Xue Wang Feasibility and Challenges in Microgrids for Marine Vessels Master Thesis Electrical Systems Department, ABB Corporate Research, Sweden Department: EEH – Power Systems Laboratory, ETH Zürich Division of Electric Power Engineering, Chalmers University of Technology Supervisors: Dr. Ritwik Majumder, ABB Corporate Research, Sweden Dr. Petros Aristidou, ETH Zürich Mr. Stavros Karagiannopoulos, ETH Zürich Examiners: Prof. Dr. Gabriela Hug-Glanzmann, ETH Zürich Prof. Dr. Massimo Bongiorno, Chalmers University of Technology Västerås, December 2016 PREFACE i Preface This master thesis is carried out in ABB Corporate Research Center, Västerås Sweden. It is also supported and co-supervised by ETH Zürich and Chalmers University of Technology via IDEA League Grand. First, my greatest thanks to my supervisor Dr. Ritwik Majumder at ABB CRC who supervises me to work on this interesting topic with continuous help and support. I would like also to express my gratitude to Prof. Dr. Gabriela Hug-Glanzmann, ETH Zürich and Prof. Dr. Massimo Bongiorno, Chalmers University of Technology for enabling this collaboration and kind helps during the scholarship application. Furthermore, I would like to give thanks to Mr. Stavros Karagiannopoulos and Dr. Petros Aristidou for the kind discussions on the project. I also wish to express my special thanks to Rickard Lindkvist and Michael Lundh for their continuous helps with technical advises. Also, I should not forget to thank Dr. Mehanathan Pathmanathan who was always available to answer my questions. I am also very thankful to Shiva Sander-Tavallaey and Cathy Yao Chen for providing me with this precious opportunity to be involved in an interesting working environment and achieve unforgettable experience in Sweden. Last but not least, I would like to thank my family for their support throughout my entire studies. They are always there supporting me wherever I am and how far I am away from home. The fulfillment of this project is a part of the master’s degree programme “Energy Science and Technology” at ETH Zürich. Disclaimer This report uses ABB’s Microgrid Plus System and Marine Solutions as a basis for analysis. Although it uses the functionality from a high level perspective, this thesis does not bare semblance on the performance or the technical functionality of ABB’s system and solution. This report is purely conceptual and only takes inspiration from the solutions. ABSTRACT ii Abstract Due to the development of distributed generation (DG) and power management technologies, an islanded marine power system, namely microgrid in marine, becomes a promising option for marine power systems and gains an increase in research interest. Microgrid solution provides integration of renewables, energy storage systems and existing generation units. It enhances energy efficiency, reduces CO2 emissions, and improves dynamic responding to load fluctuations. In this way, the overall performance of vessel information and control system is optimized. The goal of this thesis is to investigate the feasibility of operating microgrids in marine vessels. Power management strategies are formulated with integration of energy storage and renewable sources, like photovoltaics (PV), to the existing diesel generators within a small-islanded network. Before the system level analysis, modeling of the energy resources, energy storage, and power electronic converters is obtained. In order to weigh the benefits and assess the potentials of implementing the shipboard DC microgrid system, this work facilitates a detailed MATLAB simulation-based study accounting for the factors of realistic vessel operation load profiles, stability and reliability of DC microgrids. Chapter 1 and 2 provide the introduction and prior art from industrial solutions and academic research. The prior art covers shipboard power network structure, load profile requirement, energy storage system (ESS), power management system (PMS), and academic research methodologies of the marine vessels. Chapter 3 analyzes the feasibility of implementing a marine microgrid based on ABB solutions of land-based microgrids. An integration of the inland microgrid solutions to the existing marine automation and control platform is proposed. After that, the integrated functions of the proposed marine microgrid are formulated in Chapter 4. The system configuration and the microgrid PMS for marine applications are developed. Control and energy storage functions and stability of operating the proposed shipboard microgrid system (SMS) are verified in Chapter 5 by testing four specific scenarios. The simulation results show that the SMS works in stable operational modes under various loading conditions. Additionally, with proper power management strategies chosen for different operation modes, the operating of SMS can be shifted between different states without significant impacts on the power quality and stability of the system. CONTENTS iii Contents List of Acronyms v List of Figures vii List of Tables ix 1 Introduction and Prior Art 1 1.1 Introduction to Electrification in Marine, Power Network and Demand of Power Management ...................................................................2 1.2 Common Practice in Marine Power System ................................................3 1.2.1 Electric Propulsion Solutions ..........................................................3 1.2.2 Energy Storage System ...................................................................3 1.2.3 Electrical Fault ................................................................................4 1.3 Power System Solutions from ABB Marine ................................................5 1.3.1 ABB’s Advisory Suite ....................................................................6 1.3.2 Onboard DC Grid............................................................................ 6 1.3.2.1 Design Principle & Benefits .......................................... 6 1.3.2.2 DC Grid Configuration ................................................. 6 1.3.2.3 Protection & Safety ........................................................ 8 1.3.3 Hybrid Power Plants Enabled by Batteries ..................................... 8 1.3.4 Variable Frequency Drive for Shaft Generator (PTO/PTI) .............9 1.3.5 Dynamic AC (DAC) System ....................................................... 11 1.4 Solutions from Other Industrial Companies.............................................. 12 1.4.1 Siemens ........................................................................................ 12 1.4.1.1 BlueDrive PlusC .......................................................... 12 1.4.1.2 Waste Heat Recovery System (WHRS) ....................... 13 1.4.1.3 SISHIP EcoMain .......................................................... 13 1.4.2 MAN Diesel & Turbo .................................................................. 14 1.4.3 GE ................................................................................................ 14 1.4.3.1 GE New Power Take Off/Power Take In (PTO/PTI) .. 14 1.4.3.2 Exhaust Energy Recovery System: Echogen System .. 14 1.4.3.3 SeaStreamTM DP System.............................................. 15 1.4.3.4 Latest C-Series Vessel Control System (VCS) ............ 15 1.4.4 Rolls-Royce Marine ..................................................................... 15 1.5 Academic Prior Art ................................................................................... 16 1.5.1 Shipboard DC Grid Configuration ............................................... 16 1.5.2 Optimal Loading Condition of Hybrid Power System ................. 18 1.5.3 Pulse Load Compensation with Hybrid Battery/Ultracapacitor ... 19 1.5.4 Pulse Load Compensation with Flywheel and Prime Mover ....... 19 1.5.5 Energy Management Strategy for Load Variation ....................... 20 1.5.6 Stabilizing Power Fluctuation with Energy Storage System ....... 20 2 Literature Review – Microgrid for Marine 23 2.1 Microgrid and Microgrid for Marine ........................................................ 23 2.2 Academic Research in Microgrid for Marine ........................................... 23 2.2.1 Simulation Models of Shipboard Electric Power Systems .......... 23 2.2.2 Voltage Regulation and Power Sharing Control in Shipboard DC CONTENTS iv Power System ............................................................................................ 29 2.2.3 Real Time Implementation of Microgrid Reconfiguration ........... 30 2.2.4 Optimal Sizing of a Shipboard Microgrid with PV and ESS ....... 32 3 Microgrid Feasibility for Marine 35 3.1 Problem Definition .................................................................................... 35 3.2 ABB Storage and Control Systems ........................................................... 36 3.2.1 Storage Solutions ......................................................................... 36 3.2.2 Control Solutions ......................................................................... 37 3.2.3 Marine Vessel Information and Control – System 800xA ........... 37 3.3 ABB Microgrid Solutions - Microgrid Plus System and PowerStore ....... 39 3.3.1 Microgrid Plus System ........................................................................ 39 3.3.2 PowerStore ................................................................................... 41 3.3.3 ABB Microgrid – Real World Examples ..................................... 42 3.4 Feasibility of ABB Microgrid Solutions for Marines ............................... 43 4 Proposed Methodology 46 4.1 System Configuration ............................................................................... 46 4.2 Operation Load Profile and Sizing of ESS & PV ..................................... 50 4.2.1 Operation Load Profile........................................................................ 50 4.2.2 Sizing of ESS and PV .................................................................. 52 4.3 Power Management System ...................................................................... 53 4.3.1 Peak Shaving ........................................................................................ 53 4.3.2 Strategic Load Sharing ................................................................. 54 4.3.3 PV Control ............................................................................................ 58 4.3.4 Supervisory Mode Control ........................................................... 58 5 Component Modeling and System Simulation 61 5.1 Component Modeling ............................................................................... 61 5.1.1 Synchronous Generator-Rectifier System ........................................ 61 5.1.2 Diesel Engine ............................................................................... 61 5.1.3 Energy Storage System ....................................................................... 62 5.1.4 PV System ................................................................................... 63 5.1.5 Constant Power Load .......................................................................... 63 5.2 Test Scenario Definition ........................................................................... 64 5.3 Simulation Results .................................................................................... 65 5.3.1 Scenario 1 – SMS Operating under AH Mode ................................ 65 5.3.2 Scenario 2 – SMS Operating under TS Mode .............................. 67 5.3.3 Scenario 3 – SMS Operating under TT Mode ................................. 69 5.3.4 Scenario 4 – SMS Operating under HDP Mode .......................... 71 6 Conclusion and Future Work 74 Bibliography 75 Appendices 80 A Summary of the feasibility analysis .................................................................... 