A Survey of Concepts for Electric Propulsion in Conventional & Ice Breaking OSVs

7/28/2019 A Survey of Concepts for Electric Propulsion in Conventional & Ice Breaking OSVs

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30th Propulsion & Emissions Conference 2008

Radisson SAS Hotel, Sweden, Gothenburg

on 21-22 May 2008.

A Survey of Concepts for Electric Propulsion in

Conventional and Ice Breaking OSVs

Alf Kre dnanes, Dr. Ing., MScEE

ABB AS, Business Unit Marine,

P.O. Box 94, NO-1375 Billingstad, Norway

ABSTRACT

The use of electric propulsion in both open water and ice breaking OSVs has become a marine industrystandard for a wide range of applications, and is increasing to new applications and into more geographicalregions. The technologies develop continuously, and today there are several approaches to reach the "optimal

design" that reduces fuel consumption and environmental footprint, simplifies design and construction withbetter utilization of the on-board space, and creates a better working environment for the crew.This paper presents the various concepts at the market, and summarizes their technical characteristics and

limitations. It is aimed to give yards, designers, and ship owner necessary technical information in order to makea proper selection of system topology within the specifics of a vessel design.

Important aspects regarding operational safety and availability of the propulsion and station keeping plant isalso discussed.

INTRODUCTION

Since mid 1990s, OSVs have been equipped with electric propulsion, Fig. 1, where the main propulsorsand station keeping thrusters have been driven by variable speed electric motor drives, being supplied from thecommon ship electric power plant with constant frequency and voltage. Thrusters and propulsors are normally

of fixed pitch propeller design (FPP) that reduces the mechanical complexity of the units, and the electric poweris normally supplied from fixed speed combustion engines; diesel, gas, or dual fuel.

During this period, there has been a continuous development of solutions for the electric power plant forvessels with electric propulsion. The development can be considered as an incremental evolution of concepts,where the building blocks of the electric plant is adapted from the general industry applications, which has a farbigger volume of installations than the marine applications and to a large extent gives the premises for basictechnology developments.

Even though the suppliers of electric power and propulsion plants utilize building blocks that are based onprincipally the same fundamental concepts, there is a range of different configurations and preferences in themarket. As the technical arguments for the concept appears to be biased and naturally to some extent influencedby a driving force to pursue a sales, it is necessary for ship owners, yards, and designers to be able to evaluateand compare this information to make the decision.

As the author represents one of the vendors, this paper should not be considered to be an objective andneutral assessment of competing concepts; however, it is the aim of the author to give a unified description ofthe most applied concepts. Further, it is the authors objective that decisions shall be made on understanding ofthe characteristics and resulting effects of the technology, rather than the technology itself.

The author has based his information on own and public available information, and hence, actual solutionsmay deviate from what has been assumed in this paper. It is therefore necessary to supplement the informationherein with the detailed information in each project and from each supplier.


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M

BowThruster

M

BowThruster

M

BowThruster

M

BowThruster

Port Sidepropulsion

and shaft gen.

Stbd Sidepropulsion

and shaft gen.

Aux gen.Emgcy

gen.

440V, 60Hz

99kVA

230VDistribution

99kVA

230V

M MM MM

MainPropulsor

MainPropulsor

Thruster ThrusterAzimuthThruster

Fig. 1: Conventional direct mechanical propulsion, and electric propulsion concept for OSV.

THE IMPORTANCE OF TECHNOLOGY DEVELOPMENT

For many engineers, researchers, and developers, working with technology gives the daily bread and butter.Technology development is both challenging and interesting for those involved, as well as essential for the

continuous improvement of the vessels earnings and safety.In evaluation of concepts, the overall performance and characteristics should be assessed rather than the

individual component. The advantages of e.g. an efficiency improvement in one component may make no senseif it requires a non-optimal operation of the prime movers in order to function as intended. Utilizing fastchanging technologies to obtain some improvement is normal in consumer industry, but in a life cycleassessment of a vessel, it may be painful if that particular technology is obsolete and unavailable after few years.

The driving forces for technology development and assessment should be based on the effects and results ofthe technologies. Generally; the following criterion will be important for the comparison of products, systems,and services, although their weighting and importance may vary over time and between various applications:

Cost efficient building and installation Flexibility in design that improves ship utilization

High safety for crew High safety for operations Continuous availability to propulsion and station keeping systems Reduced fuel consumption Reduced impact on the external environment, lower emissions Improved working environment for the crew Low maintenance costs Availability to maintenance during the life cycle of the ship Availability to maintenance in the region of operation, often world-wide Spare parts availability Remote and on-board support

Minimizing constraints of operations leading to non-optimal performance Reduce negative consequences for other equipment High ice braking and ice management performance for ice breakers

Technology can never become the goal itself, but those who are leading in technology development andcapable to utilize this competence in a sustainable and commercially viable context should be better fit to meettodays and futures solutions that meet the users changing demands. Understanding the needs and requirementsof the users requires a continuous cooperation in the global maritime network, and continuous improvements.The optimal solution today does not necessarily meet the new requirements in the future.


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VARIABLE SPEED DRIVES FOR ELECTRIC PROPULSION

The variable speed drive (VSD) for propulsors and thrusters is one of the most essential components in apower plant for electric propulsion.

