MARINE POWER PLANT CONTROL SYSTEM
POWER / ENERGY MANAGEMENT OF MARINE POWER SYSTEMS
Part of the project: Energy Efficient All Electric Ship Department of Marine Technology Norwegian University of Science and Technology - NTNU Trondheim, Norway
Technical Report by Damir Radan, PhD-student E-mail:
[email protected] Web: www.ntnu.no/~radan
Damir Radan
Power/Energy Management of Marine Power Systems
1. Introduction - Historical review of power/energy management system Power management has grown as a necessity for automated starting and synchronizing the generators and as an overall aid to ship’s operator in control of electrical power systems. Traditionally, ship’s operator performed power management manually i.e. starting and stopping generators with feedback (in human-machine control system) established by observing current situation on the ship’s network i.e. available power, kW, kVar, kVA, frequency, etc. At the beginning, but not so much long time ago, synchronization of generators was performed manually (dark lamp test) and that was one of the reasons for various blackouts that occur by non properly trained personal. That is the reason why manual synchronization can still be trained on various marine power plant (machinery space, engine room) simulators. However, power management, found today, has grown much beyond that, especially on electrical power propulsion installations. The major breakthrough for electric propulsion in 1990’s dominating in cruise vessels, ice breakers, and offshore oil exploration put the new demand for power/energy management. All installed power equipment becomes interconnected in the power distribution system and any load variations, starting transients, and network disturbances affect the load and generators that interact and influence each other. The new generation of vessels have a complex power system configuration with advanced protection and relaying philosophies that are in close connections with the design and functional operation of power/energy management control system (PMS/EMS). In that respect, optimum operation and control of the power system becomes essential for safe operation of the vessel. One important consequence of such functional philosophy is better energy utilisation that results in lower fuel consumption. Fuel consumption is one of the major operational costs of the vessel and that is the one which always attracts the owner. Equipment that becomes available on reasonable price and standards that assure compatibility between various suppliers involved in ship design reflected also to lower automated marine applications (tankers, ro-ro, bulk-carriers) that now have much better control systems and become more resistant to blackout. Power management system (PMS) has become an integrated element of totally integrated power, automation and positioning system. That is one of the new challenges for PMS. Traditionally, power management system has been analyzing only current situation on the network and has been considering limited number of consumers and their control systems. In recent years, advanced functions have been added to power management system to be able to control the power generation and consumption by optimizing the instantaneous power flow and use. That is the reason for calling the same control system the Energy management system (EMS). Usually, different terminology has been used for the same control system so there is not big difference whether we call it energy management system or power management system. However, it is important to recognize the difference between conventional power management functions and modern advanced functions that open possibilities for further improvements in blackout prevention, overall safety and fuel consumption.
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2. Marine power system The Power Management System (PMS) is a critical part of the control equipment in the ship. It is usually distributed on various control stations that can operate together and share information between each other or independently in case of special emergency situations when vessel have to operate with open tie breakers – DP class 2, 3. PMS serves to provide the power when it is needed and to prevent blackouts. The equipment within the PMS includes the engines, generators, switchboards and controls along with the automation equipment that perform the calculation algorithms. Before considering detailed functionality of PMS it is necessary to give an overview of the marine power system. Marine power system is consisted of power generators, consumers and distribution system. Fig. 2.1. shows basic marine power system configuration with medium voltage switchboard (mostly 3.3., 6.6. or 11 kV). Medium speed diesel engines with speed of 500 to 1000 rpm are commonly used as prime movers in marine electrical installations. Robustness, reliability, fast load acceptance, low deviation in frequency and redundancy obtained with relatively large number of units installed onboard (4 to 8) are factors that, besides fuel consumption, have the greatest influence in prime mover selection. Transformers are used to achieve proper voltage for variable frequency propulsion drives and electro motors that usually operate on 1500 V and powers up to 25 to 30 MW. Low voltage switchboard feeds various small power consumers such as pumps, compressors, ventilation, etc with powers lower then 100 to 200 kW and 690V or 440V. Special sensitive equipment should be feed by means of uninterruptible power supply UPS that provides clean sinusoidal power supply with very low harmonics and current/voltage distortion. The figure 2.1. regards two independent power systems – one on the port side (left) and the other on the starboard side of the vessel (right). Each system has its own generator, medium voltage switchboard, low voltage switchboard and thrusters with frequency converter - variable speed propulsion drive switchboard. All parts of power system onboard marine vessel with or without its own controllers are interconnected with control system – hardwired or field bus. That provides enormous possibilities for advanced overall control system functionality.
Fig. 2.1. Marine power system (ABB Marine)
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The power system is divided into the following main parts: 1 2 3 4 5 6 7 8 9
Power generation system with prime movers (diesel or gas turbine) and generators. Power distribution system consisting of medium voltage switchboard and tie breakers, usually split into two (on ships), four (4) or more sections (on drilling rig/ship). Transformers for feeding of alternate voltage levels. Low voltage switchboards and motor control centres. Frequency drives for propulsion motors and other users (bow thrusters, cargo pumps, AC compressors, etc.). Filters for reducing the harmonic currents. Rotating converters for clean power supply. Uninterruptible power supply (UPS) of sensitive equipment and automation system. High and low voltage motors for various services (propulsion, thrusters, AC compressors, fans, etc.).
2.1. Power system redundancy Configuration with two generators on each network represents better solution since second generator significantly increases redundancy when vessel operates with open bus-tie. Fig. 2.2. shows single line diagram with a 4 split, 11 kV network that is typical configuration for drilling vessel. Each network has 2 diesel generators - 5.2 MW each and 2 thruster drives - 3.2 MW each [3]. Fig. 2.3. shows various levels of power system redundancy for arrangement with two engine rooms [2]. Such engine room arrangement is common for ships that are able to change the position in case of emergency. Contrary, there is an increasing trend of multiplying engine rooms on drilling vessels. Class requirements put the demand for vessel redundancy: the single fault must not be the cause for lose more than one engine. In case of fire or flooding the vessel must posses redundant engine room which will be completely independent and isolated with waterproof and fireproof bulkhead A60. The vessel with one engine room out of operation must be able to maintain the position on design storm conditions. Various configurations may exist that will give the same required redundancy. If the required power is found to be 35 MW in a nominal design conditions (design storm) and only two (2) separate and independent engine rooms are allowed by the design, the total installed prime mover power must be 70 MW (35 MW per engine room). But, if the number of engine rooms is increased on three (3) engine room, the total installed prime mover power is 52.5 MW, see figure 2.4. The amount of installed power decreases with number of split networks or number of engine rooms according to following simple equation [4]:
Pinst = Preq ⋅
n n −1
where: Pinst Preq n
– installed power – required power – number of engine rooms
Therefore, the design team should decide the number of engine rooms that will give the optimal 3
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installed power for the same required power by taking care about increased complexity of the plant and overall costs of duplicating auxiliary and control systems. Power plant redundancy is one example how design constraints may influence power management demand. Power management control system strongly depends on power system configuration, installed power and overall vessel operational conditions. If these factors have not been properly addressed in design stage it might be unreal to believe that PMS will be able to correct the same in the operation.
