Transformerless High-Power Medium-Voltage Multi-Module PV Converters

This thesis is focused on the modular multilevel converter (MMC) for Photovoltaic (PV) applications. It is an attempt to address the issues associated with the modeling, control, and power mismatch elimination of the MMC-based PV systems. Firstly, a new real power reference generation scheme is proposed that creates a linear relationship between the real power reference of the system and the dc link voltage of the submodules. Further, a new power mismatch elimination strategy is proposed for the MMC-based PV system which ensures balanced currents are delivered to the host grid regardless of leg and arm power mismatches. The thesis also proposes a new configuration for embedding battery energy storage(BES) systems into the arms of the converter. Then, an enabling control scheme is proposed for the MMC-based PV-BES system which employs the embedded BES systems to eliminate power mismatches and smoothen the output power of the PV generators. To mitigate large power mismatches which are outside the power rating of the BES systems, the proposed strategy uses a combination of power exchange with BES systems and power transfer between the legs of the converter. Finally, a modified power mismatch elimination strategy is proposed to ensure balanced grid currents even if the grid voltage is unbalanced. To achieve this goal, unequal powers are delivered to the grid from the phases of the system according to the voltage magnitude of each phase

Photovoltaics, Batteries, and Silicon Carbide Power Electronics Based Infrastructure for Sustainable Power Networks

The consequences of climate change have emphasized the need for a power network that is centered around clean, green, and renewable sources of energy. Currently, Photovoltaics (PV) and wind turbines are the only two modes of technology that can convert renewable energy of the sun and wind respectively into large-scale power for the electricity network. This dissertation aims at providing a novel solution to implement these sources of power (majorly PV) coupled with Lithium-ion battery storage in an efficient and sustainable approach. Such a power network can enable efficiency, reliability, low-cost, and sustainability with minimum impact to the environment. The first chapter illustrates the utilization of PV- and battery-based local power networks for low voltage loads as well as the significance of local DC power in the transportation sector. Chapter two focuses on the most efficient and maximum utilization of PV and battery power in an AC infrastructure. A simulated use-case for load satisfaction and feasibility analysis of 10 university-scale buildings is illustrated. The role of PV- and battery-based networks to fulfill the new demand from the electrification of the surface transportation sector discussed in Chapter three. Chapter four analyzes the PV- and battery- based network on a global perspective and proposes a DC power network with PV and complementary wind power to fulfill the power needs across the globe. Finally, the role of SiC power electronics and the design concept for an SiC based DC-to-DC converter for maximum utilization of PV/wind and battery power through enabling HVDC transmission is discussed in Chapter six