The VSD consists of: Electric motor, normally asynchronous (induction) motors, but also synchronous motors for the

high power range. Other types of motors used in special applications; such as permanent magnet

motors and DC motors. Frequency converter, converting the fixed voltage and frequency of the network to a variable

voltage and frequency needed to adjust the speed of the electric motor. Optionally line filters or transformers, depending on configuration for reducing the harmonic

distortion of currents flowing into the network and voltage adjustment where applicable. A control system, consisting typically of a motor controller and an application controller for the

propulsion / thruster control, taking care of the control functions as well as monitoring andprotection of the VSD.

For the power level needed for OSV propulsion, the Voltage Source Inverter (VSI), Fig. 2, is the dominatingtopology of frequency converters and used by most suppliers to this market. DC drives, Current SourceInverters (CSI) and Cycloconverters are rarely used and being phased out from new buildings of OSVs.Therefore, this paper only considers the VSI in various configurations.

The voltage source inverter consists of a rectifier, a DC link with voltage smoothing capacitors, and aninverter unit as the main components. The DC link may where required be equipped with a breaking chopper todissipate wind-milling power from the propeller in rapid speed variations or in crash stop conditions of thevessel.

The motor controller technology has developed from the scalar U/f control system used since the earliestvariable speed AC drives. Field oriented control of AC motors was developed already in the 60s /1/ but did notestablish as an industrial standard before in the mid 80s when digital controllers with sufficient capacity andspeed became commercially accepted. The introduction of field oriented control significantly improvedperformance and efficiency of the VSD, however, the control method was still sensitive to the motor parametersand time variations. A new method of vector control that was based on stator flux oriented control and directtorque control of the motor was developed around 1990 /2/, and was first introduced in large scale commercialdrives production in the early 90s by ABB under the name of DTC. DTC enables ultra fast control of the

torque of the motors, with a robust algorithm that gives high controllability and efficiency with much lesssensitivity to variations in motor parameters than the traditional field oriented control. As will be discussedlater; the dynamic performance of propulsors and thrusters for OSV is much lower than what the modern VSDmay perform, with exception of the need for fast black-out prevention where each fraction of a second isessential.

Rectifier Inverter DC Link MOTOR

Motor Controller

Propulsion Controller

Measurement andcontrol signals

Supply voltage:Fixed AC voltageFixed frequency

Constant

DC voltage

Variable ACvoltage andfrequency

External interfaces

- remote control- automation systems- propulsor and auxiliaries

Fig. 2: The basic modules for a Voltage Source Inverter (VSI).


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The key component in the frequency converter is the power semiconductor. Because of the high power, thesemiconductors will only operate in the active mode in the transition between being fully off (closed) state andfully on (open) state in order to minimize the power losses.

In VSI frequency converters, the components used are either passive components (diodes) or activeswitching components. Thyristors are only applied in special cases e.g. in configurations where it is a need tocontrol the DC link voltage or for soft charging of the DC link, and in some cases for regeneration of power.

Power semiconductors are made for low voltage applications, i.e. system voltages up to 690V between thephases, and for high voltage applications, >1000V system voltage. Depending on configuration, typical voltagelevels are 3-3.3kV and 6-6.6kV line-line voltages for high voltage frequency converters.

The IGBT (Insulated Gate Bipolar Transistor) is the dominating power semiconductor for low voltageapplications and being used by all major suppliers of frequency converters. The low voltage IGBT is normallymounted in compound modules, as shown in Fig. 3(b). Each module may consist of several IGBT switchingcomponents, in parallel or separately controlled, together with free-wheeling diodes for reverse currents throughthe switching elements.

For high voltage frequency converters, the GTO (Gate Turn Off) thyristor was for long times used asswitching component. In high voltage converters these are used as discrete switching elements with one siliconwafer being installed in press pack (hockey-puck) housing. The GTO is of a robust design, and a highly reliablecomponent itself, although it required a number of auxiliary components to achieve the robustness in operation,and these auxiliary components got quite high stresses from the switching of the GTO. Hence, a furtherdevelopment of the GTO was made by ABB during the 90s where the basic concept of the GTO was improvedto reduce the need for auxiliary components, and hence not only increase the overall reliability but also reducethe power losses in the overall system. This new component is called GCT (Gate Controlled Thyristor), andwhen integrated with its gate control system, IGCT (Integrated GCT). All ABB IGCTs are press-pack devices.They are pressed with a relatively high force onto heat-sinks which also serve as electrical contacts to the powerterminals. The IGCT's turn-on/off control unit is an integral element of the component. It only requires anexternal power supply and its control functions are conveniently accessed through optical fiber connections. TheIGCT is optimized for low conduction losses. Its typical turn-on/off switching frequency is in the range of 500hertz. However, in contrast to the GTO, the upper switching frequency is only limited by operating thermallosses and the system's ability to remove this heat.

As the IGBT requires a simpler gate control device, with less power consumption, a large effort has beenmade to make them available for high voltage. The first IGBTs were of module design, and were not consideredreliable enough for demanding propulsion applications. Since IGBTs are made with multiple parallel chips,there is a difficulty - with conventional press-packs - in assuring uniform pressure on all chips; a difficultywhich increases with the number of devices in a stack. Today, IGBT in press pack are available from severalmakers, including ABB /4/, and mainly used in power transmission and static converters, although some makershave eveluated them stable enough to be used in variable speed drives for ship applications.

IGBTLow voltage

GCT/IGCTHigh voltage

IGBTHigh voltage

(a) (b) (c) (d)

Fig. 3: (a) The development of power semiconductor switches /3/, (b) low voltage IGBT module, withintegrated IGBT switches and freewheeling diodes, (c) press pack high voltage IGBT, and (d) presspack GCT/IGCT.


