A. Qualizza, T. Perini, S. Michetti and M. Ratto - Fincantieri CNI

G. Sulligoi, S. Castellan, and R. Menis - University of Trieste

Integrated Power Systems in Cruise ships and Naval Vessels

ABSTRACT The integration of power systems has been successfully implemented onboard cruise ships built by Fincantieri in the last twenty years. The experience gained is now conveyed also in the application of this principle in the next naval vessels and in the ‘mega yachts’ projects. In this paper two philosophies of integrated power system designed by the company are presented. The next steps for innovative solutions that are actually studied by the shipbuilder further to new regulations requirements, energy saving aspects and utilization of PWM converters are evidenced. In this scenario an important role is played by the cooperation with the University of Trieste, whose research group is developing a software simulator of the integrated power system and propulsion drives. This paper focuses on two particular tasks of this study: modelling/simulation of propulsion drives and stability analysis and control systems involved into the regulation of electro-mechanic quantities of the integrated power system.

Acronyms and definitions MV / LV Medium / Low Voltage SWBD Switchboard IAS Integrated Automation System IPS Integrated Power System PMS Power Management System. Part of the

IAS managing the power plant ECR Engine Control Room ACB Automatic Circuit Breaker (or

contactor) DECS Diesel Engine Control System DOL Direct on line PEC Power electric controller AES All electric ship CODLAG Combined Diesel electric and Gas

turbine propulsion EPM Electric Propulsion Motor GT Gas Turbine

INTRODUCTION In recent years in both cruise ships and naval warships, the adoption of electrical propulsion systems makes possible the design of IPS, which connect electrical generators to propulsion and all electric power users in the ship. The logic of utilizing IPS in conjunction with the availability of high power, low speed electrical motors, provides several advantages both to shipyards and ship owners. Enhanced dynamics in propeller motion, noise and vibration attenuation, flexibility in engine room design, and podded-drive solutions are only some of the benefits obtainable from adoption of IPS. For these reasons, AES architecture is now used on practically all Fincantieri passenger vessels (Cruise and Mega-Yacht) . In the past Fincantieri naval business unit implemented few electrical propulsion systems on special or research vessels. Now, the first special IPS is in designing phase also for the Italian Navy Frigates. The following steps required now from AES are to address systems architecture in terms of fuel savings and to provide the necessary answers for the new concept of “safe return to port”. In the last 20 years Fincantieri Merchant division studied, designed and implemented the IPS architecture in more than 50 passengers ships, using different voltage levels (MV or LV) and frequencies (50Hz and 60 Hz) for ships with propulsion power from 1MW to 22 MW per shaft (or pod). In this paper the most recent IPS configurations are reported: - well known IPS for the next Fincantieri Hull 6151 (130,000 Gt, 304 mt length overall, 1823 pax cabins) - principle design for Italian Naval vessel FREMM Frigate

H. 6151 - POWER PLANT FEATURES The MV SWBD is divided in two sections, port side and starboard side, fitted in two separate rooms, and is normally operated in an interconnected network mode. In case of failure or maintenance, a split network mode operation is possible (Fig.1).

Fig.1 One line diagram of MV side of the network

Port MV SWBD Three identical generators of 14 MVA rated power at p.f. 0.9 (12.6 MW) are connected to the Port MV SWBD, which supplies the following users: - Starboard MV SWBD (via interconnecting line) - Port propulsion transformer (half motor) (14.1 MVA) - Starboard propulsion transformer (half motor) (14.1 MVA) - Port ER transformer (4.2 MVA) - Port ER ventilation transformer (3.6 MVA) - Spare distribution transformer (4.2 MVA) - Three Accommodation substation transformers (two 1.8 MVA, one 1.5 MVA) - Two HVAC compressor DOL motors (1.575 MW) - One stern thruster DOL motor (2.2 MW) - One bow thruster DOL motor (2.2 MW) - One harmonic filter (5.7 MVAR) - Galley transformer (2.6 MVA)

Stbd MV SWBD

Three identical generators of 14 MVA rated power at p.f. 0.9 (12.6 MW) are connected to the Stbd MV SWBD, which supplies the following users: - Port MV SWBD (interconnecting line) - Port propulsion transformer (half motor) (14.1 MVA) - Starboard propulsion transformer (half motor) (14.1 MVA) - Stbd ER transformer (4.2 MVA) - Stbd ER Ventilation transformer (3.6 MVA) Spare distribution transformer (interconnection with port side MV feeder) Four Accommodation substation transformers (1.5 MVA each)

Two HVAC compressor DOL motors (1.575 MW) Two bow thruster DOL motors (2.2 MW) One stern thruster DOL motor (2.2 MW) One harmonic filter (5.7 MVAR)

LV Engine room substation

The LV Engine room substation is divided into five sections: - ER substation port section - ER substation stbd section - ER substation spare section - ER ventilation substation stbd section - ER ventilation substation port section - Each section is supplied by its associated 11kV/690 V - ER distribution transformer. Port and starboard ER substations feed the following main users: - ER systems - Propulsion excitation - Power panels - Laundry, mooring, local entertainment substations - Emergency switchboard - Group starter panels - Shore connection panels - Provision stores The ER substation spare section can feed all of the other ER, accommodation and galley substations, and is designed as a back up in order to assure continuity of supply to the relevant main substation in case of transformer maintenance or failure.

