Marine AC Generators

AC Generator:

Main generator power rating range from 100KW to 2MW at 440v 60Hz AC driven by diesel,steam/gas turbine or propulsion shaft driven.An emergency generator is usually 20KW to 200KW at 440V.Basic principle of an AC generator is simple.
Pairs of electromagnetic poles are driven past fixed coils of wire on stator by a primemover.An alternating emf which has a sinusoidal wave form is induced into each stator phase winding.Size of emf generated depends on strength of magnetic flux and rate at which flux cut the coil. E=n.∅ n is rotational speed of poles, ∅ is magnetic flux.
Frequency f of emf is the no.of waveform cycles per second.This depends on no.of poles and rotational speed. f=n.p

In practice speed is maintained constant by generator's primemover which fixes the output frequency.Constant speed then allows size of generated emf to be controlled by size of pole flux.Emf generated in each phase is 120° out of phase with each other.One end of the each 3 phase windings are connected each other to form neutral or star point.Other end of the windings are connected to outgoing conductors called lines.

 Two main parts of generator are stator and rotor.

Fabricated steel stator frames support stator core and its windings.It is assembled from laminated steel with windings housed in slots around inner periphery of cylindrical core.
Rotor are of 2 types- salient pole type[<1800rpm] and cylindrical type[1800-3600rpm].Salient pole type has projecting poles bolted to shaft hub.Field excitation windings are fitted around each poles.Cylindrical rotor have excitation windings wedged into axial slots around steel rotor.

Inorder to prevent bearing damage from circulating shaft current all non driving end bearings are insulated from earth by a thin layer of insulating material [PTFE-polytetraflouroethylene or teflon] beneath bearing pedestal.Pedestal holding down bolts also must be insulated.
Pedestal bearing is lubricated by an oil ring.



Generator cooling are of two types - 1.air to water cooling [HV system] 2.air to air cooling
Cooling air is forced through ventilation ducts in stator core, between rotor poles and through air gap between stator and rotor.
Temperature detectors are used to monitor the temperature of stator windings ,bearings and the cooling air/water.
While generator is stopped space heaters are provided to avoid internal condensation forming on winding insulation.

Shaft Generator:

Shaft driven generators are driven by main engine to supply power to the mains.The mains must be supplied by constant voltage and frequency by shaft generator even at changing speed of main engine.
Advantages :

• reduced emission
• lower fuel consumption
• reduced noise levels
• reduction in maintenance and lub oil cost

Apparent disadvantage of shaft generator is that it does not have direct frequency control and is determined by ME speed.Frequency must be separately regulated.

• 3∅ rectifier converts AC into DC
• 3∅ controlled inverter converts DC back to fixed output frequency AC.[achieved by sequenced thyristor switching]
• A DC link inductor coil smoothens the current flow[reduce ripples] and act as a current limiter in the event of short circuit fault
• Inverter thyristor is turned ON by +ve current pulse to its gate.Thyristor is turned OFF when it's current is reduced to zero.This is a problem for inverter thyristors when driving into ship's inductive load[about 0.8 pf lagging].Thyristor can switch off only when p.f is unity ie, voltage and current pass through '0' point at the same time[in phase].To over come this leading p.f is injected using synchronous compensator.Addition of synchronous compensator solves problem as sytem works in unity.Hence shaft generator must only supply active power KW.At every instant leading KVAr +Q must match with lagging KVAr -Q of ship's load.Compensation must be automatically controlled and it is done by including a synchronous motor as synchronous compensator whose regulating pf is controlled by regulating it's DC field current.Synchronous compensator also dampens harmonics.Synchronous motor running at no load act as variable capacitor when it's excitation is changed hence called synchronous condensers.

Harmonics:
Harmonic components are periodic wave having a frequency that is integral multiple of fundamental frequency.They are voltages and currents at integral multiple of fundamental frequency 50Hz. ie, 100Hz-150Hz-200Hz etc.
Harmonics are designated by harmonic number.Harmonic with a frequency of 150Hz is 3rd harmonics.They super impose on fundamental frequency distorting it and changing it's frequency.
Harmonics causes overheating of transformers,cables,motors,generators,capacitors etc.Electronic displays and lighting will flicker,breakers can trip,computers may fail etc.
Harmonic filters avoid/bypasses harmonics generated.