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Fig. 5 shows the most common configurations for low voltage VSI frequency converters. The rectifier maybe of different types, depending on the requirements for each installations and makers preference; 6-pulse diode rectifier is the simplest design, with a full bridge passive rectifier or several in parallel if

necessary to achieve the desired power level. The AC supply voltage is rectified to form a DC voltage, ofapproximately 1.35 times supply line voltage at full load; i.e. a 690V supply gives approximately 930V DClink voltage at full load; depending on voltage drop and commutation impedance in the supply. The 6-pulse

rectifier does not need any supply transformer unless necessary to adapt the voltage. Hence, size andweight is minimized. However, the harmonic distortion from the line currents is high in the order of 25-25% THDi, resulting in a voltage distortion THDu of more than 10%. In order to achieve the limit asspecified in IEC and which most classification society now has adapted of 5%, harmonic filtering or cleanpower supply is necessary. A harmonic filter at the distribution switchboard will reduce the distortion atthis voltage level and below, but will hardly have any impact on the distortion at the main switchboard. Themain switchboard must hence be designed to tolerate a high level of voltage and current distortion, and bedocumented accordingly as specified in class rules.

12- and quasi 24-pulse configurations looks similar with two paralleled diode rectifiers; but the quasi-24pulse (Q24) VSD transformers are made in pairs of two and two with 15 deg phase shift between the two ineach pair. Depending on load conditions in the prospected operation profile and system parameters, 12-pulse configuration may meet the requirements to voltage distortion by class. For most cases, however, theworst case conditions will lead to a THD

uin the range of 6-8%, which is above the limit of most of

classification societies without using harmonic filters. The Q24-pulse rectifier utilizes the same rectifiertopology as the 12-pulse rectifier, but since the supply voltages are phase shifted through the supplytransformers, the resulting distortion at the main switchboard is reduced. At ideal conditions, where theloads of the VSDs in each pair are equal, the harmonic distortion will be equivalent to a 24-pulseconfiguration. Under other conditions, a partial cancellation of the largest harmonic components will occur,and the harmonic distortion will in most installations be under the 5% THDu limit in any practical operationmode. Certain constraints in operation may be necessary in order to guarantee this; however, the Q24-pulseconfiguration is regarded to be a cost efficient way to meet class requirements for most OSVs.

24-pulse configurations consists of four paralleled 6-pulse rectifiers, each supplied from phase shiftedvoltages through one 5-winding transformer, or two paralleled 3-winding transformers. This configurationwill normally always give distortion under the 5% limit, without constraints in operation. Normally, thetransformer will be larger and more expensive than a 3-winding transformer of equivalent rating, anddepending on the rating of the diode rectifiers, also the size and price of this may increase. The 18-pulseconfiguration utilizes the same concept, but with only three paralleled 6-pulse rectifiers and a 4-windingtransformer. The THDu will also here normally be within the 5% limit, however, comparing to the 24-pulsetopology, the total prize and size is at the same order due to a complex transformer design. 18-pulserectifiers are therefore rarely in use today.

Active rectifiers with switching elements have for some time been applied in demanding industrialapplications, especially where the load characteristics requires regenerative braking to such an extent that itmakes it beneficial to use this energy by feeding it back to the network. For propulsors and thrusters, theregenerative energy is normally negligible in a fuel and cost of energy assessment. However, since therectifier consists of switching devices, the current can be shaped similarly to the motor current and withmuch lower distortion than the currents of a diode rectifier. Even though the classical harmonic filters thencan be avoided without use of drive transformers, one should note that there are ample of harmonicvoltages from the switching, with high frequencies and with a high level of electromagnetic noise that mustbe filtered with high frequency (HF) filters. There is limited experience by use of active rectifiers in weakelectric power systems as found on vessels, and in complex systems with many drives and a range ofoperating conditions, it is challenging to perform a complete system analysis of any modes andconfigurations in order to detect and avoid possible resonance effects from the numerous combination ofparalleled HF filters. As the switching elements are more costly than diode rectifiers, the cost of thefrequency converter increases, and its losses will also be higher; reducing the benefit of avoiding losses inthe drive transformer.


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The DC link, Fig. 5, consists of a DC capacitor, in order to smooth the DC link voltage to reduce the voltageripple from the rectifier to an acceptable level for the output stage of the frequency converter, and to filter thehigh frequency distortion from the switching elements in the inverter and avoid that these are injected to thesupply network.

If regenerative power is expected from the load, e.g. in crash stop conditions of shaft propellers, the powerwill be absorbed by the DC link causing voltage rise of the capacitors unless the power can be fed into thesupply network by an active or regenerative rectifier, or dissipated in a load resistor bank. For thrusters andAzimuthing propulsors, regenerative voltage is seldom of a concern, as one may restrict these loads to onlyoperate in motoring mode. In shaft line propulsion, or in Azimuthing propulsors where crash stop or equivalentto crash stop maneuvering is made by reversal of the propeller RPM, there are certain conditions where thepropeller wind-milling effect may create reverse power; see Fig 4.

The inverter module is normally a full bridge IGBT inverter in low voltage VSIs, Fig 5, with 6 IGBTswitching elements each with an anti-parallel freewheeling diode for reversing the currents through theswitches. The switching elements in each of the three legs of the inverter must operate in inverse mode, whereone IGBT always is controlled off to avoid short circuiting the DC link. The objective of controlling the IGBTsis to feed a voltage vector to the stator winding that forces the currents and flux in the machine towards thetargeted amplitude and phase angle. Depending on the control scheme, there are different ways to achieve this,such as field oriented control with PWM or hysteresis control, and direct torque control. The characteristics ofthese methods will be discussed later.

