ELECTRICAL

AC switchboards

If voltages exceed 250 volts d.c. or 55 volts A.C. then the switchboard must be dead front (no exposed live parts at the front) of the metal clad type.

Bus bars

High conductivity copper rated to withstand the thermal and electromagnetic forces which would arise in the event of a short circuit at the bus bars with all the generators in parallel. The bus bars will withstand these conditions for the length of time it takes for the alternator circuit breakers to trip or back up fuse to blow.

Certain instruments and controls require a feed direct from the bus bars. Any connection between the bus bars and protecting fuses must be capable of withstanding maximum fault level. Standard practice is to provide a three phase set of fuses, known as 'Back Up' fuses, as near to the bus bars as possible. Connections are then led to the racks of the many instruments fuses fitted.

Circuit breakers

Must be capable of making and breaking under normal conditions and also abnormal conditions such as a short circuit. As the circuit breaker must be able to withstand closing onto a fault conditions without sustaining damage, it is of heavy construction. Fitted with an over current release and overloads with time lags, a circuit breaker can be used as follows;

1.      To control the output of a generator

2.      As a direct on line starter

3.      Control outgoing feeder circuits

On modern switchboards 'draw out' circuit breakers may be fitted. In the open position the whole circuit breaker can be wound clear of the bus bars, thus full inspection and maintenance can be achieved without the necessity of de-energising the bus bars so providing a separate isolating switch.

The 'plug in' contacts joining the circuit breaker to the bus bars are not capable of taking the breaking load and it is essential that the circuit breaker is in the open position before any attempt is made to withdraw it. A mechanical interlock is fitted arranged to trip the circuit breaker before the winding handle can be inserted,

The breaker also has a mid position, in this position the control circuits are still connected with the bus bar connection isolated. The electrical operation of the breaker can then be tested.

Circuit breakers are normally fitted with under voltage protection and tripping is accomplished by shorting or open circuiting the no-volt coil which releases the latching in mechanism. The no-volt coil may also be open circuited by a reverse power relay and an overload trip fitted with a time delay

Instruments

The following instruments are the minimum that must be fitted;

·         Bus bar voltmeter and frequency meter

·         Volt meter and frequency meter, with selector switch to measure incoming machine conditions

·         Ammeter with phase selector switch for each alternator

·         Watt meter for each alternator

·         Synchroscope and if check synchroscope not fitted lamps

·         Earth leakage indicator

Additional instruments that may be fitted

·         Watt hour meter

·         Power factor meter

·         Alternator excitation ammeter

·         Alternator excitation volt meter

·         kVAr meter

·         Share connection supply meter

·         Emergency batteries on discharge meter

When a check synchroniser is fitted it is there to prevent connecting an incoming machine to the bus bars whilst out of phase, it is not there as aid to synchronising. In an emergency the 'in synch' light may be used to indicate when the breaker may be closed.

When an incoming machine is selected, its no-volt coil and circuit breaker contactor relay coil are connected in series with contacts on the check synchroniser. These contacts must be closed, that is the machine in phase with the bus bars, before the breaker contactor relay may be energised. If starting from a dead ship the check synchroniser must be switched to off before the first generator is put on the board.

Protection

1.      Overload protection-fitted to circuit breakers

2.      Reverse power-When motive power is removed an alternator will try to become a synchronous motor and draw current from the circuit. A reverse power relay will operate after about 2 seconds and about 2-3% reverse power for turbines, 10-12% reverse power for diesels. The time delay prevents tripping during paralleling and taking the alternator off the board.

3.      Preference trip-automatically , and sometimes sequentially, sheds load from board to maintain supply to essential services during periods of overload.

4.      Fuses-Usually of the HRC type

5.      Discrimination-The protective device closest to the fault should operate and protect other services

6.      Group starter board-Large demand sections may be separated from the main switchboard by fuses and circuit breakers.

Automatic voltage regulators

Shall be supplied separately from all other instrument circuits. Protection should be by fuses mounted as close to the supply connections as possible.

Shore supply connections

1.      Where arrangements are made for the supply of electricity from a source on shore or other location a suitable connection box has to be installed in a position in the ship suitable for the convenient reception of flexible cables, it should contain a circuit breaker or isolating switch, fuses, and terminals of adequate size to receive the cable ends.

