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Three-phase Multi-level Inverter Topology Evaluation

Circuit topologies
Fig. 1. Bridge leg topologies in evaluation, (a)FC, (b)ANPC, (C) SMC
  To replace conventional non-electrical systems with electrical systems in the aircraft, the concept of More Electric Aircraft (MEA) was raised. Most of these systems are power drives, normally composed of an inverter, EMC filters, and a motor. Increasing power density is one of the most important challenges and goals in power electronics applications in transportation. Multilevel topologies are identified as promising approaches in motor drive systems owing to the decrease of switching frequency and the reduction of input and output filter size. However, the increased voltage level also requires more devices and floating capacitors. This paper presents the com-parison results based on a 1.1 kV/50 kVA three-phase multi-level inverter from the loss and volume point of view seeking to evaluate the performance of different topologies and voltage levels.
   Three topologies shown in Fig. 1 are selected. Flying capacitors (FC) are widely adopted in high voltage and medium voltage applications. Recently it is also proposed in lower PV applications and can achieve high efficiency due to the adoption of low voltage GaN devices. However, the power density is sacrificed with the increasing number of floating capacitors. Therefore, some hybrid topologies seven-level stacked multi-cell (SMC) and active neutral-point converters (ANPC) are also be compared. Evaluation starts from three-level to nine-level. Loss evaluation includes the semiconductor and floating capacitors. Both GaN devices and SiC devices are con-sidered to achieve the best performance. Volume evaluation includes the cooling system and floating capacitors. For the EMC filters, an initial comparison of noise spectrums of common-mode voltage and differential-mode current is simulated of each topology to avoid numerous filter designs and comparisons. All these compared parameters are related to the carrier fre-quency and modulation method of each topology. For a fair comparison, carrier frequencies un-der the same THD values are extracted as shown in Fig. 2. It can be observed that for FC, from three-level to five-level, the frequency decreases most. While for the other two hybrid topologies, the frequency decreases most from five-level to seven-level.
Carrier frequencies
Fig. 2. Carrier frequency extracted based on same THD under different topologies and voltage-level.

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