HV voltage generation, conversion , transformation and distribution in

ship

ByOladokun Sulaiman

OBJEVTIVE

At the end of the lecture students will be able to acknowledge:

-limiting value for High Voltage system

-High voltage approved for marine application

-High voltage distribution system

-Safety system and requirement for HV

-Safety measure for HV Maintenance

A. HV Application and benefit

WHY HV• REDUCE FULL LOAD CURRENT

• HENCE REDUCE SIZE OF CIRCUIT BREAKER

• REDUCE SIZE OF CONDUCTOR

• HEATING α I^2 AND R

• CURRENT REDUCE 15 TIMES MEANS, HEATING REDUCED 225 TIMES ALLOWING REDUCTION IN SIZE OF CONDUCTOR.

• THEREFORE, CONDUCTOR SIZE CAN BE REDUCED TO A LARGE

MARGIN KEEPING THE HEATING EFFECT UNDER CONTROL.

General Power Systems

• Majority of merchant ships have a 3-phase 3 wire, 440 V insulated neutral earth power systems

• This power system falls in the category of LV and meets the power demands of medium capacity motors up to 200 kW

contd

• When large loads are connected to the LV system the magnitude of current flow becomes too large resulting in overheating due to high iron and copper losses

• P = VI Cos • Copper loss = I2 R [kW]

Losses in Electrical Systems

• Copper Losses in electrical cables and machine windings

• Hysteresis Loss in magnetic cores

• Eddy current loss in conductors and cores

• All these losses are current dependent and rise to very high levels in LV machines for large power ratings

Benefits of H V Power Systems• Ships with large electrical loads now operate at high

voltage of 3.3-6.6 kV• Such high voltage reduces the magnitude of current and

thus lowers iron/copper/eddy current losses and also results in a cooler motor operation

• Low current flow reduces conductors size and hence lowers cost of electrical equipment

• Power transmission more efficient with low loss• A 6 MW motor load at 440 V and 0.8 pf will require SB

fault level to be of 90 kA and cable full load current of 3300 A

• Same power system at 6.6 kV, needs SB fault level of only 9 kA and cable full load current of 220 A

contd

• Flexibility of power system layout– Prime Mover-Generator units can be located

at convenient location away from the propellers

– Propulsion motor can be located at the astern below the water surface

– This results in shorter shaft and reduced vibration

contd• Load Diversity

– A set of prime movers can meet the demand of propulsion as well as ships electrical load

– Prime movers can be operated at economical speed at all times

– Propulsion load can be regulated to meet emergency electrical load without adversely affecting the ships passage time

• Ease of Control

-With advancement in power electronics fine speed regulation of ac motors has become common practice

Typical Marine HV Systems

• 3.3 kV • 6.6 kV• 11 kV

Principal Components of HV System• By generating electrical power at 6.6 kV instead of 440 V,

distribution and switching power about 6 MW becomes easy• -Main Generators• HV Switch Board• HV Cables• HV Transformers • HV Motors (2-12 MW)

Typical HV Equipment Rating

• Main SB = 6.6 –11 kV, Bus Cap =1250A

• Cargo SB = 6.6 kV, Bus Cap =1250 A

• Ballast Pp Motor = 6.6 kV, 330 – 2000 kW

• HD Copm = 6.6 kV, 770 kW, 3560 rpm

• Bow Thrusters = 6.6 kV, 3 MW,

• Transformer = 3400 kVA, 6600/450 V

• Reefer Transformer = 6600/450 V,

Typical HV Power System

HV/LV Power Supply system

Propulsion Plant Layout

Propulsion Plant Layout

Propulsion Plant Layout

Propulsion Plant Layout

HV Propulsion Power System

Concept of Electrical Propulsion

Typical Azipod Prime Mover

Azipod Drive Unit

Typical EL Propulsion

Twin Shaft EL Propulsion

B. HV Voltage Shipboard System

HV Electrical System

• General:• The highest voltage approved for use in marine electrical propulsion

system 6.6 KV.• The ship is equipped with high voltage of 6.6kV with 3 phase 60Hz

neutral insulated system.

• The advantage of having 6.6 KV:

• Minimizing the use of power cable (in length/quantity)e.g. cargo pumps, ballast pumps, etc.

• Smaller breaker sizing due to low ampere.

