Assessment of novel power electronic converters for drives applications

Phd ThesisIn the last twenty years, industrial and academic research has produced over one hundred new converter topologies for drives applications. Regrettably, most of the published work has been directed towards a single topology, giving an overall impression of a large number of unconnected, competing techniques. To provide insight into this wide ranging subject area, an overview of converter topologies is presented. Each topology is classified according to its mode of operation and a family tree is derived encompassing all converter types. Selected converters in each class are analysed, simulated and key operational characteristics identified. Issues associated with the practical implementation of analysed topologies are discussed in detail. Of all AC-AC conversion techniques, it is concluded that softswitching converter topologies offer the most attractive alternative to the standard hard switched converter in the power range up to 100kW because of their high performance to cost ratio. Of the softswitching converters, resonant dc-link topologies are shown to produce the poorest output performance although they offer the cheapest solution. Auxiliary pole commutated inverters, on the other hand, can achieve levels of performance approaching those of the hard switched topology while retaining the benefits of softswitching. It is concluded that the auxiliary commutated resonant pole inverter (ACPI) topology offers the greatest potential for exploitation in spite of its relatively high capital cost. Experimental results are presented for a 20kW hard switched inverter and an equivalent 20kW ACPI. In each case the converter controller is implanted using a digital signal processor. For the ACPI, a new control scheme, which eliminates the need for switch current and voltage sensors, is implemented. Results show that the ACPI produces lower overall losses when compared to its hardswitching counterpart. In addition, device voltage stress, output dv/dt and levels of high frequency output harmonics are all reduced. Finally, it is concluded that modularisation of the active devices, optimisation of semiconductor design and a reduction in the number of additional sensors through the use of novel control methods, such as those presented, will all play a part in the realisation of an economically viable system.Research Committee of the University of Newcastle upon Tyn

Soft switching techniques for multilevel inverters

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Elétrica

A New Quasi Resonant Dc-link For Photovoltaic Micro-inverters

PV Inverters have the task of tracking the maximum power point (MPP), and regulating the solar energy generation to this optimal operation point. The second task is the conversion of direct current produced by the solar modules into alternating current compatible with the grid. A new inverter approach such as a single phase micro inverter is emerging aimed to overcome some of the challenges of centralized inverters. As a counterpart to the central inverter, a micro inverter is a small compact module attached directly to each solar panel. To provide for the constantly increasing demand for a small size, light weight and high efficiency micro inverter, soft switching power conversion technologies have been employed. The switching stress can be minimized by turning on/off each switch when the voltage across it or the current through it is zero at the switching transition. With the addition of auxiliary circuits such as auxiliary switches and LC resonant components the so called soft switching condition can be achieved for semiconductor devices. Four main purposes to investigate the soft switching technologies for singlephase micro-inverter are: (1) to improve overall efficiency by creating the favorable operating conditions for power devices using soft-switching techniques; iv (2) to shrink the reactive components by pushing the switching frequency to a higher range with decent efficiency. (3) to ensure soft switching does not exacerbate inverter performance, meaning all conventional PWM algorithms can be applied in order to meet IEEE standards. (4) to investigate which soft switching techniques offer the cheapest topology and control strategy as cost and simple control are crucial for low power inverter applications. An overview on the existing soft-switching inverter topologies for single phase inverter technology is summarized. A new quasi resonant DC link that allows for pulse- width- modulation (PWM) is presented in this thesis. The proposed quasi resonant DC link provides zero-voltage switching (ZVS) condition for the main devices by resonating the DC-link voltage to zero via three auxiliary switches and LC components. The operating principle and mode analysis are given. The simulation was carried out to verify the proposed soft switching technique. A 150W 120VAC single-phase prototype was built. The experimental results show that the soft switching for four main switches can be realized under different load conditions and the peak efficiency can reach 95.6%. The proposed quasi DC link can be applied to both single-phase and three-phase DC/AC micro inverter. In order to boost efficiency and increase power density it is important to evaluate the power loss mechanism in each stage of operation of the micro inverter. Using the datasheet parameters of the commercially available semiconductor switches, conduction and switching losses were estimated. This thesis presents a method to analyze power losses of the new resonant DC link inverter which alleviates topology v optimization and MOSFET selection. An analytical, yet simple model for calculating the conduction and switching losses was developed. With this model a rough calculation of efficiency can be done, which helps to speed up the design process and to increase efficiency

