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

A PWM current source-based DC transmission system for multiple wind turbine interfacing

A pulsewidth modulation (PWM) current source wind energy conversion system based on a parallel configuration for high voltage direct current application is proposed. A comparison between the parallel and series configurations for current source-based systems is investigated, which shows the merits of the proposed system. A new control technique for the PWM current source inverter is proposed. It can effectively control the average dc-link voltage with a feed-forward loop, while independently controlling reactive power according to grid code requirements. The system simulation confirms the performance of the proposed system with no interaction between wind turbine modules and satisfying performance with grid integration. Practical implementation further verifies the proposed inverter control. Finally, a brief comparison between conventional line-commutated converter-based systems and the proposed PWM current source converter-based system is presented

A Small-Scale Standalone Wind Energy Conversion System Featuring SCIG, CSI and a Novel Storage Integration Scheme

Small-scale standalone wind turbines provide a very attractive renewable energy source for off-grid remote communities. Taking advantage of variable-speed turbine technology, which requires a partial- or full-scale power converter, and through integrating an energy storage system, smooth and fast power flow control, maximum power point tracking, and a high-quality power is ensured. Due to high reliability and efficiency, permanent magnet synchronous generator seems to be the dominating generator type in gearless wind turbines, employed for off-grid applications. However, wind turbines using geared squirrel-cage induction generator (SCIG) are still widely accepted due to their robustness, simplicity, light weight and low cost. Permanent magnet induction generator, a relatively new induction-based machine, has recently been recognized in the wind energy market as an alternative for permanent magnet synchronous generator. A thorough comparative study, among these three generator types, is conducted in this research in order to enable selection of the most appropriate generator for off-grid wind energy conversion system (WECS), subject to a set of given conditions. The system based on geared SCIG has been shown to be the most appropriate scheme for a small-scale standalone WECS, supplying a remote area. Different topologies of power electronic converters, employed in WECSs, are overviewed. Among the converters considered, current source converter is identified to have a great potential for off-grid wind turbines. Three current-source inverter-based topologies, validated in the literature for on-grid WECS, are compared for off-grid WECS application. Feasibility study and performance evaluation are conducted through analysis and simulation. Among all, the topology composed of three-phase diode bridge rectifier, DC/DC buck converter, and pulse-width-modulated current-source inverter (PWM-CSI) is identified as a simple and low-cost configuration, offering satisfactory performance for a low-power off-grid WECS. A small-scale standalone wind energy conversion system featuring SCIG, CSI and a novel energy storage integration scheme is proposed and a systematic approach for the dc-link inductor design is presented. In developing the overall dynamic model of the proposed wind turbine system, detailed models of the system components are derived. A reduced-order generic load model, that is suitable for both balanced and unbalanced load conditions, is developed and combined with the system components in order to enable steady-state and transient simulations of the overall system. A linear small-signal model of the system is developed around three operating points to investigate stability, controllability, and observability of the system. The eigenvalue analysis of the small-signal model shows that the open-loop system is locally stable around operating points 1 and 3, but not 2. Gramian matrices of the linearized system show that the system is completely controllable at the three operating points and completely observable at operating points 1 and 3, but not 2. The closed-loop control system for the proposed wind turbine system is developed. An effective power management algorithm is employed to maintain the supply-demand power balance through direct control of dc-link current. The generator’s shaft speed is controlled by the buck converter to extract maximum available wind power in normal mode of operation. The excess wind power is dumped when it is not possible to absorb maximum available power by the storage system and the load. The current source inverter is used to control positive- and negative-sequence voltage components separately. The feasibility of the proposed WECS and performance of the control system under variable wind and balanced/unbalanced load conditions are analyzed and demonstrated through simulation. Finally, the proposed WECS is modified by removing the dump load and avoiding the surplus power generation by curtailment of wind power. The operation of the modified system is investigated and verified under variable wind and load conditions

Modulation scheme investigation for high-power medium-voltage current source converter based drives

