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Maritime Electrical Installations And Diesel Electric Propulsion disadvantages of diesel electric propulsion systems
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Maritime Electrical Installations And Diesel Electric Propulsion by Alf Kåre Ådnanes ABB AS Marine ABB 2003-04-22 Preface This tutorial report is written in order to give an overview of application and solutions, as well as an introduction to components used in vessels with electric propulsion. The note is aimed to be a reference for those needing an update on recent solutions, and as a tutorial for new personnel in this exiting and fast developing area. Although it is tried to keep information updated and as far as possible free of errors, it is to be noted that the author or ABB cannot be held responsible for any faults or failures resulting from use of this content. Neither can this note be used as a contractually binding document for ABB in any commercial or other relations, regardless of referred to in writing or orally by other documents. Oslo, April 2003, Alf Kåre Ådnanes Technology Manager 2Table of Contents 1. Introduction............................................................................................................................5 1.1. Scope and Objectives ..............................................................................................................5 1.2. Motivations for Electric Propulsion ............................................................................................5 1.3. Power Flow and Power Efficiency .............................................................................................7 1.4. Historical Overview of Electric Propulsion ..................................................................................9 2. Applications.........................................................................................................................10 2.1. Passenger Vessels – Cruise Ships and Ferries ........................................................................10 2.2. Oil and Gas Exploitation and Exploration: Drilling Units, Production Vessels and Tankers ..........11 2.3. Field Support Vessels and Offshore Construction Vessels........................................................12 2.4. Dredgers and Construction Vessels ........................................................................................13 2.5. Yachts and Leisure Boats.......................................................................................................13 2.6. Icebreakers and Ice Going Vessels .........................................................................................13 2.7. War Ships .............................................................................................................................13 2.8. Research Vessels..................................................................................................................14 2.9. Trends and New Applications .................................................................................................14 3. Overview of Electric Power System.......................................................................................15 3.1. Introduction ...........................................................................................................................15 3.2. Electric Power Generation ......................................................................................................16 3.2.1. Prime Mover..........................................................................................................................16 3.2.2. Generators ............................................................................................................................17 3.3. Electric Power Distribution......................................................................................................18 3.3.1. Switchboards.........................................................................................................................18 3.3.2. Transformers .........................................................................................................................20 3.4. Motor Drives for Propulsion and Thrusters ...............................................................................20 3.4.1. Introduction ...........................................................................................................................20 3.4.2. The constant speed, Direct-on-Line motor ...............................................................................21 3.4.3. Variable speed motor drives and control strategies ..................................................................24 3.5. Propulsion Units ....................................................................................................................27 3.5.1. Introduction ...........................................................................................................................27 3.5.2. Shaft propulsion.....................................................................................................................27 3.5.3. Azimuth thrusters ...................................................................................................................28 3.5.4. Podded Propulsion ................................................................................................................29 3.6. Trends and New Concepts .....................................................................................................30 3.6.1. Electric power generation.......................................................................................................30 3.6.2. Electric power distribution.......................................................................................................31 3.6.3. Propulsion .............................................................................................................................31 4. Power and propulsion control...............................................................................................32 4.1. Introduction - Control Hierarchy ..............................................................................................32 4.2. User Interface........................................................................................................................33 4.3. High level control functionality.................................................................................................33 4.3.1. Power management – Energy Management ............................................................................33 4.3.2. Vessel Management ..............................................................................................................34 4.3.3. Propulsion control and dynamic positioning .............................................................................34 4.4. Low Level control functionality ................................................................................................35 4.4.1. Engine protection and governor ..............................................................................................35 4.4.2. Automatic voltage regulator ....................................................................................................36 4.4.3. Protection relays....................................................................................................................36 