Peak Voltage Stress in Inverter-Fed Machines and its Mitigation Measures

Greenhouse gas (GHG) levels are at their highest in 2 million years, and emissions are continuing to grow. The major source of GHG emissions is the combustion of gasoline for transportation. Today, the transport industry is the biggest GHG emission sector in the UK, accounting for 28% of total emissions. Out of which, road transport accounts for 87%. Therefore, governments are making strides in increasing the use of electric vehicles (EVs).
In this context, EVs are the future strides. The translation to a sustainable electrified powertrain requires an electric traction system to offer better economy, extended range, fast charging, and autonomous driving. These capabilities are enabled using wide bandgap (WBG) devices in the electrified traction system, which are capable of operating at a higher switching frequency and fast switching speed.
However, a high voltage system with a higher switching frequency and fast switching speed cause enormous problems such as excessing voltage stress within winding causing premature failure, high-frequency (HF) common-mode and bearing current causing electromagnetic interference (EMI). The voltage stress across the insulation of the winding may cause premature failure resulting in unexpected downtime. As a result, the WBG based traction systems would suffer substantial economic and safety consequences. Therefore, a study is required to enhance the scientific understanding of the HF voltage excitation on the machine insulation and to suggest mitigation measures.
An HF model of the stator winding of the traction motor is proposed for the prediction of the voltage stress. The distinctive feature of the model lies in its explicit representation of the mutual coupling between the turns of the single or double layer winding in time-domain. The comprehensive modelling approach proclaims that the voltage distribution is a result of the anti-resonance phenomenon which can be characterised by the well-known voltage oscillations at the motor terminals, besides the proclaimed voltage oscillations at the neutral point. It is the latter, which engenders peak voltage stress and prevails under simultaneous excitation of the phase. The existing voltage mitigation measures are ineffectual in mitigating the newly found voltage stress. Therefore, this analysis proposes a mitigation measure, which is experimentally validated on the automotive grade 60kW motor employed in Toyota Prius hybrid vehicle.