Mitigation of Common Mode Voltage Issues in Electric Vehicle Drive Systems by Means of an Alternative AC-Decoupling Power Converter Topology

Abstract

Electric vehicles (EV) are gaining popularity due to current environmental concerns. The electric drive, which is constituted by a power converter and an electric machine, is one of the main elements of the EV. Such machines suffer from common mode voltage (CMV) effects. The CMV introduces leakage currents through the bearings, leading to premature failures and reducing the propulsion system life cycles. As future EV power converters will rely on wide bandgap semiconductors with high switching frequency operation, CMV problems will become more prevalent, making the research on CMV mitigation strategies more relevant. A variety of CMV reduction methods can be found in the scientific literature, such as the inclusion of dedicated filters and the implementation of specific modulation techniques. However, alternative power converter topologies can also be introduced for CMV mitigation. The majority of such power converters for CMV mitigation are single-phase topologies intended for photovoltaic applications; thus, solutions in the form of three-phase topologies that could be applied to EVs are very limited. Considering all these, this paper proposes alternative three-phase topologies that could be exploited in EV applications. Their performance is compared with other existing proposals, providing a clear picture of the available alternatives, emphasizing their merits and drawbacks. From this comprehensive study, the benefits of a novel AC-decoupling topology is demonstrated. Moreover, an adequate modulation technique is also investigated in order to exploit the benefits of this topology while considering a trade-off between CMV mitigation, efficiency, and total harmonic distortion (THD). In order to extend the results of the study close to the real application, the performance of the proposed AC-decoupling topology is simulated using a complete and accurate EV model (including vehicle dynamics and a detailed propulsion system model) by means of state-of-the-art digital real-time simulation.Energies 2019, 12, 3349 2 of 27 the commutations of the power converter devices generate significant high-frequency CMV variations (Figure 1a). As a result, these voltage variations can produce not only high electromagnetic interferences (EMI) [6][7][8], but also shaft voltages (Figure 1b) in the electric machine [9][10][11], producing new capacitive paths [10]. These paths result in high-frequency leakage currents (Figure 1c) circulating through the motor bearings [12][13][14]. The most relevant capacitive paths are depicted in Figure 2, where the most significant bearing currents produced by CMV are the capacitive currents, electrostatic discharge currents, circulating currents, and rotor ground currents [9,13]. As bearings are critical components for the electric machine [10], a number of industrial companies are currently analyzing the degradation problems generated by such bearing currents [15][16][17].Regarding this issue, it is important to remark that although the majority of current industrial solutions rely on silicon-bas...

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