Ship Load Analysis
Load analysis of the ship’s electrical systems is a complex exercise and covers all operating conditions of the vessel, such as conditions in normal sea-going, cargo handling, harbour manoeuvring, and emergency operations.
Dimensioning the Electrical Network depicts a few considerations to be taken while the entire electrical system is being designed.
It must be understood that there are many constraints to be kept in mind, the main factors being the money involved and of course the ship’s operating profile.
Often the tentative area of operation is also decided; this also determines the vessel’s size. The vessel will also be required to comply with the stipulated safely norms and requirements of a Classification Society and other regulatory bodies.
The power rating of the generators is determined by the power demand of the electrical load. Principle design documents for the ship’s auxiliary services should include a load list, load analysis and short-circuit current analysis.
In consultation with the client, all electrical services on the vessel are identified.
Ship’s Electrical Network Ratings
Approximate horsepower 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 then 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 operating conditions of the ship.
Typical operating conditions would be with the ship, ‘in transit’, ‘at anchor’, ‘manoeuvring’, etc. For special vessels, other operating conditions would be appropriate such as ‘towing’ for a tug, ‘drilling’ for a drill ship.
How determine Ship Load Analysis
Load analysis also helps to calculate 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.
It is determined by an experienced assessment of toe estimated power during a four to six hour period when loads may be at their maximum utilisation.
For each service, data banks are searched to determine toe service toll-load current and power factor dependent upon the operating voltage.
The information thus derived, is used to determine the ‘services’ kilowatt and kilovar demand from which toe kilovolt-amps required is finally computed. Also, by applying the demand factor to each load (kW and KVAr) and summing all loads for specific operating conditions, toe expected kilowatts, kilovolt-amps and power factor can be computed.
By comparing toe expected load for toe different operating conditions of toe ship, toe number and rating of the main generators can be assessed.
Short-circuit Current Calculations on Ship
Preliminary short-circuit current calculations can be completed once toe load analysis and number and rating of generators have been determined. The principle purpose of short-circuit current calculation is to ascertain toe short-circuit rating of toe systems’ protective devices.
Several types of short-circuit current calculations are applied under different circumstances at various stages of the design process.
The major contributors to short-circuit current are toe generators and motors.
The most simple short-circuit current analysis is based on an assumed value of toe generator’s sub-transient reactance and an approximate estimate of the worst case motor loading can be obtained from toe load analysis.
The ‘second stage’ short-circuit current analysis is completed when toe electrical system’s conceptual one-line diagram is finished. For this calculation actual sub-transient data is used together with cable transformers and other system parameters. This calculation generally results in Iowa- values of short-circuit current.
When complete system information is available, a ‘third-stage’ short-circuit analysis is completed. This is toe most accurate and also determines toe decrements of toe short-circuit current over a 3 and 5 cycle period.
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