all electric ship++++

8/6/2019 All Electric Ship++++

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All Electric Propulsion System


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HV voltage generation, conversion ,transformation and distribution in ship
http://www.ship-technology.com/contractors/propulsion/abb/


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Marine Electrical System

Maritime electric systems include power generation, distribution andcontrol, and consumption of electric power on supply- service- andfishing vessels as well as offshore installations.

Electric propulsion has increased especially for vessels with severallarge

power consumers, for example cruise ships, floating production systems,supply- and service vessels. Maritime electric systems are autonomous power systems. The prime

movers, including diesel engines, gas- and steam turbines, are integralparts of the systems.

The power consumers are large compared with the total capacity of the

system, as for example thruster and propulsion systems for DPoperated vessels, drilling systems, HVAC systems on board ship


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The overall power train efficiency with DEP is around 87-90%.Use of permanent magnets in electric generators and motors aswell as general advances in semiconductor technology mayimprove this figure to around 92-95% in the near future. Electricaltransmission will consist of three basic energy conversions:

From (rotating) mechanical energy into electrical energy: E-generator

From electrical energy into (rotating) mechanical energy: E-motor Some form of fixed or controlled electrical conversion in

between: power converter


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Systematic overview of existing

types

E-generator

Mechanical ==> Electrical: E-Generators - DC Generators - AC Generators

E-Motors

Electrical ==> Mechanical: E-motors - Driving motors - Synchronous Motor - Positioning motors

Power converters

Electrical ==> Electrical: power conversion or transformation

- Fixed transformers - Controlled converters - Static converters -Inverter


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Structure of a combined power plant for

ships


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Electric Propulsion System (AES)

Electric propulsion of ships has been know for a long time to human Dynamic changes in human discovery has given several up and down in

history Recent time have seen a a lot of Passenger ships being built with all

electric system for various advantage that over the conventional primemovers

Early large passenger vessels employed the turboelectric system whichinvolves the use of variable speed, and therefore variable frequency, turbo-generator sets for the supply of electric power to the propulsion motorsdirectly coupled to the propeller shafts. Where, the generator/motor systemwas acting as a speed reducing transmission system.

Electric power for auxiliary ship services required the use of separate

constant frequency generator sets. System with generating sets to providepower to both the propulsion system and ship ancillary services. However fixed voltage and frequency system are suitable to satisfy the

requirements of the ship service loads.


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Electric Propulsion System

(AES) Other complication associated with earlier systems is difficulties in using multiple

motor per shaft when required propulsion power was beyond the capacity of asingle d.c. motor .

Developments in high power static converter equipment have presented a veryconvenient means of providing variable speed a.c. and d.c. drives at the largestratings likely to be required in a marine propulsion system.

The electric propulsion of ships requires electric motors to drive the propellers andgenerator sets to supply the electric power. It may seem rather illogical to useelectric generators, switchgear and motors between the prime-movers (e.g. dieselengines) and propeller when a gearbox or length of shaft could be all that isrequired.

In the light of the above, hybrid of gas turbine or Diesel with electric couple withdual fuelling that include natural gas, is explorable option for existing vessels, allelectric ship using natural gas is also a good option.

Currently there is interesting development for new ship need exploration ontechnologies to improve integrated full electric propulsion with advanced power

management systems:

Improved converter and power electronics technology

Improved generators and motors


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Electric Propulsion System (AES)

The AES give widespread electrification of auxiliaries and theopportunity to use upgradeable and flexible layouts. It will include a lowrisk, cost effective and comprehensive Platform Management Systemthat has a standardized Human-Computer Interface supportable for itsentire service life and the goal to be an Environmentally Sound Ship.

The fit into the goals of the Environmentally Sound Ship where :freedom of operation in MARPOL special and restricted areas;unrestricted littoral operations; port independence; minimum onboardstorage of waste and reduced manpower whilst reducing cost ofownership and port reception costs.

the also promise potential for replacing the current traditional systemsused in steering gear, fin stabilizers with compact, power-dense

actuators. They also offer potentials for possible use of electric valve actuators

that will simplify system architectures systematic integration of upperdeck to machinery.


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Power generation

A 2001 study concluded that fitting a Navy cruiser with more energy-efficient electrical equipment could reduce the ships fuel use by 10% to25%.

Ship fuel use could be reduced by shifting to advanced turbine designs

such as an intercooled recuperated (ICR) turbine. Shifting to integratedelectric-drive propulsion can reduce a ships fuel use by 10% to 25%. There is Potential alternative hydrocarbon fuels Like biodiesel and

liquid hydrocarbon fuels made from coal Recent time has seen firms offering kite-assist systems to commercial

ship operators.

