Tradeoffs between AC power quality and DC bus ripple for 3-phase 3-wire inverter-connected devices within microgrids

Visions of future power systems contain high penetrations of inverters which are used to convert power from dc (direct current) to ac (alternating current) or vice versa. The behavior of these devices is dependent upon the choice and implementation of the control algorithms. In particular, there is a tradeoff between dc bus ripple and ac power quality. This study examines the tradeoffs. Four control modes are examined. Mathematical derivations are used to predict the key implications of each control mode. Then, an inverter is studied both in simulation and in hardware at the 10 kVA scale, in different microgrid environments of grid impedance and power quality. It is found that voltage-drive mode provides the best ac power quality, but at the expense of high dc bus ripple. Sinusoidal current generation and dual-sequence controllers provide relatively low dc bus ripple and relatively small effects on power quality. High-bandwidth dc bus ripple minimization mode works well in environments of low grid impedance, but is highly unsuitable within higher impedance microgrid environments and/or at low switching frequencies. The findings also suggest that the certification procedures given by G5/4, P29 and IEEE 1547 are potentially not adequate to cover all applications and scenarios

Solar Photovoltaic and Thermal Energy Systems: Current Technology and Future Trends

Solar systems have become very competitive solutions for residential, commercial, and industrial applications for both standalone and grid connected operations. This paper presents an overview of the current status and future perspectives of solar energy (mainly photovoltaic) technology and the required conversion systems. The focus in the paper is put on the current technology, installations challenges, and future expectations. Various aspects related to the global solar market, the photovoltaic (PV) modules cost and technology, and the power electronics converter systems are addressed. Research trends and recommendations for each of the PV system sectors are also discussed.Junta de Andalucía P11-TIC-7070Ministerio de Ciencia e Innovación TEC2016-78430-

Solid state transformer technologies and applications: a bibliographical survey

This paper presents a bibliographical survey of the work carried out to date on the solid state transformer (SST). The paper provides a list of references that cover most work related to this device and a short discussion about several aspects. The sections of the paper are respectively dedicated to summarize configurations and control strategies for each SST stage, the work carried out for optimizing the design of high-frequency transformers that could adequately work in the isolation stage of a SST, the efficiency of this device, the various modelling approaches and simulation tools used to analyze the performance of a SST (working a component of a microgrid, a distribution system or just in a standalone scenario), and the potential applications that this device is offering as a component of a power grid, a smart house, or a traction system.Peer ReviewedPostprint (published version

Dual active bridge converters in solid state transformers

This dissertation presents a comprehensive study of Dual Active Bridge (DAB) converters for Solid State Transformers (SSTs). The first contribution is to propose an ac-ac DAB converter as a single stage SST. The proposed converter topology consists of two active H-bridges and one high-frequency transformer. Output voltage can be regulated when input voltage changes by phase shift modulation. Power is transferred from the leading bridge to the lagging bridge. It analyzes the steady-state operation and the range of zero-voltage switching. It develops a switch commutation scheme for the ac-ac DAB converters. Simulation and experiment results of a scaled down prototype are provided to verify the theoretical analysis. The second contribution is to develop a full-order continuous-time average model for dc-dc DAB converters. The transformer current in DAB converter is purely ac, making continuous-time modeling difficult. Instead, the proposed approach uses the dc terms and 1st order terms of transformer current and capacitor voltage as state variables. Singular perturbation analysis is performed to find the sufficient conditions to separate the dynamics of transformer current and capacitor voltage. Experimental results confirm that the proposed model predicts the small-signal frequency response more accurately. The third contribution addresses the controller design of a dc-dc DAB converter when driving a single-phase dc-ac inverter. It studies the effect of 120 Hz current generated by the single-phase inverter. The limitation of PI-controller is investigated. Two methods are proposed to reduce the voltage ripple at the output voltage of DAB converter. The first method helps the feedback loop with feedforward from inverter, while the second one adds an additional resonance controller to the feedback loop. Theoretical analysis, simulation and experiment results are provided to verify the effectiveness of the proposed methods --Abstract, page iii

Common-Mode Modeling of Neutral Point Clamped Converter Based Dual Active Bridge

Modern power converters designed with wide-bandgap semiconductors are known to generate substantial conducted electromagnetic interference as a side effect of high edge rate and high frequency switching. With the advancement in power electronic converters, the significant EMI challenges need to be addressed for distribution level power systems. The goal is to provide a computationally efficient method of EMI characterization for conducted emissions for this future generation of power distribution systems. The first step in making this possible is through creating an accurate EMI characterization platform for the neutral point clamped dual active bridge. In this thesis, a formalized common-mode modeling approach is carried out for transforming this mixed-mode power system into its common-mode equivalent circuit. The approach is validated through comparison of time-domain waveforms predicted by detailed mixed-mode and common-mode equivalent models of the representative power distribution system, with a proposed future validation using hardware measurements. The experimental studies highlight the utility of the proposed modeling approach to assess design mitigation strategies

