Study on Power Management of Triple Active Bridge DC-DC Converter for DC Grid System

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Study on Power Management of Triple Active Bridge DC-DC Converter for DC Grid System

Yu Yue

2019.09

Tokyo Metropolitan University

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List of Figures

1.1 Infrastructure of the DC grid model ... 4

1.2 Grid connected mode of DC grid operation ... 4

1.3 Island connected mode of DC grid operation ... 5

1.4 Diagram of DC grid system in Aichi project ... 7

1.5 DC distribution system with basic cells of renewable energy sources, energy storage element, DC converters and load in DC grids ... 7

2.1 Sources of electric power consumption in 2018 ... 15

2.2 Total Renewable Power Generation Capacity from 2011 to 2017 ... 16

2.3 Residential future DC grid scheme ... 18

2.4 Overview of the DC grid in Aomori project ... 19

2.5 Hybrid use of AC and DC distribution system in DC grid ... 25

2.6 Comparison of AC and DC power distribution systems in an industrial application... 26

2.7 DAB converter circuit topology ... 31

2.8 Equivalent circuit of DAB converter and phase shift signal patterns ... 32

2.9 Voltage and current of DAB converter in 4 period with different switching status ... 32

2.10 Waveforms of current iL, iP and iS in four periods ... 33

2.11 Simplified DAB converter equivalent circuit ... 34

2.12 Complex vector diagram of voltage and current in primary and tertiary side ... 35

2.13 Topology of TAB converter, and switching patterns of phase shift ... 37

2.14 Y-Δ connection of the equivalent TAB converter circuit... 37

2.15 Scheme of closed-loop phase shift control for power distribution ... 40

2.16 The waveforms of P2 in TAB converter response to kp=1, 5, 10 ... 43

2.17 Proposed DC grid and DC power distribution application using the TAB converter ... 43

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3.1 Diagram of load balancing and power flow control in the DC distribution system using the TAB

converter ... 49

3.2 Methodology flow chart of power management including power balancing and power flow control ... 49

3.3 Topology of DC power distribution system using TAB converter with the one battery, double loads ... 50

3.4 Power flow scheme of the DC distribution system with one battery, double loads during the three simulation periods ... 52

3.5 Simulation results and waveforms of the one battery, double loads for a DC distribution system ... 54

3.6 Current and voltage waveforms of the 500 W TAB converter, for steady-state simulation conditions ... 56

3.7 Two batteries and single load DC power distribution system using TAB converter ... 57

3.8 Power flow diagram of the DC distribution system with two batteries, single loads during the three periods ... 58

3.9 Simulation results and waveforms of the two batteries, single load distribution system. ... 59

3.10 Diagram of a DC power distribution system in a data center using the TAB converter ... 62

3.11 Power flow diagram of the 380 V DC power distribution system in three simulation periods ... 63

3.12 Simulation results and waveforms of 380 V DC power distribution system using the TAB converter ... 64

3.13 The prototype active H-bridge cell circuit. ... 66

3.14 Current and voltage waveforms for the 500 W TAB converter, for δ2 = δ3 = 30° ... 70

3.15 Efficiency versus phase-shift curve of the 500 W TAB converter ... 70

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3.16 Experimental results for the 500 W TAB converter in the double-load DC power distribution

system ... 71

4.1 Configuration of AC-DC rectifier ... 75

4.2 PWM gate signals for the switches ... 75

4.3 The abc-dq transform diagram ... 77

4.4 The abc-dq transform with space vector ... 78

4.5 Closed-loop control for the AC-DC rectifier ... 81

4.6 Circuit diagram of power distribution model using AC-DC rectifier and supply to TAB converter ... 82

4.7 Simulation results and waveforms of the AC-DC rectifier ... 83

4.8 Classical efficiency curve of a power converter with changing power delivered ... 87

4.9 Turn-on and turn-off pattern of the MOSFET in the TAB converter. ... 89

4.10 Overall efficiency with changing load factor dependence of the TAB converter. ... 90

4.11 The loss components with changing load factor dependence of the TAB converter ... 90

4.12 Overall efficiency with changing load factor dependence of the AC-DC rectifier ... 93

4.13 Comparison of the TAB converter and AC-DC rectifier efficiencies ... 93

4.14 Combined using of the TAB converter and AC-DC rectifier under light load condition. ... 94

4.15 Combined using of the TAB converter and AC-DC rectifier under heavy load condition ... 94

4.16 Overall efficiency of the combined operation with combined using TAB converter and rectifier ... 95

5.1 “2N” power transmission structure the data center. ... 103

5.2 System operation period and MTBF ... 105

5.3 Block diagram of connection types: (a). Series and parallel connection; (b). Combined connection ... 107

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5.4 A system compromised of assets in a combined series and parallel relationship ... 109

5.5 Reliability model of a conventional power distribution system ... 110

5.6 Reliability model of the proposed DC power distribution system ... 110

5.7 Monte Carlo simulation model of proposed 380V 3N power distribution system diagram ... 114

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List of Tables

Table 3.1 Parameters of the 500 W TAB converter for a DC power distribution system ... 54

Table 3.2 Simulation conditions for the proposed one battery, double loads system ... 55

Table 3.3 Simulation parameters, for the 500 W TAB converter in the two batteries, single load power distribution system. ... 59

Table 3.4 Parameters of the 380-V TAB converter used in the DC power distribution system. ... 61

Table 3.5 Simulation conditions for the 380 V DC power distribution system with TAB converter ... 62

Table 3.6 Specification of GaN power devices. ... 67

Table 3.7 Specification of three-winding transformer ... 67

Table 3.8 Parameters of the 500 W TAB converter for power distribution ... 69

Table.4.1 Parameters of the AC-DC rectifier and TAB converter simulation models ... 83

Table.4.2 Parameters of the TAB converter ... 88

Table.4.3 Parameters of Rohm 1200 V MOSFET applied in the TAB converter ... 88

Table.4.4 Parameters of MOSFET applied in the AC rectifier ... 92

Table 5.1 Reliability parameters of electrical components used in industrial and commercial systems ... 111

Table 5.2 Reliability results comparison between a conventional power distribution system and the proposed power distribution system ... 112

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Chapter 1. Introduction

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1.1 Research Background

1.1.1 Review of DC Grid

The pressures of energy shortage and pollution to environment have attracted broad attentions [1] [2].