4.4.4. Propulsion controller ..............................................................................................................40 5. Electric Propulsion Drives.....................................................................................................41 5.1. Introduction ...........................................................................................................................41 5.2. Variable Speed Motor Drives ..................................................................................................41 5.2.1. Full-bridge thyristor rectifiers for DC motor drives (SCR)...........................................................41 35.2.2. Current source converters ......................................................................................................43 5.2.3. Cycloconverters .....................................................................................................................44 5.2.4. Voltage source inverters .........................................................................................................45 5.2.5. Other converters ....................................................................................................................51 5.2.6. Comparison of electric motor drive alternatives ........................................................................51 6. System Design......................................................................................................................53 6.1. Introduction ...........................................................................................................................53 6.2. Life Cycle Cost Assessment of Conceptual Design ..................................................................54 6.3. Standard Network Analysis and Electrical Power System Studies .............................................55 6.3.1. Load flow calculations ............................................................................................................55 6.3.2. Short circuit calculations .........................................................................................................56 6.3.3. Ground fault calculations ........................................................................................................56 6.3.4. Relay Coordination / Selectivity Study .....................................................................................57 6.3.5. Harmonic Distortion ...............................................................................................................57 6.3.6. Voltage Drop Calculations ......................................................................................................58 6.4. Extended Analysis and Studies ...............................................................................................58 6.4.1. Transient Analysis .................................................................................................................58 6.4.2. Reliability analysis and FMEA.................................................................................................59 7. Harmonic Distortion..............................................................................................................60 7.1. Introduction ...........................................................................................................................60 7.2. Harmonics of VSI converters ..................................................................................................60 7.3. Harmonics of CSI converters ..................................................................................................62 7.4. Harmonics of cyclo converters ................................................................................................62 7.5. Limitations by Classification Societies .....................................................................................62 7.6. Harmonics of ideal 6- and 12-pulse current waveforms.............................................................63 7.7. Calculating harmonic distortion ...............................................................................................65 7.7.1. Basics...................................................................................................................................65 7.7.2. Frequency domain – harmonic injection ..................................................................................66 7.7.3. Time domain – network simulation ..........................................................................................66 7.7.4. Comparison of the frequency and time domain simulation ........................................................67 7.8. Managing harmonics..............................................................................................................67 7.8.1. Generator impedance ............................................................................................................67 7.8.2. Converter topology ................................................................................................................68 7.8.3. Design of supply transformer..................................................................................................68 7.8.4. Passive filters ........................................................................................................................68 7.9. Active filters...........................................................................................................................70 7.10. Clean power supplies .............................................................................................................71 8. Example configurations........................................................................................................72 8.1. Field Support Vessels ............................................................................................................72 8.2. Cruise Vessels and Ferries .....................................................................................................74 8.3. Oil tankers, LNG carriers ........................................................................................................75 8.4. The concept of contra rotating Azipod propulsion (CRP)...........................................................79 8.5. Semi-submersible drilling rigs and drill ships ............................................................................80 8.6. Floating Production Vessel .....................................................................................................81 41. Introduction 1.1. Scope and Objectives Electrical installations are present in any ship, from powering of communication and navigation equipment, alarm and monitoring system, running of motors for pumps, fans or winches, to high power installation for electric propulsion. Electric propulsion is an emerging area where various competence areas meet. Successful solutions for vessels with electric propulsion are found in environments where naval architects, hydrodynamic and propulsion engineers, and electrical engineering expertise cooperate under constructional, operational, and economical considerations. Optimized design and compromises can only be achieved with a common concept language and mutual understanding of the different subjects. The objective of this section is to give an introduction to electro-technology in general, and put special emphasis on installations for electric propulsion. It is the aim to give engineers with marine competence and background the necessary understanding of the most important electro-technical subjects used in design and configuration of ships with electric propulsion. After an introductory review of the history of electric propulsion (chapter 1) and application areas of electric propulsion (chapter 2), an overview of the electric power system (chapter 3) and its associated control systems (chapter 4) follows. Than, the main characteristics of the electric propulsion drives are presented (chapter 5). Important design and engineering considerations are discussed (chapters 6 and 7) before ending by showing typical arrangements – by use of single line drawings of the electrical installations in some important applications (chapter 8). The reference list should not only serve as a link to the information presented in this section, but the publications are also recommended as a source for further studies of this topic. 