Fig. 2.2. Power system configuration for drilling vessel [1], [3] Single line diagram for a DP Class 3 drill rig with a four-split power system - ring network Single system
Multiple system
Multiple system R1, R1+, R1-S, R1-S+
Multiple system R2, R2+, R2-S, R2-S+ RP, RPS
Diesel Electric Generator
Diesel Electric Generator
Diesel Electric Generator
Diesel Electric Generator
Diesel Electric Generator
Diesel Electric Generator
Diesel Electric Generator
Electric Power Distribution
Electric Power Distribution
Electric Power Distribution
Electric Power Distribution
Electric Power Distribution
Electric Power Distribution
Electric Power Distribution
Electric Propulsion System
Electric Propulsion System
Electric Propulsion System
Electric Propulsion System
Propulsor, shaft propeller
Propulsor, shaft propeller
Electric Propulsion System
Electric Propulsion System
Propulsor, shaft propeller
Propulsor, shaft propeller
Propulsor, shaft propeller
Fig. 2.3. Various levels of power system redundancy for one or two engine rooms [2]
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Fig. 2.4. Power system configuration for drilling vessel [4]
2.2. Machinery concepts and prime movers
Medium speed diesel engine has been traditionally considered as the most suitable prime mover for electric power installations. However, gas and steam turbines might be preferred for some designs but that also depends on electric installation supplier and overall price of installation that will include preferred prime movers. In recent years gas turbines have been installed usually in combination with steam turbines: COGES = Combination of Gas Electric and Steam. Such power plants have been installed on cruise ships that operate from one port to the other most of the time with constant speed and since are much smaller in size and weight, with lower level of noise and vibrations they might be preferred solution over diesel engines. On figure 2.5. COGES plant has been shown. Fig 2.6. presents high flexibility with diesel-mechanicalelectrical propulsion for various vessel types. Dynamically positioned vessels, icebreakers and cruise vessels naturally utilize all advantages of electric propulsion while DP vessel and icebreaker designs prefer diesel-electric prime mover machinery concept over gas turbine and/or COGES. The main reason for that is low power demand in dynamic positioning which is around 30% of the installed power during 90% time in the year as can be seen on figure 2.7. considering cumulative thrust demand. Figure 2.8. shows lower fuel consumption 5
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with multiple diesel engines compared to single diesel configuration in case of low power plant operation. Typical COGES and diesel electric configurations for the cruise vessel are compared bellow: Table 2.1. Diesel electric with:
COGES with:
- 5 x 12.6 MW engines burning HFO - 2 x 19.5 MW propulsion pods
- 2 x 25 MW LM2500+ gas turbine engines burning MGO - 1 x 10 MW back pressure steam turbine - 2 x 19.5 MW propulsion pods
Fig. 2.5. COGES plant
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Fig. 2.6. Various diesel engine machinery concepts [5] P o w e r, k W
2500 2000 1500
F ix e d s p e e d C P p ro p e lle r
1000 V a ria b le s p e e d F P p ro p e lle r
500
100 100
200
300
400
500
T h r u s t, k N
200 C u m u la tiv e th ru s t d e m a n d
300
Break Specific Fuel Oil Consumption, [ g/kWh ]
3000
BSFC 300
200
100 1 GEN
2 GENS
3 GENS
0
4 GENS
100 Total load, [ %, 4xMCR ]
D ays
P
Fig. 2.7. Comparison between fixed speed CP and Fig. 2.8. Lower consumption for multiple diesel variable speed FP propeller and thrust demand gen-sets compared to single unit [2] over one year (365 days) [1]
3. The Integrated Control System One of the main focus areas in the automation industry has been on connectivity ensuring physical integration of the various control systems [7]. The power management system is an integral part of the total integrated ship control system. Integrated control system consists of operator stations and distributed process control stations (PCS) connected to the data network [4]. Integrated control system can be regarded as total vessel control concept with the emphasis on communication and interaction between different systems and equipment installed. The Open Control System (OCS) concept transferred from land based industry sectors enables a seamless and real time data exchange between the different control systems installed onboard. The OCS uses well known industry standards that fulfil the requirements for [7]: 1 Compatibility - share the information with other systems and perform functions jointly 2 Interoperability - the capability for a various systems from other manufacturers to interact with each other on a field bus network without loss of functionality 3 Interchange ability - the capability to substitute a device from another manufacturer on a field bus network without loss of functionality or degree of integration These industry standards known as MS Windows, client-server interfaces, Ethernet, TCP/IP, Profibus, Foundation Field bus, etc. have brought about fundamental changes in the automation industry. Physical integration based on standardized communication protocols ensure connectivity of devices and integration of controllers and operator stations into three network levels, shown on figure 3.1.: 3 Field bus network - real time communication (data) network on low level between devices and controllers, 4 Control network - real time data network connecting controllers and operator stations, and 7
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5 Office plant network - network connecting to various office systems and information management systems. Office plant network opens up for satellite communication to land offices at ship operators or vendors. • Information Management • Simulation • Process Optimization • Remote Diagnostics
Office
SUPERVISORY LAYER
Operator Stations (Bridge, Control room)
NETWORK LAYER CONTROLLER LAYER
PLC Controllers
Dynamic Positioning System Vessel Manoeuvring Alarm and Monitoring System Mode Control Redundancy and Criticality Assessment System
Power Management System Ballast / Cargo and Bilge Control System Drilling Control System Safety Systems
Distributed Control Local Engine Control and Safety Drives Power Generation & Distribution Thrusters Propulsion
Fig. 3.1. Integrated Control System hierarchy [19]
3.1. Subsystems of the Integrated Control System
The main components of the Integrated Control System for the DP semi submersible are [4]: Dynamic positioning: • Dynamic Positioning System (triple redundant) • Dynamic Positioning System (single-fire backup) • Hydroacustic Position Reference System (double) • DGPS Position Reference System (four) Vessel Manoeuvring: • Thrusters and Propulsion Control System Vessel Control (Management) System: • Alarm and Monitoring System • Power Management System • Mode Control and Redundancy and Criticality Assessment System (RCA) • Ballast and Bilge Control System • Drill water, Brine, Mud and Bulk Material System • Cargo Control System (for cargo ships) Vessel Safety System: • Emergency Shutdown System • Fire & Gas Detection System 8
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Power/Energy Management of Marine Power Systems
Dynamic positioning (DP) is extremely important and that is the reason for having high redundancy in DP systems equipment. Accordingly, for drilling rig, vessel manoeuvring and navigating is less important. Power Management System and Alarm and Monitoring System are two the most important subsystems of Vessel Control System. Alarm and Monitoring System is still very simple and straightforward, as should be, since operator must be able to understand it. Over the last decade, Power Management System has increased in complexity significantly and started to interact with other control systems, equipment protection and relaying philosophy. However, the main concept of PMS should still be understood by the vessel operators.
3.2. Vessel Control System Redundancy and Criticality Assessment (RCA) is a tool developed to compare the current situation with preset mode definitions and to inform the vessel operators about any mismatches that are presented with clear identification of faulty or unavailable unit or system. For example, the RCA will detect whether a switch is in local position and should be in remote or the valve is left in manual mode while it should have been in automatic. Usually, there exist 3 levels of criticality – reflected by different colours.
That is similar with well established concept of criticality levels for Alarm and Monitoring System: there are several levels of alarm, some alarms are not critical, such as faulty operation of purifier, or high level in sludge tank (or similar) and these alarms do not require immediate response from the control system and/or operator, while the alarms such as high temperature on diesel engine main bearing or low diesel engine oil pressure is considered to be essential and may require response in mili seconds. Redundancy and Criticality Assessment, on the similar way as Power Management System, has become necessary aid to ship operator over one point of ever increasing complexity of power plant. Complex power systems require higher educated personal onboard which is not consistent with owner’s trend of employing lower qualified crew. Redundancy and Criticality Assessment is interconnected with Alarm and Monitoring system into Vessel Control (Management) System which will display, on a simplified way, the state of various units and their auxiliary equipment.
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3.3. Vessel Control System functional architecture and control equipment
Fig. 3.1. shows architecture of the total integrated control system. Three functional layers are the most important to consider [9]: 1 Controller layer – process and equipment control 2 Network layer – communication connection between supervisory layer and controller layer components 3 Supervisory layer – operator stations, user interfaces (HMI – Human Machine Interfaces) 3.3.1. Controller layer
Process and equipment control is performed primarily in Controller Layer. Control is accomplished in closed loop / feedback (cycle control). For power management systems the speed of control cycle should be on the order of 100 ms (milliseconds) and therefore the scan rate should be higher than 100 Hz (1/s). Programmable Logical Controllers (PLCs) are generally used in all automation applications. They were originally designed to replace hard-wired relay logic but over the years they developed strong communication capabilities and were among the first to support open control standards, such as Ethernet TCP/IP dealt at the beginning of the chapter. The most important characteristics of the controllers applied in marine applications are reliability, redundancy and robust I/O (input-output interfaces) – analog and digital interfaces with sensors (input) and actuators (output) and digital communication interfaces (with other PLC controllers, computers, etc. on network layer). One very important indication of the control equipment reliability is Mean Time Between Failure (MTBF). PLCs have the highest MTBF some of which exceed 400 000 hours (45 years). Also I/O devices and computer processor unit (CPU) may be considered as very reliable. However, shipboard equipment may be often exposed to conditions that are unusual for the land based installations. These includes: moisture, excessive heat, electromagnetic interference, vibrations, etc. These conditions are generally considered in any design but there are a lot of different circumstances that may generate the new conditions that were not taken into consideration previously. Single point failure is a special kind of failure that may generate these new conditions which equipment can not withstand. One example is battery failure for the energy supply of CPU. If the voltage on the battery is just a little bit lower than nominal, all control equipment will have fail readings from measurement and therefore false alarms and soon we might end-up in 10s of different false alarms. The leading PLC manufacturers support redundant configurations that assure the control and I/O transfer from failed PLC to a backup with a minimum CPU cycles. Redundancy is the most important so redundant controllers assure against the loss of control. 3.3.2. Network layer
Network layer is communication connection between supervisory layer and controller layer components. The network hardware and software is the bound between these two systems. The quality of that bound and transfer of information between systems and controllers (PLCs) of the controller layer is dependent on network speed, reliability and openness. The TCP/IP Ethernet is found to be the 10
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best network standard according to that characteristics. The speed and reliability with which data is delivered is a function of the network bandwidth and network loading. The highest reliability and speed will be accomplished with lightly loaded networks with high bandwidths. Bandwidth is the measure of the volume of traffic a system can accommodate. Network loading is Open Control Systems is usually within the control of system developer. 3.3.3. Supervisory layer
Components of the Supervisory Layer are user interfaces or operator stations in marine applications. Operator stations serve as communication between human and all integrated control system. In Integrated Control System the functions of some subsystems (DP system; Vessel Control System and sub-sub systems: PMS, alarm and monitoring, ballast and bilge system, cargo system; safety system) traditionally was allocated in separated units but more common concept found today is to distribute functions in several units [10]. The most important advantage of distributed control architecture is share of information between different controllers and share of the same control equipment by the functions of the same subsystem. Example is power management system (PMS) which will not be allocated in one controller but will be part of various functions and algorithms shared by different controllers. That gives enormous gains in overall control system redundancy. In case that one controller fails all functions will be distributed on other controllers. All control systems are implemented on the same hardware and software platforms and therefore user interface and documentation are unified. Most of software used today is MS Windows NT that runs on standard Intel computers. A large vessel control system for a drilling vessel can have as many as 5 000 I/O data points in its database and leading software running on powerful desktop computers can handle 10 times more than that. The components of the supervisory layer are also distributed through the vessel and connected to the Vessel Control System via the network layer. An example of supervisory layer architecture for drilling vessel follows [9]: 1 2 3 4 5 6 7
Four (4) operator stations located in the Engine Control Room One (1) operator station located in the Afterward SCR thrusters room One (1) operator station located in the Forward SCR thrusters room One (1) operator station located in the Afterward Pilot House Data Logger located in the Engine Control Room Two (2) DP System (DPS) operator consoles located in the Afterward Pilot House One (1) DP System (DPS) operator console located in the Forward Pilot House
DP System controls all propulsion thrusters in other to maintain the vessel’s position. Communication between DP System and thrusters is established via network layer, controller layer (control network) and field bus network, see figure 3.1. DP operator stations have to be synchronized with each other over the control network, in order to support dynamic positioning from any DP operator station, see figures 3.1. and 3.2. The time for the reception of feedback on DPS commands is usually 500 ms. Data Logger collects a large number of different parameters and other information and for drilling 11
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vessel the number of data points that are logged in every second is around 1500. The logger archives the data by component either on interval or change of state in database software and data is copied to removable disks for later analysis by shore personnel. It is intention that the data would be transferred via satellite connection to shore office for remote diagnostics.