Accommodation substations

One accommodation substation is provided for each of the seven main vertical fire zones. Each accommodation substation is divided in three sections (690 V , 230 V and 120 V), supplying all lighting, small power panels and A/C stations loads in the respective fire zone. Operation of these substations is performed in manual mode only.

Galley substation

The galley substation is divided in three sections (440 V , 230 V and 120 V). Operation of this substation is performed in manual mode only.

Emergency switchboard

The emergency switchboard is divided in two sections (690 V and 230V). The 230 V section is divided into two subsections by means of a bus tie breaker: one AC subsection and one AC/DC subsection. The 230 V AC subsection is supplied from the 690V section through two 690/230 V transformers, one on line and the other in stand-by as a spare. The 230 V AC/DC subsection is normally fed through the bus tie from the AC subsection. In case of power loss, it is fed from the temporary lighting battery.

Power Plant control philosophy

Control of the MV SWBD can be performed in three different ways: - Automatic remote control by the IAS; - Manual remote control from the ECR mosaic; - Manual local control by the MV SWBD control panels. Control of the plant is managed by means of interfaces from the MV SWBD and DECS local panels to the ECR and IAS; local control on the MV SWBD front panel has the highest priority on ECR and IAS. Main generators: Start/stop sequence commands are not available on the MV SWBD, but are possible from each DG local control panel, from ECR mosaics or IAS. Local manual control of each generator’s ACB are possible from the MV SWBD front panel or ECR mosaics, as well as the generator’s synchronisation sequence with the relevant MV SWBD, or DG unloading and disconnection sequences. The number of generators connected to the network can be managed automatically by the PMS, according to the actual power plant loading condition and power demands of the Propulsion plant, Thrusters and Compressors. The PMS manages DG starting requests to the DECS and generator synchronisation sequence requests within the MV SWBD, as well as the DG unloading and generator disconnection sequences. Connection of a DG to a ‘dead’ bus can be accomplished from the ECR MOSAIC, from the IAS or locally on the MV SWBD panels. Safety trips of generator ACBs are actuated by MV SWBD protection relays or requested by DECS. Interconnecting line: IAS directly controls the interconnecting line; automatic synchronisation of MV bus bars is possible . Manual synchronisation of bus

ties is also possible from ECR and MV SWBD front panels.

Harmonic filters: the number of harmonic filters connected to the network is normally managed automatically by IAS according to the number of generators connected and the status of the bus tie. Propulsion transformers: the propulsion control system remotely controls propulsion transformer ACBs when necessary; these ACBs trip on MV SWBD protection relay action or propulsion safety system order. ER transformers, Accommodation and Galley transformer: distribution transformer MV SWBD ACB control is possible from MV SWBD local panels, ECR mosaic or IAS. ER Spare transformer can be fed from port or starboard side, but not from both at the same time; the two MV SWBD ACBs are interlocked accordingly. DOL HVAC compressor motors: the HVAC compressor motor control system remotely controls associated MV SWBD ACBs in response to PMS power requests. DOL Thruster motors: the thruster control system remotely controls associated MV SWBD ACBs in response to PMS power requests; local command is possible to control these breakers in emergency conditions. Preferential tripping: preferential tripping is performed by main generator overload protection, disconnecting non-essential services and / or limiting propulsion power in case only one DG is on line.

Electric Propulsion system - overall view

The ship is equipped with two shafts. In order to ensure high system availability, two electrical propulsion motors of 22 MW each are composed of two separate windings with both independent and autonomous control. Field excitation for each propulsion motor is supplied by two independent excitation bridges (one normal, one standby). The converters are of Synchroconverter type, and are each composed of two network bridges and one machine bridge connected together on the DC loop through a DC link reactor. The network bridges convert the fixed frequency AC power into DC power with variable voltage and current. The network bridges are composed of two six-pulse Graetz thyristor bridges. Each bridge is made up of 6 thyristors supplied by one of two secondary windings of the propulsion transformer. Due to the 30° shift

between both transformer windings, a 12-pulse configuration is achieved, thus reducing the most powerful harmonic currents (Fig.2) [1]. The machine bridge converts DC power into variable frequency, variable voltage AC power to supply the motor winding. The six-pulse machine bridge is made up of 2 x 6 thyristors. Due to the 30° shift between both motor windings, a 12-pulse configuration is achieved, reducing full power torque pulsations. In half drive mode,12-pulse configuration is achieved on the network side and 6-pulse configuration on the motor side.