81 LIST OF ACRONYMS v List of Acronyms AES All Electric Ship PMS Power Management System DP Dynamic Positioning ESS Energy Storage System SOC State of Charge VFD Variable Frequency Drive SGM Shaft Generator/Motor PTI Power Take-In PTO Power Take-Off DAC Dynamic AC IPS Integrated Power System WHRS Waste Heat Recovery System TEU Twenty Foot Equivalent Units VCS Vessel Control System ZEDS Zonal Electrical Distribution System BUCESS Battery/Ultracapacitor Energy Storage System PSO Particle Swarm Optimization APHESS Active Parallel Hybrid Energy Storage System DG Distributed Generation SMS Shipboard Microgrid System SPS Shipboard Power System WTG Wind Turbine Generator SOFC Solid-Oxide Fuel Cell FC Fuel Cell AE Aqua Electrolyzer BESS Battery Energy Storage System PV Photovoltaics GA Genetic Algorithm RTDS Real-Time Digital Simulator CRF Capital Recovery Factor MOPSO Multi-Objective Particle Swarm Optimization ESM Energy Storage Module DES Distributed Energy Storage EPIC Electrical Plant and Inverter Controller PCS Power Conversion System AVR Automatic Voltage Regulator SES Static Excitation System DCS Distribution Control System LIST OF ACRONYMS vi OSV Offshore Support Vessel LDP Low Dynamic Positioning HDP Low Dynamic Positioning AH Anchor Handling H Harbor BP Bollard Pull TT Transit Towing TS Transit Supply Li-ion Lithium-ion Battery MPPT Maximum Power Point Tracking CPL Constant Power Load LIST OF FIGURES vii List of Figures 1 Energy storage devices-[5] ........................................................................... 4 2 Onboard DC grid configuration-[8] ............................................................. 7 3 Load sharing between battery and diesel generator-[7] ............................... 9 4 Battery voltage droop curve-[7] ................................................................... 9 5 A typical system configuration with a VFD installed in the shaft generator/motor system-[7] ........................................................................ 10 6 Operating modes of a shaft generator/motor system with VFD-[9] ........... 11 7 Comparison of specific fuel oil consumption between conventional AC and DAC power plants-[10] ..............................................................................12 8 An overview of Siemens waste heat recovery system for vessel-[14]. ...... 13 9 A logical overview of EcoMAIN system-[13] ........................................... 14 10 Typical power architectures of ring-bus based DC distribution systems -[3].............................................................................................................. 17 11 The process of sectionalization and reconfiguration based on the self -healing method-[3] .................................................................................... 18 12 The investigated shipboard DC power system-[24] ................................... 18 13 A regenerative energy management for pulse-loads in dual DC-AC microgrid-[25] ............................................................................................ 20 14 A simplified integrated shipboard power system-[26] ............................... 21 15 APHESS topology-[27]. ............................................................................ 22 16 Configuration proposal of the microgrid system in a sailing boat-[28] ..... 24 17 Configuration proposal of the hybrid propulsion system-[29] ................. 27 18 Configuration for system-level analysis of ship DC distribution power System-[30] ................................................................................................28 19 DC currents of the sources, ESS, fuel cell, and load-[30] ..........................29 20 Typical layout of a DC shipboard power system in a single line diagram -[32]............................................................................................................30 21 Block diagram of voltage regulator-exciter-[32] .......................................30 22 A shipboard microgrid system-[33] ...........................................................31 23 A shipboard microgrid system with PV and ESS-[34] ..............................32 24 Hourly ship load profile along routine of interest-[34] ..............................33 25 EssProTM energy storage applications-[37] ................................................37 26 ABB Marine automation and control system - 800xA-[40] .......................38 27 Overview of ABB’s microgrid solution-[41] .............................................39 28 Typical schematic of isolated PV/diesel microgrid-[42] ............................41 29 Schematic of PV/diesel hybrid power system-[44] ....................................43 30 Mplus_800xA schematic diagram ..............................................................45 31 Single bus with sectionalizing disconnectors .............................................47 32 Double bus-double isolating switch ...........................................................48 33 One-and-a-half isolating switch .................................................................48 34 Ring Bus .....................................................................................................49 35 Seven operating modes of OSV and duration percentage for each within 24 hours ......................................................................................................51 36 Scheme of peak shaving power management system ................................54 37 Scheme of load sharing power management system ..................................55 38 Controlled exciter and converter system ....................................................56 39 Deployment of MGC600 for marine DC microgrid ................................... 56 LIST OF FIGURES viii 40 Droop characteristics of two generators and a battery based ESS .............57 41 Block diagram: generator and ESS droop controllers ................................58 42 Supervisory mode for power management system ....................................60 43 Block diagram of diesel engine ..................................................................62 44 Topology of bidirectional converter ...........................................................63 45 Overall setups of the simulated SMS in Simulink .....................................64 46 SOC of energy storage system of scenario 1: AH mode ............................66 47 Power flow of scenario 1: AH mode ..........................................................66 48 Voltage level of scenario 1: AH mode .......................................................67 49 SOC of energy storage system of scenario 2: TS mode .............................68 50 Power flow of scenario 2: TS mode ...........................................................68 51 Voltage level of scenario 2: TS mode ........................................................69 52 SOC of energy storage system of scenario 3: TT mode .............................70 53 Power flow of scenario 3: TT mode ...........................................................70 54 Voltage level of scenario 3: TT mode ........................................................71 55 SOC of energy storage system of scenario 4: HDP mode ..........................72 56 Power flow of scenario 4: HDP mode ........................................................72 57 Voltage level of scenario 4: HDP mode .....................................................73 LIST OF TABLES ix List of Tables 1 Summary of competitors’ products/applications ....................................... 16 2 Functions and benefits of utilizing System 800xA .................................... 38 3 Functions of the Microgrid Plus system .................................................... 40 4 MGC600 microgrid controller firmware-[43] ................................................41 5 Functions of PowerStore storage devices .................................................. 42 6 Summary of the feasibility analysis ........................................................... 81 7 Comparison of DC shipboard microgrid configurations ............................ 49 8 Dina Star specifications-[47] ..................................................................... 50 9 OSV operating modes and load profile ...................................................... 51 10 Specifications of battery bank-[49] ..................................................................53 11 Specifications of PV-[51] ......................................................................... 53 1 1 INTRODUCTION AND PRIOR ART 1 Introduction and Prior Art In this thesis, the feasibility and challenges of operating microgrid in marine vessels are explored. Efficient power management strategies are formulated with integration of energy storage and renewable sources, e.g. PVs, to the existing diesel generators within a small-islanded network. Subsequently, time domain simulations are carried out in MATLAB Simulink platform to validate the main concepts. The thesis is structured as follows. Chapter 1 provides the introduction and prior art from industrial solutions and academic research. The prior art covers shipboard power network structure, load profile requirement, energy storage system, and power management system in the marine vessels. The industrial prior art presents marine solutions from technology companies covering control, power management and protection of the electrical network. The existing proposals for microgrids in marine are presented in Chapter 2. The presented proposals are extracted from academic research including simulation models for marine microgrids, voltage regulation, power sharing, network reconfiguration and sizing of the energy storage device. Chapter 3 describes the feasibility of implementing microgrid in marine with ABB solutions. First various ABB solutions for energy storage and control are presented. Then, the ABB inland microgrid solution concept is depicted and finally an integration of the inland microgrid solution to the existing marine control platform is proposed. The proposed marine microgrid is capable of integrating the control methods and operational principles required by marine applications. The integrated functions of the proposed marine microgrid are described in Chapter 4. The system configuration and the microgrid control functions for marine applications are developed. The power management strategies for vessels with renewables, ESS and diesels are proposed for efficient operation under different operational modes. The time domain simulations are presented in Chapter 5. The system stability is validated under various loading conditions. The concept of power sharing among the energy sources and peak shaving for significant load fluctuations is verified. The conclusions and scope of future work are presented at the end. 1 INTRODUCTION AND PRIOR ART 2 1.1 Introduction to Electrification in Marine, Power Network and Demand of Power Management The electric ship propulsion has a long history dating back to more than 100 years [1]. In recent decades, the high efficiency of the electric propulsion is achieved through development of semiconductor devices for marine utilizations. Accordingly, the equivalent mechanical solutions are challenged by shipboard electric propulsions. This trend has further driven significant fuel savings. As a result, utilization of all-electric ship (AES) or shipboard integrated power system (IPS) is promoted. Meanwhile, the increasing operation costs and stricter regulations regarding emission problems stimulate the ship designers and operators to follow more energy-efficient ways. In the early stage, the main electric power source was based on steam turbine technology, and then it is replaced by gas turbine and diesel engine. After that, more and more distributed energy sources can be connected to the shipboard electric grid through converters. In this way, the IPS is configured with distributed energy sources supplying shipboard load demands. Although the shipboard integrated power system equipped with electric power sources can be designed according to the mature principles and technologies of the land-based power plant, it still needs additional considerations due to its islanded scheme. The common power network operated on the AESs normally comprises IPS, electric propulsion module, and other marine loads [2]. As the shipboard power network is operated as an islanded system, the power system must be configured in terms of safety and the reliability of the generation sources needs to be maintained. Additionally, the stability of operating the shipboard power network should be sustained as well. In modern history, significant development in IT & Automation technology has resulted in increased reliability, integrality, and intelligence of the shipboard power network. For the purpose of monitoring the operation of AES and supporting operational stability, a power management system (PMS) is functionalized to manage network power flow, regulate voltage level and maintain operation stability. PMS monitors the power plant on a higher level and takes protective measures as load fluctuating and shedding. The PMS controls the shipboard power network through various control strategies, for instance, optimizing the number of online engines and adjusting the load sharing between the generation sources. Some of the PMSs can provide functionalities of protection, condition monitoring and weather forecasting, which increases vessel availability and safety. All the mentioned PMS functionalities and power system units are mostly stand- alone units, but are in the process of being integrated through system-level communication platform to improve feasibility, stability and reliability [3]. 