Pulse width modulated (PWM) current source converter (CSC) based drives are commonly used in high-power (1-10 MW), medium-voltage (MV) (2.3-6.6 kV) applications. These drives feature a simple converter structure, inherent four-quadrant operation capabilities, motor-friendly waveforms, and reliable fuseless short-circuit protection. PWM CSC-based drives are generally constructed using symmetrical gate-commutated thyristors (SGCTs) with reverse voltage blocking capabilities. In order to avoid exceeding the thermal limits of these SGCT devices, and to minimize switching losses, the device switching frequency used by PWM CSC-based drives is typically kept below 500 Hz. There are three main modulation schemes used in MW-level MV PWM CSC-based drives: space vector modulation (SVM), trapezoidal pulse width modulation (TPWM), and selective harmonic elimination (SHE). Of these three modulation schemes, SHE possesses the best harmonic performance as it features the ability to eliminate a number of low-order harmonics, all while retaining a low switching frequency. However, due to its off-line implementation, SHE suffers from poor dynamic performance, and in certain cases, requires a large, memory-exhaustive look-up table. To address these issues, this research investigates ways of improving the dynamic performance of conventional SHE through on-line (i.e., real-time) implementation. Two new modulation schemes are proposed: on-line SHE for modulation of the grid-side PWM current source rectifier (CSR) and SHE-TPWM for modulation of the motor-side PWM current source inverter (CSI). The proposed online SHE scheme models the independent switching angles used in conventional SHE as polynomial functions by applying curve-fitting techniques. This method of implementation improves the dynamic performance of conventional SHE, as it enables real-time computation of switching angles, and eliminates the need for look-up tables. Conversely, the proposed SHE-TPWM scheme combines the principles, while retaining the respective advantages, of both conventional SHE and TPWM. This integrative approach enables SHE-TPWM to possess SHE-level harmonic performance, along with improved dynamic performance rivaling that of TPWM

A Fault Tolerant 3-Phase Adjustable Speed Drive Topology with Common Mode Voltage Suppression

A fault tolerant adjustable speed drive (ASD) topology is introduced in this work. A conventional ASD topology is modified to address: a) drive vulnerability to semiconductor device faults b) input voltage sags c) motor vulnerability to effects of long leads and d) achieve active minimization of common mode (CM) voltage applied to the motor terminals. These objectives are attained by inclusion of an auxiliary IGBT inverter leg, three auxiliary diodes, and isolation - reconfiguration circuit. The design and operation of the proposed topology modifications are described for different modes; (A) Fault mode, (B) Auxiliary Sag Compensation (ASC) mode and (C) Active Common Mode Suppression mode. In case of fault and sag, the isolation and hardware reconfiguration are performed in a controlled manner using triacs/anti-parallel thyristors/solid state relays. In normal operation, the auxiliary leg is controlled to actively suppress common mode voltage. For inverter IGBT failures (short circuit and open circuit), the auxiliary leg is used as a redundant leg. During voltage sags, the auxiliary leg along with auxiliary diodes is operated as a boost converter. A current shaping control strategy is proposed for the ASC mode. A detailed analysis of common mode performance of the proposed topology is provided and a new figure of merit, Common Mode Distortion Ratio (CMDR) is introduced to compare the attenuation of common mode voltage with that of a conventional ASD topology for three different modulation strategies. The output filter design procedure is outlined. A design example is presented for an 80 kW ASD system and simulation results validate the proposed auxiliary leg based fault tolerant scheme. Experimental results from a scaled prototype rated at 1 hp prototype also confirm the operation. The common mode analysis is also validated with the experimental results

