes place through 3 individual cables for the 3 phases of the unit. Engine/ Generator HV Battery Power Electronics Air conditioning Compressor Engine/ Generator Image 1: Illustration of a typical HV cable harness Further HV components include, for example, the DC/DC converter, air conditioning compressor and power steering. The 2-pole or 3-pole cables required for these functions can often be designed as a jacket cable on account of the low voltage, and the connector, too, can have a comparatively compact design. If all components have been matched to each other and every component exists with a certified and available connector, the cable assembler can produce the cable harness quite easily. The connectors at either end of a cable do not necessarily have to be of the same type and by the same producer. If the components have not been matched in their entirety with regard to their connection technology, a distributor box or an adaptor component could be necessary. This will enable the non-matching components of a cable harness to be connected, or HV currents to be split. Also, safety functions or DC/DC converters can be incorporated into this type of component. Such a box can be firmly connected with HV cables as a cable harness component provided that assembly within the vehicle permits it. This means that connector solutions, which are often costly, can be avoided.

3 Further components of the HV cable harness concern the following functions: laying of cables, cable fastening, protection and shielding. As in the case of every cable harness, suitable cable conduits, sealing components and fixings must be provided. Image 2: Example of sealed cable bushing (not HV) The laying of HV cable harnesses often takes place in dedicated installation spaces, such as below the vehicle or near the combustion engine. This, together with the high safety requirements, necessitates particular efforts when laying out the mechanical components of a cable harness. Not to be forgotten is the shielding, which is either included in the cable or must be fixed around the cable harness in the form of additional shielding braids or metal tubes. With regard to safety, the HVIL (hazardous voltage interlock loop) merits particular mention. The HVIL safely switches off the HV system if, for example, a plug is pulled or a casing is opened. Signal cables, switches or sensors can form part of the system. Image 3: example of shielded HV cable (LEONI Hivocar ) The most essential components of HV connection technology are, of course, the cables and the connectors. Their characteristics and the associated context will be the main object of the next section.

4 Image 4: example of HV connector (LEONI prototype) 3. Demands placed on the HV cable harness The HV cable harness is required, in particular, to carry high currents (e.g. 50A - 350A) at high voltages (e.g. 110V - 650V). The large range mentioned results from the breadth of applications, from the small "mild" hybrid to the super sport scar with purely electric drive. In terms of cables, this requirement does not constitute a particular difficulty as the construction of cables in industrial applications or rolling stock was adapted to automotive specifications and tested accordingly. This task has been fulfilled by many reputable cable producers, and LEONI offers these products under the brand name of Hivocar. There is, however, further scope for the application-specific optimisation of cables. If slightly less dielectric strength is required, this means thinner insulation, greater flexibility and reduced costs. A slightly less dense braided shield around the conductor also leads to cost savings, at the expense, of course, of the shielding effect. Although these optimisations are beneficial, they increase the number of possible cable products, whose further parameters concern for instance their crosssection, insulation material and strand structure. Ultimately, therefore, a customer-specific optimised cable product is probably inevitable. However, such an optimised cable might not fit into a connector that is designed for specific outer diameters. This is an example of how the requirements placed on an electric system and its cables and connectors are correlated. The high voltage leads to large-construction connectors, as it is necessary to adhere to the creepage and clearance distances. The cable cross-section and the contact system used in the connector, in particular, can be deduced from the current it is required to carry. These correlations, too, are not so easy, since warming as an essential criterion of dimensioning depends, for example, on the ambient temperature and the way in which the cable has been laid. Since cables with typically large copper cross-sections are very inert thermally, even relatively long-lasting current pulses lead to no more