Solar power might offer some potential for augmenting other forms ofshipboard power. Talking about the question now the electric propulsion , especially with

hybrid system offer the best answer to problem of energy


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Power generation

Integrated electric-drive system derived from a commercially available system thathas been installed on ships such as cruise ships requires a technology that is moretorque-dense (i.e., more power-dense) .

Candidates for a more torque-dense technology include a permanent magnet motor(PMM) and a high-temperature superconducting (HTS) synchronous motor.

In addition, electric drive makes possible the use of new propeller/sternconfigurations, such as a podded propulsion ... that can reduce ship fuelconsumption further due to their improved hydrodynamic efficiency

Podded drives offer greater propulsion efficiency and increased space within the hullby moving the propulsion motor outside the ships hull and placing it in a podsuspended underneath the hull.

Podded drives are also capable of azimuth improving ship maneuverability. Indeed,podded drives have been widely adopted by the cruise ship community for these

reasons. The motors being manufactured now are as large as 19.5 MW, and could provide

the total propulsion power.


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Azipod drive unit


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Comparison of propulsion plants

efficiency


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Weight of propulsion systems


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Prime moversGas Turbines

Gas turbine have been selected as the future prime mover primarilybecause of their high power to weight ratio. 4. Weight sensitive ship designs favor gas turbines and projected light

weight fuel cell power plants such as PEM. They also provide significant reduction in the amount of routine

maintenance required when compared with diesel generators.

The other significant factor is the low emissions.

Diesel engine Diesel engines offer fuel costs savings of 50% if heavy fuels can be

used, and if emissions can be maintained at acceptable levels. Maintenance may include engine modifications such as dual fuel

capability for in-port use, water injection, and timing retard, and exhausttreatment such as selected catalytic reduction and oxidation catalysts. Heavy fuel use also requires careful selection of cylinder material and

lube oil


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Turbina

A gas turbine, also called a combustion turbine, is a rotary enginethat extracts energy from a flow of hot gas produced by combustion ofgas or fuel oil in a stream of compressed air.

It has an upstream air compressorradial oraxial flow mechanicallycoupled to a downstream turbine and a combustion chamber inbetween.

Energy is released when compressed airis mixed with fuel and ignitedin the combustor.

The resulting gases are directed over the turbine's blades, spinning theturbine, and, mechanically, powering the compressor.

Finally, the gases are passed through a nozzle, generating additionalthrust by accelerating the hot exhaust gases by expansion back to

atmospheric pressure.

A steam turbine is a mechanical device that extracts thermal energyfrom pressurized steam, and converts it into useful mechanical work.
http://en.wikipedia.org/wiki/Enginehttp://en.wikipedia.org/wiki/Air_compressorhttp://en.wikipedia.org/wiki/Axial_flowhttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Compressed_airhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Combustorhttp://en.wikipedia.org/wiki/Nozzlehttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Thermal_energyhttp://en.wikipedia.org/wiki/Nozzlehttp://en.wikipedia.org/wiki/Combustorhttp://en.wikipedia.org/wiki/Ignition_systemhttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Compressed_airhttp://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Turbinehttp://en.wikipedia.org/wiki/Axial_flowhttp://en.wikipedia.org/wiki/Air_compressorhttp://en.wikipedia.org/wiki/Engine


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Gas Turbine
http://en.wikipedia.org/wiki/Image:Brayton_cycle.svg


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Steam engine


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COGAG

Combined gas turbine and gasturbine (COGAG) is propulsionsystem for ships using twogas turbines connected to a singlepropeller shaft.

A gearbox and clutches allow either

of the turbines to drive the shaft orboth of them combined. Using one or two gas turbines has

the advantage of having twodifferent power settings.

Since the fuel efficiency of a gasturbine is best near its maximumpower level, a small gas turbinerunning at its full speed is moreefficient compared to a twice aspowerful turbine running at halfspeed, allowing more economictransit at cruise speeds.
http://en.wikipedia.org/wiki/Gas_turbinehttp://en.wikipedia.org/wiki/Propeller_shafthttp://en.wikipedia.org/wiki/Transmission_%28mechanics%29http://en.wikipedia.org/wiki/Clutchhttp://en.wikipedia.org/wiki/Fuel_efficiencyhttp://en.wikipedia.org/wiki/Image:COGAG-diagram.pnghttp://en.wikipedia.org/wiki/Fuel_efficiencyhttp://en.wikipedia.org/wiki/Clutchhttp://en.wikipedia.org/wiki/Transmission_%28mechanics%29http://en.wikipedia.org/wiki/Propeller_shafthttp://en.wikipedia.org/wiki/Gas_turbine


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Diesel engine
http://en.wikipedia.org/wiki/Image:4-Stroke-Engine.gif


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Prime movers

Electric drive

Electric drive transmissions have a higher specific fuel consumption, specificweight and volume than mechanical drive systems, but has advantages inarrangement which may compensate for these disadvantages.