1 INTRODUCTION AND PRIOR ART 3 1.2 Common Practice in Marine Power System In this section, the common practice implemented in conventional marine power system in terms of electric propulsion solution, energy storage system and fault prevention is introduced. 1.2.1 Electric Propulsion Solutions The electric propulsion solutions for marine today vary depending on vessel types, available technologies and operation profiles. According to ABB vessel segments, offshore vessels, ice-going vessels, passenger vessels, cargo vessels, and special purpose vessels account for the majority of vessels. For the passenger vessels and some cargo vessels, the electric propulsion system is designed for serving one or two (sometimes three) main propellers ranging up to 20-25 MW. The generator set usually consists of four to six engines. From the maintenance point of view, the engines are normally of the same rating and size. Due to the large shipboard power demand of this segment, the main electric equipment is operating on medium voltage level: 3.3, 6.6, or 11 kV [4]. In terms of the offshore vessels, the dynamic positioning (DP) operation is always of interest. The DP vessels can keep position with thrusters designed for giving maximum thrust at zero knots. In this case, the propulsion load profile fluctuates while the thrust force demanded by the DP control system varies. For this type of vessel, several propeller and thruster units are equipped on both its stern and bow. Five to eight generator engines are usually installed ranging from 1 to 6 MW for each. Depending on the level of vessel´s electrical load, the shipboard power plant is then operated at a medium voltage level, or at a low voltage level [4]. Since ice-going vessels are operated for both navigation in ice and sailing in open water, their shipboard power systems are normally dimensioned with podded propulsion units. ABB introduced Azipod®, a 360° steerable propulsion unit, in 1993 at first for icebreaker vessels. It later becomes an industry standard to use Azipod® on ice-going vessels. The propulsion unit for an ice-going vessel often operates up to 20 MW of a medium voltage electric system [4]. 1.2.2 Energy Storage System Advantages of implementing energy storage system (ESS) are increasingly recognized by the marine industry, due to its ability to improve operational stability, reduce fuel consumptions and eliminate emissions of marine vessels. ESS can be used for regenerating the power produced by the engines, compensating power load fluctuations and smoothing the power extracted from the diesel generation units. Combining with ESS, the propulsion units used for DP are optimized by fostering the dynamic responses of the vessels. Currently, because of the technology development and cost reduction, the most dominant energy storage devices for vessel applications are batteries, ultracapacitors and flywheels [5]. Batteries are usually suitable for energy-intensive applications because they have higher energy density (kWh/ton) compared to the others (Fig. 1). The runtime of an ESS depends on the energy density of the storage device, Because of that, the 1 INTRODUCTION AND PRIOR ART 4 ultracapacitors and flywheels can only run up to 1 minute, while the runtime of the batteries can last from 5 minutes to 8 hours. Accordingly, the ultracapacitors and flywheels are proper candidates for short-run but power-intensive applications balancing the power load fluctuations, while the batteries are suitable for medium and long-term supply of power. A fast-acting ESS can compensate for the slow dynamic responses of the diesel generation units and reduce negative impacts on power quality introduced by load variations and transients. Figure 1: Energy storage devices-[5] Since the leading battery suppliers, e.g. Corvus Energy and SAFT, start to actively provide marine energy storage solutions with Li-ion battery banks, this type of battery technology is suggested throughout the thesis. 1.2.3 Electrical Fault Generally, for the shipboard electric network, the neutral point of star-connected generators is not earthed (namely insulated electric network) in the low-voltage distribution, but is earthed through high resistance (namely grounded electric network) in the high-voltage distribution network [6]. The common electrical faults in the shipboard distribution network are Open Circuit Fault, Short Circuit Fault, and Earth Fault [6]. Open circuit fault is usually caused by bad connection or break in a wire. In the case of bad connection, open circuit fault can cause a lot of heat leading to a fire hazard. Short circuit fault occurs when two different phase conductors are connected. Consequently, a large amount of current is released. Failure of insulations and human errors are the examples of the causes of short circuit faults. Earth fault is caused by a connection between a phase conductor and an onboard earthed component. The insulated electric network continues to supply essential shipboard power services, since the tripping mechanism is not triggered off under a single-line earth fault. Due to this capability of maintaining continuity of power service supply, insulated electric system is normally adopted for low-voltage shipboard electric networks [6]. 1 INTRODUCTION AND PRIOR ART 5 Sectionalizing the electric distribution network and providing multiple power sources maintain the reliability of a shipboard electric system. The common approaches of maintaining power system reliability are to provide an emergency power system, sub- sectionalize the circuits, and implement selectivity schemes [6]. The protection relays in the switchboard need to make sure that the failures are identified and isolated through a selectivity scheme [6]: under fault conditions, the essential power loads are not interrupted by isolating the defective sections immediately after the faults are identified. The protective elements (e.g. circuit breakers and fuses) corresponding to the faulted sections must then be operating while the protective elements in the healthy circuits do not trip off. 1.3 Power System Solutions from ABB Marine ABB shows high performance in developing power systems for marine vessels. In the following, solutions initiated by ABB to electrifying the marine vessel operations are presented. In order to meet the goals of reducing CO2 emissions, ABB keeps improving existing technologies and searching for energy efficient ways of marine vessel design and operation. According to ABB Marine Energy Efficiency Guide [7], the key to improve marine energy efficiency is to make a sound design combined with power management system and data communication technology. To improve the energy efficiency, increasing the flexibility of a marine vessel is the most cost-effective way [7]. The flexibility can be improved by means of diversifying power generation sources and guaranteeing operational stability under various loading conditions. There are mainly two types of solutions for ABB shipboard power networks [7]:  Consulting services: a) ABB Marine Appraisal is an energy efficiency service. It is a guideline used to make investment decision, in terms of cost and payback time, on energy-saving solutions b) Energy Efficiency Audit is a service providing a detailed roadmap for evaluating savings in operational costs of energy efficient practices c) Energy Efficiency Training is implemented to raise people´s awareness of energy efficient solutions  Technical measures: specific technologies are selected by vessel segments, and are either for retrofit markets or for new builds (e.g. onboard DC Grid). State-of- the-art marine technical solutions of ABB will be presented in below with more details 1 INTRODUCTION AND PRIOR ART 6 1.3.1 ABB’s Advisory Suite [7] EMMA® Advisory Suite and OCTOPUS® Advisory Suite are ABB’s performance management tools for marine applications. The advisory systems are always integrated with other technical solutions and can be retrofitted freely to enable excellent fit for each vessel type and operation profile. OCTOPUS® Advisory Suite provides vessel motion- based tools, so it monitors the thruster and environmental conditions, which can supply data to PMS under various loading conditions. The benefits of implementing ABB’s advisory systems can be highlighted as: monitoring and benchmarking fuel consumption for operation process; optimal use of DP system; optimizing power plant for enabling the most economical way to supply load power; providing motion-based tools for increasing vessel availability and safety [7]. 1.3.2 Onboard DC Grid [8] Compared to conventional AC system, a shipboard electric propulsion system configured with DC grid can increase the vessel’s energy efficiency by up to 20% and reduce the number of electrical equipment by up to 30% [8]. Shipboard DC electric network is flexible to integrate different energy sources and optimize component placements. ABB Marine launched Onboard DC Grid concept in 2010, which provides a new solution for low-voltage shipboard power plant. According to ABB Marine’s operation tests, the onboard DC grid can be used for supplying installed shipboard power up to 20 MW with a nominal voltage level of 1000 V DC, and this DC grid configuration obtains a projected 1-year return of investment. 1.3.2.1 Design Principle & Benefits Distributing the power through DC grid reduces the number of switchboard and transformer needed by an AC distribution. This is one of the factors that drive the designers to adopt onboard DC grid. Additionally, in an AC distribution, the grid frequency needs to be maintained, so the diesel generators run at a constant speed and fuel efficiency is declined. In the case of using onboard DC grid, the speeds of diesel generators can be varied to achieve the optimum fuel efficiency. Most of the ABB generators, motors and drives are well known with proven performance, and these AC-based components can still be plugged into the onboard DC grid through electronic converters or inverters. Because of that, ABB Onboard DC Grid is a platform that enables “plug and play” retrofitting possibilities and adaptability to alternative energy sources. The diesel engine speeds can be adjusted corresponding to the load demands without changing the number of online generators frequently, which increases energy efficiency and operational stability. Furthermore, an expanded application of the onboard DC grid is to integrate it with ESS, so as to compensate the slow dynamic response of the mechanical components and improve DP operation. 