NB1

MB

NB2

SD7000 F4

NB1

MB

NB2

SD7000 F4

M

Fig.2 Configuration diagram of one motor drive system

The main objectives of the electric propulsion system are: - To ensure positive, safe electrical propulsion control - To ensure that the electromechanical operation system of each propulsion system is completely independent. - To ensure the availability of each propulsion system with two half-motors, with total independence of circuitry and control for these two half-motors - To ensure effective propulsion system fault management, with pre-programmed decisions for safety actions to be taken in event of system anomalies (e.g., automatic torque reduction, alarms, half-motor operation) - To prevent electrical system blackouts using the automatic Propulsion Limitation System The remote control system is provided with an Ethernet I/O network (fiber-optic ring, one per shaft) and Field data I/O; Ethernet networks are also provided for control communication. The remote control stations are independent from each other, and a failure of one will not affect operation of the other. A dual Ethernet network (copper ring) is provided for data exchanges

between propulsion controllers and communication between them, and a supervision monitoring system.

Propulsion control and monitoring system

The control of the propulsion system is fully digital and independent from the other onboard control systems. One of the main inputs for the Propulsion Control and Monitoring System is a man-machine interface made up of several remote control stations positioned at different locations (Bridge, Bridge wings, ECR and Converter rooms). Each drive control is divided into two halves, each one dedicated to the control and monitoring of one half-motor and its supply converter, and a PEC which performs the control algorithm and fast logic, as well as the communication tasks. It controls speed and torque and refines the control set point for the excitation and converter power electronics control systems. A transmission interface board is used to transmit signals from the main processor to excitation and converter power electronics interface boards via ultra-high speed fiber-optic serial links. Communications boards interface with the distributed I/O system and other associated onboard systems. The supervision system ensures the following functions: - Propulsion system monitoring - Alarm control - Log recording - Trend curves recording - Propulsion Drive auxiliaires control - Propeller synchronisation

There are three types of auxiliaries: - Auxiliaries common to the two half motors (propulsion motor bearing jacking pumps) - Auxiliaries for each half motor (transformer cooling pumps, converter cooling pumps) - Propulsion motor fans Each PEC controls the auxiliaries of its own half motor. The common auxiliaries of one shaft line can be started from both PECs and stopped only after both PECs have sent the command. Starting and the stopping of auxiliaries can be effective from ECR remote control panels or Local control panes through the Ethernet network, according to the station in control.

Origin of the speed reference

The "Remote control mode" allows the speed reference to be relayed from: - Wheelhouse (WH): Speed Pilot; Joystick, Lever located on the Bridge and bridge wings - Engine control room (ECR) - Local panels in converter rooms to the propulsion controllers. For independent control of each half motor, the principle is to have a parallel computation in each propulsion controller, but to take into account only one of two speed controllers when both controls are in operation. The active speed controller is the one whose excitation bridge is selected.

Performance level

Up to the rated speed, the propulsion drive can deliver a torque higher than the torque required by the shaft at full power. At the rated speed of 146rpm the rated torque corresponds to a power of 22MW; the torque is limited according to electromechanical limitation and the mechanical limitation given by the shipbuilder. Above rated speed, the torque decreases in order to maintain the rated power constant. In half-motor mode, the drive can deliver 50% of full motor torque. The propeller is a fixed pitch type; each propulsion motor turns in both directions. The minimum controllable speed in both directions is 20 rpm. Three sets of acceleration / deceleration ramps for shaft operation can be selected, according to the number of DGs connected to the network: - The normal ramp selection is used to economize fuel oil consumption - The fast ramp selection is used for high ship performance and response, based on the operation of at least 3 DGs connected to the network. - Crash stop ramp is automatically selected in case of Crash-Stop manoeuvre orders.

The PECs perform the following functions: - Speed reference processing: generation of acceleration and deceleration slopes. - Speed regulation: adjustment between motor frequency measurement and speed reference by changing torque reference. - Torque limitation: selection of the most critical torque limitation from:

- Electromechanical limitations: To protect the converters, power is limited corresponding to the speed of the motor: i.e.; 50% of rated torque below 10% of rated speed; proportional limitation of 50% to 100% of rated torque from 10% to 40% of rated speed. This limits the applied torque at low speeds to not exceed the converters’ design capability, and changing shaft speed to keep propulsion motor power constant (up to 105% of rated speed). In half motor mode, applied torque is maintained below 50% of rated torque. The same electromechanical limitations are applied if the torque is reversed.

- Sequential limitation In case of excessive temperature in the propulsion transformers or motors, torque must be limited to avoid overheating electrical elements. Two thresholds of current limitation are provided (66% and 33% of rated current). When a fault occurs, these limitations are put into operation sequentially. The sum of these current limitations on both half-motors results in a shaft line torque limitation. The values of these limitations do not depend on the motor speed.

