Marine Electrical Systems

 

Marine Electrical Systems Marine Generation & Distribution Systems PART 1:

 Explain why marine electrical generation and distribution employs employs an insulated  neutral: The main reason that marine electrical systems employ an insulated earth is that if an earth fault occurred on one line then the piece of equipment will not trip. Earth faults can be caused by insulation failure or a loose wire. If a second earth fault occurs on a different line a short circuit will result, causing the equipment to trip.  Describe with the aid of a circuit diagram how earth faults faults are monitored using  lamps: In 3 phase distribution 3 indicating lamps are used. They are connected in star, with the star point being earthed. If an earth fault occurs on one of the lines that lamp in that line will appear dim. The intensity of the light indicates the degree of the earth fault. Below is a circuit diagram of earth fault monitoring lamps:

 Define High Voltage In Marine Systems: High voltage in a marine system is any voltage that is 1000v (1kv) or larger. Typical marine applications are 3.3Kv, 6.6Kv and more rarely 10Kv.    Explain the basic idea of using a Neutral Earthing Earthing Resistor (NER) with marine  generators: A neutral earth resistor is there to limit the maximum earth fault current that could  be produced if the generator had an earth fault. It is connected between the star point of the generator and earth. The fault currents created could be massively larger than the generators designed full load current rating. If this current is not controlled then

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serious damage by overheating the winding insulation. The earth fault current is determined by the size of the NER. M3

 Explain two reasons why it is important to locate and and clear earth faults: If there is an earth fault on one line then due to the insulated neutral the piece • of equipment will not trip. If another earth fault occurs on another line then a short circuit is created which will cause the piece of equipment to trip, if this is an essential piece of equipment it could endanger the ship/machinery. This is one reason why it is important to locate and clear earth faults. Secondly if another earth fault occurs in the same line then it becomes • increasingly difficult to locate the earth faults.  Explain the main advantage of using using HV for marine installations:   The main advantage of using HV for marine installations is it gives greater power for  lower currents compared to LV applications. This results in being able to use smaller  gauge cable due to the lower currents, and in turn the build cost of a ship would be greatly reduced.  Investigate the IIE 6 th Edition of the regulation for installation of electrical equipment  on ships, and summarise the regulations regarding earthing of high voltage installations:

PART 2

 

Distribution Diagram on separate sheet

 Explain the likely connections for primary and secondary three phase transformer  windings: The 3 phase transformers will most commonly comprise of 3 single phase transformers. If one of the single phase transformers fails then the 3 phase output will remain but at reduced power. The primary and secondary coils will normally be connected in delta-delta so if one single phase transformer is to fail then it becomes an open or ‘v’ delta-delta transformer. This is an advantage as the output is not be lost and will still supply machinery i.e. they will not stop. Also only one phase will need isolating to repair the transformer apposed to the solid 3 phase core type, where the whole transformer needs to be isolated to be repaired.

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 Identify two examples of essential and two examples of non-essential services: Essential services are those that are required in the event of an emergency. Essential services: Steering gear is classed as an essential service aboard a ship. In the event of  •  power loss from the main switch board then the steering gear should still be able to function, when the emergency generator is supplying. Therefore one of  the hydraulic oil pump motors will be supplied by the emergency switch  board. Fire prevention is classed as an essential service. If power is lost from the • main board the emergency fire pump should still be able to operate. The emergency fire pump motor will be supplied by the emergency switch board.  Non-Essential services: Air conditioning is classed as a non-essential service. If power is lost from the • main board then the air condition will stop, so is only supplied by the main  board. Sewage system would be classed as a non-essential service. •

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Use your own symbol to show where earth lamps would be fitted:

 Explain the equipment required at a shore connection connection point: The shore connection box must have suitable terminals to accept shore supply cable including an earth terminal to earth the ships hull. It must also the have a circuit breaker  or an isolator switch and fuses to protect the cable connecting box to the main switch board. It should also contain a data plate giving details of the ship’s electrical system, showing the methods of connecting the cable. A volt meter and phasesequence indicator is fitted, to indicate the correct supply phase sequence. When the right lamp is bright and left one dark the sequence is correct. An alternative is a P.S.I. that consists of a rotary pointer driven by a small 3 phase induction motor, the direction the pointer turns shows if the phase sequence is correct. Below is a circuit diagram of the connection box:

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Problem

Motor will not start.

Possible Cause

1. Faul Faultt wit with h su supp pply ly.. 2. Mo Mottor or loa oad d locked up. 3. Wrong connections in control circuit.

