Technical notes of interest to Marine Engineers
Ship's electrical system described
Authored by: DC Marine, January 2000
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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.
Ship's Auxiliary Services
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.