Advanced technology motors can be located very close to and on line with thepropulsors, at the extreme aft end of the ship, or in external pods.

Electrical generator sets can be optimally spaced around the ship andelectrically connected. In the longer term, combined with fuel cells, SFC, specific

weight and volume are comparable with gas turbine and diesel prime movers fordirect drive systems.

Zone Concept : The concept of dividing future classes of ship into zones to maximize

survivability also extends to the power system. Each zone would be autonomous and include ventilation systems, cooling

systems, power distribution and other services which could be affected by

damage to another part of the ship. At least two supplies would be provided for all essential loads. Current classes,

using split generation and distribution, rely on the provision of normal andalternative supplies via Automatic Change-Over Switches


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Typical system with zoning


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Fuel cell

The fuel cell stack operates by utilizing electrochemical reactions betweenan oxidant (air) and a fuel (hydrogen), with two electrodes separated by amembrane.

The voltage of the fuel cell output can be controlled by a converter and it istherefore able to connect to any point in the ship service or propulsiondistribution system.

The fuel cell stack is modularity give redundancy advantage. It also has theadditional advantages of zero noxious emissions, and low thermal andacoustic signatures.

In the short term the fuel cell system is required to use marine diesel fuel.Diesel fuel will require reforming within the fuel cell stack, or using anexternal process, to produce a hydrogen rich gas which the fuel cell stack iscapable of processing.

The reformer will clearly add both size, weight and complexity to the fuel cellsystem. In the longer term technologies such as the Solid Oxide Fuel Cell(SOFC) are contenders, which are more forgiving of impurities and can usea fuel available world-wide, either methanol or gasoline.


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Storage option

The technologies being assessed for energy storage include areelectro-chemical batteries (both conventional and advanced),regenerative fuel cells (otherwise known as redox flow cells )Superconducting Magnetic Energy Storage (SMES) andSupercapacitors.

Regenerative fuel cells store or release electrical energy by means of areversible electrochemical reaction between two salt solutions (theelectrolytes). The reaction occurs within an electrochemical cell.

The cell has two compartments, one for each electrolyte, physicallyseparated by an ion-exchange membrane.

In contrast to most types of battery system, the electrolytes flow into andout of the cells and are transformed electrochemically inside the cells.

The power is therefore determined by the size of the cell but theendurance is determined by the size of the two electrolyte tanks


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Storage system


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Prime movers

All primemovers are potentially compliant with emerging emissionrequirements, however, complexity for achieving compliance varies withprime mover and fuel type.

Diesels require the most attention to emissions control followed at some

distance by gas turbines, where ultra low emissions levels have beenachieved for land-based systems. Fuel cells emit the lowest levels of pollutants of all the primemovers Heavier fuel cell systems and diesels represent larger machinery and

structural weight. Fuel cells can be used as a prime mover in an Integrated Full Electric

Propulsion (IFEP) system providing DC electrical power output, and arebeing developed as a replacement for diesel generators and gas turbinealternators.


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Sail and solar power ship


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Skysail


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Propulsion motor

For efficient operation of propulsion motor there is arequirement for a compact, power dense, ruggedelectrical machine to be utilized for the propulsionmotor.

For the full benefits of electric propulsion to berealized the machine should also be efficient,particularly at part load,

In order to achieve suitable compact designs rareearth permanent magnet materials may be required.

The machine topologies available for PMM aredeemed to be those based on radial, axial andtransverse flux designs.


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PMM


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Power for LNG ships

These alternatives are more economical and offer greater overall efficiency withan added advantage of providing greater flexibility and redundancy

Diesel plant also raises are inherited with problem of vibration on membrane LNG carrier it is necessary to understand the interaction between the structural

resonance that is excited by the diesel engine and the separate resonance thatis created within the membrane containment system interacting with LNG.

The traditional application of gas fired boilers for steam turbine propulsionsystems is no longer the only available option for LNG Carriers,

Direct drive, slow speed diesel plants, coupled with an on-board liquefactionplant to handle the cargo boil off, or 4 stroke medium speed diesel electricpropulsion or gas turbine with diesel electric drive appear to offer the greatestoperational efficiencies for the new designs of large LNG carriers.