1.2. Motivations for Electric Propulsion The concept of electric propulsion is not new, the idea originated more than 100 years ago. However, with the possibility to control electrical motors with variable speed in a large power range with compact, reliable and cost- competitive solutions, the use of electrical propulsion has emerged in new application areas during the 80’s and 90’s. Electric propulsion with gas turbine or diesel engine driven power generation is used in hundreds of ships of various types and in a large variety of configurations. Installed electric propulsion power in merchant marine vessels was in 2002 in the range of 6-7 GW (Gigawatt), in addition to a substantial installation in both submarine and surface war ship applications. By introduction of azimuthing thrusters and podded thrust units, propulsion configurations for transit, maneuvering and station keeping have in several types of vessels merged in order to utilize installed thrust units optimally for transit, maneuvering and dynamically positioning (dynamic positioning - DP). At present, electric propulsion is applied mainly in following type of ships: Cruise vessels, ferries, DP drilling vessels, thruster assisted moored floating production facilities, shuttle tankers, cable layers, pipe layers, icebreakers and other ice going vessels, supply vessels, and war ships. There is also a significant on-going research and evaluation of using electric propulsion in new vessel designs for existing and new application areas. 5 Fig. 1.1. Three comparative concepts of a Ropax vessel showing how space can be better utilized with electric propulsion and podded propulsion. The following characteristics summarize the main advantages of electric propulsion in these types of vessels: - Improved life cycle cost by reduced fuel consumption and maintenance, especially where there is a large variation in load demand. E.g. for many DP vessels a typically operational profile is equally divided between transit and station keeping/maneuvering operations. - Reduced vulnerability to single failure in the system and possibility to optimize loading of prime movers (diesel engine or gas turbine). - Light high/medium speed diesel engines. - Less space consuming and more flexible utilization of the on-board space increase the payload of the vessel, see Fig. 1.1. - Flexibility in location of thruster devices because the thruster is supplied with electric power through cables, and can be located very independent on the location of the prime mover. - Improved maneuverability by utilizing azimuthing thrusters or podded propulsion. - Less propulsion noise and vibrations since rotating shaft lines are shorter, prime movers are running on fixed speed, and using pulling type propellers gives less cavitation due to more uniform water flow. These advantages should be weighted up against the present penalties, such as: - Increased investment costs. However, this is continuously subject for revisions, as the cost tends to decrease with increasing number of units manufactured. - Additional components (electrical equipment – generators, transformers, drives and motors/machines) between prime mover and propeller increase the transmission losses at full load. - For newcomers a higher number and new type of equipment requires different operation, manning, and maintenance strategy. High availability of power, propulsion and thruster installations, as well as safety and automation systems, are the key factors in obtaining maximum operation time for the vessel. The safety and automation system required to monitor, protect, and control the power plant, propulsion and thruster system, becomes of increasing importance for a reliable and optimum use of the installation (Blokland and van der Ploeg 18). 61.3. Power Flow and Power Efficiency In any isolated power system, the amount of generated power must be equal to the consumed power including losses. For an electric system consisting of an electric power generation plant, a distribution system, including distribution transformers and a variable speed drive, the power flow can be illustrated in Fig. 1.2. P O W E R F L O W P out P in Power losses P P in out Trans- Frequency Electric Generator Switchboard former converter Motor Fig. 1.2: Power flow in a simplified electric power system. The prime movers, e.g. diesel engines or gas turbines, supply a power to the electric generator shaft. The electric motor, which could be the propulsion motor, is loaded by a power from its connected load. The power lost in the components between the shaft of the diesel engine and the shaft of the electric motor is mechanical and electrical losses which gives heat and temperature increase in equipment and ambient. P P out out The electrical efficiency of the system in Fig. 1.2 is: h = = P P + P in out losses For each of the components, the electrical efficiency can be calculated, and typical values at full (rated) power are for; generator: η = 0.95-0.97, switchboard: η = 0.999, transformer: η = 0.99-0.995, frequency converter: η = 0.98-0.99, and electric motor: η = 0.95-0.97. Hence, the efficiency of a diesel electric system, from diesel engine shaft, to electric propulsion motor shaft, is normally between 0.88 and 0.92 at full load. The efficiency depends on the loading of the system. Since the additional components between the prime mover and the propeller shaft in a diesel electric propulsion system contributes to a total of approximately 10% losses, the fuel savings potential is not due to the electrical component. One must regard the hydrodynamic efficiency of a speed controlled propeller compared to a fixed speed controllable pitch propeller (CPP), and the fuel efficiency of the prime mover when installed in a diesel electric system with constant speed and high loading, compared to in a mechanical propulsion system with strongly varying load. The differences may be significant, especially on low thrust operations as DP and maneuvering. Fig. 1.3 shows the fuel efficiency of a typical medium speed diesel engine, and a power vs. thrust comparison of a variable speed and a controllable pitch propeller (CPP). The hydrodynamic losses will vary significantly dependent on the operational condition for a CPP used in direct driven diesel solutions compared to variable speed fixed pitch propellers (FPP), which normally are used in electrical propulsion. In low load condition it is a rule of thumb that the zero-load hydrodynamic losses for a CPP is about 15%, while it is close to 0 for a speed controlled FPP, see Fig. 1.3(b). Notice that in most CPP configurations the propeller speed has to be kept constant on quite high rotations per minute (RPM) even though the thrust demand is zero. For FPP the variable speed drive will allow zero RPM at zero thrust demand. The advantage with CPP is that the propeller pitch ratio will be hydro-dynamically optimized for a wider speed range. 