An Enhanced Virtual Space Vector Modulation Scheme of Three-Level NPC Converters for More-Electric-Aircraft Applications

Multilevel converters have been used for aircraft electric starter/generator (ESG) systems due to their high power qualities. One of the desirable topologies is the three-level neutral-point-clamped converter. Our studied ESG system operates at a high speed during its generation mode, which results in high modulation index and puts some specific challenges on neutral-point voltage balance, especially under low power factor load conditions. Moreover, common-mode voltage (CMV) needs to be addressed properly as it leads to irreversible damage of motor shaft bearings, thereby degrading the efficiency and reliability of the entire system. Compared with the conventional nearest-three virtual space vector technique, the proposed modulation scheme employs three adjacent medium vectors to synthesize a new medium vector and two pairs of large vectors to compose new small vectors. This allows the presented modulation scheme to achieve balanced capacitor voltage and reduced CMV at the same time. In addition, the torque ripple of the proposed modulation strategy is thoroughly compared and analyzed. Meanwhile, in order to overcome the heavy computational burden, a fast calculation approach is adopted to simplify the algorithm. Simulation results obtained from Simulink/PLECS and experimental results obtained from a 45 kW, 32 kr/min aircraft ESG system verify the effectiveness of the proposed strategy in more-electric-aircraft applications.