HV ELECTRICAL SYSTEM ON BOARD

A) Bus bars:• Made form high conductivity copper having sufficient cross section

such that max tem rise is 45K• All bare areas are coated, plated with tin to protect against corrosion• Color markers provided to distinguish between phases

B) Phase Arrangement (view from front panel):

• Phase R (Red) Left Top Front• Phase S (White ) • Phase T (Blue) Right Bottom Rear

CONTINUED

C) Earthing:• For earthling the switchboard to the vessel’s hull, earthling bars and

terminals are provided through out cubicles.

• All incoming and outgoing circuits are provided with switch of earthing and short circuiting for maintenance purpose.

D) Characteristic• Rated voltage 7.2kV• Service Voltage 6.6kV• Rated Frequency 60 Hz• Rated insulation20 kV RMS (one minute)• Protection Degree IP32• Busbar Current 1250 A

POWER GENERATION

• 1 Unit of 6.6 kV Diesel Generator (3450 kW)• 2 Unit of 6.6 kV Turbo Generator (3450 kW)• 1 Unit of 440 V Emergency Generator (550 kW)• Battery Supply• 1 bank of 24V dc ship’s battery• 1 bank of 24 V dc Radio Battery- GMDSS Console

Consumers

Consumers ranging from:

• 6.6 kV- Cargo Pumps, Ballast pumps, HD Compressors

• 440V- Most of machineries in E/R such as Main Sea Water Circ. Pump, Central Fresh Water Cooling Pump, etc

• 220V- Power outlets, lighting, navigation equipments, etc

• 110 V dc- Control circuit for MSB

• 24 V dc- Navigation Equipments

SWITCHBOARDS ARRANGEMENT

Consists of:

• High voltage main switchboard

• High voltage cargo switchboard

• Low voltage main switchboard

• Low voltage cargo switchboard

• Emergency switchboard

• Group starter & Individual Panels

SIMPLIFIED POWER DIAGRAM

DGTG1TG2

HVMSB HVMSB

PORT STBD

LVCSB

HVCSB HVCSB

6.6/ 440V 6.6/ 440V

LVMSB LVMSB LVCSB

SC EDG

CONTINUED

DGTG1

HVMSB HVMSB

PORT STBD

LVMSB

HVCSB HVCSB

6.6/ 440V

LVMSB

TG2

#1 BALLAST PUMP

#2 BALLAST PUMP

#3 BALLAST PUMP

CONTINUED

HVCSB

LVCSB

6.6/ 440V

LVCSB

#2 TANK PUMP

#2 CARGO PUMPS

HVCSB

#1 TANK PUMP

#4 TANK PUMP

#3 TANK PUMP

#2 TANK PUMP

#1 CARGO PUMPS

#1 TANK PUMP

#4 TANK PUMP

#3 TANK PUMP

#2 COMPRESSOR #1 COMPRESSOR

PROTECTION SYSTEM FOR DG & TG

• Protection being provided for the DG and TG to avoid any unsafe occurrences.

• Basically the protection is done by a protection device known as HIMAP and provided at MSB.

• Type of Protection• Under Voltage- setting at 60% / 2 seconds (3960 KV)• Over Voltage- setting at 110% / 5 seconds (7260 KV)• Reverse Power- setting at -10% / 10 seconds (-345 KW)• Instantaneous- • STD setting at 250% (942A) • Instantaneous setting at 1000% (3770A)• Long Time Delay setting at 110% (415A)• Earth Fault

OVERCURRENT SETTING FOR BUS TIE, CSB AND TRANSFORMER

• STD LTD

• BUS TIE 200% (1800A) 0.7s 100% (900A) 30s

• CSB 250% (1500A) 0.57s 100% (600A) 30s

• HV Trans. 300% (630A) 0.57s 105% (221A) 60s

MOTOR PROTECTION

Protection provided by protection device MPR6-DGF

• Over current

• Earth fault

• Phase fail

• Motor Protection provided by independent relay

• Low current

• High current

• Arc fault

EMERGENCY SWITCHBOARD

LVCSB LVMSB LVMSB LVCSB

SC EDG

Essential consumers are being feed by ESB such as steering gear, control air compressor, electric whistle, UPS for IAS, emergency fire pump, etc.

Back feeding operation is possible to supply to the LVCSB or / and LVMSB

During blackout, DG and EDG will start up, and if DG ACV fails to close then EDG ACB will be closed.

OPERATION

• OVERVIEW ON CONTROL SYSTEM

Power management done by IAS:

• Synchronizing: PMS shall select which generator to be synchronized to the bus and then the synchronizer unit (installed at MSB) will control the motor, checks the voltage and frequency, then gives breaker-close signal.