It is noticed that, the conventional electrical power plants, such as thermal power plants and so on, are commonly in large-scale and caused severe air pollution emission globally [3][4]. On the other hand, the renewable energy sources such as solar power, wind power, etc. are usually in small-scale, and their power outputs are unstable [5] [6].

Utilization and marketization of the renewable energy resource is a main motivation to renovate the conventional power systems. DC grids that employs renewable energy resources, including solar, fuel cell, wind, tidal energies, etc., contributes to assuage the burden of energy insufficiency, and to reducing the environment pollution. Small-scale generations generating power from renewable resources and equipped with power devices are installed nearby the loads to satisfy the demand for power distribution [7] [8]. Moreover, DC grids provide effective solution in utilizing the renewable resources, and show great benefits in aspect of high efficiency because the DC transmission lines are organized between renewable resource to electric input[9]-[12], and local appliance. That means in the DC grid systems can reduce the stages of power conversion loss between DC lines and AC lines [13] [14].

Typical example of a DC grid infrastructure is shown in Fig. 1.1 [15]. It includes the loads, immediate storages, controllers of sensors, communication and control techniques with distributed power generation systems. This structure forms an efficient and dependable power system in DC grid that has capability of delivering power especially in the case of failures and breakdowns happen in the main utility supply. Clean energy generation in this DC grids system commonly including photovoltaic (PV) panels, wind turbines, fuel cells and so on. Since distributed renewable energy

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sources are arranged, to solve the desultory power output of the renewable sources, the backup systems, such as batteries are generally equipped to remedy the transitory gap between sources and appliances. Meanwhile, as shown in Figs. 1.2 and 1.3, DC grid shows grid connected mode and island mode, depends on the operation of AC grid. When DC grid is connect to utility grid and distribution lines, basically power interconnected through cables and distribution-level transmission lines to form unerring and reliable power systems for applications. When AC grid is off the power network, the power storages and DC generations can support the overall system with DC buses and converters.

At first, Japan originally designed a smart DC grid aimed at maintaining electricity's quality, and promoted the involvement of renewable energies at the same time [16]. Meanwhile, control technologies for DC grids is consequently needed since the DC grids are connected to wind and solar power generators from where the power supplies are not stable and rely on the weather conditions.

At the same time in the United States, the installation of a smart grid was firstly discussed as part of approaches for restraint the electricity demand from severe power shortage caused by regional power network failure. Therefore, the objective was to reduce the gap between power consumption and amount of power generation during peak demand hours, by using smart power meters that could reassign various power consumption rates based on the time of one day [17].

With Japan and US originally started to implement the DC grid for different situation and circumstances, other countries are now begin to explore and utilize DC grid, so that both power demand shortage restrain and promotion of energy conservation by renewable energy can be achieved at the same time.

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Fig.1.1 Infrastructure of the DC grid model.

Fig.1.2 Grid connected mode of DC grid operation.

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Fig.1.3 Island connected mode of DC grid operation.

Development of DC Grid in Japan

The energy shortage dilemma happen not only in the other countries, but also in Japan, especially after the March 11 earthquake suddenly. To solve this urgent energy shortage situation, the society, the researchers, and the whole industry have expectations to the high performance smart grid. The designed smart DC grid, could utilize the data and communication techniques to the renewable energy system, so that the disparity between power supply and needs can be effectively eliminated. In the beginning, Japan built the DC grid mostly appeal to promote the power quality of electricity network, and introduce the renewable energy to against energy shortage in the same time [18].

Recently, the DC grid projects are promoted and leading by NEDO (The New Energy and Industrial Technology Development Organization) in Japan. The Aichi project shown in Fig.1.4 is the regional power grids with various new energies, and demonstrated in the year of 2005 for Aichi Expo and

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Central Japan Airport City. In this Aichi project, the grids utilizing PV cells, fuel cells, and battery storage system, and controllable inverters is constructed. The fuel cells applied in the system include two molten carbonate fuel cells, solid oxide fuel cells, and phosphoric acid fuel cells. The PV system installed adopted multi-crystalline silicon, amorphous silicon and a single crystalline silicon bifacial type with 330 kW total capacity [18].

1.1.2 Review of Power Distribution System

DC distribution system is the most indispensable constituent part to interface with various electrical components and subsystems that utilize renewable energy sources and storage devices in DC grid.

DC power distribution system can facility multiple power conversion and transmission efficiency due to the reduction of devices in conversion stages. Moreover, it can enable the integration of different DC appliances including electrical vehicles (EV), LED lighting and other DC loads into the DC distribution system, which is more feasible compared to AC distribution system [19], [20]. Further, momentous modern electric network and its applications, not only residential power supply, but also telecommunication buildings, Internet companies and data centers utilized DC distribution concept for a have been presented recently [19]–[23]. Power distribution systems perform a fundamental interconnection in the modern distribution system, in where the generated electricity from renewable resources is managed and transmitted to end-consumers. Recently, impelled by policies in countries for energy conservation and pollution reduction, DC power distribution systems have been more extensively utilized in industries and becoming large-scale gradually. However, different from conventional types of thermal generations from fossil resources, renewable energy systems sometimes struggle with intermittent power supply and fluctuations.

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Fig.1.4 Diagram of DC grid system in Aichi project.

Fig.1.5 DC distribution system with basic cells of renewable energy sources, energy storage element, DC converters and load in DC grids.