Fig. 3.2. Example of Integrated Control System hierarchy [9]
Fig. 3.3. Data Logging (Marine IT Company AS – www.maritco.com)
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3.4. Integrated information management system
Today, the automation has changed its role from pure process control and monitoring to that of a ship’s integrated information management system with goes even beyond the concept of integrated control system. Lately, the automation vendors, mainly in land based industry sectors, have started the next step to physically and functionally integrate the real time control systems with the operational management systems, see [6] and [8]. This has been denoted as industrial IT, shown on figure 3.4. The introduction of industrial IT into marine applications has yet only started, and is still an area of research and development.
Fig. 3.4. Industrial IT architectures
3.5. Integration of Power Management System
In proceeding text it was mentioned that power management system (PMS) found today is not anymore allocated in separated unit but more common concept found today is to distribute functions in several units [10]. Figure 3.5 shows operator station and figure 3.6 shows concept of integrated control system with network between operating stations, PLCs and thruster drives. Process Control Stations (Units) (PCS, PCU) or Remote Processor Units (RPU) are PLCs that share information with each other and some of them take part of the PMS/EMS. All are part of the total Integrated Control System. Fig. 3.7 shows distribution of power management system for the vessel described in [11]. All PMS substations i.e. Process Control Stations (PCS) that take part in the PMS are performing PMS calculations based on information received from its own bus segment and they also share information from the other PMS substations. In case when vessel must continue to operate with open bus-tie (power system is divided on two or more subsystems which are isolated), each power system will have its own PMS substation and each PMS substation can act as autonomous unit. In this way the system is not dependant on central computer.
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Figure 3.5. Operating station (ABB Marine, www.abb.com/marine) Operator may through the PMS perform a complete monitoring and manual or semiautomatic control of the power system including power generation and Fig. 3.6. Integrated Control System (ABB Marine, www.abb.com/marine) distribution system
Figure 3.7. Functional integration and information distribution in PMS [11]
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4. Conventional functionality of power management system The main function of Power Management System (PMS) is to ensure that there is sufficient available power for the actual operating condition. PMS should prevent vessel’s blackout by constant monitoring available power on the network and starting and stopping the generators accordingly. If the available power becomes too small the PMS will automatically start the next generator in the start sequence, connect it to network and synchronize with other generators.
4.1.
Blackout restoration (restart), split network and operational modes
Blackout is a complete loss of vessel’s power and therefore it is the one of the most critical situations that may occur for the vessel. Without power, vessel can not maintain position, speed or course which may lead to collision and grounding. For vessels involved in offshore operations, working with oil and gas any such occurrence may lead to sea pollution, fire and other major accidents. Blackout restoration – restart
If blackout happens, and it might still happen, then it is of the highest importance that PMS automatically restore normal functionality of the vessel. Without PMS operator would have to start engines, synchronize them together on network, one by one, and then start manually or remotely all consumers onboard in all systems that are interconnected and complex. That would take a lot of time (aprox. 30 minutes) and also put a lot of stress on crew and they might not be concentrated on the real cause of the blackout. Therefore, it is essential to have a blackout restoration program that can restore normal power system functionality in aprox. 3 minutes. Split network
In the event of full blackout the PMS will split the bus bars into two sections. That is for safety reasons since the program does not have detailed information about the cause of the blackout. For instance, the cause of blackout may be fire or flooding in one engine room. To prevent any large disturbances on the network, the PMS will start all available generators, one at the time. All transformers/distribution breakers and interconnection breakers will be closed in sequence provided they were closed before the blackout, according to present operational mode. Usually, reconnection of bus ties is a manual operation. Operational modes
For vessels involved in offshore operations several operational modes should be defined. Each operational mode compensates some level of operational risk of the vessel. Therefore, higher mode should be selected for higher operational risk of the vessel. In addition to operational risk overall vessel risk strongly depends on weather conditions. For instance, the supply ship will have a low mode of operation when waiting on the open sea since the operational risk is low and ship does not need to maintain accurate position – ship can basically drift without engines working if there are no other ships around observed by navigational system. Contrary, when supply ship has to load or unload some cargo from the drilling rig, operational risk becomes high 15
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accordingly and operational mode should be changed. In that case, ship will need full power from the thrusters and obviously everybody onboard like to have all generators running online. The following table describes the importance of operational modes [4]. To be able to analyze the data in the table it is necessary to list following vessel data: - Installed generators: 6 generators, 7 MW each - Total installed power is: 6 x 7 = 42 MW - Required nominal design power (design storm conditions) = 35 MW (corresponds to 5 generators) - Installed thrusters = 8 Table 4.1. Operational modes of the vessel [4] Mode
Generators (minimum)
Thrusters
Harbor mode:
1
0
Transit mode:
2
2
DP Class 1 mode:
1
2
DP Class 2 mode:
2
3
DP Class 3 mode:
3
4
Switchboard configuration
Ring configuration Ring configuration
DP Class 3 operation mode requires half of the installed capacity in generators (3 of 6) and in thrusters (4 of 8) as minimum. Ring configuration means operation with split network – open bus ties that separate the network into more than 2 network sections. Usually, the operator will decide desired mode of operation according to his judgment of operational risk of the vessel. After blackout, the vessel will restore the power according to preselected mode of operation.
4.2.
Load depended starting/stopping of generators – power reservation
The main function of any power management system is to start and stop generators according to available power on the network. Available power is the difference in online generators capacity (nominal power of all connected generators) and current network loading. For instance, consider the power system with 6 generators, each generator 7 MW power. Total installed power is 42 MW. Only active power is considered since the engine (diesel or gas/steam turbine) will produce the torque that will correspond to active – real power. 7 MW is installed (nominal) power and is 100% of each generator nominal power. If the network loading is 26 MW and 4 generators are running, the available power is (4 x 7) MW – 26 = 2 MW. It is obvious that next generator in sequence must be started to have some power reserve i.e. more available power. If the next generator is connected to network and started to share the network load, available power becomes 2 + 7 = 9 MW assuming the same conditions - no starting stopping of other consumers. 16
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The next generator will be started only if the power becomes bellow preset limit. In this example this means preset limit should be higher than 2 MW. Preset limit for generator starting will trigger (initiate) the timer. If the available power stays bellow preset limit for predefined period of time the next generator in sequence will be started. The predefined period of time may vary according to power system configuration, general philosophy and owner/ designer preferred solution. Power reservation program for any installation will always depend on available power i.e. number of generators online and generator load. Power reservation program also depends on modes of operation, but not directly, since minimum number of generators online is defined by current operational mode. Example of load dependent starting generators for one installation with 6 generators installed is given in the table 4.2 and corresponding diagram on figure 4.1. Table 4.2. Load dependent generator starting (illustrative) Number of generators connected 2 3 4 5
Generator load
Number of generators connected 2 3 4 5
Generator load
Number of generators connected 2 3 4 5
Generator load
70 % 75 % 80 % 84 %
85 % 87 % 89 % 91 %
105 % 105 % 105 % 105 %
Available power (Power reserve) 2 x 30% = 60 % 3 x 25% = 75 % 4 x 20 % = 80 % 5 x 16 % = 80 %
Time delay to initiate the starting sequence 10 min. 10 min. 10 min. 10 min.
Available power (Power reserve) 2 x 15% = 30 % 3 x 13% = 39 % 4 x 11 % = 44 % 5 x 9 % = 45 %
Time delay to initiate the starting sequence 10 sec 10 sec 10 sec 10 sec
Available power (Power reserve) 0% 0% 0% 0%
Time delay to initiate the starting sequence Immediately Immediately Immediately Immediately
It can be noticed from the table that available power or power reserve that initiate engine starting may not be the same for all different situations and will be dependant on number of generators online and generator load. Generator load is the one that triggers starting next engine in sequence and overall philosophy is to utilize more power of each diesel engine if more generators are online. Therefore it might be difficult to accomplish the same value of available power that will initiate engine starting for all network situations. That is the reason for representing the available power that initiate starting in time/power windows, on figure 4.1. Diesel engine can be loaded up to 115% for very limited period of time. Therefore, maximum generator load defined in table can be for instance 105 % of the nominal generator load but generally it is better not to use additional 15% power reserve in the calculation of available power.