5 than relatively small increases in temperature. Such a current pulse can be generated in an electric vehicle through, in particular, strong acceleration or braking and its subsequent energy recovery. For the effect of the thermally inert cables to be taken advantage of, however, the current scenario resulting from drive cycles must be very familiar. The simulation of conductor and contact heating can support optimum dimensioning. Too small a construction is critical in safety terms and is to be avoided. A construction that is too big results in additional costs, large inflexible cable components are difficult to integrate and produce additional weight. It is possible that the low level of practical experience at present (for example, on the European market) is prompting greater dimensions so as to avoid a safety risk. The shielding of the cable system is mainly required in order to avoid the harmonics in motor current with their steep leading edges radiating to the outside. Since electromagnetic radiation is ultimately a systems effect, it is difficult to fix the attenuation of shielding and transfer impedance, and in shielding, too, developers have tended to be on the safe side. An important question, which is the subject of some controversy, is whether the shield as an individual shield is to be designed like a coaxial cable or as complete shield around all the conductors in a connection. Both designs have their benefits and drawbacks, which influence the choice of cable and connection system. The requisite temperature resistance can vary greatly. In hybrid vehicles with exhaust and combustion engine, HV cables operate in temperatures of up to 150 C; this in conjunction with the generation of heat through the current means that silicon cables are often required. These cables, which are continuously subjected to temperatures of up to around 200 C, don't just have benefits: sealing moulds are difficult to realise, the cost of these cables is relatively high. Particularly in the case of an adaptation development of "normal" vehicles with a combustion engine in hybrid vehicles, the available installation space proves to be very limited. Thick shielded multi-conductor cables are very stiff even if all tricks using flexible insulation materials and finely twisted conductors are used during cable construction. High forces can emerge during the assembly and laying of cable harnesses, particularly if the available installation space necessitates a narrow bend radius. The connector, too, must be constructed with the installation space in mind. Construction size, cable outlet direction and insertion force are the criteria here. While in the 12V cable harness sealed connections are used only in a damp location, in particular, the HV cable harness must always be perfectly sealed (e.g. IP6K9K), as moisture in the HV components would have dangerous consequences. The sealing requirements constitute a challenge, as a design of the connector that is as compact as possible must meet requirements at high temperatures and withstand rapid temperature changes. A further requirement relates to the insertion and pulling of the HV connector. First of all, there is the question whether a detachable connection is to be used at all. The alternative of a threaded connection may have benefits, depending on the assembly procedure. If a connector is to be selected, care must be taken to avoid it becoming detached under tension, as this might lead to the creation of extreme electric arcs. In a connector, what is called a 'first-tomake-last-to-break' contact together with the HVIL (hazardous voltage interlock loop) system is used for detection, so that the high voltage is switched off by means of a relay prior to the separation of the HV contacts. Normally, the HV plugs must be equipped with a mechanical insertion and extraction aid in order to be able to overcome the forces resulting from connecting large contacts and sealing elements

6 and to achieve an effective locking during assembly. Furthermore, the connections systems have locking devices, partly coupled with the HVIL system, which are designed to prevent unintentional breaking of connections. 4. Challenges during development Some of the technical considerations that need to be taken into account in the development of HV connection technology were discussed in the previous section. In the current stage of the development of electric vehicles the greatest challenge results from the low degree of standardisation and the low availability of components. Although quite a bit of development work has taken place over the past few years and cables and connector systems have been certified and optimised, genuine availability means that productionline tools are available. Due to their high cost, they are produced as late as possible during the project, any late change can result in high costs. In the cable segment, availability is good, numerous varieties of cables are available from stock or can be produced in less than 12 weeks. LEONI, for example, has a complete product catalogue of 105 C cables up to 200 C cables under the brand name Hivocar. The fact that unit numbers are quite low in the initial phase constitutes a further problem. Amortising development and tool costs on only one or a small number of projects would lead to a sharp rise in costs; consequently, costing based on high unit numbers means a corresponding commercial risk. 5. Requirements placed on testing For a conventional cable harness with up to more than one thousand individual conductors, the correct circuitry is tested by sending testing currents across all fixed connections in a way that is controlled by a computer. 100% accuracy is very important for the cable harness, as even single errors are practically impossible to repair on the carmaker's line and result in ejection of the vehicle at high cost. With regard to the requirements placed on HV cable harnesses, the high-voltage test, in particular, is new. Such tests are carried out on car cables, for example on coaxial aerial cables, in order to identify process faults, particularly during shield contacting. For HV cable harnesses, a suitable HV test can also be used to check the correct processing of the shielded cables. Furthermore, adherence to creepage and clearance distances is checked, which may be closer than required for example as a result of the incorrect laying of a single wire.

7 Image 5: HV testing equipment by adaptronic Series testing does not replace qualification testing, where e.g. the dielectric strength is also tested after ageing. In production, every test process constitutes a substantial cost item; for this reason, it is essential that the unit under test can be contacted rapidly and reliably. As for a "normal" cable harness check, special test adapters are used, which hold and contact the connector housing without noticeable insertion or separation force. Additional functions of connectors and cable harnesses can be tested with sensors. Such would include the position of fastening points, encodings on the connector, the correct fitting of contacts and the seal of the casing.

8 Image 6: test modules for use in the modular HV tester by adaptronic A particular feature of HV tests is the safety requirement for the actual testers themselves. The HV power supplies used produce life-threatening voltages and currents. The testing equipment itself must communicate with the connected production computers by means of standardised interfaces in order to ensure that the correct test parameters are used for all versions and that a faulty cable harness is safely ejected from production. The modular standardised construction of the adaptronic testing systems allows the separate development of test adapter and high-voltage tester. This means that the user can ideally contribute, if required, their specific requirements through the in-house construction of their own test medium. Alternatively, a testing system can be designed to customer requirements complete with test bench and safety installations. In all cases, adaptronic's service and product management are ready to support the start-up of the entire testing system. The series testing is designed to ensure correct assembly of the cable harness and to identify existing damage or weaknesses that could result in malfunctions in later operation. This means that fault-free material must be fitted correctly; the mechanical, electric and geometric properties must be tested, observed and documented.