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Therefore, employing the large-scale DC power distribution systems still faces rigorous challenges in the DC grid. Endeavors and attempts have been made for improving DC power distribution performance, such as demand response control is applied for smoothen the fluctuations in DC power distribution systems especially when apply the renewable energy, provided a state-of-the-art solutions to settle the efficient an stable operation, and integration of the large-scale DC distribution system [24] [25].

1.1.3 Power Converter for Distributed System

DC-DC converters serve as essential power routers in DC distribution systems since they realize the linkage between DC sources, DC energy storage and DC busses in the DC distribution. References [26] [27] focused on developing novel DC-DC converters for DC distributed systems with PV and other renewable applications in DC grids, which is shown in Fig1.5. In the topology of this DC power distribution system with integrated multi-port DC-DC converters, those power supply cells including PV panels and wind turbines generate electricity and compose of DC grid, and restore energy into storages such as batteries. As can be understand, multi-port DC-DC converter now growing into attractive solution to integrate the renewable energy generations, energy storages and DC transmission lines with the benefits of high efficiency, cost-effective, and high power density has been discussed in literatures [28] [29]. In [28], a three-port bidirectional converter is proposed to interface with batteries and PV panels. Moreover, the power control of multi-port bidirectional converter utilize PV to charge batteries in which is designed with decoupled controller has been introduced in [29].

Another frequently used topology is the dual-active-bridge (DAB) converter which is coupled with two-port transformer for renewable energy applications has been discussed in [30-32]. DAB converter is one of the earlies isolated DC converter topologies that came into service for power transmission.

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With the benefits of small physical size and simple power management, the DAB converter is commonly used in small-scale renewable generated and distributed systems [30]. In terms of the control strategies, [32] proposed a switching control strategy to regulate and monitor the power flow in the DAB converter, and minimize the total power losses [32].

Compared to the DAB converter, TAB converter provides extra power distribution passageway with galvanic isolation to the energy storage system, which leads to better capability and higher reliability.

If replace conventional DAB converters with TAB converters, the complicated connection and power flow between DC grids, renewable sources and consumers will no longer need extra converters, which can improve conversion and overall system efficiency.

The TAB converter based on H-bridge feature has high voltage range and conversion ratio, the bidirectional power flow and wide power capacity range are suitable for integrated distribution system in DC grids[33], [34]. Using the coupled magnetic, all three ports are galvanic isolated and the direction of power flow is ensured. However, in this topology, a drawback stands in TAB converters when transferring power from one side to another assigned side, unexpected power may happen in the third side simultaneously, which means power flows among the TAB converter three ports may be coupled. Therefore, some methods using decoupling control [35], [36] have been proposed to solve these limitations.

1.2. Motivation and Objective

Even though the multi-port bidirectional converter topologies have been discussed, and some of the advanced power control of the power distribution systems in DC grids could be collected, it is observed that, only limited explorations applied the low-cost, feasible control using the sophisticated DC converter. Besides, although the applications of the power distribution systems are extensive, the case studies of power management under the certain industrial guidance and standards are not sufficient.

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This dissertation aims at proposing a highly reliable, high efficiency DC power distribution system using the TAB converter that can be used for DC grids. The DC power distribution system is equipped with precise feedback controlled TAB converter, and under the power management including power balancing and power transmission control. By using the proposed power distribution system and TAB converter, goals of higher reliability, better efficiency, and improved performance for DC grid would be achieved.

1.3 Dissertation Outline

As introduced in Objective and Motivation, this dissertation introduced power management for power distribution system in DC grid through the following chapters.

Chapter 1 mainly introduced the general information and background of the DC grid, the infrastructure of the power distribution system. The situation and project of the DC grid and power distribution system established in Japan are simply inspected. As an essential part of control for the power distribution system, and the applications of power distribution system using the TAB converter.

Chapter 2 introduced the literature review of exist researches about the severe power demand situation, and the DC grids structures aims at solving the energy urgency under development. Afterwards, the frequently used power converters including dual active bridge converter and triple active bridge converters for power distribution systems are introduced. Based on the mentioned technologies of DC grids and power converter topologies, the applications for the power distribution system are brought in and constructed.

Chapter 3 firstly derived the power flow theoretical basis, for elicitation the proposed the phase shift control, which lays the fundamental to the power management of the TAB converter. After that, the decoupling feedback control algorithm of the TAB converter based on phase shift is proposed and operated. Moreover, the feedback power balancing control is designed and proposed originally. This power balancing control methodology provides a new aspect for power distribution system in DC

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grid to achieve higher controllability and reduce the power loss. On the basis of the power management, the simulations of the 200V and 380 V power distribution systems with closed-loop control TAB converter are tested. In the end, the experiment on 200 V TAB converter prototype is tested under the phase shift control.

Chapter 4 discussed the loss analysis of the power distribution system using the TAB converter, including the loss of TAB converter body part. Firstly, an AC-DC rectifier with feedback voltage control is designed to enhance the voltage stability from the utility. Then the power loss analysis on conventional utility supplied distributed lines including the loss of rectifier using Powersim software is investigated. Afterwards total loss of the proposed DC power distribution system using the TAB converter is analyzed. Based on the power loss of system using rectifier and TAB converter, the selections of operation for industrial applications are proposed and explained to enhance the overall efficiency in the system.

Chapter 5 firstly introduced basis of reliability analysis methods, including reliability block diagram method, and Monte Carlo simulation method. The derivation equations of reliability for the complicated systems with multiple component connections are presented, based on that, the theoretical estimations of the proposed power distribution system and the conventional power distribution system are completed. Based on the structure proposed and components’ reliability parameters surveyed from references, Monte Carlo simulation model of proposed DC distribution system is constructed and simulated.

Chapter 6 summarized the achievements of each chapter, and the proposed research topic.