High voltage frequency converters, e.g. for 3-3.3kV system voltage, utilizes the same principles as the lowvoltage VSI frequency converters. Because of the higher system voltage, and maximum voltage availability ofpower semiconductors, the high voltage VSI frequency converter normally consists of series connectedsemiconductors, as shown in Fig. 6 for the rectifier and inverter. In order to reduce the voltage over eachcomponent, the converters will normally be equipped with a third voltage level, at the middle of the negativeand positive DC bus voltage, the so called neutral point. Clamping diodes ensures that each of the switchingelements never will be exposed to higher than half of the DC link voltage, and therefore this configuration iscalled three level, neutral point clamped (NPC) inverter topology. By increasing the number of seriesconnected switching elements, the number of switching combinations also increases and with a specificmaximum switching frequency of the components, the harmonic distortion in the load currents andconsequentially the torque ripple will be reduced. Some suppliers use this concept to design multilevel invertertopologies, where the ripple current can be further reduced and lower voltage switching elements may be used.The penalty is then the higher number of components in the system, reducing its availability and reliability.

RR

Torque

Speed

Quadrant II

BrakingSpeed0

Quadrant I

MotoringSpeed>0, Torque >0

Quadrant III

Motoring

Speed0

Quadrant III

Motoring

Speed


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Rectifier DC Link Inverter

6-pulse, passive diode rectifier


30

electricaldegrees

phase

shifted

voltages


20

electricaldegrees

phase

shifted

voltages

DC link withsmoothingcapacitor


15

electricaldegrees

phase

shifted

voltages

Active rectifier with IGBTs

DC link withbraking chopper

and resistor

R

Full bridge IGBT inverter

Fig. 5: Description of basic modules for typical low voltage, two level VSI frequency converters,


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315

2000

P (kW)

U (kV)

27000

5000

6000

40000

30000

Motor Voltage

1.0 2.41.5 4.5 6.03.3

MotorPower

9000

10000

16000

6.91.8

Cyclo-converters

CSI

690

VSI

IGBT

VSI: Voltage Sour ce Inverters

with PWM or DTC

CSI: Current Source Inverters

with Thyristors

Cyclo: Direct Converter

with Thyristors

VSI

IGCT

LV MV

Fig. 7: ABB drive technology map (indicative check with ABB for details).

Fig. 7 shows the drive technology map for ABB. The map is indicative, and for illustration only. For marineapplications, the VSI concept is today the preferred solution within the constraints of its ratings andapplications.

Single inverter, low voltage VSI frequency converters are available up to approximately 4.5-5MW as apractical limit. For higher power levels, two low voltage motor drives can be used in twin or tandemconfigurations, or the frequency converters should be of high voltage topology.

For other industrial applications, or for special customer demands, Cycloconverter and CSI (Current SourceInverter) topologies may be used. However, for the OSV applications, these concepts are very rarely used todaysince the VSI frequency converters covers any application needs, and offer more technical advantages than the

alternative VSD concepts.

MOTOR CONTROL METHODS

Any motor control method is based on the principle of controlling the motor torque based on a model of themotor. For the OSV applications, normally induction (asynchronous) motors are applied. Induction motors aresimple of construction and rugged in design, and are normally used for power levels up to 5-10MW dependingon motor RPM. This limit is based on the constraints of motor design and construction, where the air gap of themotor becomes so large that the amount of magnetically stored energy, which is represented by the reactivepower of the motor, becomes excessive and impractical for cost efficient use. For higher power levels, normallysynchronous motors with wound field windings and brushless excitation are used. For special applications,permanent magnet AC motors, and even DC motors are applied.

The power range of most of the OSV propulsors and thrusters is covered by the induction motor. This motor

can simplified, and stationary be modeled by the equivalent diagram shown in Fig. 8. This shows a one phasemodel of the induction motor, with the stator resistance and leakage inductance, rotor resistance and leakageinductance, and the magnetizing inductance for air gap flux. The magnetizing resistor represents themagnetizing losses in the stator and rotor magnetic steel.

From this model, it can be derived that the voltages and frequency should vary as shown in Fig. 8b whencontrolling the speed of the motor. This relation has been known since the origin of the variable speed control,and was utilized in scalar control, Fig. 9, of the induction motor before more modern concepts was developed.


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IsIs

Vs

Rs Ls Lr

LmRm

Rr/ s* *

Re

Im

Re

Im

ImIm

Im

IrIs

Flux () producing current(Magnetization current, Im)

Torque producing current(Rotor current)

2

222

+=+=

N

rmrmsT

TIIIII

Ir

Constant torqueregion

Field weakeningregion

RPM

Magnetic flux

Maximum torque boundary

Maximum stator current boundary

Stator voltage

Stator frequency

Pitching moment

limitation

Pitching moment

limitation

(a) (b)

Fig. 8: (a) Simplified equivalent diagram and vector diagram for induction motor, and (b) the correspondingvoltage, frequency, flux, and torque characteristics in variable speed drive applications.

U/f ratioRamp - limit

PIDcontroller

Ramp - limit

U

f PWM

5 10 15 25 10 15 2

RPMSetpoint

Inverter

RPMSetpoint

M

T

DCLink

+-

Fig. 9: Principle sketch of a scalar controlled VSD with induction motor.