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Power generation for LNG ships

Although slow or medium speed diesel engines have been selected for some ofthe recent LNG carriers with dual fuel installation option that uses both gas boil-off and ordinary bunkers.

Variations of the dual fuel arrangements include:

-diesel engine or gas turbine driven generators with one propulsion shafting systemand a liquefaction plant;

-diesel engine or gas turbine driven generators with two propulsion shafting systemsand a liquefaction plant;

-diesel engine or gas turbine driven generators with two azimuth thrusters and aliquefaction plant.

To date, slow speed diesel with re-liquefaction plant as well as a gascombustion unit, and medium speed dual fuel diesel with gas combustion units,are the preferred options for the new large LNG carriers recently ordered in

Korea. It would appear that gas turbine with simple and combined cycles using heat

recovery units to drive steam turbo alternators are another alternative beingexplored. Industry is currently developing the fuel gas systems for these gasturbine options.


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Power generation for LNG ships

A dual fuel diesel-electric system uses forced boil-off from the cargotanks as the primary fuel and marine diesel oil as back-up fuel. Thearrangement can also be adapted to current LNG carrier designs.

Shipbuilders and engine designers that are proponents of dual fuelsystems point out that a gas-electric propulsion plant is more compactthan the traditional steam turbine plant used for LNG carriers,

increasing cargo capacity within the same dimensioned hull. The IMO Gas Carrier Code requires two means of utilizing boil-off gas

on all LNG carriers. Conventional systems use the main boilers forgenerating steam for propulsion. When this cannot be used, excesssteam is redirected to the condensers. Similar arrangements arerequired for the diesel propulsion systems. Current industry proposals

for the alternative means of boil-off gas utilization are a liquefactionplant or a gas combustion unit. Risk assessment methods are recommended for option selection


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Power Distribution

As the demand for electrical are 3.3 kV or 6.6 kV but 11 kV isused on some offshore platforms and specialist oil/gasproduction ships e.g on some FPSO (floating production, storageand offloading) vessels.

By generating electrical power at 6.6 kV instead of 440 V the

distribution and switching of power above about 6 MW becomesmore manageable. As for electrical Power increases on ships (particularly

passenger ferries, cruise liners, and specialist offshore vesselsand platforms) the supply current rating becomes too high at 440V.

To reduce the size of both steady state and fault current levels, itis necessary to increase the system voltage at high powerratings.


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Component parts of an HV

The component parts of an HV supply system are standard equipmentwith:

HV diesel generator sets feeding an HV main switchboard. Large power consumers such as thrusters, propulsion motors, air-

conditioning (A/C) compressors and HV transformers are fed directlyfrom the HV switchboard.

An economical HV system must be simple to operate, reasonably pricedand require a minimum of maintenance over the life of the ship.

Experience shows that a 9 MW system at 6.6 kV would be about 20%more expensive for installation costs.

The principal parts of a ships electrical system operated at HV would bethe main generators, HV switchboard, FV cables, HV transformers andHV motors.

An example of a high voltage power system is shown


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Ship HV Voltage system


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HV Systems

In the example shown the HV generators form a central power stationfor all of the ship's electrical services.

On a large passenger ship with electric propulsion, each generator maybe rated at about 10 MW or more and producing 6.6 kV, 60 Hz three-phase a.c. voltages.

The principal consumers are the two synchronous a.c. propulsionelectric motors (PEMs) which may each demand 12 MW or more in thefull away condition.

Each PEM has two stator windings supplied separately from the mainHV switchboard via transformers and frequency converters.

In an emergency a PEM may therefore be operated as a half-motor witha reduced power output. A few large induction motors are supplied at

6.6 kV from the main board with the circuit breaker acting as a direct-on-line (DOL) starting switch.


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Ship high voltage systems

These motors are:

o Two forward thrusters and one aft thruster, ando Three air conditioning compressors

Other main feeders supply the 440 V engine room sub-station (ER sub)switchboard via step-down transformers.

An interconnector cable links the ER sub to the emergency switchboard. Other 440 V sub-stations (accommodation,galley etc.) around the ship are

supplied from the ER sub. Some installations may feed the ships sub stations directly with HV and

step-down to 440 V locally. The PEM drives in this example are synchronous motors which require a

controlled low voltage excitation supply current to magnetise the rotor

poles. This supply is obtained from the HV switchboard via a step-down

transformer but an alternative arrangement would be to obtain theexcitation supply from the 440 V ER sub switchboard.