Furthermore, the attractive topics and technologies can be studied continually for the future work are explained and expected.

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Chapter 2. Literature Review

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In this chapter, the literatures and papers related to the DC grids, DC distribution system, and TAB converter are reviewed and introduced. The associated knowledge and information provide the thesis with sensible selections in respect to the power flow control method, the architecture of the DC power distribution system, and the indexes need to be taken in consideration.

2.1 DC grids Background and Related Research

2.1.1 DC Grids and Renewable Energy System

Currently, the expanding demand from energy consumption, the development of electrical technology in high speed, and the over-expend power in the entire industry have brought on concerns for power generations in worldwide, and have resulted in the energy crisis and shortage [37]. The actual situation of conventional energy sources, especially the fossil fuels, are under excessive exploitation and becomes affordable. Therefore, some renewable resource power plants, such as wind farms, and photovoltaic power plants have been built up in many countries [2].

On December 2018, ISEP Energy has released the report of data ratio of energy demand and supply until 2018, investigated and the global status of renewable energies and the trends in renewable energies [39]. The original data collected from the publication are shown in Fig 2.1, expressed that the consumption in worldwide are still dominated by fossil fuels, and the share of renewable energies in worldwide power generation is 16.5 % in 2018, increased 1.5% increase compare with the data in 2017 (15%). The published date implied that the that there are still much more spaces for the contribution of renewable energies, and will be constantly increased in the future.

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Fig.2.1 Sources of electric power consumption in 2018.

In the end of year 2017, the International Renewable Energy Agency (IRENA) published Renewable Capacity Statistics 2018, it is the most up-to-date, comprehensive, and accessible data and figures on renewable energy capacity status contains around 15 thousand data points collected from over 200 countries [40]. The Capacity Statistics 2018 pointed out that during that past year, the renewable generations capacity globally has grown 167 GW and achieved 2,179 GW in worldwide shown in Fig.2.2. This volume also means the yearly growth is nearly 8.3%, and has moved forward for seven straight years consecutively.

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Fig.2.2 Total Renewable Power Generation Capacity from 2011 to 2017.

Renewable energy is the promising solution for the countries who are looking forward to supports of economic growth, energy conservation, as well as elimination of carbon emissions, reduction of air pollution and improvement of energy security. Meanwhile, the power plant using renewable resources are usually with small scale with unstable outputs, and has difficulties in terms of massive generation and sustainable supply, due to its unpredictable characteristics [41]-[43]. The optimized power systems are to introduce the renewable energy sources such as wind power, solar power, etc. into the electrical power supply system to replace the part of the conventional power plants. To solve the inconsistent and unpredictable problem of the renewable energy resources, the effective way to utilize renewable energy is a big stimulation and motivation for the renovating the conventional power supply systems.

For solving the mentioned issues, the renovation for the scheme of DC grid, has becoming more common to improve the future energy and electricity system, which can be applied for modern and smart systems with support of communication. Moreover, an effectively way utilize renewable energy resources is to build a flexible but closely interacted power network, in that way, the DC grid’s main

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elements, distributed generation (DG) systems, appliances and loads, energy storage subsystems, and controller parts, can be connected from main grids part to subsystem parts by common coupling points as a complete unit [44]-[46]. Also, DC grid [47]-[49] is a promising solution for the future clean power systems using renewable energy sources, as well as it could reduce the power conversion loss by reduce the multiple conversion stages between DC and AC.

Fig.2.3 shows the overview of an advanced future DC grid scheming, which contains a central controller, loads, distributed generation systems such as wind farms, photovoltaics and fuel cells, power converters, and backup energy storage systems [50]. The benefits and advantages of this scheme for DC grid are:

(1) The network interconnection has capability to manage the distributed power flow from main DC distribution lines generated in grid, to the appliance and loads linked to the network. Therefore, this infrastructure can realize the renewable resource optimization for power supply.

(2) The network infrastructure enables DC grid to operate in islanding mode when parts of main grid fails. This compelling function could protect some vulnerable electricity users, such as big-computing companies and hospitals which require higher power supply safeguard, the subsystems in the DC grid operation status would not interference other users on the distribution lines, and priority loads are supplied without uninterrupted, therefore the reliability and security of entire network power system are ensured.

(3) The DC grid employs renewable energy sources, in a widespread way. Even the small-scale photovoltaic panels installed on building roofs, and light-capacity wind generators can be plugged into DC grid.

(4) Energy storages and related techniques are applied to reduce the power shortage and fluctuation especially during the peak and valley demand in DC grid distribution and generation system.

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Fig.2.3 Residential future DC grid scheme.

As mentioned in the structure of the DC grid, one of the unneglectable part is the storage system and its devices. Considered the storage systems provide DC grid with stable power backup supply, continuous power flow in the distribution system, and store the energy surplus in day and night time, one of the related research associate with DC grid area is the advancement of storage systems. Refs.

[51] to [53] introduced three major energy storage devices that are commonly employed in the DC grid system: batteries, flywheels, and super-capacitor (or ultra-capacitor), and the batteries are the most convenient and effective storage subsystem. Refs. [54] [55] reviewed materials of the batteries and developed technologies for the usage and control of power in batteries.

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2.1.2 Development of DC Grids in Japan

Fig.2.4 Overview of the DC grid in Aomori project [57].

As a country suffers from a power shortage and natural disasters such as earthquakes, Japan started early studies and investigations of the DC grids from 2005 ahead of most of countries in the world, and applies renewable energy, reliable power systems to confront deficiency of energy and instability of nature. DC grids carried out in Japan including field test projects, in which distributed power generations includes but not limited to photovoltaic cells, wind power generators, natural gas engines, fuel cells, and the energy storage batteries are placed at the same time [56]-[61].

The Aomori Project of NEDO

As the largest renewable energy research and development organization, NEDO (The New Energy and Industrial Technology Development Organization) directed various large-scale DC grid projects.