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Figure 4.1. Load dependent starting of the engines - according to table 4.2 (illustrative)
Class rules require from generators 45 seconds for starting, synchronizing, and starting sharing the network load. Usually, the generator will start to share the power in aprox. 40 seconds. It is the challenge how to anticipate the network situation 45 seconds or more in advance and start the additional generator(s) before consumers start to draw the power. Almost all PMS functionality serves to solve that problem. Engine overload is the most critical situation for the power system and hence for the vessel. It is one of the main causes of the blackout. Overloaded engine will soon start to decrease the speed, and accordingly frequency, and will start motoring – the power flow direction will be altered from network to generator i.e. reverse power. In that case, the network protection system must disconnect such generator from the bus. Several easy and very difficult situations exist that can be solved with modern power management system and will be described further in text. Generator stopping
When diesel engine loading is less than 30 to 50% of nominal power a high quantity of soot starts to accumulate inside the combustion space and exhaust receiver, in addition to carbon deposits in all parts of combustion chamber and especially on injector nozzles that are very sensitive to such occurrence. Very soon, maybe in several operating hours, injector nozzles might become damaged and their spraying capabilities seriously decreased. That will lead to possible cylinder misfiring and further increase of soot/carbon deposits. In summary, the lower the engine loading is, the more soot/carbon deposits will accumulate and injector nozzles will become more and faster damaged. When several generators are connected online one generator can be stopped if its loading is less than 50 to 60% of nominal load for more than 30 minutes. That correspond to 100 to 200 % of available power, 18
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table 4.3. Table 4.3. Generator stopping Number of generators connected 2 3 4 5
Generator load 50 % 60 % 60 % 60 %
Available power (Power reserve) 2 x 40% = 100 % 3 x 40% = 120 % 4 x 40 % = 160 % 5 x 40 % = 200 %
Time delay to initiate the starting sequence 30 min. 30 min. 30 min. 30 min.
However, generator stopping is not an automatic PMS function on all installations. Some owners prefer ship operator to stop the engine instead PMS. The reason for that is safety, again. Still there are a lot of different possibilities for improvement in PMS functionality that may decrease the probability of fault occurrence from PMS.
4.3.
Power limiting of variable frequency drives
Class rules require from generators 45 seconds for starting, synchronizing, and starting to share the network load. If the network loading becomes higher than the power capacity of online generators i.e. available power < 0, the next generator in sequence will be started immediately, see table 4.2. According to table 4.2. and diagram on figure 4.1. the next generator will start even before i.e. when available power is bellow 30 to 45 % with 10 seconds delay. However, one or more heavy consumers may draw such power at once and available power may decrease from 50 % to 0 % faster then it’s needed to get the next generator online, shearing power. Time delay of 45 seconds needed for generators to start to share the load usually is too large for power system to prevent the blackout. Therefore, one way to prevent system overloading is to limit the power to consumers. Heavy consumers that draw a high amount of power from the network are the most important to consider since they will be responsible for the highest network disturbances. The maximum % loading can be individually set for each heavy consumer, allowing a priority selection between the consumers. The PMS calculates power limits by adding a portion of the available power to the actual loading (energy consumption) of the consumer. Therefore, consumer has a power limit and can not draw more power than is allowed by PMS. Variable frequency drives
Usually, heavy consumers are variable frequency drives since they utilize possibility to change the speed on the best possible way. Variable frequency drives are the most responsible for higher flexibility in power management and advanced power control functions. Variable frequency drives are capable to increase/decrease power to drives (thrusters, pumps, or drilling package) in less than 50 to 100 milliseconds (ms). Voltage source inverters (VSI) have dynamic response time less than 50 ms while DC thyristor based rectifiers (SCR), cycloconverters and current source inverters (CSI) have dynamic response time around 100 ms [1], [3], [12]. However, diesel engines may need 15 to 20 seconds to increase the load from 0 to 70 %, as shown on figure 4.2 for PMS engine emergency start test [11]. 19
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Energy accumulation of an autonomous system is very small and means that the generators will have to respond immediately to any change in the network loading. That will lead to difficult situation and one way to overcome it is to limit the speed of power change to consumers. For instance, the load increase rate of consumers may be limited 0 to 100 % load increase in 4 seconds with minimum 2 generators online, but that depends on power system configuration and Vessel Control System overall design. Such power limitation is known as a ramp limit.
Figure 4.2. PMS Engine emergency start test [11]
4.4.
Heavy consumers blocking – start acknowledge
All heavy consumers are usually variable frequency drives on modern marine power system. Generally, heavy consumers are the main application of the variable frequency drives. However, on more conventional installations there might still be some large heavy consumers with direct on line (DOL) electric motor. The largest are controllable pitch propellers driven by large cage-type induction motors that may be designed with two pole switches to allow for two operating speeds. When started direct on line, the induction motor draws a very large starting (inrush) current which may be 5 to 7 times the nominal current. That is the reason for large voltage drops in the network and torque transients. Inrush current can be lowered successfully by many methods, the most popular known as star-delta switching, but is often not the best solution [1], [10]. Even with methods described, the transient behavior of high powered direct on line motor may be the reason for too large network disturbance and the system might be temporarily overloaded, which again may easily cause a blackout. That is the reason for introducing PMS function known as consumer blocking or start acknowledge which means that a specified heavy consumer must ask permission from PMS to start. If the PMS 20
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calculates that there will be enough available power in steady state but also in transient time interval the heavy consumer may start. If not, the PMS will start the next generator in sequence and de-block heavy consumer after generator started to share the load.
4.5.
Load shedding – preference trip
Auto start, power limiting and heavy consumer blocking are functions of PMS that can ensure blackout resistant power system, assuming that all conditions on power system can be predicted fast enough. However, that is still not possible. Rapid and large loss of power generating capacity will still cause the blackout if the PMS does not have load shedding function. Load shedding is a fast executing function for disconnecting from network of heavy consumers and group of non-essential consumers. It will trigger the circuit breaker and disconnect consumer if the available power goes bellow certain level. This level may be different for each heavy consumer and is defined by consumer priority. It is also possible to define the time delay for each consumer, as shown on the figure 4.3. More important consumers will shed the power when available power becomes very low. It is important to notice that load shedding function must not interfere with auto start. Less important consumers will be first to shed the power if the available power still continue to decrease, after auto start of next generator.
Figure 4.3. Auto start/stop and load shedding [10]
4.6.
Consumer groups and shedding priority
Consumers are divided into special groups that can be referred more easily. It is important to consider various groups of consumers since some consumers are more important for vessel safe and reliable operation then others. Examples of essential consumers are: bridge nautical equipment, control systems, DP system, lights, emergency equipment, marine auxiliary machinery necessary for generator operation: fuel pumps, fuel and oil purifiers, staring air compressors, fresh water and sea water cooling pumps, lubricating pumps, 21
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etc. Less important consumers are hotel and accommodation loads, but obviously not on cruise vessels, and drilling equipment that can be stopped for a few seconds or minutes to avoid generator overloading and consequent domino effect black-out. Hotel and accommodation loads, including auxiliary equipment, draw very small power per single unit (pump, compressor, etc) which will not be greater than aprox 200 kW (ballast pump on large ship). Starting and stopping larger number of smaller units may seam to be the same as changing consumed power for one larger power consumer like thrusters or drilling drive. Different power consumers have different time constants and speed of response and will usually cause inrush transients and produce unnecessary network disturbances that can be avoided by changing power by means of only few power drives of the same size and characteristics. Since drilling equipment and thrusters have variable frequency (variable speed) drives it is possible to temporarily lower power on such consumers, for the reasons described above. For dynamically positioned vessels it is important that the vessel will not lose much of the position if the power to thrusters is limited less than 30 seconds. Today, power frequency converters can respond to power demand in less than 50 ms for voltage source inverters or less than 100 ms for conventional thyristor based converters – cycloconverters, current source inverters or DC SCR-thyristor rectifiers. 12
10
8
6
4
2
0 Hotel and accomodations
Marine auxiliaries Drilling auxiliaries
Drilling drives
Thrusters(1/3 of installed power)
Figure 4.4. Main groups of consumers on drilling vessel [6]
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4.7.
