Technical notes of interest to Marine Engineers
Ship's electrical system described
Authored by: DC Marine, January 2000
Brought to you by www.dieselduck.net, comments to webmaster@dieselduck.net
Technical notes of interest to Marine Engineers
Ship's electrical system described
Authored by: DC Marine, January 2000
Brought to you by www.dieselduck.net, comments to webmaster@dieselduck.net
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The generators form the heart of the electrical design
and their correct sizing is the key to a safe, workable
and economical system. When sizing a marine generator
cognisance must be given to the nature of the load. The
generator often works on its own and is accordingly
susceptible to large system load swings, loads causing
distortion, the connection of motors and the connection
of large heater elements for air conditioning systems. In
addition to satisfying the apparent system load
requirements, consideration must be given to the special
requirements of any large loads, unusual operational
requirements, spare capacity requirements and the
required system operating philosophy.
International maritime regulations (e.g. SOLAS), require
at least two generators for a ship's main electrical
power system. The generators are normally driven from
their own dedicated diesel engine but this can be
expensive, taking up additional space that could be used
for other purposes. For ships engaged on long sea
voyages, it can be economical to drive the generators
from the main propulsion plant. International maritime
regulations also require at least one electrical
generator to be independent of the speed and rotation of
the main propellers and associated shafting and
accordingly at least one generator must have its own
prime mover.
If a minimum of two generators is provided, one of which
is driven from the propeller shaft, failure of one of the
generators could make the ship non-compliant with the
International regulations. For this reason many owners
opt to provide three generators. One is used for the
normal sea load (e.g. the shaft generator), leaving two
available to meet any unusually high loads or to provide
security when maneuvering. Alternately, the third is
retained as a standby set able to provide power should
one set fail in service or require specific maintenance
work.
In some applications such as a generator supplying a
large SCR type load, the generator rating may be
increased well beyond its full load value, in order to
account for harmonic heating and the inductive
requirements of the SCR devices. DCMT has developed its
own software to assist in generator sizing.
The main elements of a marine distribution system are the
main and emergency switchboards, power panel boards,
motor controllers, lighting and small power panel boards.
The system is generally designed such that under all
normal conditions of operation, power is distributed from
the main switchboard. The distribution system is designed
to keep cable costs to a minimum by distributing to power
panels located close to the user services.
The main switchboard is generally located near the centre
of the distribution system and this is normally the main
engine room or machinery control room. These locations
are normally below the ship's waterline or below the
uppermost continuous deck of the ship i.e. the bulkhead
or main deck. Consequently, in the event of a fire or
flooding it is likely that the main generators and
switchboard would be disabled. To ensure that electrical
supplies are available to emergency and safety systems,
an emergency generator and associated emergency
switchboard will be located above the main deck in a
separate space, completely isolated from the main
machinery spaces.
For shipboard installations specific protective systems
are required to shut down all ventilation systems and all
fuel oil systems in the event of fire. When motor
auxiliaries are grouped together and supplied from a
motor control center or a grouped distribution panel,
this can best be achieved by providing the MCC supply
feeder circuit breaker with an undervoltage tripping
device and connecting this to the ventilation or fuel
systems trip unit. When grouped MCC's or grouped
distribution panels are not used, separate cables must be
installed for each motor controller. This leads to
increased cable costs and increases the systems proness
to failure.
It is often convenient to group motor driven auxiliaries
according to their function, e.g. fuel and lubrication
oil services, accommodation ventilation systems,
machinery ventilation systems, and domestic service
systems. The auxiliary motors would be supplied from
grouped motor controllers located either in the engine
room, in a machinery control room or in a convenient
location close to the auxiliary motors. This can often
simplify the machinery control functions and required
protection systems.
On small ships, e.g. tugs, etc., such grouping is not
economical and the major ship's auxiliaries are normally
supplied directly from the main switchboard. In this case
the motors would be provided with individual starters
located adjacent to the motor. For high speed vessels
where weight is important, minimum cable weight may be
achieved using a “non-distributed” distribution
scheme.
Auxiliary motor controls should be arranged in
consideration of the general control philosophy applied
to the machinery control systems. For ship's that do not
have automated machinery operation, the most economic
method of control is to provide local starters for each
auxiliary motor supplied from power panels located in the
same or adjacent spaces. These motors would be manually
controlled (start and stopped), locally at the motor's
controller (starter). This arrangement minimizes cable
costs.
