High-Efficiency Three-Phase Current Source Rectifier Using SiC Devices and Delta-Type Topology

In this dissertation, the benefits of the three-phase current source rectifier (CSR) in high power rectifier, data center power supply and dc fast charger for electric vehicles (EV) will be evaluated, and new techniques will be proposed to increase the power efficiency of CSRs. A new topology, referred as Delta-type Current Source Rectifier (DCSR), is proposed and implemented to reduce the conduction loss by up to 20%. By connecting the three legs in a delta type on ac input side, the dc-link current in DCSR can be shared by two legs at the same time. To increase the switching speed and power density, all-SiC power modules are built and implemented for CSRs. The switching waveforms in the commutation are measured and studied based on double pulse test. Four different modulation schemes are compared for high efficiency CSR considering the switching characteristics of different device combinations. The most advantageous modulation scheme is then identified for each of the device combinations investigated. A compensation method is proposed to reduce the input current distortion caused by overlap time and slow transition in CSRs. The proposed method first minimizes the overlap time and then compensates the charge gain/loss according to the sampled voltage and current. It is verified that the proposed method can reduce the input current distortion especially when the line-to-line voltage is close to zero. The dc-link current will become discontinuous under light load in CSRs, when the traditional control algorithm may not work consistently well. To operate CSR in discontinuous current mode (DCM), the CSR is modeled in DCM and a new control algorithm with feedforward compensation is proposed and verified through experiments. A protection scheme with fast response time is proposed, analyzed and verified to protect SiC devices from overvoltage caused by current interruption in CSRs. To deal with the harmonics and voltage sag in the input ac voltage, a new control algorithm is proposed. By adding ac current feedback control and proportional-resonant (PR) control, the proposed control algorithm can reduce the input current distortion and dc output voltage ripple under input voltage disturbance

Control strategy to share reactive power and regulate voltage in microgrids with autonomous mode operation

The objective of this thesis is to develop control strategies for distributed generation (DG) systems, that allows to achieve reactive power sharing and regulate voltage in microgrids operated autonomously. The proposed control strategies present different alternatives to improve reactive power sharing among DGs that conform the microgrid and to regulate the voltage in the nodes. Thus, it is possible to contribute to the overcoming problems caused by the continuous connection and disconnection of power loads, avoiding DG tripping under this situation. The scope of the proposed strategies covers a wide range of possibilities, from regulating voltage in the nodes of the microgrid based on electric vehicle connection and disconnection, to urban and rural residential loads. The usefulness of these control strategies is focused on DG systems that operate in isolated mode with great integration of renewables, mainly located in remote areas. Therefore, in this research we worked on the comparison of both the consumption of rural and urban communities to obtain different load variations to test the control strategy in the microgrid. The control strategies included in this research are: virtual RMS voltage, variable virtual impedance, virtual current, and virtual voltage.El objetivo de esta tesis es desarrollar una estrategias de control para los sistemas de generación distribuida (DG), que permitan lograr compartir la potencia reactiva y regular el voltaje en las microrredes operadas de manera autónoma. Las estrategias de control propuestas presentan diferentes alternativas para mejorar la compartición de potencia reactiva entre las diferente DG que conforman la microrred y para regular el voltaje en los nodos. Por lo tanto, es posible contribuir a la superación de los problemas causados por la conexión y desconexión continua de las cargas, evitando el disparo de DG en esta situación. El alcance de las estrategias propuestas cubre una amplia gama de posibilidades, desde la regulación del voltaje en los nodos de la microrred basada en la conexión y desconexión del vehículo eléctrico, hasta las cargas residenciales urbanas y rurales. La utilidad de estas estrategias de control se centra en los sistemas de DG que funcionan en modo autónomo con una gran integración de energías renovables, principalmente ubicadas en áreas remotas. Por lo tanto, en esta investigación trabajamos en la comparación del consumo de las comunidades rurales y urbanas para obtener diferentes variaciones de cargas para probar la estrategia de control en la microrred. Las estrategias de control incluidas en esta investigación son: voltaje RMS virtual, impedancia virtual variable, corriente virtual y voltaje virtual.ColcienciasThesis presented as a partial requirement to obtain the title of: Doctor en Ingeniería - Línea de Investigación en Automática.Doctorad

Experimental validation of a three-phase induction motor operating with a three-phase bidirectional variable speed drive

