High-voltage test stations and component qualification for electric and hybrid vehicles

Continuous monitoring of the insulation level (from megohms to gigohms) increases test efficiency and product quality

The manufacturers of electric or hybrid vehicles have high requirements for both the electrical insulation level of the vehicle as a whole and that of the high-voltage (HV) components. This, in turn, presents suppliers with major challenges in setting up and operating HV test stations and in HV component qualification. When delivered, values up to the giga-ohm range are required in order to be able to compensate ageing effects and wear. This requires special measurement technology, which ideally provides exact measurement results during the entire testing process. Bender GmbH & Co. KG in Grünberg offers the optimal solution for such applications with the insulation monitoring devices ISOMETER® isoHR685W or isoHR1685.

Electric or hybrid vehicles are becoming more and more common on the roads. This comes along with the constant improvement of batteries and electric drives by the manufacturers, on the one hand due to the high requirements such as the reduction of the CO2 emissions and on the other hand due to the growing competition. However, the focus is not only on optimising the main components of an electric vehicle, but also on regularly adapting and improving the individual HV components. The permanently changing demands are passed on directly from the vehicle manufacturers to the supplier industry.

Whereas in the past, the on-board networks in the automotive industry with DC 12 V, 24 V or 48 V were relatively uncritical for all parties involved, so-called HV on-board networks in electric vehicles impose completely different conditions on materials, manufacturing processes and test procedures. Depending on the vehicle type and engine power, on-board voltages of up to DC 800 V are used, which places enormous demands on the HV components. In addition, the HV components should mostly be able to be used maintenance-free over a service life (EOL) of 15 years.

Standards and regulations for more safety

The German Association of the Automotive Industry (VDA) sees it as its task to make mobility as a whole safer, more efficient, more comfortable and more environmentally friendly. In this context, the association provides manufacturers and suppliers with a variety of publications, guidelines for action and standards. For example, there are already various international standards such as the International Organisation for Standardisation (ISO) and the Global Technical Regulations (GTR) on the Electric Vehicle Safety (EVS). In addition, there are specific factory standards from manufacturers such as Mercedes MBN LV 123, BMW GS 95023 or Volkswagen VW 80303, which explicitly describe the electrical properties and safety requirements for HV components in electric vehicles. Similarly, it defines requirements for the test processes and necessary minimum limit values for the HV components of the suppliers.

Technical challenge

HV applications place very high demands on the material, the individual components and the overall system, which can only be achieved, among other things, through appropriate material selection, design decisions during construction, clearances of the air and creepage distances or additional insulation.

Just as much attention is paid to the insulation level of the application and here a simple electrical cause and effect relationship becomes a real challenge: The parallel connection of several components in the vehicle.

In electric vehicles, the individual components are connected in parallel with the HV battery. This means that as the number of HV components increases, the insulation resistance of the overall system decreases. Remember: The resistance of the overall system of a parallel circuit is always smaller than the smallest individual resistance and this also applies to the insulation resistances in electric vehicles.

The problem shows when looking at the schematic structure of an electric vehicle.

For the reasons mentioned above, vehicle manufacturers have a great interest in ensuring that the HV components from the suppliers each have a very high insulation resistance. This requirement is reflected in the factory standards of the respective vehicle manufacturers mentioned previously. Insulation resistances in the three-digit mega-ohm range are required for the tests within the scope of the service life qualifications of the manufactured HV components. This does not refer to the test result after production, but to the entire planned life cycle of 15 years.

In addition to the parallel connection of the individual components, there is also material wear and tear, moisture penetration, contamination and corrosion throughout the entire service life of an electric vehicle. Therefore, the vehicles must have much higher insulation values when delivered in order to function safely for a long time. Insulation values of up to 10 GΩ (gigohms) are therefore not uncommon. With regard to later maintenance work, it is of interest to know exactly the insulation value of the components or the entire vehicle as delivered, because only then can measured insulation values be correctly evaluated within the life cycle during inspections.

The manufacturers of HV components must therefore test how the products behave during development in order to be able to guarantee the required values over the entire service life. High-voltage tests must be carried out by trained personnel, and this very often results in the interruption of long-term tests. This costs time and money. With the insulation monitoring devices ISOMETER® of the types isoHR685 and isoHR1685, Bender now offers a solution with which these high-impedance insulation values can be monitored continuously during development.

The demand for very high insulation values and continuous monitoring of insulation values automatically leads to the fact that previously used test benches and laboratory equipment of the supplier industry as well as the test houses have to be modified or retrofitted and completely redesigned in terms of measurement technology.

Requirements for the test station

The general requirements for test stations are defined in the standard DIN EN 50191 (VDE 0104), among others. Operators as well as manufacturers of electrical test stations also have a high demand for personal protection. In addition to various measures relating to contact protection or the safety distances for test equipment, VDE 0100-410 "Protection against electric shock under fault conditions" applies to protection in the event of a fault. The following protective measures are considered:

High-voltage tests >1 kV, which are more common in electric mobility, require galvanic isolation of both the testing equipment and the test object, and allow a maximum leakage current of 3 mA. This refers to the maximum leakage current between the test set-up and the earth potential.

The test equipment manufacturer or operator must also use measurement technology that can detect and monitor insulation values in the giga-ohm range. A higher insulation value also means increased personal protection and thus offers additional safety over the entire vehicle service life.

New requirements for the testing process

Practical experience shows that DUTs are exposed to extreme conditions during the test setup, but the test sequence is often interrupted for various measurements - even in environmental and long-term tests. This leads, for example, to artificially created environmental conditions (temperature, humidity, pressure) being changed for a short time, which in turn can lead to undesirable side effects such as condensation. These side effects often falsify the test result. Such phenomena can only be prevented by continuous monitoring of both the DUT and the test setup.This kind of continuous monitoring is particularly helpful when life cycles of components or entire electric/hybrid vehicles need to be simulated.

Insulation Monitoring Devices (IMD) for gigohm applications

ISOMETER® insulation monitoring devices are used in accordance with IEC 61557-81) for monitoring unearthed power supplies (IT systems) and can be used universally in AC, 3(N)AC, AC/DC and DC systems. This means that even complex systems consisting of AC systems and DC-powered system parts (e.g. power converters, inverters, variable-speed drives) can be reliably monitored. Due to the measuring range up to 10 GΩ, a change in the insulation level trend (continuous monitoring) can be detected and signalled at an early stage. The ISOMETER®s isoHR685W or isoHR1685 monitoring devices meet all requirements and optimally support the design and test phases, because the use of an active measuring method means that HV components can also be monitored as an unconnected unit, i.e. in a de-energised state. This means that even development engineers without HV authorisation are able to test the insulation level during development with the ISOMETER® without any high voltage being present.

In complex systems, where sub-areas may be galvanically isolated, several ISOMETER®s must be used. All ISOMETER® devices within the system are networked via Modbus. Thus, the ISOMETER® is not only suitable for monitoring complex and critical industrial applications, but also for the previously described HV test benches in the field of eMobility with their extreme requirements.

1)    DIN EN 61557-8 VDE 0413-8:2015-12
Electrical safety in low-voltage systems up to AC 1000 V and DC 1500 V
Equipment for testing, measuring or monitoring protective measures

Part 8: Insulation monitoring devices for IT systems

Fig. 2: Schematic illustration of an HV test station with HV voltage

Fig. 3: Schematic illustration of an HV test station without HV voltage

2021-03-01 – BENDER, T. Püschel und M. Lehr


Präsentation Hochvoltprüfplätze Technical information 1.1 MB DE2021/05/2626.05.2021

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