• Frequency control / Load sharing: PMS shall control the speed of the DG or TG in order to keep steady bus frequency.

• Balanced / Unbalance Load sharing: PMS has two modes of load sharing where all generators connected to the bus having the same load or for the unbalanced mode, the load set points are to be decided by the operator.

• Load Dependent Start: PMS will check on the load power (consumer to be started) and measure the load on each generator. If available power is insufficient when starting any load. PMS will start the standby generator.

• Blackout Monitoring: If blackout occurs, the PMS will start up the Diesel Generator and connect it to the bus.

CONTINUED

• DIESEL GENERATOR CONTROL:• With the DG in standby mode, it shall start automatically when;

A. Blackout occurs

B. Low voltage on main busbar

C. High voltage on main busbar

D. Low frequency on main busbar

E. Low/low frequency on main busbar

F. ACB abnormal on running generator

G. Over current on running generator

H. Both boiler trip or low steam pressure at turbo generator

• For condition A, generator shall start and connect to main busbar

• For condition B, C and E the generator shall start and connect to busbar after the TG in service ACB has tripped

• For Condition E, D, F , G and H the generator shall stat, synchronize, connect automatically to the busbar and run in parallel with balanced load sharing.

HV MSB

• The operation could be in manual or auto. However, it is always to be kept in auto since control is done from IAS.

• All necessary mimics and controls are being provided inside IAS.

• Manual operation is as per normal low voltage switchboard where indication light, breaker control, meters. Governor control, etc being provided on the switchboard.

• Generators are being protected by a device known as HIMAP. All protection settings are being set inside the HIMAP.

• Vacuum Circuit Breakers (VCB) are being used for the bus tie and generator Panels.

• Vacuum combination Contactors are being used for the ballast and cargo pumps.

CONTINUED

• HV CARGO SWITCHBOARD:

• Cargo switchboard is equipped with soft starter for the cargo pumps. By pass switch is provided in order to start by direct on line.

• Soft starter panels are provided for each port and starboard cargo switchboard.

• Facility to simulate the cargo pumps is possible. This is done by disconnecting the breaker and removing the low current relay. This is done prior to discharge port.

SOFT STARTER

CURRENT CURRENT

TIME TIME

CURRENT CONTROL TORQUE CONTROL

Torque control: is the most efficient way of starting motors. Unlike voltage and current based systems the soft starter monitors the torque needed, and allows to start with the lowest possible current . Using a closed loop torque controller, linear / ramp control are possible.

Current control: the voltage ramp can be used with a current limit which stops the voltage ramp when the set maximum current level is reached. The maximum current level is the main setting and must be set by the user depending the maximum current allowed foe the application.

POWER GENERATION

• Generator particulars: Maker: Nishishiba Electric, Type: Brushless Excitation System, Voltage: 6600V Output: 3450KW rated Current: 377A Frequency: 60Hz, Full Load Speed: 1800RPM, Rotor Type: Salient Type, Protection: IP55

• A dc current is applied to the rotor winding which produces a rotor magnetic field. The rotor is then turned by prime mover producing a rotation of magnetic field. This magnetic field induces a three phase set of voltages within the stator windings, the dc current is set at 5.6A by maker.

• A brushless exciters are used to supply this dc current. A brushless exciter is a small ac generator with its field circuit mounted on the stator and its armature circuit mounted on the rotor shaft.

• The three phase output of the exciter generator is rectified to direct current by rectifier circuit, also mounted on the shaft and then fed to the main field winding (rotor winding)

CONTINUED

• By controlling the small dc field current of the exciter generator, it is possible to adjust the field current on the main field or rotor winding.

• To make the excitation of generator completely independent of any external power sources, a small pilot exciter known as PMG is included in the system.

• The PMG is actually a small ac generator with permanent magnets mounted on the rotor shaft and a three phase winding on the stator. It produces power for the field circuit of the exciter which in turn controls the field circuit of the main machine.

PROCEDURE OF MEGGERING THE CARGO PUMP

• The switchboard is designed with many interlock keys. Therefore, knowing the steps are essential:

• A) ensure the cargo pump is in stop condition.

• B) Turn the key and disconnect the breaker by using handle.

• C) Turn the other key to enable the handle for mechanical earthing switch to be inserted.

• D) Turn the mechanical earthing switch to earth the breaker

• E) Take the key and bring it to rear side

• F) Turn the key to open the bottom compartment

• G) While the compartment is in open condition, turn the key off and bring back to front panel to disable the mechanical earthing

• H) Confirm that the pump has no residual potential by using potential detector.

