An Advanced Three-Level Active Neutral-Point-Clamped Converter With Improved Fault-Tolerant Capabilities

A resilient fault-tolerant silicon carbide (SiC) three-level power converter topology is introduced based on the traditional active neutral-point-clamped converter. This novel converter topology incorporates a redundant leg to provide fault tolerance during switch open-circuit faults and short-circuit faults. Additionally, the topology is capable of maintaining full output voltage and maximum modulation index in the presence of switch open and short-circuit faults. Moreover, the redundant leg can be employed to share load current with other phase legs to balance thermal stress among semiconductor switches during normal operation. A 25-kW prototype of the novel topology was designed and constructed utilizing 1.2-kV SiC metal-oxide-semiconductor field-effect transistors. Experimental results confirm the anticipated theoretical capabilities of this new three-level converter topology

Investigation of Fault-Tolerant Capabilities in an Advanced Three-Level Active T-Type Converter

A novel fault-tolerant three-level power converter topology, named advanced three-level active T-Type (A3L-ATT) converter, is introduced to increase the reliability of multilevel power converters used in safety-critical applications. This new fault-tolerant multilevel power converter is derived from the conventional T-Type converter topology. The topology has significantly improved the fault-tolerant capability under any open circuit or certain short-circuit faults in the semiconductor devices. In addition, under healthy condition, the redundant phase leg can be utilized to share overload current with other main legs, which enhances the overload capability of the converter. The conduction losses in the original outer devices can be reduced by sharing the load current with the redundant leg. Moreover, unlike other existing fault-tolerant power converters in the literature, full output voltages can be always obtained in this proposed A3L-ATT converter during fault-tolerant operation. A 13.5-kW ATT-A3L converter prototype was developed and constructed using silicon carbide MOSFETs. Simulation and experimental results were obtained to substantiate the theoretical claims of this new fault-tolerant power converter

Hybrid Multilevel Converters with Internal Cascaded/Paralleled Structures for MV Electric Aircraft Applications

Using on-board medium voltage (MV) dc distribution system has been a megatrend for next-generation electric aircraft systems due to its ability to enable a significant system mass reduction. In addition, it makes electric propulsion more feasible using MV power electronic converters. To develop high-performance high-density MV power converters, the emerging silicon carbide (SiC) devices are more attractive than their silicon (Si) counterparts, since the fast switch frequency brought by the SiC can effectively reduce the volume and weight of the filter components and thus increase the converter power density. From the converter topology perspective, with the MV dc distribution, the state-of-the-art two-level converters are no longer suitable for next-generation electric aircraft system due to the excessive dv/dt and high voltage stress across the power devices.To address these issues while still maintaining cost-effectiveness, this work demonstrates a megawatt-scale MV seven-level (7-L) Si/SiC hybrid converter prototype implemented by active-neutral-point-clamped (ANPC) converter and H-bridges which is called ANPC-H converter in this work, and a MV five-level (5-L) Si/SiC hybrid ANPC converter prototype, which are hybrid multilevel converters with internal cascaded and paralleled structures, respectively. Using multilevel circuit topology, the voltage stress across the devices and converter output voltage dv/dt are reduced. The tradeoff between the system cost and efficiency was addressed by the adoption of the Si/SiC hybrid configuration with optimized modulation strategies. Comprehensive design and evaluation of the full-scale prototypes are elaborated, including the low-inductance busbar designs, power converter architecture optimization and system integration. To control the 7-L Si/SiC hybrid ANPC-H converter prototype, a low computational burden space-vector-modulation (SVM) with common-mode voltage reduction feature is proposed to fully exploit the benefits of 7-L Si/SiC hybrid ANPC-H converter. To further reduce the converter losses and simplify control algorithm, an active hybrid modulation is proposed in this work by applying low frequency modulation in Si cells and high frequency modulation in SiC cells, thus the control framework is simplified from the 7-L SVM to a three-level SVM. To control the 5-L Si/SiC hybrid ANPC converter prototype to overall loss minimization, the low frequency modulation and high frequency modulation are also adopted for Si cells and SiC cells respectively in 5-L Si/SiC hybrid ANPC converter prototype. Compared to the SVM-based hybrid modulation in 7-L ANPC-H converter, the hybrid modulation for 5-L hybrid ANPC adopts a simpler carrier-phase-shifted pulse width modulation for its inner-paralleled high frequency SiC cells, which extensively suppresses harmonics caused by high frequency switching. With the proposed modulation strategies, extensive simulation and experimental results are provided to evaluate the performance of each power stage and the full converter assembly in both the steady-state operation and variable frequency operations of the demonstrated hybrid converters