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Ship high voltage systems


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High Voltages solid state AC-DC-

AC conversion


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Solid State Switching Principle

The power systems engineers is interested in high voltages primarily forpower transmission, and secondly for testing of his equipment used inpower transmission in laboratory

High voltage can be obtained locally from power generating plantthrough the use of solid state

In many testing laboratories, the primary source of power is at low

voltage (400 V three phase or 230 V single phase, at 50 Hz). Fromwhich high voltage can be obtained On board ship the same technology can be used to use high voltage Laboratory test are aimed to design the required high voltage Since insulation is usually being tested, the impedances involved are

extremely high (order of M ohm and the currents small (less than anampere).

High voltage testing does not usually require high power. Thus special methods may be used which are not applicable when

generating high voltage in high power applications.


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Solid State Switching Principle In the field of electrical eng. & applied physics, high voltages are required

for several applications As:-a power supply (eg. hv dc) for the equipments such as electron microscope

and x-ray machine.

-Required for testing power apparatus insulation testing.

-High impulse voltages are required for testing purposes to simulate overvoltages due to lightning and switching.

Sometimes, high direct voltages are needed in insulation test on cablesand capacitors. Impulse generator charging units also require high dcvoltages of about 100-200kV.

Normally for the generation of dc voltages of up to 100kV, electronicsvalve rectifiers are used and the output currents are about 100mA. Therectifier valves require special construction for cathode and filaments

since a high electrostatic field of several kV/cm exists between the anodeand cathode in the non-conduction period. The ac supply to the rectifier tubes maybe of power frequency or maybe

of audo frequency from an oscillator. The latter is used when a ripple ofvery small magnitude is required without the use of costly filters tosmoothen the ripple.


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Half and Full Wave Rectifier

Rectifier circuits for producing high dc voltages from ac sourcesmaybe

a. Half-Wave

b. Full-Wave

o The rectifier can be an electron tube or a solid state devices.Nowadays, single electron tubes are available for peak inversevoltages up to 250kV and semiconductor or solid state diodes upto 250kV.

o For higher voltages, several units are to be used in series. When anumber of units are used in series, transient voltage distributionalong each unit becomes non-uniform and special care should betaken to make the distribution uniform.


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V outVinR

L

Half Wave Rectifier

VAVG

Vp

0

T

Mean Load Voltage or Average Value of half wave output

D 1


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R L

t

o

t

1

t2t

o

t

1

+

-

D 1

D 2

t o t1 t2

Vp

VAVG

Full wave Rectifier Circuit

Mean Load Voltage or Average Voltage Full-wave output

Voltage Multiplier Circuits


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Voltage Multiplier Circuits

Both full-wave as well as half-wave circuits canproduce a maximum direct voltage corresponding tothe peak value of the alternating voltage.

When higher voltages are required voltage multipliercircuits are used. The common circuits are thevoltage double circuit

Used for higher voltages. Generate very high dc voltage from single supply

transformer by extending the simple voltage doublercircuit.


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Types of high voltages;

High d.c. voltages

High a.c. voltages of power frequency

High a.c. voltages of high frequency

High transient or impulse voltages of very short

duration - lightning overvoltages

Transient voltages of longer duration switching

surges


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The voltage doubler circuitmakes use of the positive andthe negative half cycles to

charge two different capacitors.These are then connected inseries aiding to obtain doublethe direct voltage output. Figureshows a voltage doubler circuit.

In this case, the transformer willbe of small rating that for thesame direct voltage rating withonly simple rectification. Furtherfor the same direct voltage

output the peak inverse voltageof the diodes will be halved.Voltage doubler circuit


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High Alternating Voltages

Required in laboratories and a.c. tests as well as for the circuit of high d.c. and impulse voltage. Test transformer are generally used. Single transformer test units are made for high alternating voltages up

to about 200 kV. However, for high voltages to reduce the cost (insulation cost

increases rapidly with voltage) and make transportation easier, acascade arrangement of several transformers is used.

For higher voltage requirement, series connection or cascading of theseveral identical units of transformer is applied.


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Cascade arrangement of transformers


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1600 kV, 9.6 MVA Cascaded Power Transformer


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A typical cascade arrangement of transformers used to obtain up to300 kV from three units each rated at 100 kV insulation. The lowvoltage winding is connected to the primary of the first transformer,and this is connected to the transformer tank which is earthed.

One end of the high voltage winding is also earthed through thetank.

The high voltage end and a tapping near this end is taken out at thetop of the transformer through a bushing, and forms the primary ofthe second transformer.

One end of this winding is connected to the tank of the secondtransformer to maintain the tank at high voltage.

The secondary of this transformer too has one end connected to

the tank and at the other end the next cascaded transformer is fed.



