Power cables in battery electric vehicles used in underground mining : Analysis of electromagnetic dynamics in high-power cables and development of application-specific design strategies for reduction of EMI

Abstract: The electrification of the automotive industry is growing in popularity, considering the environmental impacts of the conventional diesel-powered automobile. However, from the electromagnetic compatibility (EMC) viewpoint, it is observed that the use of variable-frequency drives (VFD) and relatively high-power cables to propel electrical motors has led to a considerable rise in electromagnetic interference (EMI) within and outside the machine. EMI could come from the fast switching of the inverter, electromagnetic radiation from the high- power cables, common mode and differential mode currents as well as parasitic coupling of some of the components in the machine. The signals transmitted by near-by communication cables can be distorted as a result or, in the worst case, interference with the controller area network (CAN) bus of the machine.This thesis work aims to investigate different ways of mitigating EMI in battery-electric mine trucks used for underground mining. Having a three-phase system with power cables consisting of three conductors per phase per traction motor connecting the variable frequency drives (VFD) to the motors, the electromagnetic emission is significantly high because of the current level transmitted by the cables. This is in addition to the fast switching frequency of the inverter as the load/torque varies. Cable models are made using a finite element method (FEM) simulation tool, Ansys electronics desktop. The models are used to study how the cable shielding and material, arrangements and phase orientation can impact the radiated EMI within the machine. Experimental measurements are made in order to validate the models. Parasitic coupling between cables and components such as shield and protective earth conductors is considered to estimate the emitted magnetic fields. Results from one of the simulations show that a hybrid shield consisting of 50% Mu metal and 50% copper will give shield effectiveness up to 65% with reference to when an only copper shield is used. Mu-metal is the next most recommended shield because of the system low fundamental frequency. Steel shield gives as high as 20% better shielding than copper.Further simulations present the trefoil placement of the cable bundles, with the center bundle positioned upside-down compared to the two outer bundles, as a better option compared to when the cables within bundles are placed in a linear configuration, although the difference in the induces EMI is only approximately 5%.The major conditions for the above stated preferred arrangements include that bundles of cables within each bundle are tightly held together and the phase orientation is such that a cable is placed farthest away from the cable with the corresponding phase in the neighbouring bundle. Study on the effect of the connection of cable shield shows that common mode current is increased with the shield connected to ground through the body of the machine. This will give a considerable rise to both conducted and radiated EMI, but could help to reduce the risk of current flowing in uncontrolled parts of the machine.