7A propeller designed for high transit speed, will have reduced efficiency at low speed and vice versa. Hence, the operational profile is of major importance while designing the propulsion system. The fuel efficiency characteristics of the diesel engine, with maximum fuel efficiency in the load range of 60 to 100% load, strongly contribute to the difference in power consumption for a traditional mechanical propulsion system, and a diesel electric propulsion system. In a power plant for diesel electric propulsion, the power generation will consist of multiple smaller diesel engines, where the number of running aggregates can be selected to have an optimum loading of each engine. The rating of the engines can also be adapted to fit the intended operational profile of the vessel, ensuring that it is possible to find an optimal configuration for most of the operational modes and time. Power, kW 3000 350 2500 300 2000 250 Fixed speed 1500 CP propeller 200 1000 Variable speed 150 500 FP propeller 100 100 200 300 400 500 50 100 Thrust, kN 0 200 0 % 20 % 40 % 60 % 80 % 100 % 120 % Cumulative 300 %MCR thrust demand Days (a) (b) Fig. 1.3: (a): Diesel engine specific fuel consumption (typical), Marintek 63 and (b): Propeller bollard pull characteristics (example), Ådnanes et.al. 57. For a field support vessel, with operational profile as shown in Fig 1.4, it was found that the fuel savings by using diesel electric propulsion was in the range of 700 tons of diesel per year. With a price of approx 40 cents per liter, this gives annual savings of in the order of 280 thousand USD. As shown, the savings will strongly be dependent on the operational profile, as shown in Fig 1.4(b). Here, the operational profile is split in DP/maneuvering and in Transit, showing how an increase portion of DP operations will increase savings, and vice versa. At quay, 6 % At quay, 6 % By platform / ROV, 11 % By platform / ROV, 11 % Standby Standby Transit, 11 Transit, 11- -12 knots 12 knots 40 % 40 % 40 % 40 % Anchor handling, 6 % Anchor handling, 6 % (a) (b) Fig. 1.4: (a): Operational profile for a field support vessel. (b): Fuel consumption, compared for electric propulsion (Azipod) and conventional mechanical propulsion. 8 g/kWh1.4. Historical Overview of Electric Propulsion th After the rather experimental applications of battery driven electric propulsion at the end of the 19 century took place in Russia and Germany, the first generation electric propulsion was taken into use in the 1920’s as a result of the strong competence of reducing transatlantic crossing times for passenger liners. At that time, the high propulsion power demand could only be achieved by turbo-electric machinery. “S/S Normandie” was one of the most renowned. Steam turbine generators provided electric power that was used to drive the 29MW synchronous electrical motors on each of the four screw shafts. The rotational speed was given by the electrical frequency of the generators. The generators would normally run one propulsion motor each, but there were also possibility for feeding two propulsion motors from each generator for cruising at lower speeds. th With the introduction of high efficient and economically favorable diesel engines in the middle of the 20 century, steam turbine technology and electric propulsion more or less disappeared from merchant marine vessels until the 1980’s. The development of variable speed electric drives, first by the AC/DC rectifier (Silicon Controlled Rectifier – SCR) in the 1970’s and the AC/AC converters in the early 1980’s enabled the power plant based electric propulsion system, which is typical for the second generation electric propulsion. A fixed voltage and frequency power plant consisting of a number of generator-sets feeding to the same network was supplying the propulsion as well as the hotel and auxiliary power. The propulsion control was done by speed control of the fixed pitch propellers (FPP). These solutions were firstly used in special vessels like survey ships and icebreakers, but also in cruise vessels. “S/S Queen Elizabeth II” was converted to electric propulsion in the mid 1980’s, and later followed the Fantasy and Princess class cruise vessels, several DP vessels, and shuttle tankers. Notice that in direct driven diesel propulsion the thrust is normally controlled by a hydraulic system varying the propeller pitch angle. This is denoted as controllable pitch propellers (CPP). Podded propulsion was introduced in early 1990’s where the electric motor is installed directly on the fixed pitch propeller shaft in a submerged, rotateable pod. While this concept was originally developed to enhance the performance of icebreakers, it was early found to have additional benefits on hydrodynamic efficiency and maneuverability. After the fist application in a cruise liner, “M/S Elation”, the advantages were so convincing that podded propulsion almost over night became a standard on new cruise liners, Fig. 1.5. Fig. 1.5: Cruise vessel “M/S Elation” (lower right) equipped with Azipod propulsion frees up space compared to sisterships (upper left) that can be utilized for other purposes, e.g. grey water treatement. 92. Applications 2.1. Passenger Vessels – Cruise Ships and Fe rries Passenger vessels, cruise ships, and ferries have very high requirement for on-board comfort regarding noise and vibration. In addition, the reliability and availability is very critical for the safety of the passengers and the vessel. Consequentially, electric propulsion was early evaluated to be beneficial and taken into use. The list of cruise vessels with electric propulsion is today long and increasing. As the podded propulsion is shown to give significant improvements in maneuverability and fuel costs, with an increase in propulsion efficiency of up to 10% (Kurimo 27), a large and increasing portion of new-buildings are specified with electrical podded propulsion. As the environmental concern is increasing, the requirements of reduced emission, spill, and damages on coral reefs by anchoring of the cruise vessels are increasing. Hence, the vessel must maintain its position solely by thrusters controlled by a DP system. This will increase the need for electrical propulsion and podded propulsion in the cruise market even more. The same restrictions and tax penalties for gas emissions (COx, Nox and Sox) have resulted in that several recent new buildings of ferryboats for fjord and strait crossing have been equipped with electric propulsion. With frequent crossing schedules and quay docking, the improved maneuverability by podded propulsion has significantly reduced the fuel consumption. The propulsion power varies with the size of the vessel, from some few MW for smaller ferries up to 30-40 MW for large cruise liners. The hotel load can be a significant part of the total power installation, for a large cruise liner typically in order of 10-15 MW. Fig. 2.1 shows a schematic overview of the main electrical and automation components in a typical cruise vessel with diesel-electric podded propulsion. INTEGRATED VESSEL AUTOMATION DYNAMIC POSITIONING COMMUNICATION LINE TO SHORE POS. REF. SYSTEMS X TERMINALS PLANT NETWORK ENGINE CONTROL TUNNEL THRUSTERS AUX POWER GENERATION & DISTRIBUTION FIELDBUS NETWORK CONTROL NETWORK MAIN POWER GENERATION & DISTRIBUTION DRIVES PROPULSION/ POD Fig. 2.: Example of propulsion and control system layout for a cruise vessel. 