Supply or Started trips out at start.

1. Wron Wrong g or or loo loos se connections. 2. Mo Moto torr ov over erlo load aded ed.. 3. In Inte tert rtia ia of lo load ad to high. 4. Lo Low w Vol Volta tage ge du due e to volt drop in cables 5. Ov Over erlo load ad or ci circ rcui uitt

Tests

Solution

1. Chec Check k for for cor corre rect ct voltage at motor

1. Fit Fit new new fu fuse ses, s, re rese sett curcuit breakers, etc.

terminals. 2. Make Make su sure re mo moto torr and and load are free to turn.

2. Remo Remove ve cla clamp mps, s, loc locks ks,, etc. 3. Sor Sortt out con contro troll circ circuit uit..

3. Chec Check k to en ensu sure re contactors operate. 1. Chec Check k all all lu lugs gs are are properly crimped or soldered, and connections are tight. 2. Check load performance data against motor performance data. 3. Me Meas asur ure e volt voltag age e at

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1. Fix Fix up up con conne nect ctio ions ns.. 2. Ch Chan ange ge mo moto torr for for correct size. 3. Ch Chan ange ge cab cable les s for for correct size. 4. Corre Correct ct se sett ttin ing g of  overload or breaker or change.

 

breaker incorrectly set or sized.

motor terminals while motor starting. 4. Chec Check k sett settin ings gs of  of  overload and circuit breaker and allow for starting current

Motor starts but has no torque. Motor does not reach full speed or takes a long time to accelerate.

1. Iconnection. ncorr rre ect 2. Del elta ta wo woun und d motor connect in star. 3. St Star ar/D /Del elta ta st star arte terr staying in Star. 4. In Iner erti tia a of of loa load d to to high. 5. Mo Moto torr ov over erlo load aded ed..

1. Chec Ch eck k conn coand nnec ecti tion on diagram nameplate data. 2. Check load performance data against motor performance data. 3. Meas Measur ure e volt voltag age e at motor terminals while motor starting

1. Sort So rt ou outt and and co corre rrect ct connections. 2. Ch Chec eck k tim timer er an and d starter control circuit. 3. Ch Chan ange ge mo moto torr for for correct size. 4. Chan Change ge cab cable les s for for correct size.

6. Low Low vol volta tage ge du due e to volt drop in cables. Motor Overheating.

1. Moto Motorr ov over erlo load aded ed.. 2. In Inef efffec ecti tive ve cooling. 3. Excessive ambient. 4. Wrong connection. 5. Del elta ta wo woun und d motor in star. 6. Mo Moto torr “S “Sin ingl gle e Phasing”. 7. Wr Wron ong g vol volta tage ge or frequency.

1. Check load performance data. 2. Ch Chec eck k fan fan an and d air air flow and temperature of air. Look for build up of dirt. 3. Ch Chec eck k conn connec ecti tion on diagram and nameplate data. 4. Ch Chec eck k vol volts ts an and d amps in all three phases. 5. Ch Chec eck k nam namep epla late te

8. Supp Supply ly vo volt ltag age e unbalanced.

6. Meas Measur ure e pha phase se to phase voltage

1. Fix Fix pro probl blem em wi with th lo load ad or fit larger motor. 2. Cl Clea ean n mot motor. or. So Sort rt ou outt cooling of air temp. and flow. 3. So Sort rt out out con conne nect ctio ions ns.. 4. Re Rest stor ore e sup suppl ply y to to all all phases. 5. Co Corre rrect ct vo volt ltag age e or or frequency. 6. Bala Balanc nce e supp supply ly or or accept unbalance.

accurately No load amps in excess of  Full load amps

1. Incorr rre ect connection 2. St Star ar wo woun und d mot motor or connection Delta. 3. Vo Volt ltag age e in ex exce cess ss of nameplate.

1 & 2. Check

1 & 2. Sort out and

connection diagram

correct connections at

and nameplate data.

motor terminals. 3. Correct supply

3. Measure voltage 4. Moto Motorr supp suppli lied ed for for different voltage or frequency.

at motor terminals. 4. Compare supply voltage and frequency to nameplate.

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voltage 4. Change motor for correct voltage and frequency

 

Mechanical Noise or Vibration. Noisy bearings. Bearings overheating.

1. Thru Thrust st fr from om lo load ad or misalignment. 2. Damaged bearings, too much grease, no grease, or foreign matter in grease. 3. Ro Roto torr pul pulli ling ng or foreign matter in air gap. 4. Ou Outt of ba bala lanc nce e load, coupling or pulley. 5. Ex Exce cess ssiv ive e bel beltt pull.