- Process limitation Torque limitation due to the shaft line technical design is provided by the building yard: for direct torque: 40% of rated torque from 66% of rated speed astern to 30% of rated speed ahead, proportional limitation from 40% to 100% of rated torque from 30% to 87% of rated speed ahead; for reverse torque: 60% of rated torque from 66% of rated speed astern to maximum speed ahead.

Propulsion Limitation system

Propulsion power is the main consumer on the electrical network onboard the ship. For this reason, to prevent a total loss of power due to an overload or under load (reverse power) on the generators, a propulsion power limitation system is implemented. The limitation function is always active: even when the ship is at quay (propulsion secure) if there is too much power load for one generator, an additional generator will be required on the network.

- Anti overload limitation This function is managed by the PEC and generates a torque limitation. In the main switchboard, a dedicated

I/O rack monitors generator breaker positions, bus tie breaker positions and active and reactive power measurements for each generator. The software is designed to take into account the full operating range generator capability curve; Generator active and reactive maximum output values are calculated according to each generator capability curve. The limitation operates on the most loaded generator: in case of active power overload limitation when the active propulsion power demand is greater than the maximum active power available on the network, propulsion power is automatically limited. The threshold can be adjusted according to operator requirements on the supervision system. Similarly, in the case when the reactive power demand is greater than the maximum reactive power available on the network, the propulsion power is automatically limited. The threshold calculation is performed according to the generator power factor.

- Under load limitation In case of an active power under load limitation during speed reduction of the propulsion motor, for example during a crash-stop sequence, the load of each generator in service is maintained above a pre-determined threshold of minimum active power through the propulsion system.

- Anti black out limitation In order to avoid any overload in case of failure of the I/O rack, there is a built-in Limitation System inside each PEC which controls one half-motor. This limitation is activated as soon as the frequency or network voltage is out of a specified range. This function is fulfilled by the PEC by effecting the necessary speed changes in the converter. - Min/Max frequency limitation The main bus frequency is continually monitored. If it decreases below a certain level, in order to prevent a blackout, automatic torque limitation on the propulsion motor is implemented. If main bus frequency increases over a specified level, for example during a braking operation of the propulsion shaft when the propulsion motor is in generating mode, the generators are under loaded and are going to overspeed. In order to prevent a blackout, the automatic torque limitation on the propulsion motor in generating mode is implemented.

- Network voltage limitation The main bus voltage is also continually monitored. If its value decreases below a specified level, the generators are overloaded. In order to prevent a blackout, the automatic torque limitation on the propulsion motor is implemented.

- Other actions achieved by limitation system For all above mentioned limitations, all bus-tie breaker and alternator breaker status is continually monitored; the limitation action is tuned to comply with the actual network configuration capability. Mimics display propulsion system and network configuration, as well as status tables for critical parts of the propulsion system, and any abnormal conditions anywhere in the propulsion system. - Crash stop manoeuvre When necessary, the propulsion system will be capable of performing this manoeuvre automatically upon receiving a crash stop order; control electronics will actuate required firing sequences of converter bridges for synchronous propulsion regenerative braking down to zero shaft rotation speed, then reversing the direction of shaft rotation in order to stop the ship in the shortest time.

Interfaces with onboard systems

The propulsion control system is connected to the following onboard systems: - Ship’s IAS - MV SWBD (propulsion transformer cells, generators / bus tie cells, harmonic filter cells) - ER SWBD (excitation feeder breakers) - Group starter panels for propulsion auxiliaries - Speed pilot - Joystick - Shaft line bearing monitoring (hardwired or through IAS) - Shaft line turning gear - Steering gear system

Italian Navy Multi Mission Frigates (FREMM) – MAIN AND AUXILIARY PROPULSION SYSTEMS - SHORT DESCRIPTION The Main propulsion architecture is a CODLAG configuration [2], providing mobility and manoeuvrability of the ship. The main propulsion plant is a mixed electrical and mechanical power system with electric motors and mechanical boost power sources, including two main shaft-lines and propellers, two low speed EPMs mounted on the shaft-lines, one cross-connect Gearbox and one GT (General Electric LM 2500+G4) (Fig.3).