1.3.2.2 DC Grid Configuration In the DC grid system, all the electric power generated is fed into a common DC distribution bus either directly or through a converter [8]. At this stage, each onboard power source and consumer is controlled and optimized independently. The placement of the electric component is simple and occupies less space compared to the case of AC 1 INTRODUCTION AND PRIOR ART 7 network. The configurations of DC distribution grid are normally based on two approaches: a multidrive approach (Fig. 2a) where the converters are all placed within the same location as in the main switchboard of an AC grid, or a distributed approach (Fig. 2b) where each converter is located close to the corresponding power source or load [8]. 2a. Onboard DC Grid, multidrive approach 2b. Onboard DC Grid, distributed approach Figure 2: Onboard DC grid configuration-[8] 1 INTRODUCTION AND PRIOR ART 8 1.3.2.3 Protection & Safety There are some challenges posed for DC distribution grid. Since there is no natural zero crossing for DC current, it is more difficult to be interrupted. Besides, the costs of DC circuit breakers are higher compared to AC circuit breakers. ABB overcomes the mentioned challenges by introducing a new philosophy. Reliability of the onboard DC grid is achieved by a combination of fuses, isolating switches and controlled turn-off semiconductor power devices [8]. When a fault occurs in a module, the fuses are operated to isolate power electronic modules from the defective parts, and isolating switches of the input circuits isolate the power electronic modules from the main DC bus. The isolating switches are installed in each circuit branch to isolate faulty parts from the healthy sections. In this case, one failure of a load or a generation unit will not affect other suppliers and consumers in the distribution system. In the case of faults on the DC bus itself, the system is primarily protected by means of the isolating switches situated in the circuit branches and the controllable power electronic devices placed in the diesel generators’ output circuits. 1.3.3 Hybrid Power Plants Enabled by Batteries [7] In section 1.2.2, energy storage devices are categorized in terms of energy-intensive use and power-intensive use. Depending on the way of constructing the cathode and utilizing the materials, Li-ion battery can be applied as a high-energy/energy-intensive battery or a high-power/power-intensive battery. With the functionality provided by Li-ion battery technology, the ESS can run in parallel with other power sources. A hybrid system integrating diesel generators with batteries is now considered as a possible alternative power source for marine application. The basic frame of shipboard microgrid system proposed in Chapter 4 is also based on this concept. After implementing this hybrid system, the reliability of the marine power system is increased with instantaneous energy backups. In addition, with a proper control strategy implemented, the overall operational efficiency is also increased by smoothing the power consumption under fast and strong load fluctuations. Chapter 4 deals with selection of control strategies and relevant methodologies are presented in details later. Another question concerned for the onboard DC grid is how to regulate the DC link voltage. One of the approaches maintaining the DC link voltage at a certain level (voltage reference), is to control the AC/DC rectifier. At the same time, the power load sharing/DC voltage regulation is issued. Load sharing based on voltage droop (Fig. 3) is an effective and robust method for parallel operating power sources (battery and diesel generator in this example). The voltage at the battery terminal varies according to their state of charge (SOC) and charge/discharge current. The natural battery voltage droop curve is referred to as the “cell voltage versus discharge current” (Fig. 4). Without considering the line resistances, the DC voltage is common for both the battery terminal and the AC/DC rectifier. 1 INTRODUCTION AND PRIOR ART 9 Figure 3: Load sharing between battery and diesel generator-[7] Figure 4: Battery voltage droop curve- [7] 1.3.4 Variable Frequency Drive for Shaft Generator (PTO/PTI) [7] For a container vessel, it is typically retrofitted to have shaft generators installed to reduce the loading of diesel generator sets. The shaft generators can generate power in parallel with diesel generator sets, or provide all power demands without running the auxiliary engines. With a variable frequency drive (VFD) retrofitted to a shaft generator, the shaft generator can be used on a wider speed range and its operational flexibility is improved [7]. Figure 5 presents a typical system configuration for a container vessel with a VFD installed in the shaft generator/motor system (SGM). 1 INTRODUCTION AND PRIOR ART 10 Figure 5: A typical system configuration with a VFD installed in the shaft generator/motor system-[7] There are mainly three operating modes of a VFD shaft generator/motor system:  Full electric propulsion: shaft generator works as Power Take-In (PTI) motor (Fig. 6). It can be powered by the auxiliary engines. The main engine can be shut down during low speed operation  Normal operating condition: shaft generators supply the required electricity load for the vessel entirely by themselves  Power Take-Off (PTO) mode (Fig. 6): shaft generator/motor is used as an electricity generator when onboard power demand increases. VFD shaft generator/motor system switches to PTO mode to feed power into the shipboard grid 1 INTRODUCTION AND PRIOR ART 11 Figure 6: Operating modes (PTO/PTI) of a shaft generator/motor system with VFD-[9] 1.3.5 Dynamic AC (DAC) System [10] The technology of high-voltage DC grid, which supplies large load demand, is not mature enough to be utilized in large commercial passenger vessels. In addition, a centralized frequency converter is too large and costly to be installed to convert the voltages, which are generated by the variable speed generators, into constant frequency [10]. As a consequence, a concept of dynamic AC (DAC) system is established by ABB. The DAC concept of ABB can optimize the total fuel consumptions in large vessels by adjusting the rotational speeds of the diesel generation sets. This allows the system frequency to vary within a specified range (Fig. 7). The generators for this concept should be specified to operate within a frequency range, instead of at a fixed frequency level. The magnetic circuits, windings and the other electromagnetic devices need to be dimensioned for being operated under variable frequency conditions. 1 INTRODUCTION AND PRIOR ART 12 Figure 7: Comparison of specific fuel oil consumption between conventional AC and DAC power plants; source-[10] 1.4 Solutions from Other Industrial Companies This section introduces the technology developments by various vendors apart from ABB. The main suppliers for marine power systems except ABB are Siemens, MAN DIESEL & TURBO, GE and Rolls-Royce Marine. 1.4.1 Siemens [11] Siemens BlueDrive PlusC drive solution and Waste Heat Recovery System lead the development of shipboard electric propulsion system. BlueDrive PlusC is a comprehensive solution to diesel-electric vessels and increases safety, cuts operational cost, and decreases the environmental impact [11]. The waste heat recovery system utilizes the heat from exhaust gases to generate steam and produce additional electrical power. 1.4.1.1 BlueDrive PlusC In 2013, Siemens together with Østensjø Rederi and Corvus Energy delivered an integrated power system (IPS) for the platform supply vessel, Edda Ferd. The ESS in Edda Ferd has a capacity of 260 kWh (40 x 6.5 kWh) and the bus voltage of it is 888 V DC [12]. After that, the Danish ship owner Esvagt chose Siemens’ BlueDrive PlusC propulsion system for its two offshore support vessels in 2015. The vessels have four high-speed diesel-generators with a thruster system consisting of two main 1600 kW azimuths, two 1000 kW tunnel thrusters and a redundant fed 880 kW retractable thruster. All thrusters are frequency controlled by BlueDrive PlusC [13]. The variable speed diesel generator plays a crucial role in the BlueDrive PlusC system. This type of generator enables the engines to run within the optimal speed range. The shipboard power network is prepared for connections to renewable energy sources and ESS. This can lead to a significant reduction of fuel consumption and CO2 emission. Additionally, BlueDrive PlusC system also integrates Siemens’ leading automation 1 INTRODUCTION AND PRIOR ART 13 technology, SIMATIC. It is used for onboard power management system to monitor generators’ speeds and output voltages, track the vessel’s power demands, and manage DP operations [11]. Siemens SIMATIC shares some similar functionalities with ABB Advisory Suite. 1.4.1.2 Waste Heat Recovery System (WHRS) Siemens Solution of shipboard WHRS is a heat-to-power electric propulsion system usually for cargo vessels. Since a WHRS regenerates power from the exhaust gases, it cuts vessel energy costs and reduces the CO2 and NOx emissions of the vessel as well. PMS imbedded in the WHRS ensures a safe and reliable operation performance at any time. Figure 8 shows an overview of a WHRS. Figure 8: An overview of Siemens waste heat recovery system for vessel-[14] 1.4.1.3 SISHIP EcoMain Siemens Solution for Shipping (SISHIP) delivers an automation control system for vessel management. EcoMain is involved in SISHIP portfolio and collects data from all relevant onboard systems of an entire fleet, so as to provide a platform for vessel performance evaluation (Fig. 9). EcoMain’s uniformed database can also be accessed from onshore, which enables a comprehensive review and analysis throughout entire fleet. Data provided by EcoMain can be used for efficiency improvement, environmental compatibility, trouble-shooting and remote maintenance [11, 13]. Maersk Triple E Class container ships are integrated with Siemens VFD-based WHRS as well as EcoMain system [15]. 1 INTRODUCTION AND PRIOR ART 14 Figure 9: A logical overview of EcoMain system-[13] 1.4.2 MAN Diesel & Turbo [16] The energy from the main engine exhaust gas is attractive among the waste heat sources of a ship is because of its large heat flow and high temperature. By implementing WHRS, fuel reductions of between 4 ~ 11% are possible [16]. MAN Diesel & Turbo is a large engine designer and manufacturer. It has been involved in some research and feasibility studies on WHRS. According to its studies, an onboard WHRS significantly contributes to reducing emissions, ship operating costs and the newly adapted energy efficiency design index (EEDI). 1.4.3 GE [17] GE Global Offshore & Marine offers solutions for marine business worldwide. Based on “heat-to-power” principle, Echogen Power System is introduced with a specialty of using CO2 as the working fluid. Besides, optimization of dynamic positioning operations and vessel automations are realized by SeaStream™ DP System and Latest C-Series Vessel Control System respectively. 1.4.3.1 GE New Power Take Off/Power Take In (PTO/PTI) Technology [18] The contract with Maersk Line marks GE Global Offshore & Marine enter into the container ship industry [18]. Like some other competitors, GE also proposes a solution based on SGM. PTO/PTI solution is provided by GE to Maersk Line for eleven 2nd generation triple-E vessels. With a capacity of 19630 TEU for each, it is comprised of a SGM installed between the main engine and the propeller. This solution consists of two drives, two induction asynchronous motors and a PMS. The electrical energy converted from the vessel drive shaft is then extracted to where it is needed. In this way, it is not necessary to burn fossil fuels to power these systems. 