• I) The meggering can be done as usual for the pumps

Describe the high voltage distribution board considering electrical safety

OC

(INS

T)

OC

(I/T)

UV

UB

RP

E LE

AK

G G

M

M

M

M

G G

OC(INST)

OC (I/T)

UV

UB

E LEAK

OC(INST)

OC (I/T)

E LEAK

OTEMP

OC(INST)

UV

OFEQ

UFEQ

OC(INST)

T O LD

L ROTOR

E LEAK

6.6 KV 60 HZ MAIN SWBD

440 V SWBD 440 V SWBD

C. HV Safety

Precautions before work on HV

• Safety wearing must be used before proceeding to job.

• Isolate power

• Test HV live-line tester to prove its proper functioning.

• The circuit on which work is to be carried out, must be tested and proved dead by an HV live-line testing equipment.

• The circuit to be earthed down by connecting with hull.

• Issue EPTW.

• Minimum two people should work together.

Safety precautions associated with operation of high voltage system

• Before start of work an Electrical Permit to Work must be issued.

• EPTW is prepared and approve by the authorized officer.

• The copied permit signed by the responsible person usually has at least 5 sections with the first stating the work to be carried out.

• The next section is a risk assessment declaring where isolation and earthling has been applied and where danger/caution notices have been displayed then the permit is signed as authorized by the Chief Electrical Officer or Chief Engineer.

• In the third section, the person responsible for the work (as named in section one) signs to declare that he/she is satisfied with the safety precautions and that the HV circuit has been isolated and earthed.

• Section four relates to the suspension or completion of the designated work.

• The last section cancels the permit with a signature from the authorizing officer.

• The EPTW is valid only for 24 hours.

Marine Electrical HV System • Maritime electric systems include power generation,

distribution and control, and consumption of electric power on supply- service- and fishing vessels as well as offshore installations.

• Electric propulsion has increased especially for vessels with several large 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 integral parts of the systems.

• The power consumers are large compared with the total capacity of the system, as for example thruster and propulsion systems for DP operated vessels, drilling systems, HVAC systems on board ship

Marine Electrical HV Propulsion System

• Overall power train efficiency with EP is around 87-90%. • Employment of permanent magnets in electric generators

and motors as well as general advances in semiconductor technology may improve this figure to around 92-95% in the near future.

• Electrical transmission 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

Systematic overview of existing typesE-generator

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

E-Motors

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

Power convertersElectrical ==> Electrical: power conversion or transformation• - Fixed transformers• - Controlled converters• - Static converters• -Inverter

Structure of a combined power plant for ships

Electric Propulsion System • 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 prime movers

• Early large passenger vessels employed the turboelectric system which involves the use of variable speed, and therefore variable frequency, turbo-generator sets for the supply of electric power to the propulsion motors directly coupled to the propeller shafts. Where, the generator/motor system was 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 provide power 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.

Shuttle Tanker Electrical System Layout

Shuttle Tanker Electrical Line Diagram

Drill Ship Electrical System Layout

HV CIRCUIT BREAKERS

• Air Circuit Breaks

• Oil Circuit Breakers

• Air-ballast Circuit Breakers

• Gas (SF6- Sulphur Hexafluoride) Circuit Breakers

• Vacuum Breaker

• AIR CIRCUIT BREAKERS:

• Used for low voltage

• Arc chutes and arc contacts are incorporated

Arc Contact (make first and break last)Main Contact ( copper strips clamped together in the manner of a leaf spring)

Generator input

Bus bar

Opening spring

Close by handle

• Air blast circuit breakers:

• High pressure at about 30kg/cm^2 air is blown during the operation of circuit breaker.

• Capability: 40 KA at a line voltage of 765 KV in a matter of 3 to 6 cycles on a 60 Hz line.

• Operation is too noisy.

OIL CIRCUIT BREAKERS

• Napthenic base petroleum [(CH2)n] have been carefully refined to avoid sludge or corrosion.

• The resulting insulating oil is identified as type 10-C transformer oil.

• Excellent dielectic strength.• High thermal conductivity.• Prone to fire hazard, leakage/contamination.

• SF6 CIRCUIT BREAKERS:

• The gas is a chemically very stable, non flammable, non corrosive, non poisonous, colourless and oduorless.

• Limits the sonic velocity (1/3 of air).• Excellent dielectric strength, about twice of air.• Used in the range from 3 KV to 800 KV.• GWP (global warming potential is high)• Lifetime 3200 years.