Fig.3 FREMM Propulsion system overview Electrical power supplied by the MV power generation and distribution sub-system, is transformed into mechanical power by two EPMs. EPMs are also able to operate as shaft generators with or without any DGs running. The minimum turning speed of the EPM when operating as a generator shall be lower than the minimum shaft rotating speed for propulsion purposes. Boost mechanical power is generated by the GT. When the GT is running, suitable propeller speed is obtained with one cross-connected reduction gear. The utilisation of the two different power sources (EPM and/or GT) is performed by two clutch devices between gearbox and EPM, and one clutch device between GT and gearbox. The two shaft-lines transmit power to the propellers. The torque of each shaft is converted to thrust by a Featherable Controllable Pitch Propeller. The thrust

generated is transmitted to the ship via thrust-blocks in the shaft-lines. Associated auxiliary modules, dedicated to the main propulsion sub-system, ensure effective running of each component, taking into account noise, shock and availability requirements. GT combustion air is sucked from outside the ship with dedicated ducts and silencer. GT combustion gases are collected into exhaust ducts going through a funnel to the outside of the ship an exhaust silencer. The main propulsion sub-system is controlled and monitored by the Ship Management System (SMS). The SMS process stations performs automatic control of the shaft line interacting with the primary control systems provided for each propulsion unit (electric propulsion motors/shaft generators, gas turbine, reduction gear, Controllable Pith Propeller). Depending on requested ship speed, change of ship navigation mode (e.g., maximum speed, silent speed) is performed by a change of propulsion mode. Change of ship speed is performed by changing propeller rotating speed and/ or by pitch adjustment. The nominal operating modes or configurations of the CODLAG propulsion system are: - CODLAG mode (EPMs and GT running) for high speeds - GT mode (GT running) for intermediate and astern speed - EPM mode (two EPMs running) for low, cruise, silent and astern speeds - EPMs operating as generators when driven by the shaft-lines (with mechanical power generated by the GT. Auxiliary propulsion system The auxiliary propulsion system ensures mobility after loss of the main propulsion sub-system and participates in the manoeuvrability of the ship. The auxiliary propulsion sub-system is an azimuthing retractable thruster, which propels the ship in emergency conditions (failure of main propulsion sub-system) by transmitting thrust in any direction for manoeuvring. Power is supplied by an electric motor and transmitted to the propeller via direct-connect shaft. A hydraulic unit allows steering and retracting operations. Main electrical power system Electrical power is generated by four onboard DG or received from shore connections.

The MV power plant has the following main characteristics: - Rated voltage: 6.6kV / 60Hz, - Connection to earth 60Hz: three-phase, neutral earthed through impedance. Each MV SWBD is connected to a MV/LV transformer which feeds the LV SWBD. The two MV SWBD are located in two separate rooms fore and aft. Electric power is also generated from the EPM in GT mode. The system allows the two EPMs to operate in parallel, with or without DGs in parallel. Each MV SWBD distributes the electrical power received from the DGs or from shore to the following main users: Main propulsion Sub-System Harmonic filters MV / LV (450V) transformer Homopolar protection system The aft MV SWBD is connected through an interconnecting bus-tie to the forward MV SWBD.

Fig.4 FREMM Network one line diagram

NEXT STEPS FOR INNOVATIVE SOLUTIONS New steps are needed by IPS to attain innovative solutions in terms of increasing automation and reducing manning. New power distribution systems and propulsion drives are being considered, with particular focus on safe-return-to-port rules of the next application in the cruise sector. Moreover, improved energy management solutions (to save fuel and extend

operating range) and innovative electrical distribution systems (including MV direct current distribution) are currently being evaluated for new ships, including cruise liners, offshore supply vessels and naval vessels. MV shore connection capability is also intended to become a standard of installation for the future, on the new builds as on the refitted ships.

Safe Return to Port

The requirements for safe return to port for passenger ships contained in resolution MSC.216(82) will enter into force on 1st July 2010. Application of these requirements to new designs, which may also evolve from modified existing designs, have been shown to be challenging; Fincantieri began studies and trial applications to assess their impact and help determine the most effective way to implement them. The ship systems capability is addressing : - availability of essential systems after a flooding casualty according to SOLAS regulation II-1/8-1.2, - systems capable of supporting a ship safe return to port under its own propulsion after a fire casualty according to SOLAS regulation II-2/21.4 (including functional requirements for safe areas according to SOLAS II-2/21.5) and - systems to remain operational after a fire casualty (time for orderly evacuation and abandonment) according to SOLAS regulation II-2/22. - the basic layout of the vessel affected by the rule, and a list of all systems to be submitted for assessment, including the IPS system. The current configuration of the IPS architecture satisfactorily answers the rules requirement in terms of system functionality and redundancy. Detail study is now in progress to carefully examine all aspects related to the proper arrangement of engine room equipment.

Fuel saving

A dedicated study has recently been carried out how to improve the efficiency of the IPS. Two items have been closely examined: - The Fuel and Energy Consumption Management Tool that utilizes a range of available information including the fuels held on board, power generated and amount of power consumed to automatically carry out calculate plant status and efficiency.