Marine Electrical Distribution

Function of ship's electrical distribution system is to safely convey the generated electrical power to every item of consumer equipment.

 Advantages of AC distribution:

• high power to weight ratio
• transformers can step up and step down AC voltages
• 3∅ AC is efficiently converted into rotary mechanical power

Majority  of the ships have 3∅ AC 3 wire 440V insulated neutral system.Neutral point of star connected generator is not earthed to ship's hull.High voltage ships have neutral point earthed to ship's hull through a resistor or an earthing transformer.60Hz is most common power frequency adopted.Higher frequency means that motors and generators runs at higher speed with a consequent reduction in size for a given power ratio.

General Distribution Scheme:

Circuit breakers and switches are means of controlling the flow of electric current.Fuses and relays protects the distribution system. Electrical load is divided into essential and non essential services.Most essential services are fed via an emergency switchboard and others via main switchboard.

Emergency Power:

Many vessels regain their power system after a blackout using an emergency generator and switchboard.Emergency power source supply all essential services for safety in an emergency.

• emergency source must be independently driven and shall have an independent fuel source
• shall be started automatically in the event of a power failure
• shall connect to ESB in less than 45seconds
• shall work when ship is at a list of 22.5° either sides or 10°trim fwd or aft  

Location of emergency power source:

• emergency source of power is located in such a way that a fire in machinery space will not interfere with supply,control and distribution of emergency electrical power
• space containing emergency power source shall not be contiguous to the boundaries of machinery space
• emergency switchboard is to be installed as near as practical to emergency source of electrical power

Starting arrangements of emergency generator sets:

• shall be capable of starting in cold condition at a temperature of 0°Celsius.If lower temperature can be encountered then heating arrangements must be operational
• flash point of fuel used must be more than 43°celsius
• EG sets should have starting devices with stored energy capability of 3 consecutive starts
• in addition another source of energy is to be provided for an additional 3 starts within 30minutes.
• stored energy for starting EG sets are always maintained as - electrical system is maintained from ESB , if started by air then dedicated compressor and reservoir are provided.Starting, charging and energy storing device are to be same room.

Air circuit breaker of EG to emergency switchboard is interlocked with bus coupler from main switch board.When power from MSB is lost, ABT[ automatic bus transfer] will connect EG to ESB.This would occur only when MSB is clear of all loads.This ABT also called as bustie which prevents paralleling of EG with main generators.
Emergency generator should have fuel for 18hrs.

Periodic testing of emergency generator:


Emergency generator may be used for short period such as routine testing and dead ship start up.
Weekly testing shall be carried out during drills which include simulation of loss of main power.
Start equipment may provide a test switch button to interrupt the normal power supply from MSB.Emergency generator should be run upto its rated speed for a short time.







 


Shore Supply or Cold Ironing:

Shore supply or cold ironing is required so that major overhauling of ship's generator and primemover is possible at drydock.Moreover cooling system of prime mover cannot be run so easily.
Cold ironing was the term used with olden days coal fired engine.When ships arrives at port,engines were not required and they will cool down and eventually turn cold.
Cold ironing also avoids air pollution and reduces fuel consumption at dock.
Shore supply switch on MSB is interlocked with generator supply breakers.So it cannot be switched ON when power is available on MSB or ESB.Generator breakers will not close if shore supply breaker is ON and alive.



Isolated and Earthed Neutral System:

Isolated or insulated neutral system is one that is totally electrically insulated from earth[ship's hull].

In an insulated neutral system if one earth fault occurs, no earth fault current will flow.With such a sytem an electric motor serving an essential equipment can be kept running until stoppage for repair.
But if there occurs two earth fault and second earth fault occurs in another circuit, large fault current flow occurs which will be equivalent to a short circuit and protective devices would operate.

In tankers it is not advised to  have earthed neutral system.An earth fault current occurring could circulate in ship's hull within the hazardous zone which may cause an explosion of flammable cargo.

*Earth fault in earthed neutral system

Earthed neutral system has supply neutral point connected to earth.HV systems >1000V are usually earthed to ships hull via a neutral earthing resistor [NER] or through a high impedance transformer.Ohmic value of earthed resistor is usually chosen so as to limit the earth fault current not to go more than the full load current rating of generator.

If a single earth fault occurs in earthed neutral sytem, fault current passes through NER. An earth fault relay and CT is fixed in this case to monitor the fault current and to trip the faulty circuit.