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

Comprehensive loss optimization of induction motor drives

Extensive use of power electronics-controlled induction motor drives over the past few decades has enabled the development of loss minimization control algorithms. With the technological advancements in power semiconductor switching devices such as insulated gate bipolar transistors and gate commutated thyristors, induction motor drives are increasingly used in applications, ranging from automotive traction to more-electric aircraft, which have widely varying speed, torque and power requirements. Advances in control technology have enabled the development of various sophisticated controllers for motor drives aimed at performance enhancement. Substantial energy savings may be obtained when drive controllers are optimized for loss reduction under varying operating conditions. This dissertation addresses loss optimization opportunities in induction motor drives from system perspectives. First, a constrained loss optimization method is developed. Past work on loss minimization has focused on specific drive components such as the machine stator and rotor windings, inverter and dc-link. Component-level loss minimization, however, will not guarantee minimum total loss in the drive system. So, a system-level loss minimization method is proposed using a comprehensive loss model, to achieve true minimum total loss. Next, a lossless damping controller is proposed to suppress undesirable resonant oscillations in the machine voltages and currents due to the use of LC filters between the inverter and motor terminals. Passive damping methods employing physical resistors to suppress these oscillations, contribute to additional losses. Lossless active damping methods with virtual resistors have been explored in the literature. Conventionally, this resistance value is fixed, based on empirical rules, and left unchanged for all operating conditions. Choosing incorrect resistance values for the damping controller can result in degraded system behavior. A small-signal transfer function approach based on operating conditions and dynamic adjustment of the virtual resistance, is developed for the damping controller. The controller is designed to allow a flexible differential damping approach. Finally, power electronics loss reduction is investigated in a voltage source inverter (VSI)-based induction motor drive. It is known that low drive speeds will result in poor bus utilization and increased power electronics loss for higher link voltages. Losses can be reduced by dynamically varying the dc link voltage according to operating conditions. In addition to reducing losses, varying the link voltage also reduces the switched voltage magnitude across the inverter switches, potentially increasing inverter reliability. In the proposed method, the link voltage is varied using a front-end dc-dc buck converter according to a loss minimization algorithm. The effect of additional loss from the front-end converter on the total loss is also studied. Benefits of the proposed methods are verified by simulations and experiments

Design and Optimization of EMI Filters for Power Electronics Systems

Modern power electronics develop very rapidly. The main direction for development nowadays is increasing power density. This can be achieved by utilizing higher switching frequencies. The last generation of SiC and GaN semiconductors can reach switching frequencies up to several MHz. At the same time the number of power electronics devices connected to the power grid has grown significantly during past decades and continues to increase. These two factors result in the high level of power grid pollution with electromagnetic interferences. In order to minimize the emission level, passive electromagnetic interference (EMI) filters are employed. Within the frame of this thesis, issues related to design and improvement of EMI filters for three-phase power electronic converters are comprehensively studied. Detailed analyses of magnetic materials, filter components, mutual couplings, enhancement methods and filter topologies are presented. Insertion loss of EMI filters is analysed within a system under idealised conditions 50 Ohm reference impedance). Moreover the impact of the reference impedance used for EMI filter characterisation on insertion loss is investigated based on mathematical models. Behavioural models of EMI filters with different complexity are developed. Diverse simulation approaches are used for profound understanding of the physical processes inherent in EMI filters. Exactness of the behavioural models is proven by measurements of built prototypes. The main insertion loss degradation mechanisms are derived from the developed models. The advantages and disadvantages of models based upon the network theory, S-functions and differential equations with the help of electronic design automation tools are shown. Existing filter improvement methods are systematized and complemented. Novel improvement possibilities for conventional filters including parasitics compensation methods and mutual coupling minimization are developed. Different combinations of improvement methodologies are applied to reference EMI filters and tested. Multistage filters with enhanced topologies are proposed. Critical comparison of conventional and proposed filter topologies considering power density and costs is carried out. Obtained results demonstrate considerable advantages of enhanced EMI filter topologies over conventional ones. Three phase power electronics systems are examined from the perspective of electromagnetic compatibility. Analysis of common and differential currents' contribution into the spectrum of a conventional drive system is carried out. EMI filters are tested together with different types of power electronic converters. Effects caused by variation of converter parameters on the emitted spectrum of disturbances are assessed from the EMI filter design point if view. It is determined that essentially all investigated parameters of a power electronics converter and a drive system except DC voltage fluctuation have feasible impact on the EMI spectrum. Thus EMC behaviour of the system can be adjusted by variation of these parameters. Semiconductor speed, motor stray capacitance and concept of protective earthing are determined as the most relevant system parameters influencing EMI filter design