102.2. Oil and Gas Exploitation and Exploration: Drilling Units, Production Vessels and Tankers Still a few years ago extensive oil and gas resources were accessible in shallow waters and could be exploited by fixed drilling and production units. In the North Sea, Gulf of Mexico (GoM), and Brazil as in several other areas, those new resources that remain are found in smaller and/or less available fields in deeper waters. These fields require new cost-effective methods to obtain acceptable economy and profit. Deep-water drilling and floating production have become possible with dynamic positioning or thruster-assisted position mooring. Thruster assisted positioning is applied in the North Sea, Canada, and areas with harsh environment. In Brazil, West Africa, and the planned USGOM (US Gulf of Mexico) installations, the trend has been to rely on mooring without thruster assistance for oil production and dynamic positioning for deepwater drilling. The thrusters used for station keeping (DP operation) typically also constitutes the main propulsion in transit and maneuvering of the vessel, either all or selected units only. Typical of these vessels is their large installed thruster power, typically 20-50 MW. Together with the production, drilling, utilities, and hotel loads, the installed power is typically 25-55 MW. The typical installation has a common power plant for all these loads, enabling flexibility to operation with high energy-efficiency and high availability. Fig. 2.2 shows a schematic overview of a semi-submersible drilling rig. See Farmer 10 and Ådnanes et.al. 57 and 59. Shuttle tankers are used for transport of oil from an offshore facility (platforms, buoys, towers or FPSOs) to a processing or storage terminal onshore. There are numerous of different offloading methods in use. For most of them the shuttle tankers need to maintain a fixed position (station keeping) with high accuracy subject to varying environmental conditions. Therefore, there most of the shuttle tankers are equipped with a DP system. Most of the ships have installed electrical tunnel or azimuthing thrusters, whereof some are also having diesel-electrical systems for main propulsion, Hansa-Schiffart 14. For many applications there is a high degree of redundancy in the propulsion for transit and station keeping. The solutions have normally a redundant power generation and distribution system, with redundant propulsion converters, and a tandem or redundant propulsion motor. The introduction of podded propulsion may influence the design of the diesel-electric shuttle tankers, since it may be a more cost efficient solution to obtain redundant propulsion with to pod units than with two conventional shaft lines. BACK-UP PLANT SYSTEM NETWORK SAFETY SYSTEM EMERGENCY SHUTDOWN WIND SENSORS PROCESS CONTROL FIRE & GAS STATION MRU GYRO CONTROL NETWORK INTEGRATED THRUSTER CONTROL SYSTEM - DYNAMIC POSITIONING - POSMOOR - AUTOSAIL - OPERATOR CONTROL SYSTEM INTEGRATED MONITORING & CONTROL SYSTEM - EXTENSION ALARM - PROCESS CONTROL CONVERTER THRUSTER / POD DRILLING DRIVE SYSTEM FIELDBUS NETWORK ENERGY MANAGEMENT SYSTEM POWER GENERATION & DISTRIBUTION Fig. 2.2: Example of electrical system layout for a semi-submersible drilling unit. 112.3. Field Support Vessels and Offshore Construction Vessels For vessels with dynamic positioning (DP) as the main operating mode, such as diving support vessels, crane ships, and pipe layers, electric propulsion was early taken into use, first with fixed speed CP propellers and later with variable speed thrusters. The reduction in fuel consumption and environmental emission from diesel electric propulsion compared to conventional mechanical propulsion is significant for vessels with a diversified operational profile. Savings of 30- 40% in fuel consumption annually has been reported from ship owners, and with the increased focus on operation costs and environmental impact from the oil industry, has given a large growth in number of field support vessels, first in the North Sea, and later in other geographical areas. With the rapidly increasing need for high-speed communication system and a global fiber optic cable network, there has been established a large fleet of cable laying vessels with electric propulsion and dynamic positioning. These vessels will be configured as DP vessels, class 2 or 3 (DnV 60, Lloyds 61 and ABS 62), and most will have electric propulsion with a total power demand of 8-30 MW, depending on size and drilling/lifting capability. Fig.2.3: Offshore supply vessel with electric propulsion. Fig. 2.4: Some Offshore and Construction Vessels 122.4. Dredgers and Construction Vessels Diesel electric propulsion and station keeping facilities are also used in vessels for operations in more shallow waters, such as dredgers and construction vessels, wind mill installation vessels, etc. Accurate maneuverability with improved fuel economy, in combination with a need to move working place frequently are typical performance criteria that are beneficial for electric propulsion in these applications. In ships with a large ship service electrical installation, the power plant can also be utilized better by combining the plants for ship service and propulsion power generation and achieve better redundancy and utilization of installed power capacity. 2.5. Yachts and Leisure Boats A small and niche application area for electric propulsion is the yachting market. Comfort and environmental friendliness are essential design criteria for these ships, and electric propulsion with high requirements to low vibration and noise with high efficiency is now becoming more common. The installed propulsion power is typically in the range of 500kW to 2000kW, and for larger ships even higher. 2.6. Icebreakers and Ice Going Vessels The dynamic requirements for the frequency converters in propulsion application are low, compared to many other industrial applications. But in ice-going vessels and icebreakers, the load variations may be significant and rapid, and this implies that the propulsion system must have high dynamic performance in order to avoid over- loading of components and undesired tripping. Electric propulsion has been used in a majority of new-buildings since the 80’s. The basic configuration can be similar as for service vessels, with a redundant power generation and distribution system, although there will normally not be any DP requirement for the icebreakers. As oil exploitation in arctic regions emerges, there is also an increasing demand for icebreaking and ice going field support, escort, and shuttle tankers. The installed propulsion power may be in the range of 5-55MW, depending on ice breaking capability. Fig. 2.5: “M/S Botnica”, Icebreaker which serves as a supply vessel in summer season, equipped with Azipod propulsion. 2.7. War Ships Despite the great interest in the application of electric propulsion to warships, there are quite few conventional surface warships with pure electric propulsion, but more are being projected. For sub-marines, electric propulsion with diesel engine generation and battery storage, fuel cell or nuclear power plant is applied. Electric propulsion for war ships does not conceptually differ much from the merchandise vessels, but the solutions may differ since the requirements to availability and redundancy are normally stricter. Also, the ability to withstand shock and provide low noise signatures are prerequisites for electric drive when applied to a warship. 