Design and development of exclusive modulation techniques for effective power regulation in dual two-level-inverter-fed open-ended winding traction motors

Abstract

An integral part of an EV propulsion system is the traction drive. The existing two-level inverter drives reportedly has several drawbacks. Multilevel inverter has been proposed as an improvement over the conventional inverter drive for EVs. Dual two-level inverter topology, fed from isolated DC sources (battery packs) can be a suitable preference for the multilevel inverter in EVs. However, in this topology, the isolated DC sources obtained from battery packs may have different charge or capacities due to their manufacturing tolerances and aging over the time. This will result in different voltage levels and state of charge (SOC) in the individual battery pack. In normal operation of the drive, power flow between isolated sources need to be regulated. In first work, a simple carrier-based modulation (CBM) technique for sharing and controlling the power flow between isolated DC sources in a dual two-level inverter has been proposed . The proposed technique is extensively studied for different steady state and dynamic conditions. The effectiveness of the proposed technique for EVs is analyzed for different driving profiles and results are provided for dynamic SOC balancing between the isolated battery sources. In the second work, an output current ripple analysis is provided for the dual two-level inverter for varied power sharing ratio. This work demonstrates that the ripple in motor phase current changes when the power sharing in dual two-level inverter is changed. Current error trajectory and current ripple envelope for different switching schemes are compared for equal and unequal sharing. Polar plot representations for peak to peak current ripple in linear modulation range are also illustrated for varied power sharing. The concepts were initially simulated using Matlab/Simulink and PLECS. A 5.7 kW open-end winding induction motor was used as the load for experimental studies. The dual inverter system was build using two Semikron’s stacks having half-bridge modules of SKM50GB12T4. For the speed control and PWM, a digital signal processor (TMS320F28069M) and a real-time simulator OPAL-RT (OP8665) were used.