RIFEL - Ripple and Electromagnetic Fields in Electric Vehicles

Report, 2018

The electrical system in an electrified vehicle consists of high voltage (HV) components interacting in a complex way. The switching interaction in the power electronics results in ripple causing electromagnetic fields, disturbing other electronics and degradation of components. An overview of this can first be obtained when a physical system is built which could lead to unintentional over- or under dimensioning of HV components. This lack of information within the electrical system can lead to late verifications in the project causing substantial cost if changes are needed.

This project aims at improving early evaluation of new concepts, create tools and build the necessary competence for a virtual system model that includes the key HV components: battery, electrical motor and power electronics, a simple load along with cable and connectors. This virtual model shall be able to simulate voltage and current ripple generated by the power electronics, initially in a frequency range up to 100 kHz. Results from the simulations shall be presented both in time and frequency domain as well as be expressed in RMS values for easier comparison to measured results.

Some of the more important findings are briefly summarised below;

For the high voltage battery, the electrical characteristics up to a frequency of roughly 1000 Hz was well determined using an impedance spectroscopy instrument at cell level and then multiplied by the numbers of cells.  However for finding the impedance behaviour for frequencies above 1000 Hz, the determination must be done on the battery pack level since bus bars and other component in the complete battery pack will be dominating in this frequency range.

From measurements of differential mode impedance in high voltage cables it is found that it is important that the mutual inductance between the centre conductor and shield is included in the model to describe cable impedance below 10 kHz properly.

The control of the inverter is very important for the overall behaviour and in this project SVM was used which has been shown to give the lowest current and voltage ripple of the traditional switching schemes. And for the machine model, the temperature variations must be taken into account since the machine parameters has been found to vary with ~20 % over the specified temperature range.

The system model is found to agree well with rig measurements well up to 1 MHz with regards to both currents and voltages at the DC and AC sides. Furthermore, measurements in a real car match those in the rig. For time domain simulations, it was decided to use Ansys Simplorer since it can handle the inverter and the electrical machine simulations very well and for frequency domain simulations, it was decided to use LTspice since it is freeware, has support for AC-sweeps, improved switching compared to other SPICE-simulators, and is easy to use.

Magnetic field simulations have been calculated and compared to measurements in the driveline rig at Chalmers. It was a good match across the investigated frequency range 10 Hz to 100 kHz.

In this project, only internally developed component models were considered. To expand the functionality of the system modelling tool, international interface standards such as the Functional Mockup Interface (FMI) need to be investigated. Consequently, it would be a good idea to include additional automotive OEMs as well as suppliers and software vendors in future research collaborations.