The scalar control of the induction motor is based on the stationary motor model, and since the dynamics of

the motor is neglected, significant information is disregarded, giving a poor dynamic control of the motor.During the 60s, significant improvements in the motor modeling and control of induction motor was achieved,/X/, and this enabled control methods that were comparable to DC motor control. The principle was to decouplestator current into a magnetizing component, and a rotor current component and control each of these quantitiesin a synchronous rotating frame, oriented along the vector of the rotor flux. The new control algorithms werecalled field oriented control, or rotor flux oriented control, Fig. 10. As these control methods required digitalcontrollers for optimal performance, they did not become commercially available until the mid 80s. Still, thefield oriented control suffered from two major weaknesses: The motor model is very depending on the rotor time constant, which is time variant and since it cannot be

directly measured, it must be continuously estimated in order to obtain satisfactory performance andefficiency of the converter.

The field oriented controller requires a closed loop control of the two current components, and the speed and

dynamics of the outer loop control is limited by the bandwidth of the two current control loops.During the 80s, another approach to modeling and control was introduced, /2/. Instead of referring themodel to the rotor flux vector, the motor variables were now referred to stator flux. Also, instead of controllingthe two decoupled current components, advanced algorithms for directly controlling the switching pattern of theinverter of the VSI were developed. This resulted in a very robust algorithm, not sensitive to the highly variablerotor time constant, and with ultra fast dynamic response, only limited by the current rise time of the statorwindings. The Direct Torque Control (DTC), Fig. 11, was the first large scale industrial use of this controlmethod, and has been applied by ABB since the beginning of the 90s , made possible only by using dedicated,high speed ASICS in combination with fast signal processors.


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PWM

f0 5 10 150 5 10 15PID

controllerRamp - limit

RPMSetpoint

Inverter

RPMSetpoint

M

T

DCLink

+-

Flux

controller

PIDcontroller

PID

controller

+

-+

Motormodel

2

3ej

2

3ej

2

3

e-j

ParametersParameteradaption

Fig. 10: Field oriented, or rotor flux oriented control of induction motors.

PIDcontroller

Ramp - limit

RPMSetpoint

Inverter

RPMSetpoint

M

DCLink

+

Fluxcontroller

Direct torqueand flux

hysteresiscontrol

Motormodel

Optimalswitching

logics

Torqueref

Fluxref

Torqueact

Fluxact

RPM act

Fig. 11: Direct Torque Control (DTC) of induction motors minimizes at each sampling the deviation betweendesired and actual flux and torque in the motor. Here shown induction motor, it is also applied insynchronous and permanent magnet motors.

For VSD in propulsion and thruster applications, the dynamic performance of the modern frequencyconverters is much faster than needed for normal speed control. Typically, the speed regulation must be sloweddown by use of speed ramps in order to not overload the mechanical systems in the thruster and propulsor powertransmission, and to avoid excessive load variations in the prime movers. However, there are certain concernsthat needs fast acting and precise control algorithms to be solved;

Black-out prevention: In order to avoid black-out by power phase back of the VSD, the load power of thepropulsors and thrusters should be reduced within 300-500 ms, Fig. 12, after a fault in the power generationsystem. This can only be achieved with the fastest dynamic performance in order to avoid load shedding.

Out of water effects: When thrusters and propulsors are running at high load in rough weather conditions, airsuction and out of water effects lead to sudden loss of load torque with a high risk for tripping by over speedif the dynamics of the controller is not fast enough to avoid runaway.

Power efficiency: The minimization of power losses in the motor is based on a precise control of themagnetization and torque producing currents. Errors in the model may give unnecessary high losses in themotor.


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OPERATORWORKSTATION

MAINCONTROLLER

REMOTE I/O

CABINET

REMOTE I/O CABINET

SWITCHBOARD/GENERATOR PANEL

ENGINE SAFETY &

CONTROL SYSTEM INTERFACE-HARDWIREDENGINE CONTROL

& SAFETY SYSTEM

DIESELGENERATOR

COMMUNICATIONNETWORK


PROPULSOR

ORTHRUSTER

FREQUENCYCONVERTER


VSI Drives FPP load redVSI Drives FPP load red

PMS load reductionPMS load reduction

DP power limitationDP power limitation

Fixed speed CPP load redFixed speed CPP load red

100% 200% 300%

10 sec

1 sec

0.1 sec

0.01 sec

Load

x MCR

Risk for

Blackout

Fig. 12: Black-out prevention algorithms requires very fast dynamic response of the VSD controller for thrustersand propulsors. 300-500 ms is normally regarded as maximum response time for the complete loadreduction loop including detection, transmission, and load control times.

PROPULSION CONTROL METHODS

The essence of any motor control is to achieve a good torque control of the motor drive. When torquecontrol is achieved, the motor controller can be regarded as a torque amplifier with a time lag, and the time lagis depending on the applied motor control algorithms, and the motors own electrical dynamics.

Then, it is easy to adapt the motor drive to various types of outer control algorithms, and the most normalfor variable speed control, is to create a closed loop speed (RPM) control of the propulsor or thruster drives,where the set-point from the remote control system is regarded as an RPM set-point, as shown in Figs. 9-11.

However, one could also use other approaches to control the propeller, by interpreting the remote controlsignal as directly being the torque reference to the torque controller of the VSD, amplified by the motor controlalgorithm, and even to use the set-point as a power command to the VSD, controlling the torque to keep theshaft power equal to the remote set-point.