When a centralized machinery control system is required,
cables for the motor control functions can be installed
back to the machinery control room and the starter push
buttons located on a centralized machinery control
console. Alternatively, the motors may be grouped
together on motor control centres located inside the
control room. The motor control functions can then be
left on the motor's starter at the MCC or again wired
back to a central control desk.
When hard-wired systems are used, the installation is
prone to mechanical problems which may cause loose or
broken connections and the marine environment which may
cause corroded connections. These problems can be
eliminated somewhat by using micro-processors and digital
control systems.
When fully automatic machinery control is required, these
techniques are now in common use and micro-processor
devices control the ship's machinery through video
display units located in the machinery control room or on
the bridge. The ship's auxiliaries are generally
controlled with programmable logic controllers (plc's)
installed inside the motor control centres and linked
through a data bus to the machinery control location.
When this type of system is used, the motor control
centres can be located either together in the machinery
control room or alternatively, distributed throughout the
ship close to the motors being controlled. There is
little difference in the cabling requirements of either
method, however when motor control centers are located
outside a dry, atmosphere controlled space such as the
machinery control room, a higher degree of mechanical
enclosure is required (IP 44 instead of IP 22) and
consequently adds to the MCC costs.
Emergency services would be supplied from the emergency switchboard using distributed panels for navigation, safety and emergency lighting services. These distribution panels are also generally arranged to be above the bulkhead deck. For lighting it is important to ensure that a fire or flooding in one area will not cause loss of lighting in other areas or along escape routes and circuitry must be designed in consideration of the ships general arrangements.
DCMT's principle design documents for the ships auxiliary services include a load list, load analysis and short-circuit current analysis. In consultation with the client all electrical services on the vessel are identified. Approximate horse-power or kilowatt ratings are obtained for motors. Lighting loads are estimated from the ship's general arrangements and electronic aids are obtained from similar vessels, and a complete load list compiled.
The
electrical load analysis uses the load list in order to
estimate the expected power demand of the electrical
system under specific ship operating conditions. Typical
operating conditions would be with the ship, “in
transit," “at anchor,"
“maneuvering,” etc. For special vessels, other
operating conditions would be appropriate such as
“towing” for a tug, “drilling” for a
drill ship.
The load analysis calculates the expected power demand by
multiplying each service power by a “demand”
factor. The demand factor is a combined load factor and
diversity factor and is the ratio of the estimated power
consumption of a service to its normal full load power
consumption. The demand factor is determined by an
experienced assessment of the estimated power during a
four to six hour period when loads may be at their
maximum utilization.
DCMT's load analysis obtains load information from the
load list. For each service, data banks are searched to
determine the service full load current and power factor
dependent upon motor operating voltage. This information
is used to compute the services' kilowatt and kilovar
demand from which is computed the kilovoltamps. By
applying the demand factor to each load kW and kvar's and
summing all loads for specific operating conditions, the
expected generator kilowatts, kilovoltamps and power
factor can be computed. By comparing the expected load
for the different ship operating conditions, the number
and rating of the main generators can be assessed.
Preliminary short-circuit current calculations can be
completed once the load analysis and number and rating of
generators have been determined. The principle purpose of
the short-circuit current calculation is to ascertain the
short-circuit rating of the systems protective devices.
DCMT has developed several types of short-circuit current
calculations which are applied under different
circumstances at various stages of the design process.
The major contributors to short-circuit current are the
generators and motors. Cables and transformers act to
reduce the short-circuit current load at a specific
location. The most simple short-circuit current analysis
is based on an assumed value of the generator's
sub-transient reactance and an approximate estimate of
the worst case motor loading can be obtained from the
load analysis.
The “second stage” short-circuit current
analysis is completed when the electrical system
conceptual one-line diagram is finished. For this
calculation actual subtransient data is used together
with cable transformers and other system parameters. This
calculation generally results in lower values of
short-circuit current.
When complete system information is available a
“third-stage” short-circuit analysis is
completed. This is the most accurate calculation DCMT
completes. The calculation determines the decrements of
the short-circuit current over a 3 and 5 cycle period.