Earth Fault Monitoring System:

Earth fault indication by set of lamps -

• If there is an earth fault, when the test button is pushed lamp will not glow for the faulty phase as there is no potential difference across it
• In each phase current will flow without hindrance.In phase having earth fault current will take an easier path than through the resistance.Both ends of the lamp will be in ground potential

 Instrument type earth fault indicator:

• Instrument applies a small DC voltage into the distribution system.The resulting current being measured to indicate insulation resistance of the system.
• Instrument permits a maximum earth monitoring current of only 1mA and indicates IR directly in KΩ.It gives both visual and audible alarm.

E/F Indicator-instrument type

In HV system earth fault indicator can just be an ammeter measuring neutral current via a current transformer.

Protection in an earthed system can also be provided using current transformer[CT].If motor is healthy, phasor sum of current measured by CT is zero.If an earth fault occurs in motor,phasor sum of current will not be zero.Current monitored by E/F relay trips the contactor in starter to isolate the faulty motor circuit.










Earth Fault Detection and Clearance:

• Suppose an earth fault occurred in 220V lighting d.b, earth fault monitor for lighting d.b indicate the presence of earth fault.
• Switches A,B,C are one by one switched OFF and ON to find the circuit of fault presence.Suppose earth fault is at B.
• Circuit B supplies to a lighting distribution board where earth fault indicator is not present.Here IR tester is used to find E/F location.

•  At distribution fuse board, fuse pair no 1 is removed to isolate load.IR tester is used between B and hull.If IR is healthy the connect between A and hull.If found healthy fuse pair can be put back.Now circuit no2 fuses are removed and thereby checking is carried out.
• If circuit no2 is found unhealthy,then fault has located.In his case circuit no.2  fuses are removed, switches are opened, lamps are removed to split the circuit into different conductors which are isolated.
• Now IR test is carried out on each individual conductors to pinpoint the E/F.

MCB

Distribution Circuit Breakers:

MCB- Miniature Circuit Breakers [5-100A]

• Limited breaking capacity.
• Commonly used in final distribution boards instead of fuses
• Has thermal over current protection and short circuit protection

MCCB-Moulded Case Circuit Breakers [50-1500A]

MCCB

• Lower breaking capacity compared to main breakers
• Has adjustable thermal over current trip,adjustable magnetic over current trip,short circuit protection,under voltage trip, back up fuses.

 Transformers Onboard Ship:

Laminated steel core transformers are used.They are generally air cooled and mounted in sheet steel enclosures close to main switch board.Transformer on 3∅ system are generally interconnected by a delta-delta circuit configuration.
440V-220V Transformers are △-△ configuration

Ship's Electrical System, Safety and Checks

Electrical Power onboard - 440V,60Hz also 380V,50Hz

High Voltage ships-3.3KV,6.6KV,11KV[ To limit the size of load current thereby also fault current]. As per British standards L.V is 50V-1000V AC.

Lighting and low powered ancillary services - 110V or 220V.

Portable equipments- 55V or 24V.

Bow thrusters - 3.3KV

Batteries- 12V or 24 DC

Power Management System [PMS]:

An electrical power management system is to ensure continuous availability of electrical power and to avoid blackout.Main switch board is associated with this.

PMS is for

1. ship’s operation mode selection - harbour mode, manoeuvering mode, at sea mode
2. generator management during black out
3. load dependent starting and stopping of generators with all safeties
4. autosynchronistaion, load sharing and frequency control
5. preferential trips, various protection systems, load inhibition of heavy consumers

Electrical Diagrams:

• block diagrams - simplified form, shows interrelationship of elements in system
• system diagrams - illustrates ways of operating systems,shows main features
• circuit diagram - shows in full the functioning of a circuit, essential tool for troubleshooting
• wiring diagram - shows detailed connection between components , mainly to instruct wiring installers how to construct and connect

Electrical Safety:

To work on ship’s electrical system -

• get to know / study ship’s electrical system
• operate and maintain as per manufacturers instruction
• ensure all guards, cover and doors are fixed properly
• inform OOW before shutting down machinery
• Risk assessment and work permit to be taken
• confirm circuit is dead[ by voltage tester]
• LOTO - lock out & tag out

Electrical Shock:

• A shock current as low as 15mA DC or AC is fatal
• body resistance goes down as applied voltage goes up - typical dry full contact body resistance is 5000ohm at 25V, 2000ohm at 250V
• voltages 60V and below is safe for portable hand use. 24V portable hand lamps are generally used for confined spaces. special step down transformers are used with portable equipments for which secondary winding is centre tapped to earth [splits winding into two sides with each having only 55V with respect to earth for a 440-110V stepdown transformer.55V is still fatal but better than full 110V]

Welding equipment safe practices:

• insulate yourself from work piece and return cable by wearing dry insulated clothes
• use rubber sole shoes and gloves in good condition
• use good lighting
• wet working condition to be avoided, even person’s perspiration can lower body resistance
• terminals on welding machine to be tight and clean
• do not use damaged / worn cables and lugs
• do not touch return line as it carries as much current as welding cable
• there should be good earthing arrangement made close to the job
• avoid using hull as return conductor as this will circulate current in hull through parts like bearings causing damage.
• do not wrap cables carrying current around body
• electrodes should not be inserted into a live holder
• use fully insulated electrode holder
• do not touch energized electrode with bare hands
• handtools and welding equipment must be inspected periodically[once in 3months]
• ELCBs to be installed in extension boards for providing temporary power supply
• check condition of quick detachable connection of 1.5metres, insulated wheels of machine, insulation of line connection

Insulation Resistance:

Insulation prevents leaking away of current from conductors and prevents contact with live wires.Insulation resistance is measured between

• conductor & earth
• conductors

Surface deposits reduce insulation resistance.Flow of leakage current through surface deposits [dirt,oil,moisture] on insulation is called tracking.Insulation resistance is also affected by creepage and clearance between terminals.

Equipments must be maintained clean to prevent tracking and to maintain high value of insulation resistance.Insulation is also affected by humidity,temperature,electrical and mechanical stress,vibration,chemical,oil,dirt,grease and ofcourse aging.All marine equipments are constructed to work satisfactorily at a temperature below 45degree celsius.

Various classes of insulation at which they are safe to operate :

These are steady surface temperature measured with equipment stopped and no flow of cooling air. Hotspot temperature of 105degrees-classA,130degrees-classB is accepted as normal temperature at centre of coils and windings with surface temperature as mentioned in table.Machines operated continously with above hotspot temperature has life expectancy of 15 to 20yrs.However every 10degree rise of hotspot temperature reduce life by half.

Insulation testing: IR tester is used

Resistance is measured between

• conductor and earth
• conductors

500V DC producing megger is used. In H.V ships test voltage produced is at the range of 5000V and above.

To prove basic operation switch to megaohm , short two probes together and hit test button.Pointer should indicate around 0ohm.Before testing, equipment must be LOTO. To test if equipment is live IR tester is to be switched to megohm and probes are connected to pairs of terminal. Do not press hit button. Meter will now indicate if the circuit is live or not.

For an IR test of a 3 phase machine 3 readings are measured as U-V, V-W, U-W.

Phase to earth is also measured as U-E,V-E,W-E.

Insulation becomes more leaky at high temperature. So testing while at its working temperature shows realistic value.Regular recording of the test gives a trend which if downwards indicates impending trouble.

Continuity Testing: IR Tester/Multimeter is used

• Prove correct operation by shorting probes and hitting test button.Should show no resistance.
• Equipment to be LOTO
• Prove whether equipment is dead
• Switch instrument to ohm or continuity
• Connect probes to circuit and hit test button
• Check indication of pointer on scale

Measuring AC Voltage:Multimeter is used

• Set selector switch to appropriate voltage range
• Hold insulated probes and connect the tips of the probe to a known live voltage source like AC 220V socket and verify working of meter
• When not aware of the voltage set it to maximum range
• Connect the probes across the point to be measured and note the value

Measuring current using clamp meter:

This equipment is used against live circuit, atmost care is to be taken.Meter must be certified and be capable to measure current in that circuit.If not aware of the range then set it to maximum.

Hold meter over the conductor by pressing the button to open the spring loaded clamp and check reading.

Latest clampmeters measures power and power factor too.For multicore cables it shows zero as net flux is zero.

Live Line Testers are simple devices to check whether or not a voltage exist at terminals.They are of various types-some light up,some make noise or operate LED etc.