RPM control, torque control, and power control have all been applied in various applications. Theirdifferences can be observed by the characteristic behaviors in Fig. 13. Since a closed loop RPM control uses the

PID controller to minimize the deviation between reference speed and actual speed, load variations at thepropeller will instantly result in torque and hence power variations at the shaft. These power variations willimmediately be reflected on the power taken from the power plant, and result in frequency variations andpossible dynamic overload of the prime mover if the load variations are high. This is primarily a problem ofconcern for ice breaking OSVs, where load variations are sudden and may be very high, but also for shipsoperating in high waves the load may vary quite significantly.

If using the remote set-point as a torque reference, the load torque variations will not be reflected in thetorque produced by the VSD, which is controlled to follow the set-point only. As seen in Fig. 13, this may givelarge variations in the propeller RPM and will therefore also give power variations at the shaft and hence in thepower plant, since the load power is proportional to load torque and RPM.

Propeller load torque

Max torque limit

Torque Control

RPM Control

Power Control

To

rque

Maxpower limit

RPM 0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10

Speed (RPM) Control Torque Control Power Control

rpm

load torque

el torque

power

(a) (b)

Fig. 13: Characteristics of RPM (speed) control, torque control, and power control of a propulsor or thruster, (a)in RPM-torque frame; (b) in time frame.


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The speed is not constant in Power Mode.At this point the ship is turning and due to increased

propeller load, the speed is decreasing.The torque increases inverse proportionally

to speed in order to keep constant power;Power = Torque x (2 x x RPM/60).

Fig. 14: Logging from sea trial, showing that the load power is constant when operating in power control mode,even though the load torque may change significantly by effects from waves and course changes.

Power control is based on a control algorithm where the remote set-point is regarded to be the commandedpower (kW) set-point to the propeller, and the VSD controller will then control the load torque to produce apower to the motor shaft which equals this set-point. Any load variations will be counteracted by keeping the

motor shaft power constant, as seen in Fig. 13, and the load power of the prime movers will therefore beminimized. The power control works well within the constraints and limitations of the components in the VSD,as shown in Fig. 14; which is a real logged characteristics of a propulsion drive system in heavy seas and duringcourse changes where the load varies significantly without causing load power variations of the prime movers.

Power control is found advantageous at high propeller loads in most ship applications, such as free sailingand transit modes. RPM control gives some faster dynamic performance, and is normally used in operationmodes with lower load on the propellers, and at high dynamic requirements, such as maneuvering mode andstation keeping. For ice braking vessels, power control is a necessity, since the loads of the propeller may varyexcessively and very fast, e.g. when the propeller hits ice blocks. Torque control is rarely used today, as speed(RPM) and power control are better alternatives for the various operational modes.

Research has shown that the use of power control may also give significant achievements in power stabilityof the network and reduction in load variations for the prime movers in station keeping conditions. Until today,

there has not been reported any successful use of such control schemes in DP systems, and the slower responseof the VSD drive in power control mode may be one explanation to this.

Note that it is necessary to include additional control and monitoring functions to achieve safe and reliablecontrol of the propulsors and thrusters in all operating modes, however, it is beyond the scope of this paper todescribe all these in details.


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SYSTEM CONFIGURATIONS

As previously shown, the basic topologies for the VSD are relatively similar among the various suppliers tothe application of electrical propulsion.

From a ship application perspective, one of the main technical differences are related to how these productsare put together in a system configuration for electric power generation, distribution, and propulsion / stationkeeping. Several system configurations are applied, of which the most common ones are shown in Fig. 15. For

each main configuration, there may be several variants for optimization to the actual requirements for eachvessel.

The main challenge in system design is to meet the class requirements and ship specific requirements at aminimum total cost including equipment and installation costs, and with a best possible life cycle economy.Each vessel may have its own specific requirements, e.g. whether space is a scarce resource or not in the design.Propulsion transformers are large and heavy equipment, and the 6-pulse, active rectifier, and the Q12-pulse withphase shifted main voltages are examples on transformer-less solutions, but not without other penalties.

The 6-pulse and Q12-pulse solutions normally will require some kind of harmonic filter installations, unlesssignificant restrictions and constraints of operations shall be applied, which may cause deterioration of the fueleconomy of the prime movers and limitations of operational windows for the vessel. The Q12-pulse solution inparticular depends on a complex main switchboard with two feeders to each frequency converter for balancedloads that is a necessity for maximum performance. Each feeder will carry a 6-pulse current that to some extent

will enter the respective generators on the switchboard, or flow through the two primary sides of the distributiontransformers, and give additional losses that to some extent will counteract the benefits of avoiding the losses inthe drives transformer.

The active rectifiers increase the number of active components in the installation, and the complexity of theinstallation as each of the rectifiers requires a HF harmonic filter that introduces resonance modes of theinstallations that should not be excited by the switching frequency of the rectifier. Also, the size and costs of thefrequency converter itself will increase, as well as the power losses in the rectifier, counteracting at least partlythe benefits of the transformer-less design.

Hence, there exists no one ideal design for all vessels. The different solutions have differentcharacteristics, and only when considering the requirements and limitations for a vessel design, the best solutioncan be applied.

Note, that in the configurations of Fig. 15, there are no thrusters with change-over feeder from the two

switchboards or switchboard sections. Change-over supply is often used in OSVs for retractable Azimuthingthrusters in order to be able to provide power to this even with a fatal failure of one of the main switchboards.Fig. 16 shows typical solutions for such change-over arrangements for different frequency converter topologies.