9 Wiring tester Adaption Test Item Image 7: Illustration of an HV test One example of testing standards for the final inspection of HV wiring in other sectors comes from the aviation and aerospace industry, where on-board voltages of 28 VDC and 115 VAC are used. As test parameters in accordance with EN2283, a maximum of 3 amperes is required for connection and short-circuit tests, an insulation test with 500 VDC with a minimum resistance of 20 MOhms and a voltage stability test with a maximum of 1,300 VDC for 1 second. When applying EN50343 to a typical HV wiring system for a hybrid vehicle, an on-board voltage of 400 VDC works out to test voltage of e.g. 2,550 VAC. LEONI has been guided by the standard EN60664 in a recent hybrid project in the commercial vehicle segment and stipulated a test voltage of 3,500 VDC / 2,500 VAC with a maximum nominal voltage of 750 VDC in the test item. High-voltage tests for insulation and dielectric strength require a correspondingly costly structure and maintenance of the testing systems so as to ensure their lasting safety and accuracy. The relevant components of the testing equipment include, in particular, generators, test point cards and the wiring. To function reliably, the design of the adapters must be observed particularly with respect to cleanliness and to the ambient conditions. The maintenance intervals selected must be sufficiently short and requirements with regard to cleanliness of the workplace (dust, flakes, strand cuttins ) need to be included in the planning. When generating the test programmes and parameters, the physical conditions of the HV system, such as rise times, residence times, discharge times and parasitic currents, need to be taken into consideration. High-voltage tests with corresponding voltage ramps (rise time, residence time, fall time) substantially extend the length of the overall test and thus the test costs; consequently, it is important to agree meaningful figures with the respective customer. For testing voltages and currents above 1,500 VDC/ 2 ma or 1,050 VAC/3mA, the equipment is no longer intrinsically safe even if operated by trained specialist personnel (EN 61010, EN 50191) and

10 requires extensive protection against accidental contact (protective hood, two-handed operation, light curtain). Operating personnel must be trained and instructed in the handling of high-voltage test equipment and occupational safety. 6. Future trends and developments Just as the whole market of electric and hybrid vehicles currently appears uncertain, so does the further development of the HV cable harnesses that will be required to this end. Consequently, we will here venture on a few forecasts that are without doubt subject to some uncertainty: At the moment, a few solutions using threaded connections can be found on the market. Attempts will be made to dispense with them, as they are not considered to be particularly process-secure on the production line and make assembly considerably more difficult. On the other hand, a threaded connection can be developed relatively quickly and flexibly to suit specific customer requirements and can transfer the highest currents. Threaded connections will continue to play an important role for some of the required connections in the future. The trend towards the standardisation of cables and connectors will continue. Nevertheless, the large number of different electric drive concepts will necessitate many customer-specific developments, so that the HV cable harness and its components will always require a particularly intensive development process. For example, some of the requirements currently placed on conductor cross-sections or the ambient temperature will be lowered as experience grows and optimisation advances. This effect will make an important contribution towards the attainment of future cost targets. In order to realise this optimisation, the connection technology will be set early on, and the development of the connection technology will take place together with the other HV components within the system. EMC requirements will be reduced to the bare minimum based on readings taken in actual vehicles. The electric components will make an essential contribution towards improvement, so that cables and connectors will, in future, require less expense on shielding. 7. Summary The role of connection technology in the development of electric and hybrid vehicles should not be underestimated. It is only through the early and system-oriented collaboration between the component suppliers, OEMs, cable harness suppliers and testing specialists that an optimised cable harness can be developed in good time with regard to the quality gates set by the project in question.

11 The development processes of the HV cable harness should be adapted versus the traditional approach, as interplays that have not been sufficiently taken into account in the system can be time-consuming and expensive. The series testing of HV cable harnesses is particularly important as faults in the cable harness can have dangerous consequences on account of the high voltages and high available currents. A 100% test of all safety-relevant features such as insulation, dielectric strength and sealing is required. There are suppliers on the market who are able to provide and maintain mature testing technology. The standards and experience gained in other sectors constitute useful aids in the installation of new test stations. Literature [1] Hybrid-, Batterie- und Brennstoffzellen-Elektrofahrzeuge, Dietrich Naunin and 18 co-authors, Expert Verlag [2] ELKS 2008, contributions to 1. ELKS Symposium, October 08th and 09th, 2008, TU Braunschweig About the LEONI group The LEONI Group, which is market-listed in the German MDAX index, employs about 56,000 people in 34 countries and, with 87 subsidiaries, generated consolidated sales of 2.96 billion in Contacts for journalists LEONI Bordnetz-Systeme GmbH Dr. Helmut Kalb Dr. Michael Frommberger Stieberstraße 5 Flugplatzstraße Roth, Germany Kitzingen, Germany Phone: Phone: helmut.kalb@leoni.com michael.frommberger@leoni.com