1.4.3.2 Exhaust Energy Recovery System: Echogen System [19] 1 INTRODUCTION AND PRIOR ART 15 GE provides Echogen Power Systems to deliver efficient heat-to-power plants for the marine vessels. Echogen is a closed loop system using CO2 as the working fluid to convert exhaust energy into electricity, with an approximate system efficiency of 50% [19]. The properties of CO2 ensure a more compact, more efficient and cheaper waste heat recovery system. 1.4.3.3 SeaStream™ DP System [20] SeaStream™ DP system enhances efficiency and safety of the DP operations. It provides flexibility for effective maritime DP operations. It is an energy-efficient marine system to reduce operational costs and emissions. It is fully integrated and configured for optimizing electric propulsion performance. 1.4.3.4 Latest C-Series Vessel Control System (VCS) [21] The Latest C-Series Vessel Control System is a centralized monitoring, automation and control system providing fully remote services. A coherent system is obtained by implementing the VCS which integrates and unifies the sub-systems on an individual vessel. As a result, machinery monitoring, automation control and power management are carried out via the latest C-series . 1.4.4 Rolls-Royce Marine Rolls-Royce marine is an experienced marine technology and service producer. Its UT 700-series for platform supply vessel is recognized as a worldwide benchmark within the offshore industry [22]. Rolls-Royce marine promotes IPS in recent years. The IPS with SAVe label is configured for vessels with energy saving systems [23]. The SAVe Safe system is an example of Rolls-Royce marine’s IPS. The number of generators installed depends on the total shipboard power requirements or the vessel operating profiles. When power demand gets reduced, some engines can be turned off. As a conclusion of analysis on solutions from industrial companies, table 1 presents a summary of the mentioned industrial companies’ products and applications. The trend in the marine industry is moving towards to heat-to-power solution, automation of vessel information and control system, and integrated power systems. In terms of shipboard integrated power systems, Siemens and Rolls-Royce are the most active players among the competitors of ABB. 1 INTRODUCTION AND PRIOR ART 16 Application & Product Partnership Vessel Type Siemens BlueDrive PlusC Østensjø Rederi and Corvus Energy Edda Ferd (2013) Platform supply vessel Danish Esvagt Offshore support vessel Waste Heat Recovery System (WHRS) UMM SALAL, United Arab Shipping Container vessel SISHIP EcoMAIN Maersk Triple E Class Ships Container vessel MAN Diesel & Turbo Waste Heat Recovery System (WHRS) GE New Power Take Off/Power Take In (PTO/PTI) Maersk Line 2nd generation Triple-E Container vessel Exhaust Energy Recovery System: Echogen SeaStream™ DP Systems Latest C-Series Vessel Control System (VCS) S.A. Agulhas II Icebreaking polar supply ship Rolls-Royce Marine Integrated Power Systems (IPS) SAVe label SAVe Safe System UT 700-series Industrial Standard Platform supply vessel ABB Onboard DC Grid System Dina Star (2013) Platform supply vessel Table 1: Summary of competitors’ products/applications 1.5 Academic Prior Art Some of the academic prior arts of marine power systems are presented in this subchapter. They investigate on grid configurations, fault protections, hybrid battery/ultracapacitor ESS and VFD-based control. Most of the methodologies provided in the studies are focused on developing individual component, independent module or local control topology, but system-level control, holistic communication and power management strategy have not been discussed extensively at this stage. 1.5.1 Shipboard DC Grid Configuration According to the advantages of using shipboard DC grid aforementioned, utilization of a shipboard DC distribution network is focused in this thesis. Zheming Jin and Giorgio Sulligoi propose an optimized configuration of AES featuring onboard DC network [3]. Figure 10 demonstrates a ring-bus DC gird configuration designed for the crucial loads with higher security requirements. Onboard DC grid configuration, especially the ring bus, obtains more compact structure compared to AC grid, which fits the size and weight requirements predefined by the structure of the vessel. 1 INTRODUCTION AND PRIOR ART 17 10a. Single-ring-bus DC distribution 10b. Dual-ring-bus DC distribution Figure 10: Typical power architectures of ring-bus based DC distribution systems-[3] The reliability of the system is sustained by implementing specific fault solutions. In figure 11, the switches around the ring bus are deployed to isolate faults that may occur at the buses. The ring-bus-based distribution system on marine vessel under Zheming Jin and Giorgio Sulligoi’s study is named as zonal electrical distribution system (ZEDS). It should be noticed that the onboard loads in the zones are fed from both sides of the ship. As shown in Fig. 11, the restoration mechanism of the ZEDS is based on self-healing method. After a success in isolating a fault, the power converters of the healthy buses are reenergized to allow the effective sections to be operated normally. For the dual- ring-bus DC distribution (Fig. 10b), the system applies different current levels on the two buses that are connected to the loads with high power quality on the inside and lower power quality on the outside. Solid state circuit breakers may be able to isolate faults for the dual-ring-bus DC configuration. 1 INTRODUCTION AND PRIOR ART 18 Figure 11: The process of sectionalization and reconfiguration based on the self- healing method: (a) faults occur, (b) fault1 clear, (c) fault2 clear, and (d) fault3 clear-[3] 1.5.2 Optimal Loading Condition of Hybrid Power System Bijan Zahedi and Lars E. Norum conduct a detailed efficiency analysis of a shipboard DC hybrid power system [24]. The power system under investigation consists of four generation units, propulsion loads, auxiliary loads, and an energy storage system configured by Li-ion battery and full-bridge bidirectional converter (Fig. 12). Figure 12: The investigated shipboard DC power system-[24] A local control system in the generation unit including a local prime-mover optimizer and a governor is implemented to control the rotational speed of each prime mover. In terms of managing power sharing among the generators, controllers based on voltage droop are used to coordinate all the exciters of the diesel engine systems. The bidirectional DC/DC converter of the ESS enables charging and discharging the battery 1 INTRODUCTION AND PRIOR ART 19 in a locally controlled manner. The proposed ESS operates at a continuous mode or a periodical charge/discharge mode. It is observed that under continuous mode the DC source operates with constant number of engines defined for each operating mode, and in the case of periodical mode, the DC energy source works with variable number of online diesel engines. In order to achieve an energy-efficient system, the optimal loading conditions are found with the purpose of minimizing fuel consumption. However, the power quality, in terms of the DC link voltage level, of this proposed hybrid power system is not tested. 1.5.3 Pulse Load Compensation with Hybrid Battery/Ultracapacitor Battery and ultracapacitor combination systems applied for future land vehicles have been broadly explored, but similar applications in shipboard electric systems need to be further studied. Yichao Tang and Alireza Khaligh verify the feasibility of Hybrid Battery/Ultracapacitor energy storage system for naval applications [5]. A 500 V to 1 kV Battery/Ultracapacitor Energy Storage System (BUCESS) is designed for a 100 ~ 500 kW propulsion system. For both charging and discharging modes, feasibility of 100 kW transmission capacities for batteries and 1 MW for ultracapacitors is investigated. In the research of Yichao Tang and Alireza Khaligh, the DC-distribution power system of a navy ship has two dc voltage levels: one is high voltage level (between 4.7 kV and 10 kV) and the other is medium voltage level (between 700 V and 1 kV). The medium voltage level is implemented mainly for critical loads like energy storage system. Batteries can charge ultracapacitors through a bidirectional converter and another converter realizes the transmission between DC bus and batteries. Similar scheme is utilized for 1 MW charging and discharging of ultracapacitors. The batteries and the ultracapacitors are also capable of being combined to discharge at the same time. This paper verifies the feasibility of applying the proposed BUCESS for pulse load compensation, but further analysis on system-level transient behavior when the operational profile changes or pulse load occurs is needed. 1.5.4 Pulse Load Compensation with Flywheel and Prime Mover Mahdi Saghaleini and Behrooz Mirafzal set up a concept of Regenerative Energy Management for pulse-loads in dual DC-AC microgrids [25]. Pulse loads can cause voltage sags and degrade the system stability. In this work, with the consideration of pulse loads, it demonstrates an approach that can be executed to regulate the voltage level of a DC-bus in the power distribution network. Kinetic energies of flywheels and motor drives, e.g. prime movers, on an electric ship are used in the proposed method. Figure 13 presents the proposed system. To some extent, the whole power network can be used to feed the pulse loads in addition to the flywheel. The technical challenge lays in the fact that the impacts of the pulse loads are easily leaked to the network. In order to eliminate the impacts, the prime mover of the shipboard power system works as the main compensator of the pulse loads, and the flywheel motors are used to suppress the fluctuations caused by the variations of the prime-mover (a huge motor) speed. 1 INTRODUCTION AND PRIOR ART 20 Figure 13: A regenerative energy management for pulse-loads in dual DC-AC microgrid-[25] 1.5.5 Energy Management Strategy for Load Variation As discussed before in section 1.3 and 1.4, ABB and some other technical companies in marine industry have done some research on VFD-based applications, in order to compensate the load variations and reduce fuel oil consumption. Spyridon V. Giannoutsos and Stefanos N. Manias optimize energy management and diesel fuel consumption in marine power systems through VFD-based flow control [26]. This energy flow management strategy is applied as retrofit installation in a typical tanker vessel’s power system. Experimental results are compared to the previous operating conditions without retrofitting, so as to verify the effectiveness of the optimization. Four engine room ventilation fans and one CSW pump in the tanker vessel are retreated with the proposed system. Under various operating load demands, the energy management system calculates optimal number of online diesel generators based on the proposed topology. In this way, annual diesel fuel consumption is reduced especially during water-going period with main engine running at low speed (55% ~ 60% of rated speed). Note that this work proposes a feasible approach for optimizing the onboard fuel consumptions, but the reliability and stability of using this system when there are significant changes in different operational modes are not verified. Thus, problems could occur when it is implemented on offshore support vessel that often requires dynamic positioning operations. 