The use of change-over circuits is an issue for discussion with class. Although class rules allows for change-over supply, class does not necessarily allows thrusters with change-over to constitute a part of the stationkeeping capability after a single failure, if the power source must be automatically or manually reconnectedafter the failure. For the configurations in Fig. 16 (b) and (c) this is necessary to avoid paralleling of the bus tieor transfer feeder via the thruster cables.

The configurations in Fig. 16 (a) and (d) could meet the requirement for continuous availability providedthat each branch of the rectifier parts is rated appropriately. Dual feeding through diode rectifiers normally willlead to a 2x100% rating of the rectifiers, as the rectifier loads are given mainly by the total load and thedifference of the supply voltages. Even a small difference in voltage supply is a source for large unbalance in

the rectifier loads. Also the dual feeding of active rectifiers must be rated 2x100% if full load capability shall beobtained after a worst case fatal fault of one switchboard section. In addition to the rating, there are also othercomplicating effects for such designs, as also all auxiliary and control voltages must be tolerant andcontinuously available after a fault on any of the main switchboard, and likewise for the critical signal interfacesto the power management system and other automation systems. This fault tolerance may be achieved, but it isnot necessarily straight forward and must be discussed with, and approved by class in each case. However, if theresult is that e.g. a retractable thruster can be considered as available for the DP capability after a single fault, itmay be worth the efforts.


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INV

M

INV

M

MM MM

INV

M

AR AR

INV

M

AR AR

INV

M

AR AR

INV

M

AR AR

FC FC

DP 3 Change-over6-pulse 12- / Q24-pulse 24-pulse Active rectifier

(a) (b) (c) (d) (e)

Fig. 16: (a)-(d) Alternative for thrusters with change-over from two switchboard sections. Note, that in DP3applications, the change-over circuits must be equipped with isolating switches also at the load end ofthe feeding cables (e).

HARMONIC FILTERNING AND CLEAN POWER SUPPLIES

Frequency converters are inherently non-linear components due to the switching characteristics of therectifier components, meaning that they do not draw sinusoidal currents from the network, even though beingfed by sinusoidal voltages.

The non-sinusoidal currents into the rectifier consist of a fundamental voltage, and a series of harmoniccomponents with a wide content of frequencies which depends on the rectifier type and system configuration.For the type of converters that have the highest level of distortion in the currents, typically those with 6-pulse,12-pulse, and Q12-pulse rectifiers, the level of harmonic distortion in the currents may lead to voltage distortion

above the class limits. Most class societies now have adapted the IEC 60092-101 requirement, such as ABS,American Bureau of Shipping:

When the limit of the applicable regulation will be exceeded with the decided frequency converter andsystem configuration also after optimizing the design of the generators and transformers in the plant, there arestill several ways to manage the harmonic distortion level.

A harmonic filter can be applied. There are two main different types for harmonic filters; passive LC filters(alternatively damped LCR), Fig. 17 and active filters, Fig. 18. For ship applications, passive filters are more

commonly applied, due to their lower costs; especially since they can be used at lower voltage levels in thedistribution system to filter the voltage distortion not necessarily in the complete installation, but for thesensitive equipment only.

Active filters are, similarly to the active rectifier in frequency converters, a topology with switchingelements like IGBT for low voltage systems. However, while the active rectifier is designed to draw sinusoidalcurrents from the network, the active filters measures the distorted load currents, and cancels the harmoniccomponents of the VSD by injecting harmonic current with 180 deg phase shift to the power plant.

Both methods are efficient filters; it is normally a matter of economy to select which solution to apply.


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One stage passive LC filter; un-damped

L

C

First order

undamped LC filter

Lg

Agg reg ated

Generator / motor model

Z()

Two stage passive LC filter; un-damped

Z()

L

C

First order 11th harmundamped LC filter

Lg

Agg regat edGenerator / motor model

L

C

First order 5th harmundamped LC filter

Frequency scan:

Generator

2 . . f . LgGenerator

2 . . f . Lg

FilterFilter

ResultingResulting

f / f1

Z

f / f1

Z

11 13 23 25

Frequency scan:

Generator

2 . . f. Lg

Generator

2 . . f. Lg

FilterFilter

f / f1

Z

f / f1

Z

11 135 23 2511 135 23 25

ResultingResulting

Fig. 17: Passive LC filters, left: one stage for filtering of one (5th) harmonic frequency;right: a two stage filter, here for filtering 5th and 11th harmonic components.

Introducing capacitive components in an inherent inductive network, as done with passive filters, must bedone with care as it introduces not only the desired series resonance to achieve low impedance current paths forthe harmonic currents of concern, but also parallel resonances that may cause excessive distortion globally orlocally in the installation if they are excited by harmonic currents. Therefore, when including harmonic passive

filters, an extensive system analysis must be made to ensure that such parallel resonances that always will exist,are not excited. Active filters are less vulnerable to such, however, as for the active rectifiers, they also need HFharmonic filters in their feeders to avoid high frequency harmonics and common mode noise to distort thenetwork.

Non-

linear load

G

M

Acti ve

filter

i1 i2ig = i1+i2

3 5 7 911

13

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

47

49

51

53

55

57

59

61

63

65

67

69

71

73

75

77

79

81

83

85

87

89

igi1

i2

Fig. 18: An active rectifier can measure the non-linear current and compensate instantaneous its harmonicdistortion. Some filters need one or more periods to calculate the harmonic components that shall becompensated.