- The software tool, graphic pages and data storage integrated in the IAS that can be loaded into existing dedicated desktop computers and utilized for the Reporting System. The information used is: • fuels held on board (fuel quantities and characteristics, location of fuels in tanks, fuel temperatures), • power produced and consumed . The goal is to automatically carry out a number of calculations that allow Operators to implement a variety of operating, safety, voyage planning and environmental related features that are extremely useful for efficient ship operation. The core components of the Management Tool are the Level Sensors, and the system operating data already available in the IAS. The Level Sensors must provide accurate repeatable readings; this is fundamental to avoid the need for a flow meter, and consequently reduce system cost. This software tool will not merely provide a status of power generation and consumption but makes a series of calculations that until now ship operators have had to perform manually using graphs and tables. It will give ship operators accurate real-time data regarding power generation and consumption and will allow them to keep a close check on delicate fuel bunkering operations. It also will monitor the quantities of sludge oil, treated bilge water and wastewater produced, and will provide a variety of other useful information important for the efficient manning of the ship.

Propulsion systems employing PWM voltage-source converters

The use of PWM voltage-source converters may be sufficiently mature for propulsion system application [3], [4]. This technology is commonly used for Medium voltage (MV) motors in many fields of application and now seems ready for low speed MV propulsion motors [5]. One of the most widespread PWM converters for medium voltage applications employ the Neutral Point Clamped (NPC) topology, represented in Fig.5 [6].

Fig.5 NPC converter circuit diagram This topology, compared with the common two-level bridge topology, has the advantage to halve the voltage stress of each device and to synthesize a three-level (+Vdc/2, 0, −Vdc/2) output voltage, thus reducing harmonic content and voltage derivatives (harmful for motor windings). PWM converters are able to feed the motor windings with almost sinusoidal current (THD ~ 4 - 5%) while synchroconverters provide a classical Graetz bridge current, causing pulsating torque and inducing higher structure borne noise. PWM converters can be coupled with the shipboard power network by a Diode Front-End (DFE), that is a 12 or 24 pulse diode rectifier, or an Active Front-End (AFE), that is an AC/DC NPC voltage-source converter. The AFE, in spite of its higher cost, offers the following important benefits: - almost unity power factor operation, that is the line current is almost sinusoidal and in phase with the supply voltage. Even if a DFE is employed reactive power balance is more favourable than a synchroconverter because the displacement factor [7] is almost unitary (for a synchconverter it ranges from 0.2 to 0.9 according to the load), but, like for a synchroconverter, current harmonics of order p⋅h±1 (p=number of converter pulses, h=1, 2, …) are absorbed from the power network. Unity power factor of propulsion loads imply that the Power Factor of the IPS is a consequence of the Hotel Load only, and may increase up to 0.85 or 0.9. Eventually, the generators might be optimised with a possible reduction in size and weight. - Filter-Less solution. The advantage is to combine a filter-less solution without over sizing the generators. - possibility of transformer-less connection to the grid, which take an important reduction in size and weight. Only a series inductive filter is needed. It is worth mentioning another possible positive feature of the AFE, that is the possibility, if adequately sized

and endowed with a suited control system, to work as a reactive power compensator. Both synchronous and induction motors are employed in medium voltage electric drives for marine propulsion. Synchronous motors are widely used in high power applications because of their higher efficiency. However induction motors are simpler motors; they have no excitation system (rotating transformer / rotating rectifier / rotating connections / insulated field coils / rotating diode and rotor protecting devices), no excitation converters (4 converters per vessel) and no excitation control system. Consequently, they require less maintenance and inspection work for the whole excitation system and less associated spare parts stock. On the other hand the problems of lower efficiency and higher size are mitigated by the development of improved efficiency and high torque density induction motors [3]. Particular concern should be addressed to the asynchronous EPM, especially for the reduced air gap values to be respected on high power machines during shaft operation onboard in every sea condition.

SIMULATION OF AES The research unit of the power electronics and electric drives laboratories of the University of Trieste is involved into a three-years-long research project funded by RINAVE (Consortium for Naval and Marine Research of Trieste – Italy – Fincantieri Partnered) aimed at developing a software simulator of the IPS and propulsion drives of AES. The employed simulation software is Matlab/Simulink with its tool SimPowerSystems, dedicated to simulation of power systems and electric drives. Research activities are focused on: developing an innovative voltage control system [8], exploring possibilities of better exploiting alternator capability [9], coordinating control systems and protection tuning into a combined design approach, modeling and simulating propulsion drives [10], stability analysis and control systems involved into the regulation of electro-mechanic quantities of the IPS [11]. The last two topics are more strictly related to development of the simulator and are described in the following. The developed simulator has first concerned a propulsion drive of a cruise liner [10], constituted by a double three-phase stator winding synchronous motor supplied by two 12-pulse synchroconverters, like the

one represented in Fig.2. Indeed, the couple of simulated converters has a resulting 24-pulse behavior. It is obtained adding auxiliary windings in series to the primary windings of the transformers, in order to achieve respectively a +7.5° and a –7.5° phase shift in the output voltages of the two transformers. The developed model can simulate the behavior of the drive from its starting, where the converter is operated in a force-commutated way with the so called “pulse mode”, to speeds high enough for the counter e.m.f. of the motor to allow self-commutated operation, up to nominal speed. The simulator is endowed with a user friendly graphic interface, composed by a main screen and three sub-screens for parameter settings of transformers, motor and synchroconverters. The main screen is represented in Fig.6. It gives the possibility to start and stop simulation, to vary the motor speed reference while the simulation is running and to monitor motor and converter quantities such as input currents, dc-link currents and voltages, electromagnetic torque and motor speed.