1.5.6 Stabilizing Power Fluctuation with Energy Storage System In the previous studies of Yichao Tang and Mahdi Saghaleini, the regulation approach against power volatility is set up for the pulse loads (e.g. weapons) in the shipboard power system instead of considering the propulsion load fluctuations. The pulse loads can be controlled via forecast, but due to the complex marine conditions, the propulsion load fluctuations cannot be predicted that easily. Accordingly, in [27], hybrid energy 21 1 INTRODUCTION AND PRIOR ART storage is implemented to keep the propeller’s speed constant and compensate propulsion load fluctuations. Jingnan Zhang and Qiang Li propose a method for stabilizing the power fluctuation of the shipboard electric propulsion system, and establish a dynamic simulation model of the integrated system [27]. In this energy management strategy, battery combined with a super capacitor is considered as a hybrid energy storage system. The hybrid energy storage capacity is optimized using Particle Swarm Optimization (PSO) algorithm. At the same time, the active parallel hybrid energy storage system (APHESS), including two levels of charge and discharge controller, establishes a local control function to enable the energy storage system generating precisely and responding promptly to the load fluctuations. The load fluctuation is studied according to a LNG ship’s experience. Figure 14 shows the topology of the shipboard power system under study. This electric propulsion system implements an AC-DC-AC converter to control the propulsion motor M. Pulse load is connected to the AC bus through a rectifier, and the hybrid energy storage unit (Li-ion battery with super capacitor) connects to the system via a DC/DC converter. Figure 14: A simplified integrated shipboard power system-[27] In order to obtain an optimized capacity of the hybrid energy storage unit, PSO algorithm is used. Furthermore, the APHESS is designed to carry out a real-time adjustment and stabilization for output power fluctuations. The objective of the optimization problem is to minimize the configuration cost of the storage system. Operation characteristics of the Li-ion battery and super capacitor, operation constraints of the generators, and variations of system power loads are considered as complementary conditions. Detailed equations can be referred to [27]. In terms of the local energy storage control, an APHESS is set up with two levels of bidirectional DC/DC converters (Fig. 15). 22 1 INTRODUCTION AND PRIOR ART Figure 15: APHESS topology-[27] When the sum of generators’ output power is greater than the maximum power set point, APHESS will be controlled to release power. On the other hand, when the DC bus voltage increases due to load demand decreases, the energy storage system is controlled to absorb power from the system. As a consequence, the total power reserved in APHESS can be distributed flexibly between the batteries and super capacitors through this control strategy. This study of Jingnan Zhang and Qiang Li provides a simple prototype of shaving the propulsion load peaks, but operational windows of the generation units need to be investigated and optimized. Moreover, since the realistic loading profile is more complex than the test case defined in this study, consideration regarding the power flowing properties and functionality of the proposed hybrid energy storage system need to be expanded. As a summary for Chapter 1, it first presents the solutions, from both ABB and other industrial companies, of electrifying the marine vessel operations based on the common practice implemented in the existing marine power systems. After that, some of the academic prior arts of marine power systems are introduced. Most of the methodologies aforementioned are concentrated on developing individual component, independent module or local control topology, but system-level control, communication and power management system for a network have not been discussed extensively in this chapter. In Chapter 2, a literature review is conducted regarding to different topics that are investigated in this thesis. This overview of theoretical research methodologies and mechanisms lays a foundation for developing actual system design, control and energy storage concepts of this thesis. 23 2 LITERATURE REVIEW – MICROGRID FOR MARINE 2 Literature Review – Microgrid for Marine In this chapter, some theoretical research methodologies and mechanisms proposed in academic literatures are investigated. This overview lays a foundation for developing the actual concepts of this thesis. First, a definition of a microgrid is presented with an extension to scheme of microgrid for marine. After that, the current framework, theoretical knowledge and academia prior art within the area of microgrid for marine are reviewed to analyze the possibility of implementing and adopting these research methodologies and mechanisms. 2.1 Microgrid and Microgrid for Marine The ABB definition for microgrid refers to operating distributed energy resources and loads in a controlled and coordinated manner. The generation units and loads can optionally be connected to the utility grid or operate in “islanded” mode. Microgrid solution can be enhanced via advanced power network control and various power management strategies. Due to the development of distributed generation (DG) and power management technologies, an isolated or islanded power system, namely microgrid in marine, becomes a promising option for marine power system and gains an increase in research interest. In other words, a shipboard power system is, in every extent, a microgrid because it contains DG, power network control, and loads within an islanded frame. It is an isolated and self-sufficient power system in the sea. Note that this thesis mainly investigates the “islanded” mode of the marine microgrid, and the option of plugging it to the onshore utility grid is not studied. Overall, the microgrid in marine restructures the shipboard electric system and meets the increasing demands of improving marine energy efficiency and reducing fuel consumption. Based on an overview of the recent studies and applications of microgrid for marine, there are significant amounts of investigations focusing on DC grid implementation. This is mainly because that the number of power components based on DC distribution, e.g. DC load consumptions, storage systems and some DG sources, is increased. DC-based microgrids can offer additional benefits of saving configuration space and simplifying grid design. DC microgrid systems are becoming more competitive and popular in the areas of data center security, renewable power generation & storage, vehicles and marine vessels. 2.2 Academic Research in Microgrid for Marine Currently, in the study area of marine microgrid, to understand the reactions of shipboard microgrid system (SMS) operated under different load profiles is one of the most popular topics. These topics include power system analysis, fuel consumption estimation, novel power system evaluation and fault insertion combined with system restoration. In the following examples, modeling and simulation methodologies with respect to the topics aforementioned are presented. 2.2.1 Simulation Models of Shipboard Electric Power Systems The electric propulsion solutions for marine nowadays vary depending on vessel types, available technologies and operation profiles. According to ABB vessel segments, offshore vessels, ice-going vessels, passenger vessels, cargo vessels, and special purpose vessels account for the majority of marine vessels. 24 2 LITERATURE REVIEW – MICROGRID FOR MARINE Li Wang has established the simulation models for simulating the dynamic performance of a microgrid system feeding the electrical loads in a sailing boat [28]. The design of the shipboard microgrid system consists of a diesel generator, a wind turbine generator (WTG), two solid-oxide fuel cells (SOFC), a seawater aqua electrolyzer (AE), a battery energy storage system (BESS), a DC/AC inverter and an AC/DC converter (Fig. 16). Figure 16: Configuration proposal of the microgrid system in a sailing boat-[28] In this study of Li Wang, the general principle of the microgrid system and the relevant mathematical models are discussed. Based on time-domain steady state and dynamic simulations, the proposed microgrid system is validated in terms of stable supply of required power loads under both high load demand and low load demand conditions. As shown in Fig. 16, the microgrid system includes four subsystems (in dashed boxes). The frequency of the loads can be maintained by those subsystems. Since the normal load can be interrupted while the emergency load needs to be constantly supplied, the diesel-engine generator should be started at proper times to compensate the lack of supply from the three-phase DC/AC inverter. The power flows on DC bus of this microgrid system is the combination of the output of AC/DC converter fed by WTG, the absorbed power of AE, the outputs of two SOFCs, the charge or discharge power of BESS, and absorbed power of DC/AC inverter. To conduct power system analysis, the mathematical models of some components in the microgrid system are presented below. 25 2 LITERATURE REVIEW – MICROGRID FOR MARINE 1) Battery Energy Storage System: In this paper, the demand of power load and the power generation of the SOFC(s) determines the operation of the BESS during the simulation period. If 𝑃𝐹𝐶(𝑡) > 𝑃𝐿𝑜𝑎𝑑, the BESS is under charging mode and the battery state of charge (SOC) at time t is expressed by: SOCBESS (t) = SOCBESS(t − 1) + (PFC(t) − PLoad(t))∆tηch (1) The 𝑆𝑂𝐶𝐵𝐸𝑆𝑆 (𝑡) and 𝑆𝑂𝐶𝐵𝐸𝑆𝑆 (𝑡 − 1) are the capacities of the BESS at time t and t-1. 𝜂𝑐ℎ is the charging efficiency. If(𝑃𝐿𝑜𝑎𝑑(𝑡) − 𝑃𝐷𝑖𝑒𝑠𝑒𝑙(𝑡)) ≤ 𝑃𝐹𝐶(𝑡) < 𝑃𝐿𝑜𝑎𝑑(𝑡), the BESS is under charging and the battery capacity is given by: SOCBESS (t) = SOCBESS(t − 1) + (PFC(t) + PDiesel(t) − PLoad(t))∆tηch (2) If (𝑃𝐿𝑜𝑎𝑑(𝑡) − 𝑃𝐷𝑖𝑒𝑠𝑒𝑙(𝑡)) > 𝑃𝐹𝐶(𝑡) , the BESS is under discharging mode and starts to compensate the power shortfall. The battery capacity of the BESS is given by: SOCBESS (t) = SOCBESS(t − 1) + 1 ηdis (PFC(t) + PDiesel(t) − PLoad(t))∆t (3) Except the conditions mentioned above, the SOC of the BESS at any time should follow the constraints of 𝑆𝑂𝐶𝐵𝐸𝑆𝑆(min) ≤ 𝑆𝑂𝐶𝐵𝐸𝑆𝑆 (𝑡) ≤ 𝑆𝑂𝐶𝐵𝐸𝑆𝑆(max) . The upper and lower limits are the maximum and minimum allowable working window of the BESS respectively. 2) Solid-Oxide Fuel Cell: The SOFC output voltage 𝑉𝑑𝑐 is expressed in terms of partial pressure of hydrogen, oxygen and water as: Vdc = NE0 + NRT 2F ln [ PPH2 PPO2 0,5 PPH2O ] − rintIFC (4) PP refers as the partial pressure, 𝑟𝑖𝑛𝑡 is ohmic-loss resistance of SOFCs, F is the Faraday Constant, and 𝐸0 is the reaction-free voltage of a cell. 3) DC/AC Inverter: This power electronic unit manages the different dynamics of the SOFCs, BESS, WTG, and onboard load demands. The power line between inverter and load is approximated as purely inductive, and the inverter is assumed as lossless. Accordingly, the AC voltage and active power at the inverter output are given by: 26 2 LITERATURE REVIEW – MICROGRID FOR MARINE Pac = mVdcVL X sin δ (5) The corresponding 𝑃𝑎𝑐 = 𝑃𝑑𝑐 = 𝑉𝑑𝑐𝐼𝑑𝑐 and the molar flow of hydrogen is 𝑞𝐻2 = 𝑁(𝐼𝑑𝑐+𝐼𝑏) 2𝐹𝑈 , so the phase angle 𝛿 is calculated as: δ = sin−1 { [( 2qH2 FU N ) − Ib] X mVL } (6) As a result, the output voltage of the inverter can be controlled by the modulation index m, and the output active power can be controlled by adjusting the hydrogen flow 𝑞𝐻2 . 