QUE-ANS:10-30 words
[1] What are the sources of electrical power on ship?
- Main alternator
- Emergency generator
- Emergency batteries
- Shore power
[2] What is understood by ‘essential loads’?
Essential loads or services are those required for the safety of personnel and for safe navigation and propulsion of ship.
[3] How are the power factors of two alternators running in parallel are maintained equal?
By automatic voltage regulators.
[4] What is varied to balance alternator load?
The governor setting is varied to balance generator load.
[5] What is used to adjust the power factor?
To adjust the power factor, the excitation of the alternator is adjusted.
[6] When voltages are equal in frequency where does the synchroscope pointer lie?
When the voltages are equal in frequency, the synchroscope pointer lies at the noon position.
[7] What is the maximum time a synchroscope should be on line for?
The maximum time a synchroscope should be on line for is 20 minutes, as it is not continuously rated.
[8] When must the circuit breaker be closed when paralleling?
The circuit breaker must be closed when paralleling, when the voltages and frequencies are the same and the pointer on the synchroscope is at the 11 o clock position.
[9] When using a synchroscope in which direction should the pointer be travelling before closing the circuit breaker?
When using a synchroscope, the pointer should be travelling in a clockwise direction at no more than 1 rev / 5 seconds, this ensures the incoming machine is slightly fast and it will immediately assume load.
[10] Using lamps, when is the correct moment to close the circuit breaker?
When using synchronizing lamps, the correct moment to close the breaker is when the key lamp is dark (top light when in triangular formation) and the other two lamps are equally bright.
[11] What should be done after successful synchronization?
After successful synchronization, the generator load should be shared equally.
[12] What is Load Sharing?
Load Sharing is the equal balance of loads between Generators irrespective of Load Changes.
[13] What Conditions is Necessary to Parallel two Generators??
The Speed of both Machines must be the same i.e. the Frequency and the Voltages must be the same and in Phase.
[14] What Loads are considered Non-essential?
Loads considered as Non-essential are: Air Conditioning and Ventilation, Laundry, Galley, Refrigeration and Deck Equipment.
[15] What is Sequential Starting?
Sequential Starting is the automatic starting of essential equipment when power is returned after a total power failure, i.e. equipment such as Steering Gear.
[16] Where on the ship is the Emergency Generator Located?
The Emergency Generator is located remotely from the Engine Room, usually on the Accommodation Deck or at Weather Deck Level or above.
[17] At what value of voltage drop does the under voltage trip operate?
At around 70% value or less, the under voltage trip will operate.
[18] Which alternator would trip and which trip would operate?
In the event of the above happening, the alternator that would trip would be the one with the loss of excitation and the relay that would operate would be the loss of excitation relay.
[19] What kind of relays is used for under voltage protection?
The kind of relay used for under voltage protection are time delayed relays to permit the prior operation of a feeder breaker
[20] What does a reverse power relay monitor?
The reverse power relay monitors the direction of power flowing between the generator and the switchboard.
[21] Why is a steam turbine over speed trip connected to the circuit breaker trips?
A steam turbine over speed trip is connected to the circuit breaker trip because very little reverse power is required to cause over speeding, as little as 3%.
[22] What is the reverse power delay setting for steam turbine sets?
The reverse power delay setting for steam turbine sets is between 1.5 – 3 % of full load settings.
[23] Why is there a delay on the relay?
There is a delay on the relay to prevent inadvertent operation of the reverse power trip due to power surges, particularly during synchronizing
[24] What do you understand by the term ‘dead switch board’?
A dead switch board is the one in which there is no supply of power available, whatsoever.
[25] What is the setting of reverse power relay?
10% of generator [power rating with 5 secs time delay for diesel driven alternator. 2% to 5% of generator power rating with 5 secs time delay for turbo alternator.
[26] What is the use of ‘Kvar’ meter fitted on the MSB?
It is used to check the reactive power.
30-60 words
[1] What are the reasons for installing fault protection in electrical distribution system?
- To disconnect and isolate faulty equipment in order to maintain the power supply to the remaining healthy circuits in the system.
- To prevent damage to equipment from thermal and magnetic forces that occur during short circuit and overload faults.
- To protect personnel from electric shock.
[2] What is meant by Preference Tripping?
Preference Tripping is the tripping of non-essential loads when an overload condition occurs on a Generator.
If a Generator Overload develops the Preference Trip Relay operates an Alarm and acts to trip selected non-essential loads. These loads may trip at set intervals depending on how essential the load is i.e. 1st trip 5 seconds, 2nd trip 10 seconds.
[3] What is the Function of the A V R?
The function of the A V R is to regulate the Exciter Field Current automatically, comparing the Generator Terminal Voltage with the Standard Set Voltage and thus maintaining the Generator Terminal Voltage to a Constant Value, regardless of Load Changes.
[4] Explain what would happen if you were to lose a Generator.
First the Standby Generator set would start up and automatically put itself on the Board. If this did not happen, the Ship would “Black Out” and after a time delay, usually 30 seconds, the Emergency Generator would start up and supply the Emergency Switchboard, which supplies essential equipment.
[5] State where an AC shores supply is connected to a ship’s distribution system.
An AC shore supply s connected to a ship’s distribution system at the entrance to accommodation or in the emergency generator room to accept shore supply power. Ashore power supply is required so that the ship generators can be shut down during dry docking for maintenance.
[6] State what is likely to occur if the driving power of an AC generator suddenly fails when two generators are operating in parallel.
When prime mover of a generator fails while it is running parallel with another, the generator of the failed prim mover acts as synchronous motor placing additional load on the other generator.
At the preset value, reverse power protection detects this fault and trips failed generator circuit breakers.
