Abstract
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.
Highlights
THE more-electric-aircraft (MEA) brings many potential benefits in the design and carbon emissions of future civil aircraft for the aerospace industry [1]-[5]
As large back electromotive force (EMF) is required to handle when the machine speed runs above 14 krpm, a large negative d-axis current must be injected to make permanent magnet synchronous machine (PMSM) operate in the flex-weakening region, which results in a low power factor
The original modulation scheme fails to consider the common-mode voltage (CMV). This is critical for high-speed PMSM fed high-switching-frequency converter in the electric starter/generator (ESG) system since that value will bring in EMI issues for avionics of aircraft
Summary
THE more-electric-aircraft (MEA) brings many potential benefits in the design and carbon emissions of future civil aircraft for the aerospace industry [1]-[5]. Compared with two-level counterpart, 3L-NPC multilevel converter is preferable due to its mature topology with lower voltage stress on power devices and better output harmonics. When the engine speed accelerates to 20 krpm, the generation mode starts. In this mode, the engine cranks the PMSM that works as a generator and 3L-NPC converter operates as a rectifier to convert AC power into DC for supplying onboard loads. To achieve average zero NP current, the concept of nearest-three virtual space vector (NTV2) modulation scheme is firstly presented in [10], which is capable of balancing capacitor voltages in full range of modulation indices and power factors.
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