4) Wind Turbine Generator System and Frequency Variation: The WTG generates power of 𝑃𝑊𝐺 = 1 2 𝜌𝐴𝑆𝐶𝑃𝑉𝑊 3 . The system frequency variation is expressed as ∆𝑓 = ∆𝑃𝐿 𝐾𝑠𝑦𝑠 . ∆𝑃𝐿 is the imbalance between power load demand and power generation, and 𝐾𝑠𝑦𝑠 is a system characteristic in terms of frequency. Tiffany Jaster and Andrew Rowe model a hybrid electric propulsion system of a marine vessel in MATLAB Simulink with SimPowerSystems library [29]. In their work, the term “microgrid” is not used, but it provides methodology for the design of a shipboard “hybrid” power system and power management system. Because of that, the proposed system can also be regarded as a shipboard microgrid system to some extent. The proposed hybrid propulsion system is beneficial within marine areas with strict environmental regulations. With the all-electric ship (AES) mode, the emission free target is achievable during low speed cruising or DP. The studied shipboard power system is modeled with the aim of evaluating the novel design and exploring the capability of the hybrid propulsion system. The target is to investigate the effectiveness of utilizing multiple energy sources. The design of this hybrid system consists of three 215 kW marine diesel generators, a 150 kW fuel cell (FC), a 232 kWh ESS (three parallel Li-ion battery blocks), and two DC/AC inverters (Fig. 17). The hybrid generation system is connected to a 460 V AC bus. Ship propulsion system is configured with two 200 kW azimuth thrusters, and a 90 kW fixed pitch bow thruster supplementing DP operations. 27 2 LITERATURE REVIEW – MICROGRID FOR MARINE Figure 17: Configuration proposal of the hybrid propulsion system-[29] In the study of Bijan Zahedi and Lars E. Norum, DC distribution system is implemented and a simulation platform of carrying out power sharing between the two diesels, a fuel cell module, and an ESS is developed in MATLAB/Simulink [30]. The choice of distribution voltage type (AC and/or DC) is of primary importance to the design of microgrids in electric vessels. In order to pursue a system-level analysis of a shipboard DC distribution power system (Fig. 18), nonlinear properties of the power converters and interactions among the components should be analyzed for the system model. 28 2 LITERATURE REVIEW – MICROGRID FOR MARINE Figure 18: Configuration for system-level analysis of ship DC distribution power system-[30] Prior to the system-level simulation, appropriate individual models in the system are achieved. Since the interfaces among connected models must be bidirectional and based on nonlinear averaging techniques, the study [30] emphases the use of nonlinear average value model for the bidirectional converters. In the simulated hybrid power system, the two synchronous diesel generators are rated at 300 kW each, the ESS is based on Li-ion battery, and the propulsion motor is a three-phase 460 VL-L induction motor (rated at 400 hp). Regarding the load profile, three sailing modes are proposed: 1) high speed 2) moderate speed 3) low speed. In addition, ship auxiliary and hotel load is rated at 125 kW and modeled by a constant impedance load. DC currents of the different energy sources and loads (Fig. 19) represent their power contributions. According to the simulation results, with high vessel speed, the power load reaches the highest demand compared to those of the other two speed modes. Changing from the high speed mode to the moderate speed mode, the power load demand is reduced by 70%. During this time, the generator 2 is shut down leaving generator 1 and fuel cell to supply the power demand. In the mode of moderate speed, the ESS can be charged and discharged depending on the load level, and SOC is sustained by starting an idle diesel when it reaches the lower boundary, and shutting down an online generator when SOC hits its upper limit. Under low vessel speed, the power demand is mainly supplied by the fuel cell, and the generator 1 can be shut down if the ESS is not being charged. Through the entire simulation, the fuel cell is controlled on its rated value (20 A/s) so as to prevent from fuel starvation. 29 2 LITERATURE REVIEW – MICROGRID FOR MARINE Figure 19: DC currents of the sources, ESS, fuel cell, and load-[30] In this paper [30], the DC link voltage is regulated by using voltage droop control method [31]. More details about the control method for power sharing and DC voltage regulation in shipboard power distribution system is further discussed in section 2.2.2. In short, Bijan Zahedi and Lars E. Norum establish a system-level simulation platform using the derived models for different components. 2.2.2 Voltage Regulation and Power Sharing Control in Shipboard DC Power System Bijan Zahedi and Lars Einar Norum investigate voltage control and also power sharing between the generation sources in a shipboard DC power system [32]. Main DC sources in this system are fuel cells and ESS. The power system under study includes two diesel generators, one fuel cell module, and one energy storage system. The diesel engine, propellers and other mechanical components are also taken into consideration. Figure 20 illustrates the typical layout of a DC shipboard power system in a single line diagram. The “Clean Energy Source” element can be fuel cells and photovoltaics (PV) modules most probably. The “Energy Storage” can be based on battery or super capacitor. 30 2 LITERATURE REVIEW – MICROGRID FOR MARINE Figure 20: Typical layout of a DC shipboard power system in a single line diagram-[32] The exact power sharing control cannot be finalized only through executing primary voltage droop. Thus, an advanced power sharing control is derived by implementing a higher level compensator to dispatch the load demand precisely according to the optimal loading range of a diesel engine. A compensation voltage is introduced by the compensator to adjust the DC voltage reference, so as to maintain the load sharing of the generator units within their optimal range. The described voltage regulator-exciter is depicted in Fig. 21. When the load demand increases, the compensator increases the voltage reference (𝑣𝑑𝑐 ∗ ) to deliver more power to the grid. As the load gets lower, the compensation voltage (𝑣𝐶𝑜𝑚𝑝) reduces the power delivered from the generators. Figure 21: Block diagram of voltage regulator-exciter-[32] 2.2.3 Real Time Implementation of Microgrid Reconfiguration Operating an islanded distribution power system requires a better consideration of uninterrupted power supply and an efficient reconfiguration when a fault happens. Reconfiguration is a control action which includes load or generation shedding and other measures to make the remaining loads unaffected under fault conditions. The work of F. Shariatzadehand and R. Zamora in 2011 uses genetic algorithm (GA) and graph theory based methodologies to reconfigure a shipboard microgrid in a real-time 31 2 LITERATURE REVIEW – MICROGRID FOR MARINE case study [33]. The proposed GA is used on an 8-bus shipboard power system to find out optimal configuration for meeting the predefined objectives and system constraints. The microgrid system is simplified by using graph representation, and then the corresponding matrix representation is established. Based on the matrix representation, optimal solutions for balancing the remained system after fault isolations can be found. Figure 22: A shipboard microgrid system-[33] Figure 22 depicts a shipboard microgrid system. It consists 6 switchboards (bus 1, 2, 3, 5, 6 and 7), two cables (bus 4 and 8), 4 generators (G1, 2, 3 and 4), and 18 breakers. Those components can be plotted by implementing graph representation in order to formulate the system mathematically by means of its matrix. Based on the established reconfiguration algorithm, a real-time implementation is carried out using real-time digital simulator (RTDS) and dSPCAE-DS1104 R&D controller board. RTDS simulates the shipboard microgrid system and sends fault signals to dSPCAE controller. If a fault is detected by the controller, dSPCAE will run the reconfiguration algorithm and send back the status of load breakers to RTDS for maintaining the unaffected loads. In the real-time test, all load breakers are closed in normal operation condition, but after faults occur, e.g. on bus 1 and 5, only two load breakers remain closed. As the results shown in [33], GA reconfiguration algorithm performs nicely in real-time implementation for a SMS. 32 2 LITERATURE REVIEW – MICROGRID FOR MARINE 2.2.4 Optimal Sizing of a Shipboard Microgrid with PV and ESS Hai Lan and Shuli Wen propose a method to determine the optimal size of a power generation system, which is integrated with diesel, PV and ESS in a stand-alone shipboard power network [34]. The optimal sizing minimizes the investment cost, fuel cost and the CO2 emissions. In order to validate the concept, the authors implement the proposed method to optimize the costs and emissions for a hybrid PV/diesel/ESS system (Fig. 23). The project is involved in “Study on the Application of Photovoltaic Technology in the Oil Tanker Ship in China”. Figure 23: A shipboard microgrid system with PV and ESS-[34] First of all, the study of Hai Lan and Shuli Wen focuses on modeling of a shipboard microgrid system. The microgrid system consists of a PV generation unit, a diesel generator supplying major power demand and an ESS storing energy surplus and improving the redundancy of the system. The tanker ship navigates from Dalian in China to Aden in Yemen. According to this navigation routine, the optimization includes 3840 hours in a year. The irradiation, temperature and the load profile are sampled every hour (Fig. 24). Variations of the power load under five operational conditions are modeled: regular cruising, full-speed sailing, docking, loading/unloading and anchoring [34]. It should be noticed that the impacts of motion fluctuations of the ship are not considered in the study. 33 2 LITERATURE REVIEW – MICROGRID FOR MARINE Figure 24: Hourly ship load profile along routine of interest-[34] The models of the system components (PV unit, diesel generator, and battery) are derived mathematically. Relevant expressions for those models can be found in [34]. According to the mathematical models, operational constraints of the studied system are listed as following, where 𝑃𝑑 (𝑠,𝑡), 𝑃𝑃𝑉 (𝑠,𝑡), 𝐸𝐸𝑆𝑆 (𝑠,𝑡) are referred to the outputs of the diesel generator, PV and ESS respectively at time t in season s. Pd min ≤ Pd(s,t) ≤ Pd max (7) PPV(s,t) ≤ PPV ≤ PPV max (8) EESS(s,t) ≤ EESS ≤ EESS max (9) Pd(s,t) + PESS(s,t) + PPV(s,t) = PLoad(s,t) (10) Based on the two objectives of this study, a multi-objective problem is set up. The corresponding multi-objective functions are: minf1 = Costfuel + CostPV ∙ CRFPV + CostESS ∙ CRFESS (11) 𝑚𝑖𝑛𝑓2 = 𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑖 = ∑ ∑ 𝐸𝑚𝑓𝑢𝑒𝑙 ∙ (𝑎 ∙ 𝑃𝑑(𝑠,𝑡) + 𝑏 ∙ 960 𝑡=1 𝑃𝑑 𝑟𝑎𝑡𝑒𝑑 4 𝑠=1 ) (12) Where, the total cost calculated in “𝑚𝑖𝑛𝑓1" consists of fuel cost, installation and replacement costs of PV and ESS. The capital recovery factor (CRF) is used for converting initial cost to an annual capital cost. The cost functions are represented in the form of net present cost: 34 2 LITERATURE REVIEW – MICROGRID FOR MARINE 𝐶𝑜𝑠𝑡𝑓𝑢𝑒𝑙 = ∑ ∑ 𝑃𝑟𝑖𝑐𝑒𝑓𝑢𝑒𝑙 ∙ (𝑎 ∙ 𝑃𝑑(𝑠,𝑡) + 𝑏 ∙ 960 𝑡=1 𝑃𝑑 𝑟𝑎𝑡𝑒𝑑 4 𝑠=1 ) (13) 𝐶𝑜𝑠𝑡𝑃𝑉 = (𝐶𝑐𝑎𝑝𝑖𝑡𝑎𝑙 𝑃𝑉 + 𝐶𝑟𝑒𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑃𝑉 ) ∙ 𝑃𝑃𝑉 (14) 𝐶𝑜𝑠𝑡𝐸𝑆𝑆 = (𝐶𝑐𝑎𝑝𝑖𝑡𝑎𝑙 𝐸𝑆𝑆 + 𝐶𝑟𝑒𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝐸𝑆𝑆 ) ∙ 𝐸𝐸𝑆𝑆 (15) After that, Multi-Objective Particle Swarm Optimization (MOPSO) is implemented to solve the multi-objective optimization problem. In the case of MOPSO optimal sizing (minimizing cost and emission), the total diesel output power is at minimum level with the application of PV and battery. Compared to the cases without optimal sizing or with other optimization techniques, the scenario with the proposed MOPSO methodology demonstrates that the selected size of the PV generation and the energy capacity of LiFePO4 ESS make the hybrid system (Fig. 23) achieve the lowest net present cost and produce the minimal emission. After reviewing the methodologies summarized in the literature review part, the system-level analyses for shipboard microgrids are well investigated. Most of the raised methodologies can be referred for developing the actual concepts in this thesis. 35 3 MICROGRID FEASIBILITY FOR MARINE 3 Microgrid Feasibility for Marine First of all, in this chapter the problems and goals defined in this thesis are recapped. Some of the methodologies from the prior art studies discussed in Chapter 1 and 2 are not specified for the microgrid scope and system-level scale. In this chapter, in order to investigate the feasibility of establishing microgrids for marine vessels, crucial technologies related to energy storage system, vessel automation & control system and inland microgrid solutions are studied. Based on that, the possibility of implementing existing solutions for marine microgrids is investigated. 