13Fig. 2.6 shows the K/V Svalbard, a coast guard vessel in service since 2002 for the Norwegian Navy, equipped with dual Azipod propulsion system, and partially fulfilling military requirements. Fig. 2.6: “K/V Svalbard”, Ice breaking coast guard vessel with podded propulsion for the Norwegian Navy. 2.8. Research Vessels Geo technical research vessels, oceanographic vessels, and fishing research vessels have in common very strict underwater noise requirements, typically several decades dB below normal levels for other applications. This has traditionally been achieved by use of direct propulsion with DC motors, special considerations for filtering and reduction of vibrations and torque variations. By use of modern frequency converters and filtering techniques, AC motors have become feasible for such high demanding applications as well, and are now taken into use in recent ship designs. 2.9. Trends and New Applications Electric propulsion is continuously being investigated and evaluated for new applications. LNG and chemical tankers, Ro-Ro vessels, container vessels, fishing vessels are typical examples of large volume markets where electric propulsion yet is not taken into use because of the increased investment costs. However, only small changes in operation and design criteria, such as increased fuel or emission costs, regulatory restrictions, and equipment cost reduction, may give a tremendous shift in technology application for several of new areas. Fig. 2.7: The world new build market of ships. Electric propulsion dominates in the sectors of construction, cable and pipe layers, offshore, icebreakers, and passenger vessels. In other segments, electric propulsion is less in use although there is a significant increase in interest for conceptual studies and designs. 143. Overview of Electric Power System 3.1. Introduction The main difference between the marine and a land-based electrical power system is the fact that the marine power system is an isolated system with short distances from the generated power to the consumers, in contrast to what is normal in land-based systems where there can be hundreds of kilometers between the power generation and the load, with long transmission lines and several voltage transformations between them. The amount of installed power in vessels may be high and this gives special challenges for the engineering of such systems. High short circuit levels and forces must be dealt with in a safe manner. The control system in a land-based electrical power system is divided in several separated sub-systems, while in a vessel; there are possibilities for much tighter integration and coordination. The design of power, propulsion and control systems for a vessel have undergone significant changes and advances over a relatively recent period of time. Because of the rapidly expanding capabilities of computers, microprocessors and communications networks, the integration of systems which were traditionally separate, stand alone systems is now not only feasible, but fast becoming industry standards. The increasing demand for redundant propulsion and DP class 2 and class 3 vessels, requires system redundancy with physical separation. The interconnections of the diverse systems on a vessel have become increasingly complex, making the design, engineering and building of a vessel a more integrated effort. Fig. 3.1 shows the schematics of the main power installations in a vessel with electric propulsion in a Single Line Diagram (SLD). This chapter describes the main components as they are applied in a marine electric installation: § Electric Power Generation § Electric Power Distribution § Variable Speed Drives § Propulsion / Thruster units Fig. 3.1: Single line diagram for a ship with podded electric propulsion; G1-G4: Generators, SWBD: Switchboard, TRANSF: Transformer, BT: Bow Thruster, ® AZ THR: Azimuthing thruster, AZIPOD : Podded propulsion. 153.2. Electric Power Generation 3.2.1. Prime Mover The source for power is most often a generator set driven by a combustion engine which is fueled with diesel or heavy fuel oil. Occasionally one can find gas engines 64, and also gas turbines, steam turbines or combined cycle turbines, especially for higher power levels, in light high-speed vessels, or where gas is a cheap alternative (e.g. waste product in oil production, boil-off in LNG carriers, etc.). In a diesel-electric propulsion system, the diesel engines are normally medium to high-speed engines, with lower weight and costs than similar rated low speed engines that are used for direct mechanical propulsion. Availability to the power plant is of high concern, and in a diesel electric system with a number of diesel engines in a redundant network; this means high reliability but also sophisticated diagnostics and short repair times. The combustion engines are continuously being developed for higher efficiency and reduced emissions, and at present, a medium speed diesel engine has a fuel consumption of less than 200g per produced kWh at the optimum operation point as seen in Fig. 3.2a). Even though this is regarded to be a high utilization factor of fuel, it represents only about 40% of the energy in the fuel, the rest of the energy being removed by the exhaust or heat dissipation. Moreover, the efficiency drops fast as the load becomes lower than 50% of MCR (Max Continuous Rating). At this working condition, the combustion is inefficient, with high NOx and SOx content, and with a high degree of soothing which increases the need for maintenance. In a diesel electric system with several diesel engines it is hence an aim to keep the diesel engines loaded at their optimum operating conditions by starting and stopping generator sets dependent on the load, as seen in Fig 3.2b), with an aim to keep the average loading of each running diesel engine closest possible to its optimum load point. For detailed description of design and functionality of diesel combustion engines, see Mahon 66. η % 50 350 45 Optimal operation area 300 diesel electric propulsion 40 250 Optimal operation area mechanical drive 200 Diesel-electric propulsion four prime movers 150 25 Mechanical propulsion 100 one prime mover 50 0 0 % 20 % 40 % 60 % 80 % 100 % 120 % P % 10 %MCR 15 50 60 100 MCR Fig 3.2: a) Example fuel consumption for a medium speed diesel engine. b) Total efficiency from engine to propeller shaft, in a single machine direct mechanical propulsion system and a four machine diesel electric propulsion system. 