High Power Density, High Efficiency Single Phase Transformer-less Photovoltaic String Inverters

abstract: Two major challenges in the transformer-less, single-phase PV string inverters are common mode leakage currents and double-line-frequency power decoupling. In the proposed doubly-grounded inverter topology with innovative active-power-decoupling approach, both of these issues are simultaneously addressed. The topology allows the PV negative terminal to be directly connected to the neutral, thereby eliminating the common-mode ground-currents. The decoupling capacitance requirement is minimized by a dynamically-variable dc-link with large voltage swing, allowing an all-film-capacitor implementation. Furthermore, the use of wide-bandgap devices enables the converter operation at higher switching frequency, resulting in smaller magnetic components. The operating principles, design and optimization, and control methods are explained in detail, and compared with other transformer-less, active-decoupling topologies. A 3 kVA, 100 kHz single-phase hardware prototype at 400 V dc nominal input and 240 V ac output has been developed using SiC MOSFETs with only 45 μF/1100 V dc-link capacitance. The proposed doubly-grounded topology is then extended for split-phase PV inverter application which results in significant reduction in both the peak and RMS values of the boost stage inductor current and allows for easy design of zero voltage transition. A topological enhancement involving T-type dc-ac stage is also developed which takes advantage of the three-level switching states with reduced voltage stress on the main switches, lower switching loss and almost halved inductor current ripple. In addition, this thesis also proposed two new schemes to improve the efficiency of conventional H-bridge inverter topology. The first scheme is to add an auxiliary zero-voltage-transition (ZVT) circuit to realize zero-voltage-switching (ZVS) for all the main switches and inherent zero-current-switching (ZCS) for the auxiliary switches. The advantages include the provision to implement zero state modulation schemes to decrease the inductor current THD, naturally adaptive auxiliary inductor current and elimination of need for large balancing capacitors. The second proposed scheme improves the system efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency within a line frequency cycle. This scheme aims at minimizing the combined switching loss and inductor core loss by including different characteristics of the losses relative to the instantaneous switching frequency in the optimization process.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

A Transformerless PCB Based Medium-Voltage Multilevel Power Converter with A DC Capacitor Balancing Circuit and Algorithm

This dissertation presents a new method of constructing a transformerless, voltage-sourced, medium-voltage multilevel converter using existing discrete power semiconductor devices and printed circuit board technology. While the approach is general, it is particularly well-suited for medium-voltage converters and motor-drives in the 4.16 kV, 500 - 1000 kW range. A novel way of visualizing the power stage topology is developed which allows simplified mechanical layouts while managing the commutation paths. Using so many discrete devices typically drives cost and complexity of the gate-drive system including its control and isolation; a gate-drive circuit is presented to address this problem. As with most multilevel topologies, the dc-link voltages must be balanced during operation. This is accomplished using an auxiliary circuit made up of the same power stage and an associated control algorithm. Experimental results are presented for a 4.16 kV, 746 kW, five-level power converter prototype. This dissertation also analyzes a new capacitor voltage-balancing converter along with a novel capacitor voltage balancing control algorithm. Analysis of the inverter system provides a new description of capacitor voltage stability as a function of system operating conditions

Analysis of two level and three level inverters

The power electronics device which converts DC power to AC power at required output voltage and frequency level is known as inverter. Inverters can be broadly classified into single level inverter and multilevel inverter. Multilevel inverter as compared to single level inverters have advantages like minimum harmonic distortion, reduced MI/RFI generation and can operate on several voltage levels. A multi-stage inverter is being utilized for multipurpose applications, such as active power filters, static var compensators and machine drives for sinusoidal and trapezoidal current applications. The drawbacks are the isolated power supplies required for each one of the stages of the multiconverter and it’s also lot harder to build, more expensive, harder to control in software. This project aims at the simulation study of three phase single level and multilevel inverters. The role of inverters in active power filter for harmonic filtering is studied and simulated in MATLAB/SIMULINK. Firstly, the three phase system with non-linear loads are modeled and their characteristics is observed . Secondly, the active power filters are modeled with the inverters and suitable switching control strategies ( PWM technique) to carry out harmonic elimination

Simple Lossless Inductive Snubbers-Assisted Series Load Resonant Inverter Operating under ZCS-PDM Scheme for High-Frequency Induction Heating Fixed Roller

This paper presents a high-frequency pulse-density-modulated (PDM) soft-switching series load resonant inverter for use in induction heating (IH) fixed roller applications, which is used in copy and printing machines. The proposed simple high-frequency resonant inverter uses an asymmetrical pulse pattern PDM control scheme to achieve complete zero-current soft-switching commutations over a wide output range of input power regulation. Additionally, when the printer toner requires operation in very light load conditions, this causes difficulty in achieving zero-voltage or zero-current soft-switching operations in the IH high-frequency resonant inverters with pulse frequency modulation or pulse width modulation control schemes. The proposed resonant inverter demonstrates the capability to accomplish highly efficient power conversions. In this work, a fixed roller for printing machines is developed for doing experiments to verify the efficiency of the proposed circuit topology and its PDM control schemes. The inverter’s steady-state and transient operating principles are analyzed based on the proposed control strategy at a high-frequency PDM. Operating conditions such as power loss analysis, power conversion efficiency and temperature rise characteristics of the proposed inverter are presented and analyzed through experimental results. Finally, from a practical viewpoint, a comparative study of a conventional halogen lamp heater and the proposed IH fixed roller is deliberated.publishedVersio