3.1 Problem Definition In this thesis, a shipboard microgrid system (SMS) is set up with integration of energy storage and solar sources to the existing diesel generators within an islanded network. In order to investigate the feasibility of microgrid operation in marine vessels, a power management system (PMS) consisting of Load Sharing Strategy and Peak Shaving Strategy is formulated. Subsequently, time-domain simulations are carried out to validate the main concepts. At first, some of the prior art, in terms of shipboard power network, electric propulsion, energy storage system, and power management system, from both industrial and academic studies are presented. Based on that, feasibility of microgrid solutions for marine vessels is investigated. After that, the actual design and control concepts are developed, and testing case scenarios are defined. In addition, time domain simulations are implemented to verify the proposed methodologies. According to the prior art studies, DC based shipboard power system is more of interest. Some recommendations for analyzing the generic microgrid architectures and the specific applications are given by the “DC Microgrid Scoping Study” [35]. These recommendations provide valuable study guidance and reveal future potentials of marine microgrid studies. In order to assess the system stability and validate the PMS when implementing this shipboard DC microgrid system with installation of ESS and renewable resources, this project facilitates a detailed MATLAB simulation-based system level study accounting for the factors of realistic load/generation profiles, stability and power quality of DC microgrids. The overall target of this project is to achieve a highly energy efficient microgrid system for marine vessel, facilitate dynamic response to load fluctuations, and enhance existing vessel information and control system. Prior to the system-level simulation, models of the energy resources, energy storage, and power electronic converters need to be obtained. After that, control functions and stability of operating the proposed shipboard microgrid system (SMS) are verified. 36 3 MICROGRID FEASIBILITY FOR MARINE 3.2 ABB Storage and Control Systems In order to improve performance and manage power flows of islanded electric power systems, control and storage subsystems are needed. Some of the ABB control and storage solutions are discussed below. 3.2.1 Storage Solutions In a power grid, volatilities caused by renewable integration and loading profile fluctuation impact the grid stability. For marine vessels, persistent load fluctuations introduced by the rotating motion of the propeller and wave-induced motions, exist throughout the normal operations. The presence of renewable energy sources also increases volatility in shipboard power systems. Accordingly, energy storage system (ESS) embedded with specific control functions is increasingly recognized by the marine industry. Energy storage and control systems for marine can take advantages of the experience and techniques from the distributed microgrid systems in other areas. ABB’s Energy Storage Module (ESM) provides a packaged solution that stores energy produced by different sources. The energy is usually stored in batteries for specific energy demands. ABB ESM portfolio includes [36]:  Community Energy Storage (CES) System: 25 kW – 100 kW / 30 min – 4 hours; 2 enclosures: Batteries and Battery Management System (BMS), and inverter & switchgear.  Distributed Energy Storage (DES) System: 100 kW – 5 MW / 15 min – 4 hours; Containerized solution with batteries, BMS, inverter, inverter PLC, switchgear, and transformer.  Grid connection equipment: 400 kVA – 20 MVA; one container with inverter, inverter PLC, switchgear, and transformer.  Battery containers: containerized solution with batteries, racks, and management systems. Battery Energy Storage System (BESS) is an ESM technology based on battery devices. BESS is a solution, which makes the power network smarter by raising power quality with better voltage and frequency regulation as well as component redundancy. Accordingly, the scheme of BESS can be applied in islanded DC microgrids for implementing load sharing, stabilizing DC link voltage and shaving load fluctuation. Within the scope of BESS, EssPro™ Battery Energy Storage System offers a distributed solution for supporting the grid (Fig. 25). The battery type includes lithium- ion (Li-ion), sodium-sulfur (NaS), nickel-cadmium (NiCd), and lead-acid [37]. At the same time, The EssPro™ electrical plant and inverter controller (EPIC) enables manual and automatic operation of all BESS components in various control modes [37]. 37 3 MICROGRID FEASIBILITY FOR MARINE Figure 25: EssProTM energy storage applications-[37] 3.2.2 Control Solutions In presence of distributed generation and load, control devices are needed to interconnect the components in the system, so as to execute system-level operations and communications. For the battery technologies, it is a common practice to use a bidirectional converter to incorporate energy storage in electric power system and provide battery control. In order to maintain the acceptable voltage level, system redundancy and power quality in DC microgrids, the active power balance in the system needs to be controlled. EssPro™ Power Conversion System (PCS) is usually used for energy storage applications. It acts as an interface between the grid and the batteries [38]. It allows energy to be stored or accessed whenever it is needed. For microgrid application, the PCS supplies critical loads in any circumstance by producing controlled local generation. UNITROL® automatic voltage regulators (AVR) and static excitation systems (SES) offer solutions for any type and size of power plant [39]. Because of its high flexibility, it enables voltage regulation for various types of applications. For instance, power plants with diesel engines, electrical propulsion for marine, and diesel electric locomotives can all implement UNITROL® control concept. Due to its design features and built-in control functions, it becomes a compact and robust regulator for both AC and DC voltage inputs. Moreover, it obtains an Ethernet-based fieldbus interface, which makes it highly probable to be connected to a larger scale communication network [39]. 3.2.3 Marine Vessel Information and Control – System 800xA 38 3 MICROGRID FEASIBILITY FOR MARINE Figure 26: ABB Marine automation and control system - 800xA-[40] System 800xA is an extended marine automation system offering services beyond traditional automation system [40]. It is an automation platform designed for users with excellent connectivity capabilities. Because of that, this integration system builds up a network where plenty of applications, plant systems, and devices are connected. All information is available for optimizing normal operations, control performance and protection under faults. Figure 26 shows the system diagram of ABB 800xA. Table 2: Functions and benefits of utilizing System 800xA Advisory Systems: performance management of the vessels •EMMA® Advisory Suite: Minimize energy consumption •OCTOPUS® Advisory Suite: Maximize availability and safety Power Energy Management System (PEMS) •An advanced control concept prepared for new and alternative energy sources •Used on DINA Star Third Party DCS Access •AC 800M control and I/O products •Modbus interface •Extend the functionality 39 3 MICROGRID FEASIBILITY FOR MARINE Table 2 summarizes some useful functions and benefits of utilizing System 800xA. To emphasize, 800xA provides connectivity to distribution control systems (DCS), as well as third party devices and applications. 3.3 ABB Microgrid Solutions - Microgrid Plus System and PowerStore Land-based microgrid technologies are more investigated and industrialized than those of marine microgrid. This section presents the ABB solutions for land-based microgrids, so as to investigate if microgrids for marine vessels can lean on some of the concepts raised from inland microgrids. The investigation is carried out from aspects of grid control strategies and system-level storage solutions. ABB’s microgrid offer energy storage solutions as well as automation and intelligent control solutions that can manage renewable energy integration in remote or islanded grids, ensuring outstanding power quality and grid stability [41]. ABB's microgrid solution consists of Microgrid Plus System™ and PowerStore™ flywheel or battery-based grid stabilizing system [41]. They build up land-based microgrids with system-level control functions and energy storage solutions. Normally, ESS is embedded to the microgrids and work synergistically with control functions. In this way, grid stability and redundancy are maintained. Figure 27 gives an overview of the ABB microgrid solutions. Figure 27: Overview of ABB’s microgrid solution-[41] 3.3.1 Microgrid Plus System ABB Microgrid Plus System™ is a distributed control system for microgrid [41]. It is a specially designed network control system. It is a technology responsible for coordinating the operations of hybrid power plants and integrating renewables into microgrids [42]. Table 3 lists most of the functions of the ABB Microgrid Plus System. 40 3 MICROGRID FEASIBILITY FOR MARINE Function Definition Renewable Energy Integration Ensure sufficient capacity is available in microgrid for renewable energy integration Storage / Stabilizing Integration Designate available assets for smoothing. As well as synchronizing sources Archiving Log important and significant events and occurrences Managed load demand Monitoring and managing connected loads Generator scheduling/ dispatch/ start-stop Maintain generator status and operation commands Generator Power Sharing (Active & Reactive Power) Set active & reactive power set points for supplying loads Generator configuration management Configure generator settings and parameters Generator overload capability Calculate the overload capability and set value in generator Feeder proactive load shedding and automatic microgrid black start; Managing feeder automation parameters for shedding and reconnection. Control of voltage, frequency in standalone/isolated grids Maintain desired operating parameters Solar PV generator power limitation set point Power generation in solar PV can be limited Determine and manage the energy storage system state of charge Monitoring capacity and managing the energy storage systems within the microgrid Table 3: Functions of the Microgrid Plus system The unit manages the energy flow within a power network to ensure sufficient power reserve, voltage stability, and balance between supply and demand in the power grid. It also optimizes the use of intermittent energy sources. The Microgrid Plus System can be implemented for different setups. Figure 28 presents a typical example of land-based isolated PV/diesel microgrid. 41 3 MICROGRID FEASIBILITY FOR MARINE Figure 28: Typical schematic of isolated PV/diesel microgrid-[42] In the microgrid plus system, MGC600 microgrid controller package (Table 4) is used to manage and automate distributed power generation systems [43]. It contains various controllers with different functions to allow communication between the microgrid devices. MGC600 controllers are deployed into the microgrid system in order to allow system-level communication. Different electrical devices select different types of firmware. Firmware / Controller Description Diesel/Gas generator (MGC600G) To control, monitor and interface to diesel generators Distribution Feeder (MGC600F) To control, monitor and interface to feeders and thei