On-Line Fault Diagnosis and Fault-Tolerant Operation of Modular Multilevel Converters –A Comprehensive Review

Modular multilevel converters(MMCs)have been one of the most broadly used multilevel converter topologies in industrial applications,particularly in medium-voltage motor drives and high-voltage dc power conversion systems.However,due to the utilization of large amount of semiconductor devices,the reliability of MMCs becomes one of the severe challenges constraining their further development and applications.In this paper,common electrical faults of the MMC have been summarized and analyzed,including open-circuit switching faults,short-circuit switching faults,dc-bus short-circuit faults,and single line-to-ground faults on the ac side.A thorough and comprehensive review of the existing online fault diagnostic methods has been conducted.In addition,fault-tolerant operation strategies for such various fault scenarios in MMCs have been presented.All the fault diagnosis and fault-tolerant operation strategies are comparatively evaluated,which aims to provide a state-of-the-art reference on the MMC reliability for future research and industrial applications.

Toward More Electric Powertrains in Aircraft:Technical Challenges and Advancements

The main purpose of this article is to provide an instructive review of the technological challenges hindering the road toward more electric powertrains in aircraft.Hybrid,all-electric,and turboelectric powertrain architectures are discussed as possible fuel consumption and weight reduction solutions.Among these architectures,the short-term implementation of hybrid and all-electric architectures is limited,particularly for large-capacity aircraft due to the low energy/power density levels achievable by state-of-the-art electrical energy storage systems.Conversely,turboelectric architectures with advanced distributed propulsion and boundary layer ingestion are set to lead the efforts toward more electric powertrains.At the center of this transition,power converters and high-power density electric machines,i.e.,electric motors and generators,and their corresponding thermal management systems are analyzed as the key devices enabling the more electric powertrain.Moreover,to further increase the fuel efficiency and power density of the aircraft,the benefits and challenges of implementing higher voltage powertrains are described.Lastly,based on the findings collected in this article,the projected roadmap toward more electric aircraft powertrains is presented.Herein,the individual targets for each technology,i.e.,batteries,electric machines,and power converters,and how they translate to future aircraft prototypes are illustrated.

Si/SiC Hybrid 5-level Active NPC Inverter for Electric Aircraft Propulsion Drive Applications

Medium voltage DC(MVDC)system is considered as a promising technology to improve the efficiency and power density of electric aircraft propulsion(EAP)drives.To adapt to the MVDC voltage level and achieve high drive performance,a five-level active neutral point clamped(5L-ANPC)inverter consisting of three-level ANPC and flying capacitor circuits is investigated,which possesses higher voltage capability,lower output harmonics,as well as mitigated dv/dt and common-mode voltage.To fulfill the requirements of high-speed operation and pursue further enhanced efficiency and power density of the inverter for the next-generation EAP drives,Silicon Carbide(SiC)semiconductor devices are considered for implementing the 5L-ANPC inverter.However,the large commutation loops associated with certain switching states of the inverter lessen the benefits of configuring all the switches as SiC devices.As a result,a hybrid Si/SiC 5L-ANPC inverter is developed with a synchronous optimal pulse(SOP)width modulation strategy for controlling the switches in cell 2 and finite-control-set model predictive controller(FCS-MPC)for those in cell 3 of the inverter.Consequently,in the proposed topology,the SiC devices are merely used for the high-frequency switches in cell 3 and the rest of the low-frequency switches are configured with Si IGBTs.This Si/SiC hybrid ANPC inverter concurrently provides high efficiency and low implementation cost at high-speed operation mode.Simulation and experimental results are provided to verify the effectiveness of the proposed hybrid inverter.

Fast Online Diagnosis of Open-circuit Switching Faults in Flying Capacitor Multilevel Inverters

Flying capacitor multilevel(FCML)inverter is an attractive power converter topology which provides high-quality staircase output voltage waveforms by cascading flying capacitor cells.However,the large number of semiconductor devices utilized in the FCML inverters degrades the hardware reliability,which may constrain such converters from being applied in safety-critical applications.Targeting at open-circuit switching faults,a fast online fault diagnostic method for FCML inverters is presented.Conventional phase-shifted PWM(PSPWM),which can naturally balance the voltage across flying capacitors,is used as the modulation method in this work.Hence,to retain the simplicity feature of the PSPWM,the proposed diagnostic method is developed so that it does not require any voltage measurements of flying capacitors.Only the output AC voltage and current data along with the switching PWM signals from the microcontroller are needed to detect an open-circuit switching fault,and all such sensory data is typically available in the inverter,requiring no additional sensors or hardware for the implementation of this diagnostic method.The detection process takes 5% of the fundamental period of the inverter output signals to diagnose the faulty switch.Simulation and experimental results are presented to verify the effectiveness of the proposed diagnostic method.