16 g/kWh Efficiency from diesel to propeller3.2.2. Generators The majority of new buildings and all commercial vessels have an AC power generation plant with AC distribution. The generators are synchronous machines, with a magnetizing winding on the rotor carrying a DC current, and a three-phase stator winding where the magnetic field from the rotor current induces a three-phase sinusoidal voltage when the rotor is rotated by the prime mover. The frequency f Hz of the induced voltages is proportional to the rotational speed n RPM and the pole number p in the synchronous machine: p n f = ⋅ 2 60 A two-pole generator will give 60Hz at 3600RPM, a four-pole at 1800RPM, and a six-pole at 1200RPM, etc. 50Hz is obtained at 3000RPM, 1500RPM, and 1000RPM for two-, four-, and six-pole machines. A large medium speed engine will normally work at 720RPM for 60Hz network (10 pole generator) or 750RPM for 50Hz networks (8 pole generator). The DC current was earlier transferred to the magnetizing windings on the rotor by brushes and slip rings. Modern generators are equipped with brushless excitation for reduced maintenance and downtime, Fig 3.3. The brush-less excitation machine is an inverse synchronous machine with DC magnetization of the stator and rotating three-phase windings and a rotating diode rectifier. The rectified current is then feeding the magnetization windings. Fig 3.3: Magnetization of the rotor winding, left: brushed; right: brush-less magnetization. The excitation is controlled by an automatic voltage regulator (AVR), which senses the terminal voltage of the generator and compares it with a reference value. Simplified, the controller has PID characteristics, with stationary limited integration effect that gives a voltage drop depending on the load of the generator. The voltage drop ensures equal distribution of reactive power in parallel-connected generators. According to most applicable regulations, the stationary voltage variation on the generator terminals shall not exceed ±2.5% of nominal voltage. Also, the largest transient load variation shall not give voltage variation exceeding -15% or +20% of the nominal voltage unless other has been specified and accounted for in the overall system design. In order to obtain this transient requirement, the AVR is normally also equipped with a feed-forward control function based on measuring the stator current. In addition to the magnetizing winding, the rotor is also equipped with a damper winding which consists of axial copper bars threaded through the outer periphery of the rotor poles, and short circuited by a copper ring in both ends. The main purpose of this winding is to introduce an electromagnetic damping to the stator and rotor dynamics. A synchronous machine without damper winding is inherently without damping and would give large oscillations in frequency and load sharing for any variation in the load. 17The stationary, transient and sub-transient models are known from the theory of synchronous machines. Simplified one could say that the flux linkages in the damper winding, which are “trapped” and resist changes due to being short-circuited, characterize the sub-transient interval. This is observed as an apparent lower inductance in the generator, which gives a stiffer electric performance during quick load variations, and helps to reduce transient voltage variations and the voltage variations due to harmonic distortion in load currents. This effect is only contributing for dynamic variations faster than characterized by the sub-transient time constant such as the first period of motor start transients and transformer inrush, and for harmonic distorted load currents. Often, the generators are connected to a propulsion engine’s shaft, i.e. a shaft generator. The shaft generators are in some applications made for two-directional power flow, which means that it can be run as motor. This principle may be called a PTI-PTO concept (Power take-in – Power take out). Shaft generators have the disadvantage of forcing the main propeller to work at fixed speed if the generator output shall have constant frequency. This will reduce the efficiency of the propeller in low load applications. Static converters may be installed to keep fixed frequency for variable speed. 3.3. Electric Power Distribution 3.3.1. Switchboards The main (or generator) switchboards are usually distributed or split in two, three, or four sections, in order to obtain the redundancy requirements of the vessel. According to rules and regulations for electric propulsion, one shall tolerate the consequences of one section failing, e.g. due to a short circuit. For strictest redundancy requirements, one shall also tolerate failure due to fire or flooding, meaning that water and fireproof dividers must be used to segregate the sections. In a two-split configuration, with equally shared generator capacity and load on both sides, the maximum single failure scenario will hence be to loose 50% of generator capacity and loads. In order to avoid a high installation costs, the system will often be split in three of four, which reduces the required additional installations. Also, change-over switches which ensures that a generator or a load can be connected to two switchboard sections will have similar cost reducing effects, e.g. for azimuth thruster in Fig. 3.1. In propulsion mode, the switchboards are normally connected together, which gives the best flexibility in configuration of the power generation plant. The load transients are distributed on a large number of diesel- generators, and the most optimal number of units can be connected to the network. Another possibility is to sail with independent switchboard parts supplying two or more independent networks. In this case the ship is often assumed to be virtually blackout proof, which could be attractive in congested waters. In this operating mode one network including it’s connected propulsion units is lost if one switchboard section fails, the other, however, remaining operable. I practice, there are also other considerations to be made in order to obtain such independence, especially all auxiliaries, such as lubrication, cooling, and ventilation must be made independent. Also, loss of propulsion or station keeping power on one part of the system, will through control systems also have impact on the remaining parts, as the total power or thrust tends to be kept the same, e.g. for dynamic positioning. The normal operation in DP vessels, in particular for class 3 operations, is to split the network in order to be tolerant to failure of one section. However, rules and regulations now allows for operation with closed tie breakers, if the protection circuits are designed to detect and isolate faulty parts without tripping the healthy parts. The NMD rules (Norwegian Maritime Directorate) has one of the more stricter practicing of these rules and will normally not accept connected networks in class 3 operations. As the installed power increases, the normal load currents and the short circuit currents will increase. With the physical limitations on handling the thermal and mechanical stresses in bus bars and the switching capacity of the switchgear, it will be advantageous or necessary to increase the system voltage and hence reduce the current levels. Medium voltage has become a necessity to handle the increasing power demand in many applications. 