The three-phase induction motor is the main electric motor used in industrial applications, contributing significantly to the industrial electricity consumption. Additionally, the traditional variable speed drivers, due to the internal constitution based on a passive rectifier, contribute to accentuate power quality problems on the grid side. In this context, this paper presents thesimulation, implementation and subsequent experimental verification of an electronic variable speed drive for three-phase induction motors, which is composed by a three-phase ac-dc converter on the grid side and by a three-phase dc-ac converter on the motor side. With the proposed solution, besides driving the motor, it is possible to mitigate power quality problems on the grid side (e.g., current harmonics and reactive power) associated with the use of diode-bridge ac-dc converters in the conventional variable speed drives. Besides, with the proposed solution, a bidirectional operation is possible, allowing to deliver to the power grid the energy generated in motor braking processes. As demonstrated along the paper, with the proposed variable speed drive it is possible to control the motor speed (including the rotation direction), and to achieve operation with sinusoidal currents and unitary power factor on the grid side. A laboratory prototype was developed, allowing to perform experimental validation and to verify the main functionalities of the variable speed driveUIDB/00319/2020, PTDC/EEI-EEE/30382/2017, PTDC/EEI-EEE/28813/2017. FCT– Fundação para a Ciência e Tecnologia within the 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 QUALITY4POWER PTDC/EEI-EEE/28813/201

Measurements, Models, Systems and Design

531 s.

Impedance Transformers

Non

Design and Control of Power Converters for High Power-Quality Interface with Utility and Aviation Grids

Power electronics as a subject integrating power devices, electric and electronic circuits, control, and thermal and mechanic design, requires not only knowledge and engineering insight for each subarea, but also understanding of interface issues when incorporating these different areas into high performance converter design.Addressing these fundamental questions, the dissertation studies design and control issues in three types of power converters applied in low-frequency high-power transmission, medium-frequency converter emulated grid, and high-frequency high-density aviation grid, respectively, with the focus on discovering, understanding, and mitigating interface issues to improve power quality and converter performance, and to reduce the noise emission.For hybrid ac/dc power transmission,• Analyze the interface transformer saturation issue between ac and dc power flow under line unbalances.• Proposed both passive transformer design and active hybrid-line-impedance-conditioner to suppress this issue.For transmission line emulator,• Propose general transmission line emulation schemes with extension capability.• Analyze and actively suppress the effects of sensing/sampling bias and PWM ripple on emulation considering interfaced grid impedance.• Analyze the stability issue caused by interaction of the emulator and its interfaced impedance. A criterion that determines the stability and impedance boundary of the emulator is proposed.For aircraft battery charger,• Investigate architectures for dual-input and dual-output battery charger, and a three-level integrated topology using GaN devices is proposed to achieve high density.• Identify and analyze the mechanisms and impacts of high switching frequency, di/dt, dv/dt on sensing and power quality control; mitigate solutions are proposed.• Model and compensate the distortion due to charging transition of device junction capacitances in three-level converters.• Find the previously overlooked device junction capacitance of the nonactive devices in three-level converters, and analyze the impacts on switching loss, device stress, and current distortion. A loss calculation method is proposed using the data from the conventional double pulse tester.• Establish fundamental knowledge on performance degradation of EMI filters. The impacts and mechanisms of both inductive and capacitive coupling on different filter structures are understood. Characterization methodology including measuring, modeling, and prediction of filter insertion loss is proposed. Mitigation solutions are proposed to reduce inter-component coupling and self-parasitics

Reduction of low-order harmonics and new topologies of current source converters

Two CSCs related issues are investigated. The first is the reductions of Low Order Harmonics (LOH) in Low Switching Frequency (LSF) CSCs. High power CSCs operate with LSF. This is not an ideal scenario for Space Vector Modulation (SVM), resulting in non-characteristics LOH, 5 th and 7 th (HD 5-7 ). Researchers have attempted to reduce these harmonics by creating new sequences of space vectors (states) or different sampling techniques. New approaches based on the fact that HD 5-7 is affected by states ON times calculation process are proposed. Results show that by using a new technique called Record Middle (RM), one can obtain a smaller HD 5-7 . Also, results for calculating states ON times as the reference vector rotates by new SVM equations show that one can get a significant reduction in HD 5-7 . Another alternative effective with large overlap periods is minimizing LOHs in LSF-SVM-CSCs with Minimum Harmonic Tracking (MHT) technique. Fuzzy logic dependent states ON times calculation give fixed states ON times adjustment during steady state and fast response. Those techniques allowed the usage of reduced size ac-side filters with high cut-off frequencies. The second issue is on new hybrid current source converters (HCSCs). Three SCRs and four IGBTs HCSC has similar characteristics, less capital cost and less power losses compared to the 6-switches CSC. Two types of modulation are proposed. The first is Hard Switching Space Vector Modulation technique (HS-SVM). HS-SVM is presenting low maximum modulation index (m a.max ), resulting in LOH distortion and requiring multiple mandatory switching inside a cycle. The second modulation technique achieves line-commutation or soft switching (SS) of the SCRs whenever possible, thus being called Soft Switching Space Vector Modulation technique (SS-SVM). SS-SVM increases the m a.max while reducing LOH, the switching frequency and switching losses. Comparisons between the 6-switches CSC and the HCSCs are provided. Cost analysis showing the importance of HCSCs is carried out. A case study is presented to show that the HCSC is economically viable in medium and high power ranges. Besides having lower capital cost, the HCSC can operate with lower power losses than 6-switches CSC