Design, Simulation and Implementation of Three-Phase Bidirectional DC-DC Dual Active Bridge Converter Using SiC MOSFETs

The use of SiC-based martials in fabricating power semiconductor devices has shown more interest than conventional silicon-based. Its promising abilities to improve the performance of power electronic systems made it a valuable choice in building high power DC-DC converters. This thesis presents the design and implementation of a three-phase bidirectional DC-DC Dual Active Bridge using SiC MOSFETs. The proposed circuit is first built in Matlab for simulation analysis. Then a phase shift modulation controller is designed in Simulink to test the simulation circuit. The controls are then integrated through an FPGA to test the prototype. Simulations and experimental results are evaluated to demonstrate the functionality and performance of the proposed circuit

A multi-port power conversion system for the more electric aircraft

In more electric aircraft (MEA) weight reduction and energy efficiency constitute the key figures. Additionally, the safety and continuity of operation of its electrical power distribution system (EPDS) is of critical importance. These sets of desired features are in disagreement with each other, because higher redundancy, needed to guarantee the safety of operation, implies additional weight. In fact, EPDS is usually divided into isolated sections, which need to be sized for the worst-case scenario. Several concepts of EPDS have been investigated, aiming at enabling the power exchange among separate sections, which allows better optimization for power and weight of the whole system. In this paper, an approach based on the widespread use of multi-port power converters for both DC/DC and DC/AC stages is proposed. System integration of these two is proposed as a multiport power conversion system (MPCS), which allows a ring power distribution while galvanic isolation is still maintained, even in fault conditions. Thus, redundancy of MEA is established by no significant weight increase. A machine design analysis shows how the segmented machine could offer superior performance to the traditional one with same weight. Simulation and experimental verifications show the system feasibility in both normal and fault operations

Analysis of AC link topologies in non-isolated DC/DC triple active bridge converter for current stress minimization

This paper presents analysis of the non-isolated DC/DC triple active bridge (TAB) converter under various purely inductor-based AC link topologies. The objective of the analysis is to find the topology that incorporates the least value of the AC link inductors which leads to reduced converter footprint in addition to minimum internal current stresses. Modelling of the TAB under each of the different topologies is presented in per unit expressions of power transfer and reactive power assuming fundamental harmonic analysis. The power expressions are used to calculate the inductor values necessary to achieve same rated power transfer of Dual Active Bridge (DAB) converter for the sake of standardizing comparison. On this basis, the topology requiring the least value of interface inductors, hence lowest footprint, is identified. Furthermore, based on phase shift control, particle swarm optimization (PSO) is used to calculate optimal phase shift ratios in each of the proposed topologies to minimize reactive power loss (hence current stress). The topology with minimum stresses is therefore identified and the results are substantiated using a Matlab-Simulink model to verify the theoretical analysis

Stability design criteria and volt var control for distribution system with single phase solid state transformers

Due to recent advancements in semiconductor technology, power electronic converters for high voltage, high power, and high frequency applications will soon be commercially available. Conventional single phase distribution transformers are replaced by solid state transformers (SST) in a distribution test system to investigate their interactive dynamics. Under certain circumstances, instabilities due to harmonic resonance are observed. A design criterion for solid state transformer during no load conditions has been proposed in order to avoid instability using an impedance-based analysis. Stability assessment is also extended to include the impact of distribution system voltages and system wide impedance analysis. It is shown that if the SST filters throughout the system are designed with regards to the proposed stability criterion, then system stability is guaranteed regardless of configuration. This leads to two resulting applications: (1) the order in which the SSTs are connected to the system will not generate instability if the criterion is satisfied, and (2) a system configuration change due to a fault will not produce instability. In distribution power systems, feeder voltages can be very sensitive to changes in load and/or distributed generation. A solid-state-transformer-based local voltage control strategy is introduced to reduce variability distribution system bus voltages. An on-line dynamic volt-var control (VVC) algorithm is proposed to regulate bus voltages by injecting or absorbing reactive power through a solid state transformer. The main goal of the algorithm is to enforce strict voltage constraints on the system voltages. The proposed control algorithm is validated in both a radial and meshed distribution system. --Abstract, page iii

Active Power Filter With Automatic Control Circuit For Neutral Current Harmonic Minimization Technique [QC446.3.H37 I98 2007 f rb].

Disertasi ini bertujuan untuk membangun penuras kuasa aktif yang cekap dan boleh dipercayai bagi meminimumkan lebihan harmonik arus neutral dan juga masalah harmonik di dalam talian neutral untuk sistem tiga fasa empat dawai. The aim of this thesis is to develop an efficient and reliable active power filter in order to minimize the excessive neutral current as well as harmonic problem in the neutral line for three phase four wire system