The initiatory field tests mainly focused on the integration of renewable energy sources into domestic distribution networks. The demonstrated DC grids projects constructed in Aichi, Aomori and Kyoto

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have been testified for integrating renewable energy generators significantly, and qualified for providing stable electricity to commercial utility network [57].

This Aomori DC grid project came into use on 2005 and now have been assessed for high power quality and reliability, low greenhouse emission, and cost effectiveness, beyond the estimation in planned stage.

Fig.2.4 describes an overview of the Aomori Project. A central components of the generation system only used renewable energy sources, contains two 50 kW and three 10 kW PV panels, totally 100 kW of wind turbines, battery energy storage, and gas engines at 170 kW, fed by biomass. The exhaust heat is also re-used by the steam boiler. The DC renewable energies produced electricity is mainly distributed to schools and governmental units. Moreover, by using private distribution lines, the DC electricity is transmitted to schools and local buildings through 300V DC lines, while the affluent electricity is delivered to business buildings with 5.4 km and 6 kV AC transmission line. The optimized power management system developed in the Aomori project focuses on value the electricity demands from buildings, and control the output of the gas turbine and boilers, together with the energy from PV panels charge and discharge the batteries.

The Kyoto Project of NEDO

The Kyoto Project, also named as Kyoto Eco Energy Project, is a municipal project directed by Kyoto Government, started to use from December 2005[58]. This DC grid demonstration project could cover 40-km interval. It includes the following distributed generation capacities: 50 kW of wind turbines and 50 kW PV in remote areas, 250 kW Molten-carbonate fuel cells (MCFCs), five 80 kW biogas generator sets, and 100 kW lead-acid battery bank. The energy control in this project is depend on the center controller, by communicate between the distributed energy resources and demand, and with using the legacy telecom network and internet protocol, energy is balanced and supply to the legacy

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distribution system. According to the recent data from Kyoto project, the unbalances can be rectified over short time-steps at 5-minute, superior to planned.

Private Sector DC grid

Besides the government and NEDO sponsored projects presented above, certain amount of private sector DC grid projects and activities are also under progress.

A Japanese private commercial building construction company, Shimizu Corporation, is now collaborating with the University of Tokyo, developing a DC grid with control system [59]. The distributed generations are two natural gas generator sets of 90 kW and 350 kW, four 100 kW and 400-kJ double layer capacitors, and a bank of 200-kW × 2-h NiMH battery. The project’s principle objective is to construct a power supply for facilities in university campuses. Further, with optimized operation and control system, the target market can be spanned to high power quality demanding appliances, such as hospitals, and data centers, etc.

2.1.3 DC grid Interfacing Converters

The power electronic area is now filled of electronic devices and topologies, however, it is not easy but essential to select and develop converters that are reliable, response fast, and efficient. In DC grid, power converters are greatly required, and are necessary to interface with multiple or hybrid sources in the power system. Power converters’ circuits could deliver the flow from electric power inputs to loads. Large number of power converter configurations, including DC–DC converters, AC–DC rectifiers, and DC–AC inverters, are fundament to attain functions in both AC distributed and DC distributed lines in grids [62]-[78].

Power converters used in the DC grids can be classified into few types based on categories. To be more specific, power converters have categories, such as isolated and non-isolated, unidirectional and bidirectional, step-up and stepdown converters, single input and multi-input converters, low voltage

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applications and high voltage power applications etc. As for the adaptive ability for the applications, the topologies of different power converters can be selected based on their operating range, efficiency, switching loss, dynamic response, gain, switch stress, voltage and current ripple, robustness, harmonics, etc., [63].

In the DC-DC converters, isolated topologies are most preferred to achieve safe galvanic isolation and protection for high voltage step-up ratio. These schemes generally employ voltage-fed converter topologies. Therefore, large capacitors connected at the terminals of input and output are required, to reduce the voltage ripple and constant voltages can be maintained. Furthermore, the capacitor at input side sometimes can be excluded by replacing output filter inductor by input side, also called as current-fed topology. Current-fed converters can reduce the components’ ratings and objective volume and size compare to the voltage-fed topologies [62], [64]. Also, most of current-fed converters have more simplified control scheme comparing to voltage-fed topologies converters [65], [66]. In [67] to [68], various current-fed topologies converters have been investigated for integrated DC-DC converters in PV module.

Consider the renewable energy sources in DC grids generally have multiple outputs with the different forms of voltage and current characteristics from DC sources, such as photovoltaic cells and fuel cells, etc. Therefore, multi-port DC-DC converter is one of the most practical used power converter [69].

Refs [70] to [73] introduced topologies of multi-port converters interact with different loads and different voltages. Multi-port bidirectional DC-DC converters constructed by multi-winding transformers are the most prominent and suitable structure for power distribution systems in DC grids [74], [75]. These converters have countless advantages, including isolation, simple soft switching, and feasible power control developed on phase shift.

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Meanwhile, renewable sources obtained from solar and wind resources are inherently DC, however, when feeding DC power to the AC distribution lines so that the AC appliances could be benefited, the DC-AC inverters are accordingly needed. Variable energy conversion systems using DC-AC inverters [76]-[78] are widely adopted for DC power generation. They can intermediate converter from DC to AC stage, so that they are suitable for small-scale renewable distribution systems, such as power delivery from PV panels on roof to household appliances, so that the AC commercial utility can be easily replaced and save the electric energy.

2.2 Power Distribution System

To understand the operation of the DC grid supplying the appliances by renewable energies, transmission and distribution system should be introduced. The entire distribution system, the architecture, and control technologies used, from production of electricity to deliver to the consumer are reviewed in this section.

2.2.1 AC Power distribution System.

AC power distribution system is the most fundamental system and has been proposed for over 40 years, and has been fully developed and perfected based on the advanced AC power technologies.

