The impact of inverter side PV plant on HVDC commutation failures

M.Phil. (Electrical Engineering in Power and Energy Systems)Abstract: The high-voltage direct current (HVDC) system is a crucial technology in transmission; however, this system suffers from commutation failure. Commutation failure is defined as an adverse dynamic event that occurs when a converter valve that is supposed to turn off continues to conduct without transferring its current to the next valve in the firing sequence. Commutation failure disturbs the power transfer, yields a large overcurrent in the converter, and causes a voltage drop in an alternating current (AC) network. Although commutation failure in HVDC systems has been studied using many other compensating devices, academic researchers have not given enough attention to evaluating the impact of distributed generation (DG) on the power system. Within this gap and based on the publications researched, no published material could be found regarding the impact of a photovoltaic (PV) plant on HVDC commutation failures. This research project seeks to focus on the impact of an inverter side PV plant on HVDC commutation failures. In this dissertation, the objective is to evaluate the impact of the inverter side PV plant on HVDC commutation failures. The case studies are done by considering the commutation failure severity, the magnitudes of the remaining voltages after different types of faults occurring, and the recovery time required to clear a fault. Case studies are performed in a network with a PV plant and also without a PV plant. The network was set up in Power System Computer-Aided Design (PSCAD) software to find the critical voltages. Further simulations were done in this study using Power System Simulator for Engineering (PSS/E) software. A network by Conférence Internationale des grandes réseaux..

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

High frequency solid-state power sources for induction heating

Induction heating and melting applications often require a power source to convert 3-phase mains input power to single-phase output power at a higher and variable frequency. Amongst various power conversion schemes, solid-state power converters using the most modern devices provide the best power control techniques available for this application. In designing for this purpose, careful consideration must be given to the characteristics of the load, which presents a very low power factor and an impedance possibly varying widely as the heating cycle proceeds. From the variety of thyristor commutation techniques commonly employed in high-power inverters, series load commutation is particularly suited to high-frequency applications, as it has an intrinsically high turn-off time for the circuit thyristors (clearly essential at high operational frequencies) and much reduced switching losses. However, series commutation circuits are load sensitive, and therefore require careful design, especially with an induction heating load. Recent developments in power conversion techniques have led to the elimination of the d.c. link in a.c. to a.c. power conversion, enabling both high operational efficiencies and substantial savings in the initial cost of the device to be achieved. This new type of converter (called a cycloinverter) power and frequency control facilities. However, in a cycloinverter, since high-frequency switching is performed simultaneously with rectification, these control schemes are dependent on the operational frequency. The direct power conversion in a cycloinverter causes, unfortunately, distortion currents in the input lines and the output circuit, and it is the designer's task to minimise these undesirable components. The project aims to investigate the potential uses, both of the series inverter in its high-frequency form and of the cycloinverter, as power sources for induction heating. Design criteria are established for each circuit, with consideration given to turn-off time, efficiency, power factor, component ratings and predicted load variations. Computer simulations of the converters are employed to investigate the different voltage and current waveforms in the circuits, and to establish how the performance of each inverter may be optimised and these are verified by results obtained an experimental prototypes

Online Switching Time Monitoring of SiC Devices Using Intelligent Gate Driver for Converter Performance Improvement

Most intelligent gate drivers designed for new state of the art WBG devices typically only focus on protection and driving capabilities of the devices. This paper introduces an intelligent gate driver that incorporates online switching time monitoring of silicon carbide (SiC) devices. For this specific case study, three timing conditions (turn-off delay time, turn-off time, and voltage commutation time) of a SiC phase-leg are online monitored. This online monitoring system is achieved through transient detection circuits and a micro-controller. These timing conditions are then utilized to develop converter-level benefits for a voltage-source inverter application using SiC devices. Junction temperature monitoring is realized through turn-off delay time monitoring. Dead-time optimization is achieved with turn-off time monitoring. Dead-time compensation is obtained with turn-off time and voltage commutation time monitoring. The case study converter assembled for testing purposes is a half-bridge inverter using two SiC devices in a phase-leg configuration. All timing conditions are correctly monitored within reasonable difference of the actual condition time. The half-bridge inverter can operate at 600 V DC input and successfully obtain a junction temperature measurement through monitored turn-off delay time and the calibration curve. In addition, dead-time control is realized to reduce device power loss and improve AC output power quality. Furthermore, the proposed online time monitoring system is board-level integrated with the gate driver and suitable for the chip level integration, enabling this practical approach to be cost-effective for end users