• VACUUM CIRCUIT BREAKER:

• Being used in the range from 5 to 38 KV.

• The arc remains in the diffused column mode until the current exceeds 15 KA.

SPECIFICATION OF A CIRCUIT BREAKER

• The maximum steady-state current it can carry.

• The maximum interrupting current.

• The maximum line voltage.

• The interrupting time in cycles

Critical Operation

• The interrupting time may last from 3 to 8 cycles on a 60Hz system.

• To interrupt large currents so quickly, rapid deionization of the arc, combined with rapid cooling is to be ensured.

Operation of a circuit breaker

• Manually: By pulling moving contacts from the fixed contacts. For large capacity circuit breaker it is not advisable.

• Because to open the contacts against heavily loaded spring will take a lengthy breaking operation.

• As a result transient recovery voltage will act across the contactors through the ionized arc.

• This may cause damage to the circuit breaker contacts.

• It may also cause instability of the system.

Automatic Operation

• The triggering action is normally done by electrically operated motor or magnetic coil that is operated by a push button.

• Opening and closing both done electrically.

Vacuum Circuit Breaker

• Dielectric Strength: The Potential gradient necessary to cause breakdown of an insulating medium is termed its dielectric strength and is usually expressed in MVs/meter of thickness. The value of DS is reduced with increase of thickness.

• For air of 0.2 mm thick DS 5.75MV/m, for 1 mm thick 4.46MV/m, For mica of 1mm 61 MV/m

• The high dielectric strength of a vacuum allows a very short contact separation.

• Unlike other circuit breaker a rapid re-strike free interruption of the arc is achieved.

Vacuum Circuit Breaker

• When an alternating current is interrupted by the separating contacts, an arc is formed by a metal vapour from the material on the contact surfaces

• And this continues to flow until a current zero is approached in the ac wave form.

• At the next instant the arc is replaced by a region of high dielectric strength which is capable of withstanding a high recovery voltage.

• Metal vapours condenses back onto the contacts.

Vacuum Circuit Breaker

METAL BELOW

CONTACTS IN VACUUM CHAMBER

CERAMIC INSULATOR

MOVING CONTACT BOLT

FIXED CONTACT BOLT

SHIELD

ADVANTAGE OF VACUUM CIRCUIT BREAKER

• COMPACT

• MINIMUM MAINTENANCE

• NON FLAMMABLE

• NON TOXIC

• LIFE- 20 YEARS

SF6 CIRCUIT BREAKER

• Sulphur Hexafluoride gas at a pressure of about 5 bar is kept in sealed contact

chamber.

MAIN CONTACTS

ARC CHAMBER

SF6

EPOXY RESIN CASE

OPERATING MECHANISM

• HV Variable Frequency Drive

Controlled Rectification• Diode, having only two terminals- they cannot control the

size of the d.c. output from the rectifier.• For controlled rectification it is necessary to use a set of

three-terminal devices such as thyristors (for high currents) or transistors (for low - medium currents).

• An equivalent three phase bridge requires six diodes for full-wave operation.

• Other single-phase circuits using a biased arrangement with two diodes and a centre-tapped transformer will create full-wave rectification

• Similarly, four diodes in a bridge formation will also produce a full-wave d.c. voltage output.

• A basic a.c.-d.c. control circuit using a thyristor switch is shown in the next slide.

Three-phase controlled rectifier bridge circuit.

Full wave controlled rectification from a three-phase a.c. supply is achieved in a bridge Circuit with six thyristors a shown

Three-phase controlled rectifier bridge circuit.

• Compared with a diode, a thyristor has an extra (control) terminal called the gate (G).

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

• Gate voltage pulses are provided by separate electronic circuit and the pulse timing decides the switch-on point for the main (load) current.

• The load current is therefore rectified to d.c. (by diode action) and controlled by delayed switching.

• In the circuit an inductor coil (choke) smooth the d.c. load current even though the d.c. voltage is severely chopped by the thyristor switching action.

• An alternative to the choke coil is to use a capacitor across the rectifier output which smooths the d.c. voltage.

Three-phase controlled rectifier bridge circuit.• 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.) voltage by using a set of thyristor (or transistor) switches.

• The inverter bridge circuit arrangement is exactly the same as that for the rectifier.

• The d.c. voltage is sequentially switched onto the three output lines.• The rate of switching determines the output frequency.• 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 with the rotor to produce torque.