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This cascade arrangement can be continued further if a stillhigher voltage is required.

In the cascade arrangement shown, each transformer needs onlyto be insulated for 100 kV, and hence the transformer can berelatively small. If a 300 kV transformer had to be used instead,the size would be massive. High voltage transformers for testingpurposes are designed purposely to have a poor regulation.

This is to ensure that when the secondary of the transformer isshort circuited (as will commonly happen in flash-over tests ofinsulation), the current would not increase to too high a value andto reduce the cost. In practice, an additional series resistance(commonly a water resistance) is also used in such cases to limitthe current and prevent possible damage to the transformer.



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What is shown in the cascade transformer arrangement is the basic principle

involved. The actual arrangement could be different for practical reasons.

In the cascade arrangement shown, each transformer needs only to be insulated

for 100 kV, and hence the transformer can be relatively small. If a 300 kV

transformer had to be used instead, the size would be massive. High voltage

transformers for testing purposes are designed purposely to have a poor

regulation. This is to ensure that when the secondary of the transformer is short circuited

(as will commonly happen in flash-over tests of insulation), the current would not

increase to too high a value and to reduce the cost. In practice, an additional

series resistance (commonly a water resistance) is also used in such cases to

limit the current and prevent possible damage to the transformer. What is shown in the cascade transformer arrangement is the basic principle

involved. The actual arrangement could be different for practical reasons.



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High D.C. Voltages

Generation of high d.c. voltages is mainlyrequired in research work in the areas of pure

and applied physics. Needed in insulation test. Use rectifier circuit (diode) to convert a.c. to

d.c. voltage. vacuum rectifiers, semiconductordiodes


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Impulse High Voltage Impulse voltages (IVs) are required in hv tests to simulate the

stresses due to external and internal overvoltages, and also forfundamental investigations of the breakdown mechanisms.

Usually generated by discharging hv capacitors through switchinggaps onto a network of resistors and capacitors.

In hv technology, a single, unipolar voltage is termed an impulsevoltage.

Rectangular and wedge-shaped IVs are normally used for basicexperiments while for testing purposes, double exponential IVs areused.

Standard test of impulse voltages can be represented as doubleexponential wave, and its mathematical equation is defined as

follows;V = Vo [exp(-t) exp(-t)]

Where and are constants of microsecond values.


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Controlled Rectification The generated three power supply on a phase a.c. electrical ship

has a fixed voltage and frequency. This is generally at M0 V and60 Hz but for high power demands it is likelv to be 6.6 kV and 60Hz.

Speed control for a propulsion motor requires variable voltage for ad.c. drive and variable frequency * voltage for an a.c. drive.

The set bus-bar a.c. voltage must be converted by controlled

rectification (a.c.--d.c.) ind/or controlled inversion (d. c. * a. c. )' tomatch the propulsion motor type.

A basic rectifier uses semiconductor diodes which can onlyconduct current in the direction of anode (A) to cathode (K) andthis is automatic when A is more positive than K.

The diode turns-off automatically when its current falls to zero.

Hence, in a single-phase a.c. circuit a single diode will conductonly on every other half-cycle and this is called half-waverectification.


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Controlled Rectification In this circuit an inductor coil (choke) smooth the d.c. load current even

though the d.c. voltage is severely chopped by the thyristor switchingaction. An alternative to the choke coil is to use a capacitor across the rectifier

output which smooths the d.c. voltage. Full wave controlled rectificationfrom a three-phase a.c. supply is achieved in a bridge Circuit with sixthyristors a shown

Other single-phase circuits using a biased arrangement with two diodesand a centre-tapped transformer will create full-wave rectificationSimilarly, four diodes in a bridge formation will also produce a full-waved.c. voltage output.

An equivalent three phase bridge requires six diodes for full-waveoperation. A diode, having only two terminals, cannot control the size ofthe d.c. output from the rectifier.

For controlled rectification it is necessary to use a set of three-terminaldevices such as thyristors (for high currents) or transistors (for low -medium currents).


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Three-phase controlled rectifier bridge circuit.


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A basic a.c.-d.c. control circuit using a thyristor switch is shown in the nextslide. Compared with a diode, a thyristor has an extra (control) terminalcalled the gate (G).

The thyristor will only conduct when the anode is positive with respect tothe cathode and a brief trigger voltage pulse is applied between gate andcathode (gate must be more positive than cathode).

Gate voltage pulses are provided by separate electronic circuit and thepulse timing decides the switch-on point for the main (load) current. Theload current is therefore rectified to d.c. (by diode action) and controlled bydelayed switching.