Power/Energy Management of Marine Power Systems
Generator/engine control – load shearing
4.7.1. Speed droop and isochronous control
Speed droop is a function of the engine controller (governor) which reduces the reference speed as load increases. This function is linear as can be seen on figure 4.5. Speed droop principle must be used to provide stable control and all engine controllers use speed droop [13]. The network frequency must be kept constant all the time. Small variations in network frequency are allowed in steady state i.e. ± 2.5%, but in transient state more frequency variation is allowed and must not be higher than ± 10%. Operation without speed droop
On every system, with or without controller, increasing the engine load will decrease the engine speed and corresponding frequency of the generator, since frequency and speed are proportional. The controller will respond by increasing the fuel until the engine speed has returned to its original value which correspond to network frequency. Due to the combined properties of inertia and power lag, the engine speed will continue to increase beyond the original speed setting, causing an overshoot in speed. The controller will respond to decrease speed to correct for the overshoot. It will over-correct the speed in the other direction causing an undershoot. This overcorrection of speed in both directions (instability) will amplify until the engine trips out on over frequency (or under frequency) as shown on figure 4.7. Speed droop control
Droop is defined as a decrease in speed setting as the load increases. Speed droop is expressed as a percentage of the original speed setting from no load to full load % can be calculated according to following simple equation: %Droop =
No load speed - Full load speed ⋅100 Full load speed
The recommended percent of droop is usually 3 to 5 % and that is the reason why frequency is allowed to change ± 2.5% in steady state (for 5% droop). Figure 4.5. shows 3% and 5 % droop which corresponds to permanent frequency variation of +3% and + 5% from 60 Hz nominal. The simple mechanical governors have the droop function built into the control system, as shown on figure 4.8. More complex hydraulic governors can include temporary droop which is the function of returning the engine to its original speed after the engine has recovered from the load change.
Single engine operation Isolated, single engine can operate in either isochronous or droop mode. The engine operation is not affected by any other engines or the utility. In isochronous operation, the speed returns back to the original value after a load has been applied up to 100% load. In droop operation, the speed decreases depends on the percentage of applied load, according to figure 4.5. If the original speed is desired, the operator must raise the speed setting to return to the original speed after the load is applied, for instance from 50 to 100 % load, as shown on figure 4.6 the speed setting of the engine must be raised for aprox 1 Hz in frequency i.e. from 61 Hz to 62.
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Figure 4.5. Speed droop of 3 % and 5 %
Power/Energy Management of Marine Power Systems
Figure 4.6. Speed setting for 3 % droop, 50 % and 100 % load
Figure 4.7. Instability on system
Figure 4.8. Mechanical governor with droop feedback
Droop is not adjustable in most mechanical governors as can be seen on figure 4.8, while more complex hydraulic governors can set the droop in operation usually between 0 and 5%. A single engine electrical generator can operate in isochronous or droop control mode. Response curves for both modes are shown on figure 4.9. Isochronous control mode will require more time in transient then droop control mode which will obtain lower frequency then required. Generators should not be paralleled isochronously with any system so big that they can not affect the frequency of the system - in land based power generation. Generator with lower frequency that correspond to droop mode can not stay connected to the utility (in land based power generation) since it will start motoring (reverse power).
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Figure 4.9. Response curves for isochronous or droop control
4.7.2. Load shearing with mechanical governor When paralleled with the bus, the utility will determine the frequency of the generator. Generator frequency i.e. corresponding engine speed must equal to the frequency of the utility. However, for autonomous power systems, as found on ships, network frequency can vary within 5 % from nominal, as explained previously. In marine applications diesel generators usually share the load equally (but not when combined with turbines) and speed setting, as well as droop should be equal for all of them. However, after connecting and starting to share, the generator load should be raised slowly by predefined program - ramp function. The same is valid for shedding the load before disconnecting the generator from the network. In the past, generator was connected and disconnected abruptly which was causing large stresses on marine electrical installation. Diesel engine will also behave much better and require less maintenance when slowly starting to share and shad the network load. For generators connected online, the load of an engine is determined by the speed setting of the mechanical governor, as shown on figure 4.6. For mechanical governors the droop in operation is fixed, see figure 4.8, but can be set only when engine is not in operation. If the generator needs to be unloaded and disconnected from the network, the speed should be slowly decreased until the generator is at the minimum power output and then the tie breaker should be opened, as shown on figure 4.10. Operator must be careful to avoid reverse power situation. That function is automatically today. Figure shows load changing form 100%, 75%, 50%, 25% to a minimum load and seams discontinuous, but load changing is continuous ramp function. If the droop is set high (up to 10% can be found) the engine will be less sensitive to speed setting but it might easily go to over speed if is suddenly removed from the bus. 4% droop is usually adequate to provide stability.
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Figure 4.10. Disconnecting the generator from the utility by adjusting the speed setting
4.7.3. Load shearing with electronic governor Electronic governor Electronic governor can change the load of an engine by changing the droop. Figure 4.11 shows load shearing for two generators operating with the same speed setting but different droop [14]. To share load equally, droop must be set equal. Diesel engines are usually equal in size, power and characteristics and optimum operation with minimum fuel consumption is obtained when they share load equally [15]. However, during connecting and load acceptance or load shedding and disconnecting, the generators have to share load unequally. On figure 4.12 electronic governor with droop and isochronous control mode represents the basis for all analogue load shearing devices on the market today [13], [14]. The balanced load bridge (see resistors R1, R2, R3, R4) is the main component and is similar to Wheatstone bridge. Resistors have equal resistance and R1 = R2, R3 = R4. When voltages in both legs are equal (VR1 + VR2 = VR3 + VR4) there is no voltage difference across the capacitor C and the signal which goes to summing point is equal to zero. This signal is the same as the one obtained with droop feedback lever on mechanical governor on figure 4.8. The main difference is that electronic governor has to sense the load from the line (active power) i.e. by measuring voltage and current in all 3 phases of the generator, while mechanical governor sense fuel rack position, which is proportional to present engine load. Load measuring is essential for droop control since droop is always proportional to actual generator loading. Potential transformers measure voltage in all 3 phases, while current transformers measure currents. That is necessary to be able to measure the displacement power factor (cos fi) and use only active power in load shearing, on the right side of the diagram. Load gain voltage is usually fixed on 6 Vdc which corresponds to generator rated load. Hence, voltages from 0 to 6 Vdc represent loads from 0% to 100% of the generator nominal load. 26
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Isochronous control with electronic governor When voltages in both legs are equal there is no voltage difference across the capacitor C and the signal which goes to summing point is equal to zero. Therefore the generator load does not affect the speed or frequency. At the summing point, the negative signal from the speed sensor is compared with the speed set adjust. The difference will be amplified in the amplifier. The PID amplifier will act to reduce the speed until the sum of the negative input signal equals the positive input signal from the speed set adjust. The control is in isochronous.
Figure 4.11. Load shearing of two generators using droop control [14]
Figure 4.12. Electronic governor with droop and isochronous control mode [13]
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Droop control with electronic governor To get the load signal on the summing point, the load bridge must be unbalanced. The resistor for droop adjust serves for that purpose. The voltage difference between two legs would exist even without capacitor C, but capacitor must assure the phase lag of load signal on the same way as droop lever will assure the phase leg only if it is closer to the speed adjust screw on the figure 4.8. The capacitor (same as inductor) is energy storage device and as such introduces inertion and time delay in the load signal. Without capacitor, load bridge would be faster then than the speed loop and that would result in oscillation of the frequency and system instability. The speed loop has its own inertion from the engine and generator side, since engine and generator (and gear, if any) coupled together can store energy as a fly wheel. Large engines have more delay in responses and larger time constants than small engines or turbines. At the summing point, the negative signal from the load bridge adds to the negative signal from the speed sensor. To obtain a summing point balance, the amplifier will act to reduce the speed until the sum of the two negative input signals equals the positive input signal from the speed set adjust. The control is in droop. The speed or frequency will decrease proportionally with addition of load. The return the system to require speed and frequency (60 Hz) it will be necessary to either increase the speed set adjust, according to figures 4.6 and 4.10, or to re-balance the bridge and return the system to isochronous control.
Isochronous load shearing with electronic governor Figure 4.13 shows the load shearing in isochronous control mode for two generators working online together. Unbalance in each load bridge is obtained by simply paralleling one leg of the bridge from the control of one engine-generator set (gen-set) with the corresponding bridge leg of the control of a second gen-set. These paralleling lines on figure 4.13 are also called the load shearing lines and are shown on figure 4.12 too. This method of connecting the load bridge between controls of multiple engine-generator sets, which are supplying the same load, can be used to obtain load sharing between almost any number of different sets. Short circuit across the load sharing lines would result in high feedback signal in all load bridges to reduce fuel in all generators. That would obviously result in a blackout. It is common for vessels to operate in droop mode with external speed and load being controlled from the PMS. This will give the highest level of redundancy.
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Figure 4.13. Load shearing with electronic governor [13]
4.8. Prime mover performance Diesel engines are the preferred prime movers for power generation on DP vessels, predominantly because of their reliability as prime movers and their ability to respond to changing power needs [16]. Changing power needs i.e. power consumption for drilling rig West Venture has been presented on figure 4.14 [10]. It is interesting to note that total active power, which has to be supplied by prime movers, is very variable with minimum of 3 MW and maximum of 20 MW in 1000 sec, having 6 peaks with variation of more than 10 MW. Gas and steam turbines can be selected according to conditions of machinery concepts dealt in chapter 2.2. Efficiency of various prime movers can be compared in following diagram, figure 4.15 [2]. Class requirements state that the transient frequency variations in the electrical network should not be in excess of ±10% of the rated frequency with a recovery time to steady state conditions not exceeding 5 seconds [17]. Steady state conditions are those at which the envelope of speed variation does not exceed +1% of the declared speed at the new power. At all loads between no load and rated power the permanent speed variation should not be more than ±5% of the rated speed – related to droop.