18Using the IEC voltage levels the following alternatives are most common selected for the main distribution system, with application guidelines from NORSOK 64: - 11kV: Medium voltage generation and distribution. Should be used when total installed generator capacity exceeds 20MW. Should be used for motors from 400kW and above. - 6.6kV: Medium voltage generation and distribution. Should be used when total installed generator capacity is between 4-20MW. Should be used for motors from 300kW and above. - 690V: Low voltage generation and distribution. Should be used when total installed generator capacity is below 4MW. Should be used for consumers below 400kW and as primary voltage for converters for drilling motors. - For utility distribution lower voltage is used, e.g. 400/230V. A few comments to these guidelines are necessary; - Where a major part of the load consists of variable speed drives with no contribution to the short circuit level, there will normally not be any problems to utilize each voltage level to significantly higher generator capacities. For optimizing the installation, one should in each case calculate load and fault currents and select the right solution. - In ships, low voltage (690V) motors are normally used for much higher power levels than 300kW. In each case, one must consider the load current, and starting characteristics for the drive, including alternative starting methods together with a comparison on overall costs. - 440V distribution is quite common in ship installations. A lot of ship equipment is available only in 440V, which means that it might be difficult to avoid this voltage level in ship applications. In US, or where the ANSI standard applies, several additional voltage levels are recognized, such as in 24: 120V, 208V, 230V, 240V, 380V, 450V, 480V, 600V, 690V, 2400V, 3300V, 4160V, 6600V, 11000V, and 13800V. 3300V is also a commonly used system voltage in IEC applications, even though not recognized in 64. Since the load current and fault current determine the limitation of the equipment, the actual power limits for each system voltage may deviate from these recommendations. This particularly applies to systems where a major part of the load is converter loads and does not contribute to short circuit power. Since these do not contribute to short circuit currents in the distribution system, it often allows increasing the power limits for the different voltage levels. Safety is an issue of concern when yards and ship owners changes from low to higher voltages, often leading to a misunderstanding effort to keep voltages as low as possible. In the context of safety, it should be regarded that medium voltage switchboards is designed to prevent personnel to get contact with conductors, even in maintenance of the switchgears. The normal and fault currents are similarly smaller, giving less forces on the conductors and cables during e.g. short circuit. Although short circuits inside the switchboards are extremely rare, arc-proof design (IEC 60298-3) is available and will prevent person injury and limit the equipment damages if worst case should occur. Circuit breakers are used for connecting and disconnecting generator or load units to the switchboards, or different parts of the switchboards together. Various circuit breaker technologies are applied. Air insulated units are the traditional solution, but today rarely applied except at low voltage levels. In the commonly used SF6 and vacuum breaker technologies, the current interruption takes place in an enclosed chamber, where the first one is filled with SF6 gas, which has higher insulation strength than air, and the vacuum breaker is evacuated by air. These designs give compact and long term reliable solutions for medium voltages. One should consider that vacuum breakers may chop the current and can cause overvoltage spikes when breaking an inductive loads with high di/dt that may require installation of overvoltage limiters. For smaller powers, fused contactors are a cost and space beneficial alternative to the circuit breaker, and are available in air (low voltage), SF6 or vacuum insulated types. The problem with switching spikes is less with fused contactors since current interruption is softer (lower di/dt). 193.3.2. Transformers The purpose of the transformer is to isolate the different parts of the electric power distribution system into several partitions, normally in order to obtain different voltage levels and sometimes also for phase shift. Phase shifting transformers can be used to feed frequency converters, e.g. for variable speed propulsion drives, in order to reduce the injection of distorted currents into the electric power network by canceling the most dominant harmonic currents. This reduces the voltage distortion for generators and other consumers. The transformers also have a damping effect of high frequency conductor emitted noise, especially if the transformer is equipped with a grounded copper shield between primary and secondary windings. There are numerous different transformer designs in use, and the most common types are; air insulated dry type, resin insulated (cast or wound), or oil/fluid insulated. Regulations, ambient conditions, and user’s, yard’s, or supplier’s preferences govern the selection of type, material, and design of the transformer. Physically, the transformer is normally built as three-phase units, with three-phase primary coils and three-phase secondary coils around a common magnetic core. The magnetic iron core constitutes a closed path for magnetic flux, normally with three vertical legs and two horizontal yokes; one in bottom and one at top. The inner winding constitutes the low voltage or secondary windings, and the outer is the primary or high voltage winding. The ratio of primary to secondary windings gives the transformation ratio. The coils may be connected as a Y-connection or Δ-connection (also called D-connection). The connection may be different on primary and secondary sides, and in such transformers, not only the voltage amplitude will be converted, but there will also be introduced a phase shift between the primary and secondary voltages. The phase shift can also be adjusted by use of Z-connected windings, normally in the primary, where the phase shift angle can be accurately determined by the ratio of turns in the segments of the Z-windings. Three- or four winding transformers with multiple secondary windings are also in use, e.g. for multi-pulse drive applications. A transformer with Δ-connected primary and Y-connected secondary is called a Dy type transformer. The first and capital letter describes the primary winding, and the second and small letter describe the secondary winding. The letter n is used to describe if the common point in a Y-connection is grounded, e.g. Dyn or Ynyn. Transformers may be designed according to IEC standards. For converter transformers, it is essential that the design accounts for the additional thermal losses due to the high content of harmonic currents. IEC also gives design rules and guidelines for such applications. 3.4. Motor Drives for Propulsion and Thrusters 3.4.1. Introduction The electrical motor is the most commonly used device for conversion from electrical to mechanical power and is used for electric propulsion, thrusters for propulsion or station keeping, and other on-board loads such as winches, pumps, fans, etc. Typically, 80-90 % of the loads in ship installations will be some electrical motors. In this chapter, a brief overview of different motors and their applications in ship installations is given and for more detailed description of design, performance, and characteristics, references to other books are made. The electrical motors in use are: - DC motors. The DC motor must be fed from a DC supply, and since the power generation and distribution system normally is a three-phase system, this means that a DC motor must be fed from a thyristor rectifier. This gives also a speed control of the motor. For detailed description of the various construction of the DC motor, see Fitzgerald et.al. 68. - Asynchronous (induction) motors. The asynchronous or induction motor is the workhorse of the industry. Its rugged and simple design ensures 20

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