2.      For three phase shore supplies with earthed neutral terminals are to be provided for connecting hull to shore earth

3.      An indicator for shore side connection energised is to be provided.

4.      A means for checking polarity or phase rotation is to be provided

5.      At the connection box a notice indicating ships requirements with respect to supply as well as connection procedure.

6.      Alternative arrangements may be submitted for consideration. 

Insulated neutral system

Advantages

1.      This system avoids the risk of loss of essential services e.g. steering gear

2.      If the neutral was earthed and a short circuit on one phase causes the fuse in that phase to blow the system would now be singled phasing and may burn out motors

3.      In an insulated neutral, one earth fault does not interrupt the supply but an earth leakage detection system will give warning.

4.      Low earth fault currents in insulated systems gives a much less fire risk.

Disadvantages

1.      On the insulated system the voltage to earth is 1.73 Vph e.g. 440v vs 250v

2.      Tracing an earth fault is more difficult because although selective tripping may trace the earthed circuit, the actual position on the circuits may still be difficult to locate. Resonant or intermittent faults in say a contactor solenoid or a transformer with an insulated neutral can cause voltages to be magnified to say 4 times the normal voltage to earth (250v x 4 = 1000v)

Note: electrical shock is not reduced by using a non-earthed neutral as large voltages are involved. Both systems are equally dangerous

Earthed neutral system

When an earthed neutral system of generation is used earthing is to be through a resistor. The resistor is to be such that it limits the earth fault current to a value not greater than the full load current of the largest generator on the switchboard section and not less than three times the minimum current required to operate any device against

Single phasing

Delta wound

Winding 'B' will receive full voltage, winding 'C' and 'A' are in series

If the motor is running when the circuit is opened it will be seen that at 100% load the current in 'B' becomes almost three times full load, the current in 'C.A' becomes about 30% overload and the line current becomes about 2 Н times full load.

At these conditions the overload protective device operate

But at 75% load, Current in 'B' is 180%, current in 'C.A.' is at 80%, and the line current is 145%. If the overload protection devices are set at 50% overload then they will fail to operate and the motor will continue to run. Under these conditions tha copper losses also increase causing additional heating.

Protection against single phasing.

The commonest cause of single phasing is the blowing of a fuse in the supply circuit. For this reason it is recommended that cartridge and not rewireable fuses are used. Cartridge fuses can be overloaded to near fusing point with out degradation. Also variations between fusing points on each fuse is small.
It is good practice that if one fuse blows then all three should be changed. Any fuse which is intact should be transferred to a lighting circuit where its performance is not vital.
No volt releases will not provide protection in these circumstances. Should the open circuit be on the motor side of the starter due to a broken connection the no volt coil will continue to be fed from the supply. If the open circuit occurs on the supply side the coil will be supplied by the voltage induced in the motor.

Star wound

Overheating can also occur with a star connected machine on single phase, but in this case the line current is the same as the windings and so there is a better chance that the protection devices will operate. In both star and delta connected motors, they will not start in the single phase condition, they will remain stalled and subjected to large currents if the protection devices are set to high or with an excessive time delay. Running indication may be connected across a healthy phase giving an incorrect indication. Therefore overload setting of 125% F.L. with suitable time lag to allow starting is recommeded.
If this is not practical, then some form of single phase protection is required. One such device is shown below.

Under overload conditions- all bi-metal strips heated equally by coils and deflect moving top bar sufficiently to operate a trip mechanism.

Under single phase conditions- two strips deflect. No deflection in the other strip. The top and bottom bars are hinged and move together. If forced apart the trip mechanism operates.

Another simple method, where it can be conveniently arranged, is to connect the overload coils in the delta circuit in series with the windings. This is possible in the case of star-delta starters. In systems with an earthed neutral, then single phasing will occur if a fuse blows due to a fault to earth in one line. It may also result from a defective contact in a switch or starter and proper maintenance is therefore most important.

Motor connection

A motor designed to be delta connected must not be connected in start otherwise the heating effect in the windings will be three times design but the line current would be normal and therefore protection devices would not operate.