Analysis of high-voltage DC relays at short-circuit currents in electric vehicles

Project: Research

Project participants

Switching the battery in an electric vehicle on and off is normally done via so-called HV-DC relays (HV = high voltage, DC = direct current). In case of a short circuit, caused by an accident or faulty insulation, an emergency shutdown must be initiated via a fuse. Due to the demand for longer ranges, more powerful batteries are necessary, which lead to a higher short-circuit current, so that current emergency shutdown concepts are questioned and new concepts have to be designed. This requires a detailed analysis of all components and especially the relays in case of a short circuit. When the current in a relay increases strongly, current flows in opposite directions at parts of the contact surfaces, which lead to a force that initiates an unintentional opening of the relay contacts: The so-called levitation. When the contacts open, an arc is created between them, which thermalizes and vaporizes the electrodes. This leads to a rapid increase in pressure in the relay until the relay housing is destroyed and must be prevented at all costs. The behavior of relays in precisely this extreme situation is to be investigated in cooperation with Panasonic Industrial Devices Europe GmbH (PIDE), Power Electronics R&D Center, based in Lüneburg. The focus is on the physical behavior and the prediction of the time-dependent pressure increase. To this end, the European Union is funding the project "Analysis of HV-DC relays in short-circuit currents in electric vehicles" in the European Regional Development Fund (ERDF) for three years from July 1, 2018. In this project, the behaviour of the relay is to be analysed in detail with multiphysical simulations, compared with measurements on the relays and a global model of the pressure increase derived from the findings as simple as possible. A complete simulation of the entire transient relay behavior is hardly possible, which is why it is divided into three phases: Before opening one contact, until the second contact opens, and then the opened relay including the arc plasmas. Each phase is simulated separately and the results of a previous phase are used as input parameters for the subsequent phase. The global model to be derived subsequently has the advantage that it can be solved relatively quickly and can be used to analyze higher-level problems, such as new shutdown concepts.