1. Check gap

1. Re-align couplings

between coupling

2 & 3. Clean bearing

halves and

housing, change

alignment.

bearings and repack

2 & 3. Turn shaft

with fresh grease.

slowly by hand and feel for roughness or

4. Fix up out of  balance items

stiffness. Check for

5. Loosen belt tension

bent shaft or fan

6. Increase strength of 

rubbing.

foundations

4. Run motor 6. Moto Motorr fo foun unda dati tion ons s not rigid.

disconnected from load and then with pulley or coupling removed. 5. Run motor without belts. 6. Check design and construction foundations

Motor amps in excess of  nameplate full load amps on load

1. Moto Motorr ov over erlo load aded ed.. 2. Low s su upply voltage. 3. Wr Wron ong g vol volta tage ge or frequency. 4. Wrong Connections. 5. Mo Moto torr ‘S ‘Sin ingl gleePhasing’. 6. Su Supp pply ly vo volt ltag age e unbalanced. 7. Moto Motorr Spe Speed ed no nott matched to load.

1. Check load and

1. Fix problem with

performance data.

load or fit larger

2. Measure voltage

motor.

at motor terminals

2. Fix problem, maybe

3. Check nameplate.

with larger cables.

4. Check nameplate

3. Correct voltage or

5 & 6. Check volts

frequency.

and amps in all three

4. Sort out and

phases.

correct.

7. Measure motor

5 & 6. Restore

speed and check load

balanced supply to all

speed requirements.

three phases. 7. Change motor for correct motor speed.

Excessive electric noise

1. Wrong connections.

1. Check connections

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1. Fix up connections

 

Wron ong g vo volt ltag age. e. 2. Wr 3. Moto Motorr ‘S ‘Sin ingl gleePhasing’.

2. Check voltage with

2. Correct voltage.

nameplate

3. Restore supply to all

3. Check volts with

three phases.

amps in all three phases. Unbalanced amps in different phases when motor loaded

1. Unb nbal alan ance ced d supply voltage

1. Meas Measur ure e pha phase se to phase voltage accurately

1. Bala Balanc nce e supp supply ly or or accept unbalance

Motor runs in wrong direction

1. Wrong connections.

1. Watch shaft rotation

1. Swop and two phases of supply.

MAJOR FAULTS IN GENERATOR  The major faults in case of generator can be classified as: (a) Failure of prime movers: Whenever there is a fault on prime mover side the conversion of mechanical power to electrical power stops. If this generator works in parallel with other generator sets it will start working as a synchronous motor running at synchronous speed and the prime mover will act as load on it. When generator starts running as a motor it is called as inverted operation. If the fault is cleared the generator will automatically pick up generation. (b) Failure of excitation: The turbo generator set under consideration works in parallel with other sets. If the field of  the generator is interrupted it will continue to operate as an induction generator-delivering load at very low power factor. So the other generators will be overloaded, as they will have to supply the load of this faulty generator. This may lead to decrease the supply voltage and the stability of the system will be affected. The fault generator can be switched off but for an automatic operation an under current relay in conjunction with time lagged tripping and time lagged reinforcing relay as shown in fig 1 can be used.

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The under current relay is a moving coil type instrument and is fed through a shunt in the field circuit and its provided with a double trip circuit contact. When there is field circuit failure under current relay is no longer energized and the action moves up the operating armature up and in doing so it stops supply to time lagged reinforcing and trip relays, thus both of theses relays are operated simultaneously. (a) Failure of insulation in the stator or interconnecting cable can be further divided into: i. Interphase short circuits – These are accompanied by large l arge fault currents, the values of which are dependent upon the capacity of the system to which the alternator is connected, the nature of inter connection to the system and the moment at which the short circuit occurs. ii. Single phase to earth fault – This will lead top the burning of alternator winding when leakage current exceeds even 5 amps. This current is dependent upon the nature of neutral connection, i.e. whether its earthed or unearthed. iii. Inter turn faults – In this type of fault short circuit exists between the turns of the same sam e phase phase or bet betwee ween n turns turns of paralle parallell branch branches es in the same same phase. phase. The magnitude of the fault current depends on the number of turns shorted. This is a dangerous type of fault iv. Earth fault on the rotor – Shorting of the rotor at one point to earth in itself does not cause any damage. But the appearance of another fault to earth may sharply disturb the magnetic flux distribution, which will lead to unequal voltages being induced in the stator and increased vibration in the alternator.

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