Generally, the developed AC power distribution systems have two categories: flexible AC transmission system (FACTS) and high-frequency AC (HFAC) power distribution system [79]-[82].

The flexible ac transmission system (FACTS) has been introduced to modern smart grids, for abilities to solve critical problems in power transmission systems. In the aspect to facilitate the transition, its potential has been explored and utilized for years. In Refs. FACTS is recommend for its flexible controllability and enhanced power transfer capability in AC transmission system [10].

For the special applications with AC appliances with critical issues, the FACTS technology could benefits to the development of the power distribution systems, mainly because:

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2 4 - Improvement of the power distribution capacity.

- Flexible utilization in the network structure system.

- Continuous control are provided for reactive power during the entire operation.

- High adaptation to restrict power quality requirements and existing standards.

- Dynamic power flow control.

The HFAC power distribution is mainly applied in fields and applications including telecommunications, lighting, electric vehicle and computers. The major advantage of HFAC power distribution systems is that connection of AC utilities lines is feasible, and facilities can be easily supplied with eliminated conversion, therefore, the following benefits can be easily achieved:

- higher power density;

- connector-less power transfer

While the DC transmission lines are more widely used, AC transmission network is still taking responsible for transporting massive power from conventional power plants to the substations and household appliances. In the year of 2015, there was around 360,000 million miles of AC distribution lines in the American power grid [84].

In the architecture of the DC grids, the AC transmission system is mainly used for hybrid systems contain both DC and AC transmission lines, so that the AC commercial utilities can be utilized. The Fig.2.5 expressed the hybrid use of AC distribution system in DC grid, where red lines represent AC power and blue lines represent DC power.

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Fig.2.5 Hybrid use of AC and DC distribution system in DC grid.

2.2.2 DC Power Distribution System

Surveys on power distribution lines illustrated that nearly 8% of the electricity generated is dissipated in transmission lines, while approximately 20% of the electric power is generated to satisfy peak demands only during a short-time period, which only counts for 5% of total operation time [85].

Moreover, almost 90% of power disruptions happen in power distribution lines and network [86].

To solve the high loss and interruptions, in today’s DC grid, DC distribution system and transmission lines are the most efficient approach to transmit substantial power especially for multiple inputs from renewable resources. Advanced DC devices and technologies, the increased use of renewable energy sources have also promoted demand and develop of DC power distribution system. DC power distribution system now becoming prospective future solution, for its higher efficiency and reliability.

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Fig.2.6 Comparison of AC and DC power distribution systems in an industrial application.

DC transmission lines can be suitable for installed both overhead lines and underground lines, even undersea lines. The uses of DC transmission lines have proved their higher efficiency compared to AC transmission lines [87], it is mostly due to the asynchronous property of DC distribution system.

Asynchronous lines signify no frequency specifics in the entire system, so that electric power can be transmitted between two lines or grids without any synchronization.

Two of the major concerns about the DC power distribution system are the electric shock risk and equipment protecting. The main reason from this concern is due to challenging to break a DC transmission line compared to an AC line. Therefore, switchgears such as fuses, circuit breakers, and grounding methods are required to applied in the DC power distribution system, so that the system can be protected from damage and ensure the safety. Comprehensive investigations have been conducted and published on enhancing the safety of DC power distribution systems [87], [88];

specific protections against lightning shock which may affect DC generation devices including PV generators are proposed [89]. Further, higher DC voltage levels could result in currents leakage in the

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transmission lines and lead to corrosion of the transformer saturation, underground equipment, and failures of protective equipment [90]-[92].

A comparison of AC power distribution system and advanced DC power distribution systems is conducted in Fig. 2.6 [93]. Both two types of power distribution systems are applied in a data center application. The internal components of ICT equipment including central processing unit (CPU), memory, etc., they usually operate under a relatively low DC voltage supply voltages, while the external backup storage such as battery generally operate under a medium voltage level. From the Fig.2.6, it is obvious that, with using an AC power supply, the power conversion from AC to DC is frequently experienced four times so that can be connected to equipment, would result in multiple conversion losses. In contrast, with a DC power supply, after the first conversion from AC to DC, the DC source can be connected to the storage batteries directly, and only limited power conversions are required, typically twice DC-DC conversion for voltage adjustment. As a result, DC power distribution achieve higher efficiency, and due to less power conversion stages and components, the failures and damage of DC power distribution system are reduced principally, compare to AC distribution system.

Not only the above example, but also many researches achievement and explorations investigated advantages of DC distribution system over AC distribution system. DC distribution system provides better efficiency and higher reliability with improved power quality. It reduces installation costs due to fewer power conversion stages, so as copper loss and assemble space are reduced. It enables uncomplicated integration of DC sources generated from renewable energy, and process of energy storage systems is simple. Unlike the complicated AC distribution system with synchronization, DC distribution system requires no synchronized plug and play subsystem so that the expansion of DC system from long-distance renewable generator to local small-localized consumers are facilitated.

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DC power distribution system also shows potential in telecommunication, big-computing and data centers areas, which is promising for developing smart and communication network in the future.

That is due to the high reliability of DC distribution system enables to supply critical, elaborate and sensitive loads. Moreover, the facilities in data centers require the use of batteries to maintain high reliability level, with the benefits from DC distribution system, no superfluous rectifiers and inverters are needed for charging and discharging of the batteries since the voltage bus is DC.

Large-scale computing companies have released universal standards for DC power distribution system, including 380V and 48V DC distribution architectures, which can satisfy both high and low voltage level in a data center [95] [96].

To be noticed, the 380 V DC distribution supply line is recommended to the connections in the DC grid, since high voltage is needed in transmission lines to reduce transmission losses, as power loss is proportional to the resistance and square of current and of the cables and conductors. [97] proposed a conceptual 380V DC power distribution system for home appliances. In that paper, the conversion efficiency of DC distributed system using renewable energy and energy storage system in the DC grid is nearly 8%, higher than the conversion efficiency in the conventional AC distribution system.