A novel backup protection scheme for hybrid AC/DC power systems

This thesis presents and demonstrates (both via simulation and hardware-based tests) a new protection scheme designed to safeguard hybrid AC/DC distribution networks against DC faults that are not cleared by the main MVDC (Medium Voltage DC) link protection. The protection scheme relies on the apparent impedance measured at the AC "side" of the MVDC link to detect faults on the DC system. It can be readily implemented on existing distance protection relays with no changes to existing measuring equipment. An overview of the literature in this area is presented and it is shown that the protection of MVDC links is only considered at a converter station level. There appears to be no consideration of protecting the MVDC system from the wider AC power system via backup - as would be the case for standard AC distribution network assets, where the failure of main protection would require a (usually remote) backup protection system to operate to clear the fault. Very little literature considers remote backup protection of MVDC links.;To address this issue, the research presented in this thesis characterises the apparent impedance as measured in the neighbouring AC system under various DC fault conditions on an adjacent MVDC link. Initial studies, based on simulations, show that a highly inductive characteristic, in terms of the calculations from the measured AC voltages and currents, is apparent on all three phases in the neighbouring AC system during DC-side pole-to-pole and pole-poleground faults. This response is confirmed via a series of experiments conducted at low voltage in a laboratory environment using scaled down electrical components. From this classification, a fast-acting backup protection methodology, which can detect pole-to-pole and pole-poleground faults within 40 ms, is proposed and trialled through simulation. The solution can be deployed on distance protection relays using a typically unused zone (e.g. zone 4).;New relays could, of course, incorporate this functionality as standard in the future. To maximise confidence and demonstrate the compatibility of the solution, the protection scheme is deployed under a real-time hardware-in-the-loop environment using a commercially available distance protection relay. Suggestions to improve the stability of the proposed solution are discussed and demonstrated. Future areas of work are identified and described. As an appendix, early stage work pertaining to the potential application and benefits of MVDC is presented for two Scottish distribution networks. The findings from this are presented as supplementary material at the end of the thesis.This thesis presents and demonstrates (both via simulation and hardware-based tests) a new protection scheme designed to safeguard hybrid AC/DC distribution networks against DC faults that are not cleared by the main MVDC (Medium Voltage DC) link protection. The protection scheme relies on the apparent impedance measured at the AC "side" of the MVDC link to detect faults on the DC system. It can be readily implemented on existing distance protection relays with no changes to existing measuring equipment. An overview of the literature in this area is presented and it is shown that the protection of MVDC links is only considered at a converter station level. There appears to be no consideration of protecting the MVDC system from the wider AC power system via backup - as would be the case for standard AC distribution network assets, where the failure of main protection would require a (usually remote) backup protection system to operate to clear the fault. Very little literature considers remote backup protection of MVDC links.;To address this issue, the research presented in this thesis characterises the apparent impedance as measured in the neighbouring AC system under various DC fault conditions on an adjacent MVDC link. Initial studies, based on simulations, show that a highly inductive characteristic, in terms of the calculations from the measured AC voltages and currents, is apparent on all three phases in the neighbouring AC system during DC-side pole-to-pole and pole-poleground faults. This response is confirmed via a series of experiments conducted at low voltage in a laboratory environment using scaled down electrical components. From this classification, a fast-acting backup protection methodology, which can detect pole-to-pole and pole-poleground faults within 40 ms, is proposed and trialled through simulation. The solution can be deployed on distance protection relays using a typically unused zone (e.g. zone 4).;New relays could, of course, incorporate this functionality as standard in the future. To maximise confidence and demonstrate the compatibility of the solution, the protection scheme is deployed under a real-time hardware-in-the-loop environment using a commercially available distance protection relay. Suggestions to improve the stability of the proposed solution are discussed and demonstrated. Future areas of work are identified and described. As an appendix, early stage work pertaining to the potential application and benefits of MVDC is presented for two Scottish distribution networks. The findings from this are presented as supplementary material at the end of the thesis

Design of power converters with embedded energy storage for hybrid DC-AC applications

The high penetration of renewable energies into power systems is leading to a revolution in the structure of modern power grids. In this context, the present thesis investigates the design of power electronics converters with extended capabilities due to the embedding of energy storage within the topologies. Thus, the research objective is to propose power converters with capabilities of integrating energy storage technologies to provide further services required for the operation of hybrid dc-ac systems. The thesis contains two parts, first part shows the work developed for low- and medium-power applications, while the second part describes the investigation performed for high-power systems. The first part of this thesis explains the design and operation of a three-port dc-dc-ac converter developed for integrating energy storage into hybrid dc-ac applications. The topology is based on a conventional two-level dc-ac converter, and it uses a single power conversion stage to control the power flow between three ports, minimising the required components. Simulation and experimental results validate the operation of the proposal, showing that a multi-variable control system allows exploiting the degrees of freedom to manage power interactions of multiple elements without needing extra power converters. Furthermore, a comparative analysis is carried on to showcase the advantages and limitations of the proposal as opposed to state-of-the-art solutions in the same context. The study concludes that the proposed topology is suitable for low- and medium-power systems with bidirectional power flow capabilities among all ports and limited voltage boost needs. Simulation analysis shows that efficiencies up to 95.94% can be reached for a 3 kW design, which compares to efficiencies of similar state-of-the-art topologies. Moreover, the operation is also validated in a reduced-scale prototype allowing to test the multi-variable control scheme in a real-time implementation. The second part of the thesis focuses on the design and operation of a Modular Multilevel Converter (MMC) topology with integrated energy storage using new parallel branches in the phases of the converter. This topology allows the integration of partially-rated Energy Storage Systems(ESS) to decouple the ac and dc sides of a High Voltage Direct Current~(HVDC) substation. Thus, it enables the provision of ancillary services such as fast frequency response, black-start capabilities and load-levelling, which are required by modern hybrid dc-ac power grids. Results show that the proposal allows the addition of up to 37% power from the ESS considering similarly rated power semiconductors in a simulated 1 GW MMC substation. Analysis shows that extra device losses remain under 1% for an additional +-10% of ESS power on top of the nominal substation-rated power. Furthermore, a laboratory-scale experimental rig was built to demonstrate the operation of the proposed design. In conclusion, two different topologies are proposed and analysed for integrating energy storage into hybrid dc-ac applications depending on the power rating required. The study is supported by simulation and experimental results obtained during the project to validate both proposals