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22
45
20
40
Electric Propulsion
Combined Cycle
18
35
Diesel Mechanical Propulsion
Efficiency [%]
16
MW
14 12 10 8
Gas Turbine
30
Steam Turbine
25 20 15
6 10
4
5
2 0
0
100
200
300
400 500 600 Time [s]
700
800
0
900 1000
0
5
10
15
20
25
30
35
Propulsion power [MW]
Figure 4.14. Changing power needs for drilling rig West Venture [10]
Figure 4.15. Efficiency of prime movers [2]
4.8.1. Prime mover transient responses Good transient response of prime movers is essential requirement for vessels that have changing power consumption. It should be emphasized that higher degree of turbocharging, which is related to higher Break Medium Effective Pressure (BMEP), for modern diesel engine is the main reason for engine transient response to develop in negative direction [16]. Break Medium Effective Pressure (BMEP) is a parameter which is often use to indicate how highly rated the engine is. It is a measure of engine volumetric power. Higher the BMEP is, higher will be the engine brake power, for the same speed and combustion space volume. BMEP also corresponds to higher efficiency, but lower transient responses. To increase the thermal efficiency and lower the fuel consumption the turbocharging pressures have increased tremendously since 1970’s. In 1970’s it was common that engines could be loaded from 0 to full power in single load step. Highly turbocharged modern diesel engine can be loaded most quickly by a successive gradual increase in load. According to class rules today, the application of electrical load should be possible with 2 load steps and must be such that prime movers – running at no load – can suddenly be loaded to 50% of the rated power of the generator followed by the remaining 50% after an interval sufficient to restore the speed to steady state. According to figure 4.16, diesel engine with BMEP higher than 18 bar can not be loaded from idling (0 % power) to 100 % even in three (3) step loads. Lower the BMEP is, the engine can increase more power in fewer steps in load. Load responses for gas turbine with rated power of 25 MW are shown on figures 4.17 – 4.19. Comparing these with diesel engine responses generally represented by figure 4.16 it can be concluded that diesel engine will assure much better transient response behavior and load acceptance in general. Due to low inertia and high power per single unit, gas turbines are not capable to winstand the power increase of 28 % from idling (0 %), see figure 4.19. Regarding figure 2.5 in chapter 2.2 modern COGES power plant will have 2 gas turbines. If each unit has 25 MW rated power, total installed power will be 50 MW in addition to power of steam turbine, see figure 2.5. Machinery plant with diesel engines would be optimal with 5 x 12.6 MW engines, see table 30
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2.1. Therefore, 6 MW of load increase, shown on figure 4.17 for gas turbine corresponds to less than 50% of rated diesel engine power. According to figure 4.16, diesel engine can accept the load from 0 to 80 % or 100 % in 2 or 3 load steps.
Figure 4.16. Required steps in load for modern diesel engine [17]
Figure 4.17. Gas turbine response to load from full load (25 MW) to 0% load
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Figure 4.18. Gas turbine response to load: 6 MW - 0 MW - 6 MW (24 % of full load)
Figure 4.19. Gas turbine response to load: 7.5 MW - 0 MW – 7.5 MW (28 % of full load)
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4.8.2. Methods to improve diesel engine transient response In addition to the time required to accelerate the turbocharger and hence get enough air into the combustion chambers the performance is affected by rotational inertia of the whole generator set, the speed governor adjustment and behaviour, generator design, alternator excitation system, voltage regulator and nominal output of the unit [18].
Engine preheating If the engine is preheated it will respond better to any kind of load variation. Still, it is a challenge to predict when the engine should be prepared for starting. Engine can be heated by cooling water from the cooling system of engines that operate in the same time or by its own steam/electric cooling water heater. According to figure 4.20 engine can be started and loaded according to the following procedure [19]:
A: Normal start without preheated cooling water. Only on marine diesel oil (MDO) B: Normal start with preheated cooling water. On MDO or heavy fuel oil (HFO) C: Stand-by engine. Emergency start, with preheated cooling water, intermediate prelubricating or continuous prelubricating. On MDO or HFO.
Figure. 4.20. Engine pre-heating [19]
The engine shall always be prelubricated 2 minutes prior to start if no intermittent or continuous prelubricating is installed. Intermittent prelubricating is taking place 2 minutes every 10 minutes.
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Air injection into the turbocharger Air nozzles directed at the turbocharger rotor are used to accelerate the turbocharger and hence reduce the lag. The system is simple and practical but the challenge is how to control the system. Most systems are triggered by engine speed i.e. frequency dip but this is in most cases too late, see figure 4.21. More advanced systems uses load feedback from a large consumer of feed forward signals from other control systems. Fig. 4.21. illustrates the operation of Lambda controller [19]. In case of a momentary load increase, the speed governor will increase the index on the injection pumps and hereby the regulator arm (1) is turned, the switch (2) will touch the piston arm (3), whereby the electrical circuit will be closed. Thus the solenoid valve (4) opens. The air injection into the turbocharger is activated, the turbocharger accelerates and increases the charge air pressure, thereby pressing the piston (3) backwards in the lambda cylinder (5). When the lambda ratio is satisfactory the jet system will be deactivated. function. In case of a shutdown situation (e.g. overspeed, low lubricating oil pressure or high temperature for H.T. water), the safety system will activate the shutdown valve (6) and thereby compressed air will press the piston in the Lambda controller downwards and force the index at the fuel pumps to zero.
Figure. 4.21. Air injection into the turbocharger MAN B&W Lambda Controller [19]
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The jet air consumption at sudden step load can be calculated according the following equation [19]: Air consumtion =
(step load % - C1 ) ⋅N C2
C1 and C2 are constants that will generally be different for each engine. For MAN B&W L16/24 C1 = 25 and C2 = 179. N is the number of cylinders. Example: For MAN B&W L16/24 with 8 cylinders: -
(50 % - 25) ⋅ 8 = 1.117 m3 179 (75 % - 25) For engine step load percentage is 75 %: Air consumtion = ⋅ 8 = 2.23 m3 179 For engine step load percentage is 50 %: Air consumtion =
If load changes are frequent, the air consumption is high and requires large air vessels and compressors. Jet assist also put additional stress on the compressor wheel of the turbocharger. For frequently changing load levels, such as often seen in drilling operations, air injection has proven fairly unpractical. The lagging response may in worst case cause larger speed variations.
Mechanical air blowers Two stroke diesels use mechanical air blowers to provide enough air for combustion at low loads i.e. loads less than 50%. The same principal is sometimes used on 4-stroke engines. This method may be unpractical as a means to improve transient response on higher loads due to the high air mass flows and pressures. Mechanical blowers can not provide high pressure air which is required at higher loads.
Force the rotation of the turbocharger A more direct way of avoiding combustion air deficit is to force the rotation of the turbocharger, even in conditions where the exhaust gas flow is not able to maintain turbocharger speed. This concept was used already in the 1960’s in icebreakers, preventing the engines from stalling when operating in sever ice conditions. It involves either an electric motor or a power turbine on the turbocharger rotor shaft. Using electrical high speed motors would maybe offer the most convenient control interface, but represents a fairly challenging mechanical interface due to the high rotational speed of the turbocharger.
Feed forward control On the controls side of things, there are numerous innovative approaches that can be put to work in improving diesel genset loading response. For certain applications, using a load feed forward signal, may be very useful, especially if the load changes fall in an established pattern. In most cases the measured generator load is not so useful. It would be necessary to start compensating just before the load change.
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Higher speed of the actuators Actuators that translate the governor output signal into a mechanical movement of the fuel rack are normally hydraulic. Electric actuators have entered the market and represent potential for faster response in the actuating portion of the system but the mechanical levers and link rods remain.
Variable geometry turbochargers Variable geometry turbochargers (VGT) can provide variable performance of the diesel engine during operation. However, adjustments are relatively slow and VGT can not be considered as a method to improve diesel engine transient behavior.
Figure 4.22. ABB turbocharger for 4 stroke diesel engines
Additional systems and features may improve loading response, but hardly any of the available technologies have been commercialized as selectable options from the engine builders [16]. New technologies such as common rail fuel injection and load pulsing control marks potential for future improvements in diesel engine loading performance.
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5. Advanced functionality of Power Management System 5.1.
Functional Integration of Power Management System with DP System
Power management system per definition is an integral control system onboard ship. Considering subsystems of total Integrated Control System described in chapter 3.1 and conventional PMS functionality described in chapter 4 it can be concluded that PMS must be an integral part of the overall vessel control system. For instance, the number of engines started online will depend on operational mode, which depends on overall operational risk of the vessel. Higher operation risk assumes higher power consumption of thrusters and other consumers which depend on weather and vessel operations. Overall protection philosophy, selectivity and concept of tolerance to faults will strongly depend on integration and interconnection with PMS and overall integrated control system. Therefore, functional integration is a natural extension of PMS functionality. Functional integration of PMS with DP System is one of the most important and will be described in this chapter [10]. When the vessel has to operate with open bus-ties i.e. split network, conventional DP System would calculate the thrust for each thruster at the same way as the network is connected. One example for the sway movement of the barge is shown on figure 5.1 a) and represents simple situation with equal power on each thruster. Now, assume that each network, represented on figure 2.2 has a different power loading from other consumers. Then each generator, in individual engine room would have a different power represented on figure 5.2 for conventional DP-System. The dotted lines on the figure 5.2 show the generator load with a conventional DP system in 4-split operation. Due to different load on the four switchboards, two generators have crossed the limit of available power represented with dotted red line, around 60 sec. Two generators next in sequence will start to share the power, equally with other two at 70 sec. Two generators will be started completely independently, each in its own engine room, and each will share the power with the previous working unit in its own engine room. Two generators in one engine room will share around 40 % of power. With functional integration (solid line), uneven thruster power, see figure 5.1.b, is used to get equal load on each switchboard, see figure 5.2. Hence, it is not necessary to connect two next generators in sequence. This example shows that generators are not optimally utilised with conventional DP-system. DP-system and PMS are integrated on the way that DP-System takes into account power situation on each switchboard (2, 4) when calculating the thrust on each individual thruster. All advanced power management functionality goes toward better utilisation of diesel generators while lowering the risk level of blackout, at the same time. Better utilisation of generators provides lower engine fuel consumption.
