Multiport DC-DC Converters for Hybrid Energy Systems

Renewable energy sources (RESs) like solar and wind have gained attention for their potential to reduce reliance on fossil fuels and mitigate climate change. However, integrating multiple RESs into a power grid is challenging due to their unpredictable nature. Power electronic converters can manage hybrid energy systems by controlling power flow between RESs, storages, and the grid. Conventional single input dc-dc converters have limitations such as low efficiency, bulky designs, and complex control systems. Multiport dc-dc converters (MPCs) have emerged as a solution for hybridizing multiple sources, storages, and load systems by providing a common interface. Existing MPCs have limitations such as high component count, limited operational range, complex control strategies and restrictions on the number of inputs to list a few. Thus, there is a need to develop new MPCs that combine the advantages of existing designs while overcoming their limitations. Isolated MPCs with unipolar or bipolar outputs are needed that can accommodate any number of inputs, offer high voltage gain, use fixed magnetic components for galvanic isolation (regardless of the number of ports), and have a simplified control strategy. Additionally, new non-isolated MPCs with unipolar or bipolar outputs are required, featuring reduced component count, simultaneous power transfer and power flow between input ports, high voltage gain, low control complexity, and modular design allowing for arbitrary increase in the number of input ports. There is also an opportunity to apply MPCs in the integration of RESs and storages to ac grids through multilevel inverters for low component count, high efficiency, low harmonics, and higher power density. Further, advances in bipolar MPCs provide the chance to balance the dc bus without requiring a complex control system.acceptedVersio

Development of Multiport Single Stage Bidirectional Converter for Photovoltaic and Energy Storage Integration

The energy market is on the verge of a paradigm shift as the emergence of renewable energy sources over traditional fossil fuel based energy supply has started to become cost competitive and viable. Unfortunately, most of the attractive renewable sources come with inherent challenges such as: intermittency and unreliability. This is problematic for today\u27s stable, day ahead market based power system. Fortunately, it is well established that energy storage devices can compensate for renewable sources shortcomings. This makes the integration of energy storage with the renewable energy sources, one of the biggest challenges of modern distributed generation solution. This work discusses, the current state of the art of power conversion systems that integrate photovoltaic and battery energy storage systems. It is established that the control of bidirectional power flow to the energy storage device can be improved by optimizing its modulation and control. Traditional multistage conversion systems offers the required power delivery options, but suffers from a rigid power management system, reduced efficiency and increased cost. To solve this problem, a novel three port converter was developed which allows bidirectional power flow between the battery and the load, and unidirectional power flow from the photovoltaic port. The individual two-port portions of the three port converter were optimized in terms of modulation scheme. This leads to optimization of the proposed converter, for all possible power flow modes. In the second stage of the project, the three port converter was improved both in terms of cost and efficiency by proposing an improved topology. The improved three port converter has reduced functionality but is a perfect fit for the targeted microinverter application. The overall control system was designed to achieve improved reference tracking for power management and output AC voltage control. The bidirectional converter and both the proposed three port converters were analyzed theoretically. Finally, experimental prototypes were built to verify their performance

Analysis and Design of a Hybrid Dickson Switched Capacitor Converter for Intermediate Bus Converter Applications

By 2020 it is predicted that 1/3 of all data will pass through the cloud. With society\u27s growing dependency on data, it is vital that data centers, the cloud\u27s physical house of content, operate with optimal energy performance to reduce operating costs.Unfortunately, today\u27s data centers are inefficient, both economically and environmentally. This has led to an increase in demand for energy-efficient servers. One opportunity for improved efficiency is in the power delivery architecture which delivers power from the grid to the motherboard. In this dissertation, the main focus is the intermediate bus converter (IBC), used for the intermediate conversion, typically 48-12V/5V, in server power supplies. The IBC requires compact design so that it can be placed as close to the load as possible to enable more space for computing power and high efficiency to reduce the need for external cooling. Most commonly used converter topologies today include expensive bulky magnetics hindering the converter\u27s power density. Furthermore, high output current of an IBC makes the efficiency very sensitive to any resistance, such as magnetic parasitic resistance or PCB trace resistance. In this work, analytical loss models are used to review the advantages and disadvantages of frequently used IBC topologies such as the phase-shifted full bridge and LLC. The Hybrid Dickson Switched Capacitor (HDSC) topology is also analyzed. The HDSC\u27s high step-down conversion ratio and low dependence on magnetics due to the reduced applied volt-seconds, provides a new opportunity for applications such as the intermediate bus converter. The HDSC designs the on-time of devices in order to achieve soft-charging between flying capacitors. Other advantages of the HDSC include low switch stress, small magnetics and adjustable duty cycle for voltage regulation. Challenges, such as minimizing parasitic inductance and resistance between flying capacitors, are addressed and recommendations for PCB layout are provided. In this paper, a 4:1 24-5V and 8:1 48-5V, 100W GaN-based HDSC is designed and tested. The influences of capacitor mismatch and limitations placed on soft-charging operation for the HDSC is also modeled. This analysis can be used as a tool for designers when selecting flying capacitors

Morphing Switched-Capacitor Converters with Variable Conversion Ratio

High-voltage-gain and wide-input-range dc-dc converters are widely used in various electronics and industrial products such as portable devices, telecommunication, automotive, and aerospace systems. The two-stage converter is a widely adopted architecture for such applications, and it is proven to have a higher efficiency as compared with that of the single-stage converter. This paper presents a modular-cell-based morphing switched-capacitor (SC) converter for application as a front-end converter of the two-stage converter. The conversion ratio of this converter is flexible and variable and can be freely extended by increasing more SC modules. The varying conversion ratio is achieved through the morphing of the converter's structure corresponding to the amplitude of the input voltage. This converter is light and compact, and is highly efficient over a very wide range of input voltage and load conditions. Experimental work on a 25-W, 6-30-V input, 3.5-8.5-V output prototype, is performed. For a single SC module, the efficiency over the entire input voltage range is higher than 98%. Applied into the two-stage converter, the overall efficiency achievable over the entire operating range is 80% including the driver's loss

Power factor-corrected transformerless three-phase PWM converter for UPS applications

This thesis describes the research of a new transformerless three phase PWM converter for uninterruptible power supplies (UPS) applications. The removal of the bulky three phase transformer in larger power UPS can provide a significant saving in weight and cost of the overall system. The converter consists of a new four-wire rectifier coupled with a four-wire inverter via a dc bus. The supply and load neutral may be connected together without any neutral current flowing into the utility regardless of the load on the inverter. This allows the load to be at the same potential as the utility. The rectifier, inverter and complete UPS and control system are described in detail and simulation results are used extensively to back up the theory. An experimental prototype of the four-wire rectifier provides further confirmation of the principles. A further proposal to digitize the system is given. This would reduce the size of the required control circuit and simplify the hardware requirements

Grid Integration of DC Buildings: Standards, Requirements and Power Converter Topologies

Residential dc microgrids and nanogrids are the emerging technology that is aimed to promote the transition to energy-efficient buildings and provide simple, highly flexible integration of renewables, storages, and loads. At the same time, the mass acceptance of dc buildings is slowed down by the relative immaturity of the dc technology, lack of standardization and general awareness about its potential. Additional efforts from multiple directions are necessary to promote this technology and increase its market attractiveness. In the near-term, it is highly likely that the dc buildings will be connected to the conventional ac distribution grid by a front-end ac-dc converter that provides all the necessary protection and desired functionality. At the same time, the corresponding requirements for this converter have not been yet consolidated. To address this, present paper focuses on various aspects of the integration of dc buildings and includes analysis of related standards, directives, operational and compatibility requirements as well as classification of voltage levels. In addition, power converter configurations and modulation methods are analyzed and compared. A classification of topologies that can provide the required functionality for the application is proposed. Finally, future trends and remaining challenges pointed out to motivate new contributions to this topic

Analysis of a new family of DC-DC converters with input-parallel output-series structure

There is an increasing trend of development and installation of switching power supplies due to their highly efficient power conversion, fast power control and high quality power conditioning for applications such as renewable energy integration and energy storage management systems. In most of these applications, high voltage conversion ratio is required. However, basic switching converters have limited voltage conversion ratio. There has been much research into development of high gain power converters. While most of the reported topologies focus on high gain and high efficiency, in this thesis, the input and output ripple currents and reliability are also considered to derive a new converter structure suitable for high step-up voltage conversion applications. High ripple currents and voltages at the input and output of dc-dc converters are not desirable because they may affect the operation of the dc source or the load. A number of converters operating in an interleaved manner can reduce these ripples. This thesis proposes a dc/dc switching converter structure which is capable of reducing the ripple problem through interleaved action, in addition to high gain and high efficiency voltage conversion. The thesis analyses the proposed converter structure through a dual buck-boost converter topology. The structure allows different converter topologies and combinations of them for different applications to be configured. The study begins with a motivation and a literature review of dc/dc converters. The new family of high step-up converters is introduced with an interleaved buck-boost as an example, followed by small-signal analysis. Experimental verifications, conclusions and future work are discussed

Analysis of a new family of DC-DC converters with input-parallel output-series structure

There is an increasing trend of development and installation of switching power supplies due to their highly efficient power conversion, fast power control and high quality power conditioning for applications such as renewable energy integration and energy storage management systems. In most of these applications, high voltage conversion ratio is required. However, basic switching converters have limited voltage conversion ratio. There has been much research into development of high gain power converters. While most of the reported topologies focus on high gain and high efficiency, in this thesis, the input and output ripple currents and reliability are also considered to derive a new converter structure suitable for high step-up voltage conversion applications. High ripple currents and voltages at the input and output of dc-dc converters are not desirable because they may affect the operation of the dc source or the load. A number of converters operating in an interleaved manner can reduce these ripples. This thesis proposes a dc/dc switching converter structure which is capable of reducing the ripple problem through interleaved action, in addition to high gain and high efficiency voltage conversion. The thesis analyses the proposed converter structure through a dual buck-boost converter topology. The structure allows different converter topologies and combinations of them for different applications to be configured. The study begins with a motivation and a literature review of dc/dc converters. The new family of high step-up converters is introduced with an interleaved buck-boost as an example, followed by small-signal analysis. Experimental verifications, conclusions and future work are discussed

Soft-Switching Solid-State Transformer (S4T) With Reduced Conduction Loss

© 2020 IEEESolid-state transformers (SSTs) are a promising solution for photovoltaic (PV), wind, traction, data center, battery energy storage system (BESS), and fast charging electric vehicle (EV) applications. Traditional SSTs are typically three-stage, i.e., hard-switching cascaded multilevel rectifiers and inverters with dual active bridge (DAB) converters, which leads to bulky passives, low efficiency, and high EMI. This paper proposes a new soft-switching solid-state transformer (S4T). The S4T has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc-link, and controlled dv/dt which reduces EMI. The S4T comprises two reverse-blocking current-source inverter (CSI) bridges, auxiliary branches for ZVS, and transformer magnetizing inductor as reduced dc-link with 60% ripple. Compared to the prior S4T, an effective change on the leakage inductance diode is made to reduce the number of the devices on the main power path by 20% for significant conduction loss saving and retain the same functionality of damping the resonance between the leakage and resonant capacitors and recycling trapped leakage energy. The conduction loss saving is crucial, being the dominating loss mechanism in SSTs. Importantly, the proposed single-stage SST not only holds the potential for high power density and high efficiency, but also has full functionality, e.g., multiport DC loads integration, voltage regulation, reactive power compensation, unlike traditional single-stage matrix SST. The S4T can achieve single-stage isolated bidirectional DC-DC, AC-DC, DC-AC, or AC-AC conversion. It can also be configured input-series output-parallel (ISOP) in a modular way for medium-voltage (MV) grids. Hence, the S4T is a promising candidate of the SST. The full functionality, e.g., voltage buck-boost, multiport, etc. and the universality of the S4T for DC-DC, DC-AC, and AC-AC conversion are verified through simulations and experiments of two-port and three-port MV prototypes based on 3.3 kV SiC MOSFETs in DC-DC, DC-AC, and AC-AC modes at 2 kV.This work was supported by Power America Institute, ARPA-E under DE-AR0000899, and Center for Distributed Energy, Georgia Institute of Technology