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