Feasibility study of the dual active bridge as a low-frequency sine wave inverter

A dissertation submitted n fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, 2018The conventional Sinusoidal PulseWidth Modulation (SPWM) inverter is limited by the fact that it does not allow for Zero Voltage Switching. This means that the switching frequency is kept low to reduce the switching losses. As a consequence of holding these switching frequencies low, the distribution of power over the frequency spectrum is kept closer to the fundamental frequency (compared to higher switching frequencies) leading to larger reactive components to filter out these harmonics. The use of high-frequency switching, Zero Voltage Switching, and different modulation schemes can lead to higher power densities. This research investigates under what conditions the use of these techniques in a Dual Active Bridge (DAB) inverter might lead to a higher power density than the SPWM. Volumetric approximations for the different circuit components in the investigated inverter topologies are demonstrated. These approximations are used to design circuits using physical volume as the cost function where possible. Additionally, a loss model is derived to determine the expected efficiency of each topology being investigated. This model is related to the power density since it is directly proportional to the size of heat sink required to cool the inverter. The techniques for improving power density mentioned above are presented, and the impact that they have on power density is shown using the volumetric approximation function. From this approximation, the volumes between the DAB and the SPWM are compared and investigations into where the DAB may have a higher power density have been performed. It was found that the DAB was not smaller than the SPWM for frequencies less than 72kHz. When simulating the converters operating at different frequencies, the general trend is that the SPWM increases in volume as the frequency increases, whereas, the DAB decreases in volume as the frequency increases. An exact frequency at which the DAB would be smaller than the SPWM was not found in this research. However, many conclusions have been drawn around the use of a DAB as an inverter and the strengths and shortcomings it provides. The modulation scheme would need to be modified to reduce the losses and provide a more competitive volume. Additionally, multi-level and multi-stage techniques could be used to reduce the volume further.MT 201

A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple

Design Approaches to Enhance Power Density in Power Converters for Traction Applications

This dissertation presents a design strategy to increase the power density for automotive Power Conversion Units (PCUs) consisting of DC-DC and DC-AC stages. The strategy significantly improves the volumetric power density, as evident by a proposed PCU constructed and tested having 55.6 kW/L, representing an 11.2 % improvement on the Department of Energy’s 2025 goal of 50 kW/L for the same power electronics architecture. The dissertation begins with a custom magnetic design procedure, based on optimization of a predetermined C-core geometrical relationship and custom Litz wire. It accounts for electrical and thermal tradeoffs to produce a magnetic structure to best accomplish volume and thermal constraints. This work is coupled with a control strategy for the DC-DC converter whereby a variable-frequency Discontinuous Conduction Mode (DCM) control is used to further reduce the required values of the passive components, to provide an increase in power density and a large improvement of low-power-level efficiency, experimentally demonstrated at full power through an 80 kW Interleaved Boost Converter. Integration of this enhanced DC-DC stage to the DC-AC stage requires a DC-Link capacitor, which hinders achieving power density targets. Increasing the switching frequency is an established method of reducing the size of passives. However, it is the RMS current sizing requirements that diminishes any gains achieved by raising the switching frequency. A synchronous carrier phase shift-based control algorithm, that aligns the output current of the boost stage with the input current of an inverter, is proposed to reduce the RMS current in the DC-Link capacitor by up to 25% and an average 20% smaller capacitor volume. Lastly, a new electrothermal platform based on paralleled discrete devices is presented for a 50 kW traction inverter. Embedded capacitors within the vacant volume of the hybrid material thermal management structure enables higher power density (155 kW/L) and significantly reduces cost

Design Approaches to Enhance Power Density in Power Converters for Traction Applications

This dissertation presents a design strategy to increase the power density for automotive Power Conversion Units (PCUs) consisting of DC-DC and DC-AC stages. The strategy significantly improves the volumetric power density, as evident by a proposed PCU constructed and tested having 55.6 kW/L, representing an 11.2 % improvement on the Department of Energy’s 2025 goal of 50 kW/L for the same power electronics architecture. The dissertation begins with a custom magnetic design procedure, based on optimization of a predetermined C-core geometrical relationship and custom Litz wire. It accounts for electrical and thermal tradeoffs to produce a magnetic structure to best accomplish volume and thermal constraints. This work is coupled with a control strategy for the DC-DC converter whereby a variable-frequency Discontinuous Conduction Mode (DCM) control is used to further reduce the required values of the passive components, to provide an increase in power density and a large improvement of low-power-level efficiency, experimentally demonstrated at full power through an 80 kW Interleaved Boost Converter. Integration of this enhanced DC-DC stage to the DC-AC stage requires a DC-Link capacitor, which hinders achieving power density targets. Increasing the switching frequency is an established method of reducing the size of passives. However, it is the RMS current sizing requirements that diminishes any gains achieved by raising the switching frequency. A synchronous carrier phase shift-based control algorithm, that aligns the output current of the boost stage with the input current of an inverter, is proposed to reduce the RMS current in the DC-Link capacitor by up to 25% and an average 20% smaller capacitor volume. Lastly, a new electrothermal platform based on paralleled discrete devices is presented for a 50 kW traction inverter. Embedded capacitors within the vacant volume of the hybrid material thermal management structure enables higher power density (155 kW/L) and significantly reduces cost

Fast DC Fault Current Suppression and Fault Ride Through in Full-Bridge MMCs via Regulation of Submodule Capacitor Discharge