60-100 words
[1] Which distribution system is more efficient in maintaining supply?
The distribution system that is more efficient at maintaining a supply is the insulated distribution system (used in most marine electrical supplies), if one earth fault occurred in this system, it would not cause any protective gear to operate and the system would continue to function normally. Important equipment still operates. The single earth fault does not provide a complete circuit, so no earth fault current can exist. It would take two earth faults on two different lines to cause an earth fault current.
[2] The short time rating; the rms value of the current that a circuit breaker is capable of carrying for the started time. What course of action needs to be taken when a generator is overloaded?
The course of action to be taken when a generator is overloaded is that the preference trip s should be operated; these will normally operate automatically by means of a timing relay disconnecting non essential loads in a definite order and at definite time intervals.
Example:
1st trip Air conditioning, galley, laundry, ventilation.
2nd trip Refrigeration plant
3rd trip Deck equipment
[3] Describe how Synchronizing Lamps are used to parallel a Generator.
this is normally used as a back-up or alternative way to the Synchroscope, the Lamps are connected between the Incoming Generator and the Bus-bars. The “Sequence Method” is the preferred one as it displays a rotation of Lamp Brightness, which indicates, whether the incoming machine is running fast (clockwise) or slow (anti clockwise). As with the Synchroscope, the Lamp Sequence must appear to rotate slowly clockwise. Correct synchronization occurs when the top or “Key” lamp is dark and the two bottom lamps are equally bright.
[4] State what would be the effect on the current, voltage and frequency of the machine if an AC generator slowed down slightly during operation.
If one machine running alone:
Speed slow down, frequency decreases. N a f/p.
Voltage is not affected as it controlled by AVR.
Impedance reduces (XL = 2PfL) with voltage being constant, current would therefore increase.
If two alternators in parallel:
If one generator slows down slightly, it would drop / shed part of its kW loading which would be taken up immediately by the other one. However, there will be no change in the frequency, voltage and current in the system as both are locked electrically.
100-150 words
[1] On medium voltage, 440V marine, AC, 3 phase, distribution system, the neutral wire is invariably insulated, whereas in shore practice it is otherwise. Briefly list and give reasons for :
(a) The advantages of insulating the neutral, and
(b) The disadvantages of insulating the neutral.
Advantages of insulating the neutral:
Protect the essential service in case of earth fault occurred.
Fewer fire risks.
Less prone to single phasing.
No danger of triple current (3rd harmonic current), overheat will activated earth fault relay which may trip off the generator.
Disadvantages:
Earth fault detection harder.
Phase voltage at line voltage (if one phase is earthed, insulation highly stressed due to star connected).
Less suitable in high voltage system.
Required step down transformer, double pole switches/fuses for/and lower voltage sub circuits.
[2] Explain the circumstances under which electrical machinery onboard ships tend to run hotter than usual when supplied from ashore and suggest a remedy.
Electrical machinery onboard ship tends to run hotter than normal when supplied from ashore:
The shore supply may have a different frequency and / or voltage to that ship system. A higher frequency will cause motors to run faster thus overload and overheat while with a higher voltage will generally cause equipment to take excess current and overheat. It will also cause motor to accelerate more rapidly and this may overstress driven load.
With a lower voltage is generally not so serious but may cause motor to run slower and overheat and may cause motor to stall which is more dangerous. In order that problems to overcome, its best that to supply the voltage and frequency at the ship operating voltage and frequency.
[3] Briefly explain the statement, ‘three phase, three wire distributions with insulated neutral is favored’.
3 phase three wire distribution with insulated neutral is favored because:
one earth fault occurred on an equipment does not interrupt in supply but only raise a warning on the leakage system, thereby avoiding loss of essential services.
This allows the operator to search for and clear the fault at a convenient time.
Further advantage is that the electric shock risk is reduced, since (in 440V system) the capacitive return current is usually only a few milliamps.
Thus with such values of earth fault current, the risk of fire due to such fault is reduced.
Thus in insulated systems at 440V, the risk of failure due to over voltage are not regarded as serious.
[4] State what are the main considerations when designing a ship’s main electrical power supply system, with respect to the type of services supplied, and state the minimum number of generators that may be utilized?
Main consideration for electrical circuit design with respect to type of sources supplied:
Electrical load is divided into essential and non-essential sources for safety of personnel and safe navigation. Essential services are necessary for loads which required handling in emergency situation.
In case of fire in ER or blackout, essential services will receive power from emergency sources.
The distribution system provides power to essential services which are duplicated from separate section of switchboard.
Lighting in machinery spaces:
2 sets to prevent darkness.
Switches of double pole type.
Emergency lighting.
Undesired restarting after trip due to low voltage except for essential services e.g. steering gear.
Remote stop of vent fan and pumps delivery oil to boilers and main engine.
Electrical power to starting fire pump not passing thru engine room.
Heating and cooking equipment is controlled by multiple switch mounted reset to equipment.
Fire detection system from main and emergency sources.
[5] Explain how the correct order of phase connection from three phase supply lines to motor terminals can be deduced from standard markings, and state a necessary precaution in guarding against incorrectly marked terminals.
Standard markings with British Standard, if generator is driven with clockwise rotation when looking at its driven end, the terminal phase sequence will be in the order A, B, C. (For colour marking, the sequence in Red, Yellow, Blue). If the motor is connected to a supply terminal hase sequence in the order of A, B, C, and then the motor would rotate in clockwise direction.
A necessary precaution is guarding against incorrectly marked terminal is by run the motor with coupling to mechanical equipment disengaged. Ensure correct running direction. In case of air compressor if wrong direction, compressed air still can be produced but LUB oil pump engaged to compressor will not working, no lubrication cause bearing damage.