In this circuit an inductor coil (choke) smooth the d.c. load current eventhough the d.c. voltage is severely chopped by the thyristor switchingaction.

An alternative to the choke coil is to use a capacitor across the rectifieroutput which smooths the d.c. voltage. Full wave controlled rectificationfrom a three-phase a.c. supply is achieved in a bridge Circuit with sixthyristors a shown


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The equivalent maximum d.c. voltage output is taken to be about 600 V

as it has a six-pulse ripple effect due to the three-phase input waveform. Controlled inversion process - A d.c. voltage can be inverted (switched)repeatedly from positive to negative to form an alternating (u.c.) voltageby using a set of thyristor (or transistor) switches. A controlled three-phase thyristor bridge inverter is shown

The inverter bridge circuit arrangement is exactly the same as that forthe rectifier. Here, the d.c. voltage is sequentially switched onto the

three output lines. The rate of switching determines the outputfrequency.

For a.c. motor control, the line currents are directed into (and out of)the windings to produce a rotating stator flux wave which interacts withthe rotor to produce torque.

The processes of controlled rectification and inversion are used in

converters that are designed to match the drive motor.


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Three-phase inverter circuit and a.c. synchronous motor


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Converter Types

The principal types of motor control converters are:

- >a.c.-d.c. (controlled rectifier for d.c. motors) . a.c.-d.c.-a.c. (PWM forinduction motors)

- >a.c.- d.c.-a.c. (synchroconverter or synchronous motors) .

-> d.c.-a.c. (cycloconverter for synchronous motors)

These are examined below:

a.c.- d.c. converter This is a three phase a.c. controlled rectification circuit for a d.c. motor

drive. Two converters of different power ratings are generally used for the

separate control of the armature current and the field current whichproduces the magnetic flux . Some systems may have a fixed field current which means that the field

supply only requires an uncontrolled diode bridge


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Converter Types

Shaft rotation can be achieved by reversing either the field current orthe armature current direction.

Ship applications for such a drive would include cable-laying, offshoredrilling, diving and supply, ocean survey and submarines.

a.c.- d.c.-a.c. PWM converter This type of converter is used for induction motor drives and uses

transistors as the switching devices. Unlike thyristors, a transistor can be turned on and off by a control

signal and at a high switching rate (e.g. at 20 kHz in a PWM converter). The input rectifier stage is not controlled so is simpler and cheaper but

the converter will not be ablg to allow power from the motor load to beregenerated back into the mains supply during a braking operation.


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Controlled rectification converter and d.c.

motor


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PWM converter and a.c. induction motor


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Converter Types

From a 440 Y a.c. supply, the rectified d.c. (link) voltage will besmoothed by the capacitor to approximately 600 V.

The d.c. voltage is chopped into variablewidth, but constantlevel, voltage pulses in the computer controlled inverter sectionusing IGBTs (insulated gate bipolar transistors).

This process is called pulse width modulation or PWM. Byvarying the pulse widths and polarity of the d.c. voltage it ispossible to generate an averaged sinusoidal ac. output over awide range of frequencies typically 0.5-120Hz.

Due to the smoothing effect of the motor inductance, the motorcurrents appear to be nearly sinusoidal in shape.

By sequentially directing the currents into the three statorwindings, a reversible rotating magnetic field is produced with itsspeed set by the output frequency of the PWM converter.


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Converter Types

Accurate control of shaft torque, acceleration time and resistive brakingare a few of the many operational parameters that can be programmedinto the VSD,usually via a hand-held unit.

The VSD can be closelv tuned to the connected motor drive to achieveoptimum control and protection limits for the overall drive.

Speed regulation against load changes is very good and can be made

very precise by the addition of feedback from a shaft speed encoder.

VSDs, being digitally controlled, can be easily networked to othercomputer devices e.g. programmable logic controllers (PLCs) for overallcontrol of a complex process.


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Converter Types

a.c.*d.c.+a.c. synchroconverter This type of convert is used for large a.c. synchronous motor

drives (called a synchrodrive) and I is applied very successfullyto marine electric propulsion.

A synchroconverter has controlled rectifier and inverter stages

which both rely on natural turn-off (line commutation) for thethyristors by the three phase a.c. voltages at either end of theconverter.

Between the rectification and inversion stages is a current-smoothing reactor coil forming the d.c. link.

An operational similarity exists between a svnchrodrive and a

d.c. motor drive. DC link synchroconverter and a dc motor drive.

Synchroconverter circuit


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Synchroconverter circuit.