Fig.6 Main screen of the graphic interface for simulation management As an example of simulation results, in Fig.7 it is shown the input current of the motor during the starting phase. It can be noticed that after about 5s there is the transition between the pulse mode operation and the self-commutated operation. Pulse mode operation can be distinguished from self-commutated operation because the current becomes zero in the middle of each conduction interval.

Fig.7 Input current of the motor at starting phase Fig.8 shows the input current of the motor and the current absorbed from the line during the self-commutated operation. THD of the current absorbed from the line is 12.9%.

a)

b)

Fig.8 a) Motor current, b) line current (24-pulse) To achieve voltage stability analysis, a model of shipboards power station voltage control system has been developed, moving from dynamic models of

alternators, voltage regulators and loads. Reduced order models of alternators have been used in order to simplify the analysis. A typical set of three generators operating in parallel on the same half-busbar has been modelled, together with the related voltage regulators (both AVR and Master AVR). Typically, for symmetry of the plant layout, the power station is made by two of these half busbars, which can be paralleled through a tie-breaker. Therefore, the analysis carried out on a single half-busbar is representative of the full system. To complete the system, the dynamic model of an equivalent RL load has been inserted into the system. Finally, an admittance matrix, representing the dynamic coupling of the alternators with the load has been calculated. The overall model so obtained is reported in Fig.9.

Fig.9 Dynamic model of power station and voltage controls Variables and subsystems shown in Fig.9 are: U(t) busbar voltage reference; V(t) busbar voltage; Ui(t) voltage reference of the ith alternator; Vi(t) machine voltage of the ith alternator; Vfi(t) field voltage of the ith alternator; Bi algebraic block representing droop reactance; ZP(p) block representing the

coupling of alternators and loads into the busbar;

M(s) transfer function of the Master AVR; Geni(s) Vi(s)/Vf(s) transfer function of the ith

alternator; Avri(s) transfer function of the ith automatic

voltage regulator (cascade of PI voltage regulator and rotating exciter compensated models).

Examples of simulation results with third order reduced generator models are shown in Fig.10 and Fig.11: - Fig.10 displays the time responses of the system at no load conditions, at design and stability limit conditions respectively. - Fig.11 displays the time responses of the system feeding an equivalent RL shipboard load, at design and stability limit conditions respectively

a)

b)

Fig.10 Time responses at design (a) and limit of stability (b), 3rd order model generators, no-load

a)

b)

Fig.11 Time responses at design (a) and limit of stability (b), 3rd order model generators, RL load

CONCLUSIONS The innovative technologies available on the market about rotating machine &drives have given Fincantieri the chance to design passenger and naval vessel with the AES concept. The onboard power systems design activity is now called to face the compliance to new regulations and energy saving needs, as well as the evaluation of the implementation of new technologies for large electric propulsion motors and drives. In this terms, the respective research activities dedicated to the finalization of a simulator of the complete onboard power system could be challenging in developing the guidelines of the future AES’s architectures.

REFERENCES

[1] B.Wu, J.Pontt, J.Rodriguez, S.Bernet and S.Kouro, “Current-source-converter and cycloconverter topologies for industrial medium-voltage drives”, IEEE Trans. on Industrial Electronics, Vol.55, No.7, pp.2786-2797, July 2008.

[2] T.Mazzucca and M.Torre, “The FREMM architecture: a first step towards innovation”, Proc. of Int. Symposium on Power Electronics, Electrical Drives, Automation and Motion, pp.574-579, Ischia (Italy), June 2008.

[3] P.Manuelle, G.Flury, E.Leleu, F.Terrien and J.C.Mercier, “A new step in high power electrical propulsion systems with PWM converters and large induction motors”, Proc. of Int. Symposium on Civil or Military All-Electric Ship, Paris, October 2005.

[4] S.Castellan, G.Sulligoi and A.Tessarolo, “Comparative performance analysis of VSI and CSI supply solutions for high power multi-phase synchronous motor drives”, Proc. of Int. Symposium on Power Electronics, Electrical

Drives, Automation and Motion, Ischia (Italy), pp.854-859, 11-13 June 2008.

[5] P.Bordignon and L.Vucetich, “Large medium voltage drives for industrial and marine applications: state of the art”, Proceed. of International Symposium on Power Electronics, Electrical Drives, Automation and Motion, pp.397-402, Capri (Italy), June 2004.

[6] J.Rodriguez, S.Bernet, B.Wu, J.O.Pontt and S.Kouro, “Multilevel voltage-source-converter topologies for industrial medium-voltage drives”, IEEE Trans. on Industrial Electronics, Vol.54, No.6, pp.2930-2945, December 2007.