• The processes of controlled rectification and inversion are used in converters that are designed to match the drive motor.

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

Controlled three-phase thyristor bridge inverter is shown

Converter TypesThe principal types of motor control converters are: - >a.c.-d.c. (controlled rectifier for d.c. motors) . a.c.-d.c.-a.c. (PWM for

induction 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 which produces the magnetic flux .

• Some systems may have a fixed field current which means that the field supply only requires an uncontrolled diode bridge

Converter Types• Shaft rotation can be achieved by reversing either the field current

or the armature current direction. • Ship applications for such a drive would include cable-laying,

offshore drilling, 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 Also, the converter do not allow power from the motor load to be regenerated back into the mains supply during a braking operation.

Controlled rectification converter and d.c. motor

PWM converter and a.c. induction motor

Converter Types• PWM involve conversion 440V a.c. supply To 600V, the

rectified d.c. (link) voltage will be smoothed by the capacitor to approximately 600 V.

• The d.c. voltage is chopped into variable width, but constant level, voltage pulses in the computer controlled inverter section using IGBTs (insulated gate bipolar transistors).

• By varying the pulse widths and polarity of the d.c. voltage it is possible to generate an averaged sinusoidal ac. output over a wide range of frequencies typically 0.5-120Hz.

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

• By sequentially directing the currents into the three stator windings, a reversible rotating magnetic field is produced with its speed set by the output frequency of the PWM converter.

Converter Types• Accurate control of shaft torque, acceleration time and resistive

braking are a few of the many operational parameters that can be programmed into the VSD,usually via a hand-held unit.

• The VSD can be closelv tuned to the connected motor drive to achieve optimum 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 other computer devices e.g. programmable logic controllers (PLCs) for overall control of a complex process.

Converter Typesa.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 successfully to marine electric propulsion.

• A synchroconverter has controlled rectifier and inverter stages which both rely on natural turn-off (line commutation) for the thyristors by the three phase a.c. voltages at either end of the converter.

• 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.

Inverter current switching sequence

Converter Types• This view considers the rectifier stage as a controlled

d.c. supply and 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 considered to 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 of the rectifier thyristors.

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

• The six inverter thyristors provide six current pulses per cycle (known as a six-pulse converter)

Converter Types• A simplified understanding of synchroconverter control is

that the current source (controlled rectification stage) provides the required motor torque and the inverter stage controls the required speed.

• To provide the motor e.m.f. which is necessary for natural commutation of the inverter thyristors, the synchronous motor must 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) to assist the line commutation of the inverter thyristors.

• The d.c. rotor field excitation is obtained from a separate controlled thvristor rectification circuit.

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 torque during a crash stop of the ship.

Cycloconverter circuit and output voltage waveform.

Converter Typesa.c.- a.c. cycloconverter• While a synchroconverter is able to provide an output frequency

range typically up to twice that of the mains input (e.g. up to 120 Hz), 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 of converter produces the a.c. output voltage waveform.

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

• Power regeneration from the motor back into the main power supply is available. A conventional three phase converter from a.c. to d.c. can be controlled so that the average output voltage can be increased and decreased from zero to maximum within a half-cycle period of he sinusoidal a.c. input.

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 the line converters have to be isolated from each other to operate correctly to obtain line commutation (natural) switching of the thvristors.

• The converters may be directly supplied from the HV line but it is more usual to interpose step-down transformers. This reduces the motor voltage and its required insulation level while also providing additional line impedance to limit the size of prospective fault current and harmonic voltage distortion at the main supply bus-bar.

Twin Shaft EL Propulsion

FPSO Electrical system Layout

Shuttle Tanker Electrical System Layout

Shuttle Tanker Electrical Line Diagram

Drill Ship Electrical System Layout

The future• Propulsion of ships by help of standard diesel

engines usually gives a non-optimal utilization of the energy.

• Today an increased use of diesel electrical propulsion of ships can be seen. New power electronics and electrical machines 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 for replacement of hydraulic systems (top-side as well as sub-sea) are becoming areas of need.

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

Future electrical ship• Future HV ships systems at sea may require voltages up

to 13.8 kV to minimize fault levels • It is therefore essential that all Marine Engineering

personnel are trained in safe working practices for these voltages.

• The Electrical officers of the near future must be fully trained to carry out maintenance and defect rectification on Medium Voltage (MV) systems.

• This will mean a considerable increase in the electrical content of all training.

• Training will also need to be given to non-technical personnel to ensure everybody is aware of the dangers of these higher voltages.

Available systems