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Rectifier

(AC to DC)

Inverter

(DC to AC)

Rectifier

(AC to DC)

Inverter

(DC to AC)

Inverter

(DC to AC)Rectifier

(AC to DC)

Inverter

(DC to AC)Rectifier

(AC to DC)

M GM G

Dirty side Clean side

Fig. 19: Clean power supplies; from top: Rotating motor-generator set; Static frequency converter; Staticfrequency converter with battery back-up (UPS)

Earlier, it was quite common to install clean power supply, where the distorted network was decoupled fromthe supply to sensitive equipment by use of either rotating or static converters, as shown in Fig. 19. Withexception of the UPS supply, which is class requirements for a range of navigation and control equipment, theclean power supplies are less used today when class rules normally requires the general power distributionsystem to fulfill stringent requirements to harmonic distortion, unlike what was common practice only a fewyears ago.

POWER PLANT CONTROL, ACTIVE AND REACTIVE LOAD SHARING

Vessels with electric propulsion will normally have a relatively advanced power management system (PMS)to ensure optimal operation and reliable access to propulsion power. For DP vessels with class 2 or 3 notations,

a PMS is also required by class, and should then also be redundant in accordance with the redundancyrequirements of the class notation.

There are basically two types of PMS configurations applied to OSVs; the mirror image configuration,where the PMS configuration reflects the power system design with one control system responsible to interfaceand control each of the redundancy segregated parts of the power system; and the duty-standby system whereone PMS control system controls the complete plant, with a second control system in hot stand-by to take overafter failure of the duty controls. Both configurations fulfil the class requirements, as long as functionality andfault integrity is met. As a general comment, the more standard off the shelf systems tend to be of the duty-standby design, with less and simpler functionalities than the more advanced, engineered solutions.

Related to the design of the electric power plant, load sharing is an important function. Both active power(kW) loads and reactive power (kVAr) loads shall be equally shared between paralleled generator sets,according to class.

The active power load sharing is either passive by droop control (Fig. 20) or active isochronous, bycommunication between the governor controllers to enable a regulation towards the average load of thegenerator sets. Passive load sharing has the benefit of not having a common communication line, with less riskfor common failures in the system compared to the isochronous load sharing. However, the frequencyadjustment is slower, with larger frequency variations for load changes. Especially for OSVs, where thepropulsion load may vary much in DP operations and in heavy sea conditions, this may cause problems forsynchronization of new generators to the system and increase the risk for excessive frequency variations andentering into undesired load reduction conditions. Therefore, isochronous load sharing is often used in OSVs,however, special considerations on the fault tolerance to avoid single point of failure is then required.


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19 f 19

Active Power Load

Frequency / RPM

Nominal RPM

Engine 1

Engine 2Paralleling- Equal fuel supply- Active load sharing

+2.5%

-2.5%

+2.5%

-2.5%

Reference RPM

Equal droop

100%

Equal droopEqual droop

100%

Reactive Power Load

Line-voltage

Nominal Voltage

Generator1

Generator 2Paralleling

- Equal magnetization- Reactive load sharing

+2.5%

-2.5%

+2.5%

-2.5%

Reference Voltage

Equal droop

100%

Equal droopEqual droop

100%

(a) (b)Fig 20: Droop control of (a) governor and (b) automatic voltage regulator, showing load sharing of two

paralleled generator sets.

The same principles apply also for the sharing of reactive power among the paralleled generators; however,droop control mode, Fig. 20 (b), does normally not give voltage variations that are large enough to createproblems for synchronization and protection of the plant. Use of cross compensation, and active equalization of

reactive loads are available technologies in voltage regulators, but only exceptionally used in ship applicationssince such solutions requires common signal interfacing and hence inherently increase the risk for single pointfailures without otherwise contributing to a more safe and reliable system.

CONCLUDING REMARKS

In design of the electric plant for OSVs with electric propulsion, it may appear that there are large variationsin the basic technologies being offered by the different makers. However, this is not necessarily the case. Mostvendors utilize similar basic components and solutions, even though the composition of systems and preferencesin design may differ to some extent often depending on the makers available technologies.

In order to understand and assess the technical argumentation for the different solutions, it is the authorsopinion that rather than preferring a specific technology above another, the important issues of the effects of the

technology on safety, performance, and life cycle economy should be considered.This paper has presented the most commonly applied solutions in electric propulsion with the objective to

give the decision makers background information to better understand the concepts and to make the mostbeneficial selection for the specific vessels requirements.

REFERENCES

1.F. Blaschke, "Das Verfahren der Feldorientierung zur Regelung der Asynchronmaschine" [The FieldOrientation Method for Controlling an Asynchronous Machine], Siemens Forschungs undEntwicklungsberichten [Siemens research and development reports] 1972, pp. 184 et seq.*.2. Depenbrock, M., Direct self control for high dynamics performance of inverter feed a.c. machines. ETZ Arch.v7 i7. 211-218.

3. Presentation:http://www.spec.ncsu.edu/selected%20presentations/Electric-power%202005-Talk.pdf4. Simon Eicher, Munaf Rahimo, Evgeny Tsyplakov, Daniel Schneider, Arnost Kopta, Ulrich Schlapbach, EricCarroll, 4.5kV Press Pack IGBT Designed for Ruggedness and Reliability;ABB Switzerland Ltd, Semiconductors, CH-5600 Lenzburg, Switzerland, IAS 2004, Seattle October 3-7.5.Product information:http://www.akerkvaerner.com/Internet/IndustriesAndServices/OilAndGas/PowerandAutomation/AKPASElectricPropulsion.htm

Documents

A Survey of Concepts for Electric Propulsion in Conventional & Ice Breaking OSVs