Electric mobility (e-mobility) is undoubtedly one of the most interesting innovations popped up in the last 15 years. The fundamental motivation that is driving the sector consists of a substantial reduction of the environmental emissions: the traditional OEM manufacturers, initially against such development, are now competing to be front-runners.

There are several ongoing R&D initiatives that typically refer to the following strands :

• development of vehicles with increasingly light efficient batteries and, novelties of recent years, available for exchanging energy with electrical grids;
• setting up of a charging infrastructure characterized by suitable large presence on the territory, ease access (smart cards) and short charging time;
• improving safety of critical components installed on board the vehicles, especially the ones most exposed to electrical and fire risks, and again batteries are usually taken as examples.

But batteries, despite being the core element of e-mobilty technologies, are not the only new electric components of electric cars.

In this regard many manufacturers are developing electrical protection equipment which sometimes exploit well-known technologies, such as switches or direct current contactors, while in other cases they use more innovative solutions, among which the so-called hybrid switches. Especially the new components, that in the electric vehicles can make the difference, require at the current stage new testing capabilities, on one side different from those of the manufacturers because of the need of independent verifications and, on the other side, different from those of the traditional testing laboratories because of specific technical requirements and performances.

CESI group, in its Kema Labs, thanks to its historical presence in Germany (Berlin), has been at the side of the main manufacturers of these components since the first requests for R&D tests. During those initial tests, technical limitations of the test circuits emerged, to the extent that they were not able to support further development of the components under evaluation.

Purpose of the project

The R&D activities of the manufacturers have highlighted a fundamental technical requirement for the DC components of e-vehicles, considering that those components will be used not only in cars, but also for trucks and public transport: circuits crossed by short currents in automotive applications show a very low inductance, sometimes equal to or less than 10 μH, which must be simulated by the test benches to properly test devices such as fuses, switches and contactors. The usual direct current (DC) test benches in Kema Labs have a circuit inductivity of more than 10 μH, which leads to overloading the interrupting capacity of the tested devices.

For that reason, OEM manufacturers started using their own test benches, but they are generally equipped with lower ratings than necessary in terms of voltage and current (below 800 VDC and 20 kA), due to the complexity to deal with high power ratings. Industrial solutions in this sector have been quickly designed to scale up to 1500 VDC and short currents up to 30-35 kA. The availability of test benches according to this development framework is practically non-existent. These are the basic concepts behind the construction of a new test bench with the characteristics above mentioned, with the idea to exploit all the existing experiences, skills and capabilities, but also opening the door to new unique testing opportunities.

Moreover, as soon as the project was started, it appeared clear that this new test bench would have allowed to verify also other components in niche markets such as aerospace.

Scientific / technological uncertainties to overcome

Once the high-level characteristics for the new test circuit had been identified, technical work teams were organized, aimed at the engineering study of the solution.

These teams were formed by Kema Labs test engineers supported also by potential component suppliers and manufacturers. The most critical issue that emerged from these studies consisted of the dangerousness of the test circuit, which requires energy storage solutions, either through batteries or capacitor banks, in order to deliver the high required capacity (2.0 F).

Both potential solutions present critical aspects, from technical, safety and, last but not least, economic point of view. Using lithium batteries appeared the most economical choice, but safety implications (high risk of fire), maintenance and end-of-life (disposal of the same), led the technical team to evaluate this solution as very risky and therefore not feasible.

The solution with capacitor banks looked immediately safer but showed decidedly critical aspects both from an economic point of view and, above all, in terms of availability of components and ease of installation, considering the high number of capacitors to be used to reach the required capacity. Being an innovative project, unique on the market, another challenge has been to identify qualified suppliers, able to provide innovative reliable solutions, never realized before.

Innovative contents of the project

The new system consists of 14 control cabinets with bidirectional DC power supply for charging and discharging a capacitor bank up to a maximum of 1500 VDC. The capacitor bank  has a capacity of 2 F to provide the necessary energy for a “shot”. The resulting impulse generator also includes resistors and safety systems to safely and reliable set the current up to 35 kA.

What makes this system unique is its ability to deliver a high current (up to 35 kA) at a nearly constant voltage (up to 1500 VDC) for several milliseconds with an inductance < 10 µH and up to 20 µH. Furthermore, the device under test can be mounted in an explosion-proof climatic chamber to ensure testing under severe environmental conditions (from -50°C to 150°C). The ability to perform destructive tests on switching devices directly under temperature conditions is unique in the world.

This new test system allows to switch the full current “on” and “off” several times without completely discharging the capacitor bank; in case the test object fails the circuit absorbs the entire energy of the capacitor bank without risks.

For further information please write to alessandro.bertani@kema.com

CESI is a world-leading technical consulting and engineering company in the field of technology and innovation for the electric power sector.

In particular, through its Division KEMA Labs, CESI is the world leader for the independent Testing, Inspections and Certification activities in the electricity industry. With a legacy of more than 60 years of experience, CESI operates in 40 countries around the world and supports its global clients in meeting the energy transition challenges. CESI also provides civil and environmental engineering services.

The company’s key global clients include major utilities, Transmission System Operators (TSOs), Distribution System Operators (DSOs), power generation companies (GenCos), system integrators, financial investors and global electromechanical and electronic manufacturers, as well as governments and regulatory authorities. In addition, CESI works in close cooperation with international financial institutions such as, among others, the World Bank Group, the European Bank for Reconstruction and Development, the European Investment Bank, the Inter-American Development Bank, the Asian Development Bank.

CESI is a fully independent joint-stock company headquartered in Milan and with facilities in Arnhem, Berlin, Prague, Mannheim, Dubai, Rio de Janeiro, Santiago de Chile, Knoxville (USA) and Chalfont (USA).