Inverter current switching


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Inverter current switching

sequence


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Converter Types

This view considers the rectifier stage as a controlled d.c. supplyand the inverter/synchronous motor combination as a d.c. motor.with the switching inverter acting as a static commutator.

The combination of controlled rectifier and d.c. link is consideredto be a current source for the inverter whose task is then to

sequentially direct blocks of the current into the motor windings The size of the d.c. current is set by the controlled switching ofthe rectifier thyristors.

Motor supply frequency (and hence its speed) is set by the rateof inverter switching.

The six inverter thyristors provide six current pulses per cycle

(known as a six-pulse converter)


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Converter Types

A simplified understanding of synchroconverter control is that thecurrent source (controlled rectification stage) provides therequired motor torque and the inverter stage controls therequired speed.

To provide the motor e.m.f. which is necessary for natural

commutation of the inverter thyristors, the synchronous motormust have rotation and magnetic flux in its rotor poles. During normal running, the synchronous motor is operated with a

power factor of about 0.9 leading (by field excitation control) toassist the line commutation of the inverter thyristors.

The d.c. rotor field excitation is obtained from a separate

controlled thvristor rectification circuit.


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Converter Types

As the supply (network) and machine bridges

are identical and are both connected to a

three-phase a.c. voltage source, there roles

can be switched into reverse. This is useful to allow the regeneration motor

power back into the mains power supply

which provides an electric braking torqueduring a crash stop of the ship.

voltage waveform


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voltage waveform.

Con erter T pes


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Converter Typesa.c.- a.c. cycloconverter

While a synchroconverter is able to provide an output frequencyrange typically up to twice that of the mains input (e.g. up to 120Hz), a cycloconverter is restricted to a much lower range.

This is limited to less than one thtird of the supply frequency (e.g.up to 20 Hz) which is due to the way in which this type ofconverter produces the a.c. output voltage waveform.

Ship ropulsion shaft speeds are typically in the range of 0-145rev/min which can easily be achieved by the low frequency outputrange of a cycloconverter to a multi-pole synchronous motor.

Power regeneration from the motor back into the main powersupply is available. A conventional three phase converter froma.c. to d.c. can be controlled so that the average output voltage

can be increased and decreased from zero to maximum within ahalf-cycle period of he sinusoidal a.c. input.

C t T


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Converter Types By connecting two similar converters back-to-back in each line an a.c.

output frequency is obtained. The switching pattern for the thyristors varies over the frequency range

which requires a complex computer program for converter control. The corresponding current waveform shape (not shown) will be more

sinusoidal due to the smoothing effect of motor and line inductance. The output voltage has ripple content which gets as the output

frequency it is this feature that limits useful frequency. There is no connection between the three motor windings because theline converters have to be isolated from each other to operate correctlyto obtain line commutation (natural) switching of the thvristors.

The converters may be directly supplied from the HV line but it is moreusual to interpose step-down transformers. This reduces the motorvoltage and its required insulation level while also providing additional

line impedance to limit the size of prospective fault current andharmonic voltage distortion at the main supply bus-bar.


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Twin Shaft EL Propulsion


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FPSO Electrical system Layout


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Shuttle Tanker Electrical System Layout


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Shuttle Tanker Electrical Line Diagram


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Drill Ship Electrical System Layout


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The future

Propulsion of ships by help of standard diesel engines usuallygives a non-optimal utilization of the energy.

Today an increased use of diesel electrical propulsion ofships can be seen. New power electronics and electricalmachines will be developed for propulsion and thrusters, as

well as other application on board. Knowledge has to be developed about how such large motor

drives will influence the autonomous power systems on-board.

Even development of new integrated electrical systems forreplacement of hydraulic systems (top-side as well as sub-sea) are becoming areas of need.

Typical system of all electrical


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Typical system of all electrical

ship

Generator sets complete with prime movers and engine controls HV/LV Switchboards, distribution systems and group starter boards Propulsion and thruster motors complete with power electronic

variable speed drives Power conversion equipment Shaft braking Power factor correction and harmonic filters (as necessary) Power management Machinery control and surveillance Dynamic positioning and joystick control

Machinery control room and bridge consoles Setting to work and commissioning Operator training


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Future electrical ship

Future HV ships systems at sea may require voltages up to 13.8kV to minimize fault levels

It is therefore essential that all Marine Engineering personnel aretrained in safe working practices for these voltages.

The Electrical officers of the near future must be fully trained tocarry out maintenance and defect rectification on MediumVoltage (MV) systems.

This will mean a considerable increase in the electrical content ofall training.

Training will also need to be given to non-technical personnel to

ensure everybody is aware of the dangers of these highervoltages.


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Available systems

all electric ship++++

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