2.2.3 Power Balancing

Due to the different scale of distributed generation (DG) in DC grids supply in put power with fluctuation, and the load power from the end of transmission lines may rise and fall in different hours of one day, the power unbalance frequently happen in power distribution system:

The unbalanced in power distribution lines including the following scenarios:

Unbalanced power between power inputs and consumers;

Unbalanced power in transmission lines formed by unbalanced loading;

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Unbalanced power from input sources, especially the three phases of AC utility.

The above scenarios often cause the power dissipation in the transmission lines, the more serious results and drawbacks caused by an unbalanced power flow lead to overload on transmission cables, higher transmission power loss, or insufficient power supply. Moreover, the overload caused by unbalanced power results in overvoltage and over current, which may lead to damage of components, and the entire system reliability would be pull down accordingly [98]-[102].

There are some advances in techniques for solving the unbalanced power and load flow. Most of these improvements are concentrated on the inclusion of distributed generations and controls for power flow calculations.

Ref [99] reviewed an unbalance case study in power systems. The unbalance situation in system and its related studies, including the quantification indices, limits unbalance causes, effects and

mitigation techniques.

2.3 Active Bridge Converters

In a power distribution system, power conversion functions are widely employed for transmission lines and subsystems to achieve the galvanic isolation as well as voltage matching among the buses [103]. One of the important elements to realize power transmission and conversion is transformers, for the past decades, line-frequency transformers have been developed for distributed generations and storage systems. However, the lousy, bulky, and noisy line-frequency transformers have encumbered the transmission efficiency and power density of power conversions [104]. In late years, high- frequency transformers have taken the place of traditional line-frequency transformers, and regarded as the evolving trend for advanced generation of power conversion. The advantages of high-frequency transformers are lightweight, low volume, low power loss, and low cost, besides, they can also eliminate voltage and current distortion caused by the core saturation compare to line-frequency

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transformers. Further, when the switching frequency is operated between a high frequency range, the power conversion noise can be significantly reduced.

In the research fields and applications of power conversion and transmission, active bridges converters are under rapid expansion, with the support and technologies of high voltage and switching power devices including insulated gate bipolar transistors (IGBTs), and metal-oxide-semiconductor field-effect transistors (MOSFETs). Wide bandgap materials for semiconductor, such as silicon carbide (SiC) and gallium nitride (GaN), have also been attractive topics in the areas of power electronics by reasons of their higher voltage and thermal ratings, so as the lower turn-on energy, which are suitable for high frequency switching converters’ applications [105], [106].

Active bridge converters are the emerging topologies developed upon high-frequency transformers and active power converters. The converter topologies can be classified to isolated bidirectional and unidirectional DC-DC converters, half-bridge and full-bridge DC converters, or the types of applications categorized by voltage range or direction of power flow. Moreover, decision on the number of switches, there are dual-flyback converter, dual-push-pull converter, dual-half-bridge converter, dual active bridge converter, and triple active bridge converter, etc.

In this thesis, it is mainly focus on the study of dual active bridge (DAB) converter and triple active bridge (TAB) converter, for achieving bidirectional power flow and isolation.

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2.3.1 Dual Active Bridge Converter

Fig.2.7 DAB converter circuit topology.

The circuit of the DAB converter [107], [108] is shown in Fig.2.7 consist of two converter cells and connection via a two-winding high-frequency transformer and external inductors, in each port, converter cell is composed of two full bridges of four switching devices (MOSFET, IGBT, etc.) in a total. In addition, a direct current voltage is connected to each port of the full bridge circuit, and bidirectional power transfer is realized by controlling the current flowing from this voltage source. As a gate drive signal of each switching element, a square wave of 50% duty cycle of several kHz to l hundreds kHz can be used. The gate drive signal Sj (j = 1, 2) are applied to switches in Fig. 2.7, and S_j̅ (j = 1, 2) indicates the 180 ° inversion of Sj. Further, it is mentioned that power transmission control can be performed by advancing or delaying the phase of the rectangular wave applied to S2 with respect to the rectangular wave applied to S1.

The power transmission and flow control of the DAB converter is manipulated by phase shift control, which is depend on gate signals given to the full bridge circuits at both ports. In Fig.2.8 the switching patterns of two controlled square wave in primary and secondary side in the DAB converter are described, and the steady-state operation with simplified circuit are explained. For simplicity, the

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input DC voltages of two ports are considered as constant, and each switching device is considered as ideal switch, while capacitor CDC, all the losses, parasitic resistance, and leakage inductance of transformer are neglected.

Fig. 2.8 Equivalent circuit of DAB converter and phase shift signal patterns.

Fig.2.9 Voltage and current of DAB converter in 4 period with different switching status.

According to the ON/OFF status, the operation of DAB converter has four period, as shown in Fig.2.9.

In period (1), S1 is ON while S2 is OFF, the voltage vL is a negative value on the primary side.

Therefore, the current iL that has been flowing in the negative direction will eventually flow in the opposite direction and becomes to the positive value.

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Fig.2.10 Waveforms of current iL, iP and iS in four periods.

In period (2), both S1 and S2 are turn-on, at this time, the voltage vL on the external inductance is 0 V, due to the current storage effect of the inductor, the current iL flows through the inductance and maintains a positive value. In period (3), the S1 is OFF while S2 is ON, at this time, since the current path is changed compare to period (2), a voltage is across to the external inductance. Meanwhile, the voltage vL on the external inductance is a positive value on the primary side. Therefore, it can be conclude that the current iL flows in the forward direction and flows in the opposite direction and becomes a negative value eventually. In last period (4), the both S1 and S2 are OFF, similar to period (2), the voltage vL on the external inductance is 0 V. Since the energy of the current iL at the end of period (3) is stored by the inductor, a negative value is maintained. The inductor current and converter currents in primary and secondary side, iL, iP and iS are summarized for the four periods shown in Fig.2.10. The time-average value of the current iP directly affects power transmission, therefore, it can be confirmed that power transmission from the primary to the secondary side is performed when the switching on the secondary side has phase delay.