High Voltage Direct Current (HVDC) is more cost-effective than High Voltage Alternating Current (HVAC) for transmitting power over long distances, and therefore is ideal for bulk power transfer from wind, solar, hydroelectric, and tidal power plants located in offshore or remote locations to load centers. The use of Voltage-Sourced Converters (VSCs) in HVDC transmission systems offers greater flexibility when compared to their counterpart, Line Commutated Converters (LCCs), due to their smaller footprint, improved power quality, as well as decoupled active and reactive power control, voltage support, and black start capabilities. The most recent advancements in VSC technology have led to the emergence of a new converter topology known as the Modular Multilevel Converter (MMC). The simplest and most economical MMC cell structure is the Half-Bridge Submodule (HBSM), which is unable to prevent AC side contribution to DC side faults in HVDC systems. Therefore, DC fault protection in the HB-MMC requires either installation of expensive DC Circuit Breakers (DCCBs) or the opening of AC side breakers that are not adequately fast. Adding two extra switches to the HBSM results in the Full-Bridge Submodule (FBSM) configuration which ensures that, in the event of a DC side fault, there is a reverse voltage in the path of AC side current feeding the DC side fault through the antiparallel diodes in the SM switches. In addition, such fault blocking SMs capable of bipolar voltage generation equip the MMCs with Fault Ride Through (FRT) ability, thus allowing them to remain connected to both AC and DC networks during DC faults while operating as Static Compensators (STATCOMs) and exchanging reactive power with the AC network. A comprehensive review of notable fault blocking SM configurations and fault ride through techniques is presented in this thesis. In the event of a DC side fault, the fault current contributions are initially made by SM capacitor discharge, which occurs before the fault is detected, followed by the AC side contribution to the DC side fault. While the AC side currents can be regulated using fault blocking SMs with bipolar voltage generation capability, the initial discharge of the SM capacitors results in high DC fault currents, which can take several milliseconds to be brought under control. A method to actively influence the rate of rise of the DC fault current by regulating the discharge of SM capacitors in an HB-MMC system has been presented in the literature. In this thesis, the approach has been modified and adapted to a FB-MMC system. The discharge direction of the FBSM capacitors is inverted following the detection of a DC side fault which leads to a reversal in the fault current direction and a fast drop-off towards zero. The conventional FRT procedure where the DC fault is cleared by making adjustments to the MMC arm reference voltages followed by STATCOM operation of the MMC is initiated after the detection of zero-crossing of the DC fault current. The proposed control scheme provides significantly faster DC fault current suppression compared to the case where the conventional FRT procedure is initiated immediately upon DC fault detection. Simulations performed on a point-to-point FB-MMC test system are used to verify the theoretical analysis and to evaluate the DC-FRT performance of the proposed scheme

Enhanced Performance Bidirectional Quasi-Z-Source Inverter Controller

A novel direct control of high performance bidirectional quasi-Z-source inverter (HPB-QZSI), with optimized controllable shoot-through insertion, to improve the voltage gain, efficiency and to reduce total harmonic distortion is investigated. The main drawback of the conventional control techniques for direct current to alternating current (DC-AC) conversion is drawn from the multistage energy conversion structure, which implies complicated control, protection algorithms and reduced reliability due to the increased number of switching devices. Theoretically, the original Z-source, Quasi-Z-source, and embedded Z-source all have unlimited voltage gain. Practically, however, a high voltage gain (>2 or 3), will result in a high voltage stress imposed on the switches. Every additional shoot-through state increases the commutation time of the semiconductor switches, thereby increasing the switching losses in the system. Hence, minimization of the commutation time by optimal placing of the shoot-through state in the switching time period is necessary to reduce the switching loss. To overcome this problem, a combination of high performance bidirectional quasi-Z-source inverter with a sawtooth carrier based sinusoidal pulse width modulation (SPWM) in simple operation condition for maximum boost control with 3rd harmonic injection is proposed. This is achieved by voltage-fed quasi-Z-source inverter with continuous input current, implemented at the converter input side which can boost the input voltage by utilizing the extra switching state with the help of shoot-through state insertion technique. This thesis presents novel control concepts for such a structure, focusing mainly on the control of a shoot-through insertion. The work considers the derivation and application of direct controllers for this application and scrutinizes the technical advantages and potential application issues of these methodologies. Based on the circuit analysis, a small signal model of the HPB-QZSI is derived, which indicates that the circuit is prone to oscillate when there is disturbance on the direct current (DC) input voltage. Therefore, a closed-loop control of shoot-through duty cycle is designed to obtain the desired DC bus voltage. The DC-link boost control and alternating current (AC) side output control are presented to reduce the impacts of disturbances on loads. The proposed strategy gives a significantly high voltage gain compared to the conventional pulse width modulation (PWM) techniques, since all the zero states are converted into shoot-through states. The simulated results verify the validity and superiority of the proposed control strategies

Modulation and Control Techniques for Performance Improvement of Micro Grid Tie Inverters

The concept of microgrids is a new building block of smart grid that acts as a single controllable entity which allows reliable interconnection of distributed energy resources and loads and provides alternative way of their integration into power system. Due to its specifics, microgrids require different control strategies and dynamics of regulation as compared to ones used in conventional utility grids. All types of power converters used in microgrid share commonalities which potentially affect high frequency modes of microgrid in same manner. There are numerous unique design requirements imposed on microgrid tie inverters, which are dictated by the nature of the microgrid system and bring major challenges that are reviewed and further analyzed in this work. This work introduces, performs a detailed study on, and implements nonconventional control and modulation techniques leading to performance improvement of microgrid tie inverters in respect to aforementioned challenges