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1st solution conventional
2nd solution
a)
b)
Figure 5.1. a) Thrust allocated for the sway movement of the barge with conventional and b) Functional integration of PMS with DP System
Figure 5.2. Generator load in 4 split network configuration with conventional operation and functional integration [10]
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5.2.
Power/Energy Management of Marine Power Systems
Fast Load Reduction
One of the features of frequency converters for ship propeller drives and drilling drives is the possibility to change the power very fast in less than 50 to 100 ms [3], [12]. This feature has no practical use for ship propellers regarding propeller acceleration and change in thrust demand since too high acceleration would be the reason for propeller cavitation and thrust could not be provided. However, high speed of frequency drives can provide a means for very fast load reduction and unloading of power system network. By using all PMS conventional functionality, described in proceeding text, it is still not possible to simulate any cause of blackout except the engine-generator trip. The traditional power management system monitors the total energy demand and compares it to the available supply. The system can automatically start and stop generator sets to coincide with load changes in accordance with the pre-set load dependent start-stop tables, see tables 4.2, 4.3 and figures 4.1. to 4.3. In addition load limiting and load shedding function can help up to some point, as described in sections 4.3 to 4.5. In case of an engine-generator trip, the main problem is the time aspect. In case of few generators running, a generator trip may result in generator load in the range of 140 – 160%, and thus the load must be reduced very fast to avoid trip of the remaining generators and resulting blackout.
5.2.1. Diesel engine shutdown and generator-set trip The most common types of faults, which cause a generator set to shutdown, are: • fuel system failures (clogged fuel lines, fuel pump failure, water in the fuel), • mechanical failures (loss of oil or water pressure, over speed, high cooling medium temperature), • control system failures (false indication of low lube oil pressure, false oil mist detectors, crankcase overpressure), or loss of an I/O signal, or • operator / human error - usually occurs in the set-up and synchronizing of generator sets and load balancing where engines can be tripped by reverse power protection An example of diesel engine shut-down safety stop control is shown on figure 5.3. On table 5.1 all alarms and operating limits for MAN B&W medium speed engine has been shown.
Figure 5.3. Diesel engine Safety Stop(C1-DSS) – (Kongsberg Maritime http://www.km.kongsberg.com/)
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Table 5.1. Operating limits, alarm set points and shut-down safety stop limits for MAN B&W L16/24 engine
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From the table 5.1. can be seen that engine auto-stop is initiated by low pressure in lubricating oil system and high temperature (HT) in cooling water and lubricating oil system. Engine overspeed limit is of special importance and always must be included in auto-stop functions. Here will be triggered on 15% above normal value at full load. Engine protection system can vary for different engines so auto-stop can be also initiated by low pressure in jacket cooling water system. Larger engines usually have more measuring points, and accordingly more alarms and auto-stop parameters.
Pre-warning alarm A very important function, that helps in avoiding sudden engine loss situation is called pre-warning alarm. Pre-warning alarm is a function which automatically starts the next available generator if any engine conditions which will lead to a shut down of the engine are getting critical near the shut down limit. Advanced engine monitoring and diagnosing, implemented in modern local engine controllers, can provide possibilities for better functionality of pre-warning alarm. In that respect, communication between PMS and local engine controller must be fast and efficient.
5.2.2. Diesel engine inertia and dynamic responses If diesel-engine trip can not be avoided by pre-warning alarm or this function is not the part of vessel’s PMS, then fast load reduction functions become necessary. Generally, medium speed diesel engines have a higher mass moment of inertia then gas-turbines or high speed engines (speed > 1000 RPM). Higher the power, larger the mass moments of inertia will be. That provides larger time constants. Larger time constants will provide more resistant engine to underfrequency protection trip due to sudden abrupt load increase. However, the consequence of high inertia is that the time for frequency recovery will be increased. Experience has shown for a 5 to 7 MW medium speed diesel operating at 720 RPM with a load of 140 %, the load must be reduced in less than 1-2 seconds to avoid under-frequency protection trip (on 54 Hz) and resulting blackout [21]. For low powered engines, this time can be shortened to 0.5 seconds.
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5.2.3. Fast load reduction systems Fast load reduction system with communication with DP-System Advanced power systems are equipped with fast load reduction system with various successes. The existing DP and PMS systems sense the power system parameters, (frequency, kVA, kW, and kVar) in the medium voltage switchboards. These signals are hardwired to the PMS process station. The PMS process station performs computations and transmits this data via Ethernet to the DP Controller (DPC). The DP controller calculates the revised thruster speed signals and transmits these via Ethernet to the thruster process stations. Multiple thruster process stations (6 to 8 on drilling rig) convert these signals to analogs. These analog signals are hardwired to the Thruster PLC. The time required for the DP and PMS to calculate a thruster load limit and transmit it to the Thruster PLC is 1 to 2 seconds. (See Figure 1 below.) The time it takes a thruster to reduce power from receipt of this decreased speed signal is 2-3 seconds. Testing has shown the systems response varied from 3 to 6 seconds [21]. Therefore, such system can not prevent blackout in case of engine trip.
Conventional fast load reduction
+ 2 to 3 sec
Figure 5.4. Conventional fast load reduction [21]
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Fast load reduction system with event-based algorithm Event-based load reduction algorithm can be fast enough to prevent blackout. If the thruster control is coordinated with fast load reduction, then ramp loading function, which normally must be used with large electric motors, can be overridden. That is very important and will save maybe 2-3 seconds, see figure 5.4. The function is triggered by generator breaker on the switchboard, which is hardwired to the remote I/O unit located near the switchboard, figure 5.5. So, immediately when first generator trips the remote I/O will transmit the signal to the PMS controller (substation of the total distributed PMS system) via fieldbus and made available for the event based load reduction program. This time is negliable. The fast load reduction function is also initiated by I/O unit located near the generator-set. Both signal transmition lines are shown on figure 5.5. with red lines. The PMS program initiating load reduction on the thrusters are dedicated to this task and its execution time is set to 50 milliseconds, which is five times faster than the rest of the PMS. Consequently, worst case scenario from signal is received to command is sent to converter is 100 milliseconds. The command to the frequency converter of the thruster is send via fieldbus. The converter will reduce the load to zero in less than 100 ms. With VSI this time can be reduced in less than 50 ms. Hence, the load should be reduced in less than 200 milliseconds, and thus well within the required 0.5 seconds. After a configurable number of seconds the “reduce load signal” is reset and thrusters are allowed to use the available power. Extensive lab experiments from ABB have verified that the fast load reduction algorithm is fast enough to avoid blackout in case of generator trip, and it has been installed in recent ABB deliveries.
REMOTE I/O CABINET
ENGINE SAFETY & SWITCHBOARD/ CONTROL SYSTEM INTERFACE GENERATOR PANEL ENGINE CONTROL - HARDWIRED & SAFETY SYSTEM
REMOTE I/O CABINET
COMMUNICATION NETWORK
DIESEL GENERATOR
FREQUENCY CONVERTER
MAIN CONTROLLER
PMS
COMMUNICATION NETWORK
OPERATOR WORKSTATION COMMUNICATION NETWORK
PROPULSOR OR THRUSTER
Figure 5.5. Fast load reduction with event-based algorithm [10] ABB Marine
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Fast load reduction with frequency sensitive - fast phase back system (FPBS) With the frequency sensitive – fast thruster phase back modification, the system frequency is sensed directly by each of the thruster PLC controllers (6 to 8 on drilling rig), see figure 5.6. There is no lag time for computations or transmission of data, as for system on figure 5.4. The fast load reduction is initiated by frequency decreasing signal from the network and not by the switchboard breaker, as for the system represented on figure 5.5. When the system frequency dips in the 58 to 55 hertz range, the FPBS system is initiated. Testing has shown the power reduction in the thruster motor occurs in 300 to 500 milliseconds after the frequency drops below the 58 hertz threshold [21]. Each thruster PLC is programmed to initiate phase back of the thruster over a range of frequencies of 58 to 55 hertz. The maximum 50% thruster current limit occurs only when the frequency is equal to or less than 55 hertz, see figure 5.7. If the initial frequency drops to 56.5 Hz, the thruster current limit would be 50 % · 50 % = 25 % of the initial current.
Figure 5.6. Fast load reduction with frequency sensitive - Fast Phase Back System [21]
Figure 5.7. Initial phase back current limit
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The rate of initial current (power) phase back is set at 200% per second (100% per 0.5 sec) while the frequency is decreasing. After the frequency starts to increase, as detected by the df/dt being positive, the power to the thruster drive is allowed to increase at 5% per second, until it equals 100%, and the spillover turns off, see figure 5.8. If the frequency starts to drop again, as detected by df/dt becoming negative and does not go below 55 Hz, the power in the drive will be reduced at the rate of 5% per second. Should the frequency drop below 55 hertz, a second major frequency spillover will occur. The current limit will be recalculated based on an even lower value of the drive current, see figure 5.7. The thruster drive manufacturer refers to the frequency sensitive – fast thruster phase back as “frequency spillover”.
Figure 5.8. Diesel engine response to step load changes
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5.3. Prime mover utilization by advanced energy management Based on the improved ability of the prime movers to support additional short time load increases with fast load reduction system, a new load dependent start table can be designed to define engine starting limits and improve overall engine utilization.
Auto-start table dependence on operational risk levels As mentioned previously, operational modes reflect the level of operational risk. Similarly, the number of engines online will reflect the blackout risk, after loosing one gen-set. The same level of risk will not be experienced with 5 engines connected on network and with only 2 engines online, see tables 4.1 and 4.2. In addition, the level of risk with low number of engines working is increased, even more, if the available power (power reserve) to initiate the start is lower with less engines operating. Table 5.2. represents different cases and reflects different operational risk levels while analyzing load dependent starting with time delay of 10 seconds. In the first case, shown on table 5.2, the risk is highest since only 30 % power is available when starting third engine. In the second case, the risk is lower since 60 % is available just before the third engine is started. Each engine is loaded up to 70%. Loosing one engine, when only two engines are working, will result in 140% loading of the remaining engine. In the third case philosophy is changed allowing the engines to start with more available power on network (90 %) when working in lower operational mode. In this case loosing one engine when only two engines are working (each with 55 %) will result in 110% loading of the remaining engine. Operational risks levels according to different load dependant start tables are shown on figure 5.9 for the first and third cases in the table 5.2.
Table 5.2. Different operational risks with different load dependant start tables st
1 case Number of generators connected 2 3 4 5
Generator load 85 % 87 % 89 % 91 %
Available power (Power reserve) 2 x 15% = 30 % 3 x 13% = 39 % 4 x 11 % = 44 % 5 x 9 % = 45 %
Time delay to initiate the starting sequence 10 sec 10 sec 10 sec 10 sec
Available power (Power reserve) 2 x 30% = 60 % 3 x 25% = 75 % 4 x 20 % = 80 % 5 x 16 % = 80 %
Time delay to initiate the starting sequence 10 sec 10 sec 10 sec 10 sec
Available power (Power reserve) 2 x 45% = 90 % 3 x 32% = 96 % 4 x 20 % = 80 % 5 x 16 % = 80 %
Time delay to initiate the starting sequence 10 sec 10 sec 10 sec 10 sec
2nd case Number of generators connected 2 3 4 5
Generator load 70 % 75 % 80 % 84 %
3rd case Number of generators connected 2 3 4 5
Generator load 55 % 68 % 80 % 84 %
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Figure 5.9. Auto-start philosophy and operational risks levels Figure represents 1st (30 to 45 %) and 3rd (80 to 90 %) cases in the table 5.2.
Dependence of blackout resistance on number of engines online The network load increase in case of one engine loss is dependant on number of engines online. This can be represented according to following equation: ∆P % =
n ⋅ 100 n −1
where ∆P is the system load increase and n is the number of gen-sets online. The system load increase for 2 to 5 generators connected is calculated in the table 5.3. Table 5.3. System load increase according to number of generators online Number of generators connected
System load increase
2 3 4 5
200 % 150 % 133 % 125 %
∆P %
From the table above, it can be concluded the more generator sets online, the less the effect of a sudden generator set loss. Therefore; the most severe test is to operate two engines at their maximum load as allowed by the systems load dependent start table and trip one generator set off-line.
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Now, according to table 5.2. third (3rd) case, it is interesting to note that two generators can be loaded up to maximum 55 % each, before next generator in sequence is started. But, having the fast load reduction system, there is no need to protect one engine from overloading which results that each generator could be loaded up to 75 %. Loosing one engine when only two engines are working (each with 75 %) will result in 150% loading of the remaining engine. Figure 5.8. shows that such a situation should not be the problem for fast load reduction system. The DP power limiting system has the ability to limit per cent load based on the number of engines online, see section 4.3. Theses setting can be coordinated with 150% overload capabilities or: 2 engines on-line = 75% DP power limit 3 engines on-line = 86% DP power limit 4 engines on-line = 100% DP power limit 5 engines on-line = 100% DP power limit 6 engines on-line = 100% Dp power limit
Described philosophy significantly improves engine utilisation and has following advantages:
•
there is no need for starting the next generator is sequence and to share equal lower load with 3 gen-sets at 50 % for each gen-set – instead working with only two gen-sets online, each 75 % loaded, with the same or lower risk level Î each engine will work closer to 85 % loading where is the lowest specific fuel Consumption Î less overall fuel consumption for the same network loading Î Less lubricating oil consumption for the engines
•
the quantity of accumulated soot will be much lower – since engines do not work on low load Î Lower maintenance costs Î Higher probability of starting for that engine and lower risk of engine fault
•
each engine will have less working hours Î Maintenance costs of the engines will be lower
•
reduced emissions due to lower fuel consumption
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6. References 1. Ådnanes, A.K. (2003), Maritime Electrical Installations and Diesel Electric Propulsion, Tutorial Report/Textbook, ABB Marine AS, Oslo, Norway, 2003 2. Ådnanes, A.K., Maritime Electrical Installations Lecture Slides, Marine Control Systems, Marine Cybernetics, Department of Marine Technology, NTNU, Trondheim, Norway, 2004 3. Ådnanes, A.K., Asgeir J. Sørensen, Thomas Hackman (1997), Essential Characteristics of Electrical Propulsion and Thruster Drives in DP Vessels, DYNAMIC POSITIONING CONFERENCE, 1997
4. John J. May and Halvard Foss (2000), Power Management System for the "Deepwater Horizon" a Dynamically Positioned All Weather Semisubmersible, Dynamic Positioning Conference, 2000 5. Laurilehto, M., Gas as fuel in Ships, 9th International Conference on Marine Engineering Systems, Helsinki University of Technology (HUT), Ship Laboratory and on board MS SILJA SERENADE, 19-21 May 2003 6. Rensvik , E., Sørensen, A.J., Rasmussen, M., Maritime Industrial IT, International Cooperation On Marine Engineering Systems, 9th International Conference on Marine Engineering Systems, Helsinki University of Technology (HUT), Ship Laboratory and on board MS SILJA SERENADE, 2003 7. Stefani, A., Information Technology for Efficient and Safe Ship’s Operation, AM SNAME, 9th International Conference on Marine Engineering Systems, Helsinki University of Technology (HUT), Ship Laboratory and on board MS SILJA SERENADE, 2003 8. Lauvdal, T., A. J. Sørensen, A. K. Ådnanes, J. P. Strand,J. F. Hansen and O. J. Sørdalen (2000). Marintronics: Optimizing marine power and automation systems through Industrial IT. ABBReview, No. 1/2000 9. Boaz, J., Osburn, D., Sims C., Weingarth, L., Drilling Vessel Power Plant Control Systems, Dynamic Positioning Conference, October 17 – 18, 2000 10. Lauvdal, T., Ådnanes, Å. K., Power Management System with Fast Acting Load Reduction for DP Vessels, Dynamic Positioning Conference, Huston, 2000 11. Savoy, S., ENSCO 7500 Power Management System Design, Functionality and Testing, Dynamic Positioning Conference, 2002 12. Radan, D. Power Electronic converters for ship propulsion electro motors, Tech. Report, Department of Marine Technology, NTNU, Trondheim, Norway, 2004 (http://www.ntnu.no/~radan/papers/ship_power_converters_radan.pdf) 13. Woodward Co., Governing Fundamentals and Power Management, 2004 (http://www.woodward.com/power/default.cfm) 14. Russell Hodge, Single Point Failures in Traditional Implementations of Power and Load Management Systems, Dynamic Positioning Conference, 2003 49
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15. Hansen, J.F., Modeling and Control of Marine Power Systems, PhD thesis, Report 2000:9-W, Department of Engineering Cybernetics, Norwegian University of Science and Technology-NTNU, Trondheim, Norway, 2000 16. Miemois, M., Power Generation Stability and Response in DP Applications – An Overview of Modern Diesel Engine Performance, Dynamic Positioning Conference, 2003 17. IACS, Requirements Concerning Machinery Installations, International Association Of Classification Societies, 2004 (http://www.iacs.org.uk/ureqs/URM.PDF) 18. Laurilehto, M., Prime Movers for Power Generation of Dynamically Positioned Vessels, Dynamic Positioning Conference, Marine Technology Society, 1997 19. MAN B&W, Project Guide for Holeby generating sets 20. Ådnanes, A.K., Optimization of Power and Station Keeping Installations by a Total System Design Approach, Dynamic Positioning Conference, DPC, October 1999, Houston 21. John J. May, Improving Engine Utilization on DP Drilling Vessels, DYNAMIC POSITIONING CONFERENCE, 2003
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