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3 4 Steady State Analysis of DAB Converter

Fig.2.11 Simplified DAB converter equivalent circuit.

After the power transfer direction controlled by phase shift is explained, the specific control method and power control equations will be derived for power flow control. First, define the switching frequency of the rectangular wave for switches is f [Hz], the DC voltages V1 and V2 [V] are positive values, and the switching pattern is as shown in Fig.2.2. Here, the phase difference is δ [rad], and the range is 0≤δ≤π. At this time, it is assumed that there is no loss in the circuit, and a simplified circuit of equivalent converter voltages u1 and u2, transformer, external inductances Le1 and Le2 are considered as ideal conditions, shown in Fig. 2.11.

Besides, since the equivalent converter voltage is a square wave, it is difficult to solve the circuit equation directly. Therefore, in order to make the converter voltage easy to process, the square wave function is subjected to Fourier transform, and only the fundamental wave component is considered.

Therefore, the converter voltage u1 can be written with the form under Fourier transform and can be expressed as the Eqn. (2.1):

𝑢_1(𝑡) = {𝑉₁(0 ≤ 𝑡 ≤^𝑇₂)

𝑉1(^𝑇₂≤ 𝑡 ≤ 𝑇) (2.1)

When conduct Fourier transform to the converter voltage u1 based on the Eqn. (2.1), the following equation can be obtained.

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Fig.2.12 Complex vector diagram of voltage and current in primary and tertiary side.

𝑢_1(𝑡)=^4𝑉¹

𝜋 ∑ sin(2𝑛−1)𝜔𝑡 2𝑛−1

∞𝑛=1 (2.2)

Assuming that the influence of the third harmonic can be ignored from the above equation, then:

𝑢1(𝑡) =^4𝑉_𝜋¹sin(𝜔𝑡) (2.3)

From the equations obtained above, then the converter voltage u1 with phasor can be expressed:

𝑢̃ =₁ ^4𝑉¹

𝜋 ∠0 (2.4)

Also, consider the converter voltage u2 is delayed by δ from u1, then, 𝑢̃ =₂ ^4𝑉¹

𝜋∠ δ (2.5)

The converter voltages u2 ̃ and u1 ̃obtained from the above can be drew on a complex plane as shown in Fig.2.12. Accordingly, the voltage 𝑢̃₂₁applied to the external inductance can be written in coordinate form as follow:

𝑢̃ =₂₁ ^4𝑉¹

𝜋 −^4𝑉¹

𝜋 cos 𝛿 + 𝑗^4𝑉²

𝜋 sin 𝛿 (2.6)

Therefore, the power transmission of the primary side to secondary side can be expressed as:

P =¹

2𝑢̃ 𝑖₁ ̃ =₂₁ ¹

2|𝑢̃ ||𝑖₁ ̃ | cos 𝜃 (2.7)₂₁

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Using the Eqn. (2.7), (2.8) and Fig.2.12, power transfer P from primary side to tertiary side can be expressed specifically:

𝑃 =_𝜔(𝐿^𝑉¹^𝑉²

𝑒1+𝐿_𝑒2)𝛿(1 −^𝛿_𝜋) (2.8)

Eqn. (2.8) is a basic derived equation in the DAB converter. However, this equation cannot

represent bi-directional power transmission because the value of the phase difference δ is limited to positive value. Therefore, when certain positive value of δ is desired, power transmission can also be written as follow:

𝑃 =_𝜔(𝐿^𝑉¹^𝑉²

𝑒1+𝐿_𝑒2)𝛿(1 −^|𝛿|_𝜋) (2.9) 2.3.2 Triple Active Bridge Converter

As introduced before, the topology of the TAB converter is constructed based on the DAB converter.

The basic operation and the power flow derivations of the TAB converter are conducted in this section.

The topology of the TAB converter and the switching patterns of control signals for switches are as shown in Fig.2.13. If use f [Hz] to represent the switching frequency of each rectangular wave and voltages V1, V2 and V3 [V] to represent the input DC voltages of three ports. The phase difference between the ports on the secondary and tertiary side is δ2, δ3 [rad], and the range is 0≤δ2≤δ3. First, in the TAB converter, consider a simplified circuit of equivalent circuit voltages u1, u2, u3 and external inductances Le1, Le2, Le3, the equivalent circuit is based on switches and transformer are under ideal conditions, as shown in Fig.2.14. After simplified the circuit, the main elements are inductances connected as Y configuration. By performing Y-Δ conversion on circuit inductances in Δ connection, a closed circuit can be described with each port voltage.

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Fig.2.13 Topology of TAB converter, and switching patterns of phase shift.

Fig.2.14: Y-Δ connection of the equivalent TAB converter circuit.

The L12, L23, L13 represent the inductance among the three Δ-connection points, and decided by external inductances, expressed as:

𝐿₁₂=^𝐿¹^𝐿²^+𝐿²_𝐿^𝐿³^+𝐿³^𝐿¹

3

𝐿₂₃=^𝐿¹^𝐿²^+𝐿²_𝐿^𝐿³^+𝐿³^𝐿¹

1 (2.10) 𝐿₁₃=^𝐿¹^𝐿²^+𝐿²^𝐿³^+𝐿³^𝐿¹

𝐿₂

By Y-Δ transform, a closed circuit can be made for to derive the voltage in each port. The power transmitted from the node i to the node j (i, j = 1, 2, 3) is noted as Pij, and the power flowing into the node i is denoted as Pi. Therefore, P12, P13, and P23 can be obtained as follows by using the power flow equations derived from DAB converter: