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

A review on power electronics technologies for power quality improvement

Nowadays, new challenges arise relating to the compensation of power quality problems, where the introduction of innovative solutions based on power electronics is of paramount importance. The evolution from conventional electrical power grids to smart grids requires the use of a large number of power electronics converters, indispensable for the integration of key technologies, such as renewable energies, electric mobility and energy storage systems, which adds importance to power quality issues. Addressing these topics, this paper presents an extensive review on power electronics technologies applied to power quality improvement, highlighting, and explaining the main phenomena associated with the occurrence of power quality problems in smart grids, their cause and effects for different activity sectors, and the main power electronics topologies for each technological solution. More specifically, the paper presents a review and classification of the main power quality problems and the respective context with the standards, a review of power quality problems related to the power production from renewables, the contextualization with solid-state transformers, electric mobility and electrical railway systems, a review of power electronics solutions to compensate the main power quality problems, as well as power electronics solutions to guarantee high levels of power quality. Relevant experimental results and exemplificative developed power electronics prototypes are also presented throughout the paper.This work has been supported by FCT-Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020. This work has been supported by the FCT Project DAIPESEV PTDC/EEI-EEE/30382/2017 and by the FCT Project newERA4GRIDs PTDC/EEIEEE/30283/2017

Design of SiC-Si Hybrid Interleaved 3-Phase 5-Level E-Type Back-to-Back Converter

In modern applications, such as variable frequency electric drives, aircraft propulsion, electric vehicles, and uninterruptible power supply units, high power-dense, and efficient AC-AC power converters are the key to reducing power losses, thus limiting the overall costs, and improving the system's reliability. Power electronic equipment can be enhanced thanks to the continuous evolution of conversion topologies and advancements in power semiconductor technology. The design and the optimization strategy of the AC-AC 5-Level converter, called Interleaved 3-Phase 5-Level E-Type Back-to-Back Converter (I3Φ5L BTB E-Type Converter), has been proposed in this paper. The converter is analyzed and experimentally characterized to prove the configuration's high efficiency and high-power density. An introduction to the characteristics of the I-3Φ5L BTB E-Type Converter is described, and afterward, the optimization methodology to design the multilevel converter is presented. The converter prototype is illustrated, which achieves a peak efficiency of 98.2% and a total weight of 6.18 kg using hybrid technology for power semiconductor

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

Dc Line-Interactive Uninterruptible Power Supply (UPS) with Load Leveling for Constant Power and Pulse Loads

Uninterruptable Power Supply (UPS) systems are usually considered as a backup power for electrical systems, providing emergency power when the main power source fails. UPS systems ensure an uninterruptible, reliable and high quality electrical power for systems with critical loads in which a continuous and reliable power supply is a vital requirement. A novel UPS system topology, DC line-interactive UPS, has been introduced. The new proposed UPS system is based on the DC concept where the power flow in the system has DC characteristic. The new DC UPS system has several advantageous with respect to the on-line 3-phase UPS which is extensively used in industry, such as lower size, cost and weight due to replacing the three-phase dual converter in the on-line UPS system with a single stage single phase DC/DC converter and thus higher efficiency is expected. The proposed system will also provide load leveling feature for the main AC/DC rectifier which has not been offered by conventional AC UPS systems. It applies load power smoothing to reduce the rating of the incoming AC line and consequently reduce the installation cost and time. Moreover, the new UPS technology improves the medical imaging system up-time, reliability, efficiency, and cost, and is applicable to several imaging modalities such as CT, MR and X-ray as well. A comprehensive investigation on different energy storage systems was conducted and couple of most promising Li-ion cell chemistries, LFP and NCA types, were chosen for further aggressive tests. A battery pack based on the LFP cells with monitoring system was developed to be used with the DC UPS testbed. The performance of the DC UPS has also been investigated. The mathematical models of the system are extracted while loaded with constant power load (CPL) and constant voltage load (CVL) during all four modes of operation. Transfer functions of required outputs versus inputs were extracted and their related stability region based on the Routh-Hurwitz stability criteria were found. The AC/DC rectifier was controlled independently due to the system configuration. Two different control techniques were proposed to control the DC/DC converter. A linear dual-loop control (DLC) scheme and a nonlinear robust control, a constant frequency sliding mode control (CFSMC) were investigated. The DLC performance was convincing, however the controller has a limited stability region due to the linearization process and negative incremental impedance characteristics of the CPL which challenges the stability of the system. A constant switching frequency SMC was also developed based on the DC UPS system and the performance of the system were presented during different operational modes. Transients during mode transfers were simulated and results were depicted. The controller performances met the control goals of the system. The voltage drop during mode transitions, was less than 2% of the rated output voltage. Finally, the experimental results were presented. The high current discharge tests on each selected Li-ion cell were performed and results presented. A testbed was developed to verify the DC UPS system concept. The test results were presented and verified the proposed concept

Fault detection in a three-phase inverter fed circuit: Enhancing the Tripping capability of a UPS circuit breaker using wave shape recognition algorithm

Uninterruptible power supplies (UPS) are electrical devices that protect sensitive loads from power line disturbances such as source side overcurrents caused by overvoltage and power surges. The critical load in a double conversion UPS system is supplied from an invert-er. When overcurrents occur on the load side of double conversion UPS systems, both the UPS system’s inverter and the critical load connected to it stand a high risk of damage. Load side overcurrents due to short circuits, ground faults and motor/transformer start-up are very damaging to power electronic components, electrical equipment and cable connections. There exists circuit breakers on the load side designed to trip when a huge overcurrent occurs, thereby clearing the fault. A circuit breaker is normally sized and installed based on the maxi-mum capacity of the host system and trips when a predetermined overcurrent is recorded within a specific period of time. The UPS system’s inverter has a pre-set current limit value to protect insulated-gate bipolar transistors (IGBTs) from damage. During an overcurrent, invert-ers can supply a fault current whose peak value is limited to the IGBT current limit value. This inverter supplied fault current is not high enough to trip the circuit breaker. After an extended period of overcurrent, UPS internal tripping will be activated and all loads lose power. Opera-tion of the UPS in bypass mode supplies the required fault current but exposes the sensitive load to power line distortions. Therefore, it is desired to always supply the critical load via the inverter. This study targets to design a detection algorithm for short circuits and ground faults with a detection time faster than the UPS system’s internal tripping in order to isolate the faulted ar-ea, when the inverter is supplying the critical load. To achieve this, first, a MATLAB model was designed to aid in preliminary studies of fault detection through analysing the system behaviour. Secondly, literature review was conducted and a fault detection method selected with the help of the MATLAB model. Next, laboratory tests on a real UPS system were carried out and compared to the MATLAB results. Lastly, the detection algorithm was designed, im-plemented and tested on a real double conversion UPS system. The test results indicate that the implemented detection algorithm successfully detects short circuits and ground faults well within the desired time. It also successfully distinguishes short circuits and ground faults from other sources of overcurrents such as overloading and transformer inrush current. Future development of this study includes additional features such as a fault classification method proposed for implementation to improve the UPS debugging process during maintenance. Moreover, the detection algorithm will also be refined and devel-oped further to activate a circuit that discharges a current pulse to increase the fault current fed to the circuit breaker