High Power Density and High Efficiency Converter Topologies for Renewable Energy Conversion and EV Applications

This dissertation work presents two novel converter topologies (a three-level ANPC inverter utilizing hybrid Si/SiC switches and an Asymmetric Alternate Arm Converter (AAAC) topology) that are suitable for high efficiency and high-power density energy conversion systems. The operation principle, modulation, and control strategy of these newly introduced converter topologies are presented in detail supported by simulation and experimental results. A thorough design optimization of these converter topologies (Si/SiC current rating ratio optimization and gate control strategies for the three-level ANPC inverter topology and component sizing for the asymmetric alternate arm converter topology) are also presented. Performance comparison of the proposed converter topologies with other similar converter topologies is also presented. The performance of the proposed ANPC inverter topology is compared with other ANPC inverter topologies such as an all SiC MOSFET ANPC inverter topology, an all Si IGBT ANPC inverter topology and mixed Si IGBT and SiC MOSFET based ANPC inverter topologies in terms of efficiency and cost. The efficiency and cost comparison results show that the proposed hybrid Si/SiC switch based ANPC inverter has higher efficiency and lower cost compared to the other ANPC inverter topologies considered for the comparison. The performance of the asymmetric alternate arm converter topology is also compared with other similar voltage source converter topologies such as the modular multilevel converter topology, the alternate arm converter topology, and the improved alternate arm converter topology in terms of total device count, number of switches per current conduction path, output voltage levels, dc-fault blocking capability and overmodulation capability. The proposed multilevel converter topology has lower total number of devices and lower number of devices per current conduction path hence it has lower cost and lower conduction power loss. However, it has lower number of output voltage levels (requiring larger ac interface inductors) and lacks dc-fault blocking and overmodulation operation capabilities. A converter figure-of-merit accounting for the hybrid Si/SiC switch and converter topology properties is also proposed to help perform quick performance comparison between different hybrid Si/SiC switch based converter topologies. It eliminates the need for developing full electro-thermal power loss model for different converter topologies that would otherwise be needed to carry out power loss comparison between different converter topologies. Hence it saves time and effort

HVDC Transmission and Energy Storage for Wind Power Plant

This thesis will investigate the effects of an energy storage system incorporated into the submodules of a modular multilevel converter connected to a HVDC line. A simulation has been made to see the effects of energy storage on the transmission of power from a generator connected to the grid via a HVDC line with two MMCs connected in each end, one of which incorporates the energy storage system

Grid-Connected Energy Storage Systems: State-of-the-Art and Emerging Technologies

High penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.This work was supported in part by the Office of Naval Research Global under Grant N62909-19-1-2081, in part by the National Research Foundation of Singapore Investigatorship under Award NRFI2017-08, and in part by the I2001E0069 Industrial Alignment Funding. (Corresponding author: Josep Pou.

Analysis and design of a modular multilevel converter with trapezoidal modulation for medium and high voltage DC-DC transformers

Conventional dual active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique where staircase approximated trapezoidal voltage waveforms are produced; thus alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. Therefore, the analyzed converter is a modular multi-level converter operated in a new mode with no common-mode dc arm currents as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft switching characteristics will be shown to be comparable to those of the two-level dual active bridge. Experimental results from a scaled test rig validate the presented concept

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