Predictive control of a direct series resonant converter with active output voltage compensation

Modern high power supplies are based on resonant converters in order to use high frequency reactive elements (for reduced size) without sacrificing converter efficiency. In an effort to achieve compact high power supplies, direct power converter topologies have been considered, since these are mainly characterised by their high power density. A direct resonant converter topology combines matrix converters with conventional resonant converters. This work focused on achieving high quality input/output power and high efficiency. Thus, this thesis presents the research on the control of a direct series resonant converter. Since the resonant converter allows a sinusoidal high frequency output current to be generated, zero current switching (ZCS) was considered to reduce the power losses. Hence, since the converter is switched at every zero crossing of the output current (fixed period), model predictive control was considered. Different predictive approaches for controlling the input and output currents were developed and analysed, however, owing to the characteristics of the topology, these strategies resulted in a suboptimal control. Therefore, in order to improve the input and output qualities (reduce distortion), an output voltage compensation strategy was proposed. This compensation approach is based on adding an H-bridge converter in series between the matrix converter and the resonant tank. This converter improves the voltage applied to the resonant tank, thus, reducing the distortion at the output and, as a consequence, also the distortion at the input. The H-bridge converter utilises only a small capacitor on the dc side in comparison with conventional resonant converters and operates at a low voltage. Simulations were carried out using MATLAB/Simulink and an experimental prototype was built to validate the strategies proposed, achieving a reduction of the input current THD from 4.4% to 2.7%, a reduction of the output current distortion of approximately 40% and an analytically derived efficiency of 89.5%

A matrix converter drive system for an aircraft rudder electro-mechanical actuator

The matrix converter is an attractive topology of power converter for the Aerospace Industry where factors such as the absence of electrolytic capacitors, the potentiality of increasing power density, reducing size and weight and good input power quality are fundamental. The matrix converter potential advantages offers the possibility to achieve the aim of the More Electric Aircraft research which intends to gradually re- place, from the aircraft architecture, the hydraulic power source and its infrastructure with electric power generation and a more flexible power distribution system. The purpose of this work is to investigate the design and implementation of a 40kVA matrix converter for an Electro Mechanical Actuator (EMA) drive system. A SABER simulation analysis of the candidate matrix converter drive systems, for this application, is provided. The design and implementation of the matrix converter is described, with particular attention to the strict requirements of the given aerospace application. Finally, the matrix converter PMSM drive system and the EMA drive system are respectively assembled, tested and commissioned

Real-Time Machine Learning Based Open Switch Fault Detection and Isolation for Multilevel Multiphase Drives

Due to the rapid proliferation interest of the multiphase machines and their combination with multilevel inverters technology, the demand for high reliability and resilient in the multiphase multilevel drives is increased. High reliability can be achieved by deploying systematic preventive real-time monitoring, robust control, and efficient fault diagnosis strategies. Fault diagnosis, as an indispensable methodology to preserve the seamless post-fault operation, is carried out in consecutive steps; monitoring the observable signals to generate the residuals, evaluating the observations to make a binary decision if any abnormality has occurred, and identifying the characteristics of the abnormalities to locate and isolate the failed components. It is followed by applying an appropriate reconfiguration strategy to ensure that the system can tolerate the failure. The primary focus of presented dissertation was to address employing computational and machine learning techniques to construct a proficient fault diagnosis scheme in multilevel multiphase drives. First, the data-driven nonlinear model identification/prediction methods are used to form a hybrid fault detection framework, which combines module-level and system-level methods in power converters, to enhance the performance and obtain a rapid real-time detection. Applying suggested nonlinear model predictors along with different systems (conventional two-level inverter and three-level neutral point clamped inverter) result in reducing the detection time to 1% of stator current fundamental period without deploying component-level monitoring equipment. Further, two methods using semi-supervised learning and analytical data mining concepts are presented to isolate the failed component. The semi-supervised fuzzy algorithm is engaged in building the clustering model because the deficient labeled datasets (prior knowledge of the system) leads to degraded performance in supervised clustering. Also, an analytical data mining procedure is presented based on data interpretability that yields two criteria to isolate the failure. A key part of this work also dealt with the discrimination between the post-fault characteristics, which are supposed to carry the data reflecting the fault influence, and the output responses, which are compensated by controllers under closed-loop control strategy. The performance of all designed schemes is evaluated through experiments