[7] H.Akagi, E.H.Watanabe and M.Aredes, Instantaneous power theory and applications to power conditioning. Hoboken (New Jersey - USA): Wiley-IEEE Press, 2007, Chap. 2.

[8] A.Arcidiacono, R.Menis and G.Sulligoi, “Improving power quality in all electric ships using a voltage and VAR integrated regulator”, Proc. of IEEE Electric Ship Technologies Symposium, pp.322-327, Arlington (VA), May 2007.

[9] A.Da Rin and G.Sulligoi, “A cost-effective approach to reactive power management in all electric cruise liners”, Proc. of Int. Symposium on Civil or military All-Electric ship, London, September 2007.

[10] S.Castellan, R.Menis, M.Pigani, G.Sulligoi and A.Tessarolo, “Modeling and simulation of electric propulsion systems for all-electric cruise liners”, Proc. of IEEE Electric Ship Technologies Symposium, pp.60-64, Arlington (VA), May 2007.

[11] G.Giadrossi, R.Menis, G.Sulligoi and A.Tessarolo, “Voltage stability analysis of all-electric cruise liners”, Proc. of Int. Symposium on Power Electronics, Electrical Drives, Automation and Motion, pp.586-590, Ischia (Italy), June 2008.

Authors Ing. Andrea Qualizza in 1994 took the degree in Electronical Engineering with the specialization in Automation and Control. He worked 2 years in the steel industry as automation engineer. In 1997 he joined Fincantieri and has been electrical manager for the building of cruise vessels for HAL and Carnival brands. Since 2002 he has been in charge of Fincantieri’s Electrical and Automation design department for the Merchant shipbuilding business unit. He is also the secretary of TC 18 Italian electro technical comity regarding the electrical installation of ships and of mobile and fixed offshore units. Under IEC, he is also a member of Maintenance Team in charge of automation design (IEC 60092-503, 60092-501). Ing. Tommaso Perini received the Dr. degree in Electrical Engineering at the University of Trieste in 2004. He joined Fincantieri in 2005 and is actually

responsible of MV Power systems and Electric propulsion system design and integration in its Electric Systems and Automation Technical Department of Merchant Ship Business Unit, located in Trieste (Italy). Ing. Marco Ratto received the Dr. degree in Electrical Engineering at the University of Genoa in 1996. He worked about one years in the electrical industry as electric transformers designer. He joined Fincantieri at the end of 1997 and he has been in charge of Fincantieri’s Electrical and Automation design department for the Naval Vessel shipbuiding business unit, located in Genoa (Italy) . He is actually responsible for the automation design activities in the above said department. Ing. Stefano Michetti received the degree in Electronic Engineering with the specialization in Microelectronics. He worked in the electrical industry as process and automation engineer and then as responsible of electrical design department. In 1998 he joined Fincantieri as responsible for Electrical and Automation Design Department in Merchant Ship Business Unit. Since 2003 he has been responsible for Automation Design Department in Naval Vessel Business Unit and since 2006 he was in charge also as responsible for the Electrical Design Department in the same business unit. He is a member of the TC 18 Italian electro technical comity, involved in the electrical installation of ships and of mobile and fixed offshore units. Under IEC, he is also a member of Maintenance Team in charge of automation design. Dr. Giorgio Sulligoi is Assistant Professor of Electric Generators Modeling at the Dept. of Electrical, Electronic and Computer Engineering of the University of Trieste (Italy). He obtained his master degree, with honors, at the University of Trieste, and his Ph.D. at the University of Padua (Italy), both in electrical engineering. He is active in the fields of shipboard power systems, all electric ships, alternators modeling and voltage control. Dr. Simone Castellan received the Dr. degree in Electrical Engineering and the Ph.D. degree in Electrotechnics from the University of Padova, Padova, Italy. In 2000 he joined the Department of Electrical, Electronic and Computer Engineering of the University of Trieste, (Trieste, Italy) as a researcher in the scientific discipline group “Power converters, machines and drives”. He is currently assistant professor of Power Electronics at the University of Trieste. His main research interests are in the field of power converters for: harmonic and flicker compensation, medium-voltage drives, fault tolerant drives, renewable energy sources, all-electric ships. Prof. Roberto Menis received the Laurea degree in electronic engineering from the University of Trieste, Italy, in 1982. From 1982 to 1984, he was a member of

the technical staff of an aeronautic industrial company. In 1984, he joined the Department of Electrical, Electronic, and Computer Engineering, University of Trieste, where he is currently an Associate Professor of Electric Drives and the Head of the Electric Drives and Power Electronics Laboratories. His research interests are in the field of electric machines and drives, which include modeling and identification of ac machines, control of synchronous generators for diesel-alternator groups, control of ac and direct current motors, and industry applications of the drives.