Performance improvement of three-level neutral-point-clamped electric-starter-generator system for more electric aircraft

The more-electric-aircraft concept is receiving more and more attention in the development of next-generation large, commercial aircraft due to its high reliability, low maintenance costs and good environmental impact. To satisfy these expectations, aerospace systems are undergoing a transition from pneumatic, hydraulic and mechanical power to electrical counterparts. With the help of cutting-edge active rectification technology, a cutting-edge aircraft electric starter-generator system tends to substitute the typical three-stage wound-field synchronous generator for avoiding mechanical failures, especially for high-speed operation. Since the three level neutral-point-clamped (3L-NPC) converter plays an essential role in the energy conversion path, this thesis primarily focuses on the following advanced pulse-width modulation (PWM) strategies that aim to enhance the reliability and efficiency of the entire system without any additional hardware. • A fast PWM implementation approach is studied to simplify dwell-time calculation and sector/subsector identification, which is significant for lowing the cumbersome computational effort of the space vector modulation technique. Thus, all proposed modulation strategies are established based on this method. • Because of the split DC-link capacitors, the 3L-NPC topology has an inherent neutral-point voltage imbalance problem. To address the disadvantage, a capacitor voltage balancing control method is developed based on the virtual space-vector concept which exhibits superior neutral-point voltage balance performance compared with other modulation strategies. • Given that voltage stress of the motor would be substantially increased when the common-mode voltage is imposed on machine terminals, and also its steep changes can induce damaging ground leakage, bearing currents and emission of common-mode noise, a modified virtual-space-vector modulation strategy is proposed, which can reduce the common-mode voltage in the drive systems. • Considering the wide-speed range operation of the electric starter-generator system, a hybrid modulation process is designed according to the operating points of the target systems, which can attain switching loss reduction in motoring mode and common-mode voltage suppression in generation mode. • In order to improve the three-level converter-fed high-speed aerospace drives’ output electromagnetic torque capability and reduce additional machine copper losses caused by the large flux-weakening current, an advanced overmodulation strategy is proposed, together with the analysis of achievable modulation index, and the incorporation of an effective neutral-point voltage balancing control method. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed approaches are proved by both simulations and experimental results