The seven-level flying capacitor based ANPC converter for grid intergration of utility-scale PV systems

The installed capacity of grid connected PV plants has increased significantly over the past few years and is expected to continue its growth over the next decade due to the continuous demand for renewable energy and distributed generation systems. This paper presents a seven-level flying capacitor (FC) based active neutral point clamped (ANPC) converter for the connection of utility-scale PV system to the electricity grid. The multilevel voltage output of the topology provides high quality waveforms while maintaining the operational characteristics of NPC based converters for PV systems. The converter topology together with a method to regulate the FC voltages to their reference values and an optimal third harmonic injection for utilization of the DC-link voltage are presented. Simulation results for the operation of the grid connected converter under steady state and transient operation are provided in order to demonstrate the operation and performance of the topology in grid connected applications. © 2012 IEEE

Advanced Silicon Carbide Based Fault-Tolerant Multilevel Converters

The number of safety-critical loads in electric power areas have been increasing drastically in the last two decades. These loads include the emerging more-electric aircraft (MEA), uninterruptible power supplies (UPS), high-power medical instruments, electric and hybrid electric vehicles (EV/HEV) and ships for military use, electric space rovers for space exploration and the like. This dissertation introduces two novel fault-tolerant three-level power converter topologies, named advanced three-level active neutral point clamped converter (A3L-ANPC) and advanced three-level active T-Type (A3L-ATT) converter. The goal of these converters is to increase the reliability of multilevel power converters used in safety-critical applications.These new fault-tolerant multilevel power converters are derived from the conventional ANPC and T-Type converter topologies. The topologies has significantly improved the fault-tolerant capability under any open circuit or certain short-circuit faults in the power semiconductor devices. In addition, under healthy conditions, the redundant phase leg can be utilized to share overload current with other main legs, which enhances the overload capability of the converter. The conduction losses in the power devices can be reduced by sharing the load current with the redundant leg. Moreover, unlike other existing fault-tolerant power converters in the literature, full output voltages can be always obtained during fault-tolerant operation. Experimental prototypes of both the A3L-ANPC and A3L-ATT converters were built based on Silicon Carbide (SiC) MOSFETs. Experimental results confirmed the anticipated performance of the novel three-level converter topologies.SiC MOSFET technology is at the forefront of significant advances in electric power conversion. SiC MOSFETs switch significantly faster than the conventional Silicon counterparts resulting in power converters with higher efficiency and increased switching frequencies. Low switching losses are one of the key characteristics of SiC technology. In this dissertation, hard and soft switching losses of a high power SiC MOSFET module are measured and characterized at different voltage and current operating points to determine the maximum operating frequency of the module. The purpose of characterizing the SiC MOSFET module is to determine the feasibility of very high frequency (200kHz-1MHz) power conversion which may not be possible to be implemented in the conventional Silicon based high power conversion. The results show that higher switching frequencies are achievable with soft switching techniques in high power converters

Novel modulator for the hybrid two-cell flying-capacitor based ANPC converter

Among the multilevel converters present in the industry, the hybrid flying-capacitor based active-neutral-point-clamped converter is very recent. It presents good features such as high quality output voltage, high modularity and easy extension to achieve a high number of levels with reduced number of power devices. This paper introduces a simple modulator for the single-phase two-cell hybrid flying-capacitor based active-neutral-point-clamped converter. The modulation technique is based on the determination of a switching sequence formed by two switching states which generate the two nearest voltage levels to the reference phase voltage. Some extra calculations are added to the modulation method to control the dc-link capacitors and the floating capacitor voltages. The computational cost of the modulation technique is low only including simple comparisons and mathematical expressions. Simulation results show the high quality output voltages and currents including the dc voltage control