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Electricity has to be distributed with minimal losses after generation. Unlike a shore based transmission system, where the lengths of the conductors run throughout a country, a shipboard electrical distribution system is short and simple. In this article, the general layout of the main electrical distribution system, along with the main switchboard and emergency switchboard arrangements, will be discussed.
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The Main Bus Bar
The main bus bar can be found inside the main switch board. Onboard a merchant ship, the supply is usually 440 volts, 60 Hertz, however higher voltages of 6.6kV are possible on LNG tankers, special purpose ships, and passenger vessels having electrical propulsion. The rating of the main bus bar is decided by the type of ship, the critical machinery on board required for cargo operations, and the machinery required for normal sea service.
If a ship is steam propelled, it might have steam turbine driven alternators and a diesel driven alternator as a back-up. If the propulsion is by electric motors, then the vessel might require huge diesel driven generators which usually operate at 6.6kV.
Ship Type -------------------------------------------------------Power Requirement
A 10,000 dwt general cargo ship-----------------------------1 MW.
Tankers---------------------------------------------------------- 1.5 to 5 MW.
Containerships (10000 TEU) (3.3kV)-----------------------8 MW
A Modern LNG carrier (Q-max/Q-flex) (6.6kV)------------12 MW
The main bus bar has three heavy, thick bars of conductor (usually copper), running horizontally throughout the length of the main switch board. Each of the conductors is insulated between each other and wherever it is supported or clamped to the switchboard. The bus bar lies at the bottom most portion of the switchboard such that it can be easily supported and insulated. At times, there may be two divisions of the bus bar which are connected to each other by either a circuit breaker or a clamping (sliding contact) device.
The main switchboard is the main power distribution center of the ship. Thus the main bus bars are contained within the switchboard feeding various ship board auxiliaries. A ship may contain two or more generators connected to the main bus bar via the circuit breaker. Various protection for the generators like overload, reverse power, etc. are connected to circuit breaker such that the faulty generator is electrically isolated from the main bus bar. From the main bus bar, the electrical power is supplied to various ship board auxiliaries like pumps, blowers, compressors, etc. The main switch board has various measuring and monitoring devices like ammeters, voltmeters, frequency meters, watt meters, synchroscope, and power factor meters.
A generic layout of a shipboard distribution system is attached here. Refer to the diagram below for the following explanation; however this is just a simplified version of the main power distribution. This may become complex as the size of the vessel and amount of machinery increase in number.
The main switch board gets its supply from the main generators located in the engine room. The distribution system can be divided into Feeder side and the Load side. The feeders are the generators and the loads are various ship board auxiliary machineries. The Load side is further divided into Essential and Non-Essential services. Essential services are very critical auxiliaries which are directly affecting the safety of personnel, ship in terms of navigation and propulsion. They include supply to navigational aids like radars, communication equipment, navigational lights, and steering gear motors. These essential services may be supplied directly from the main switch board or via sectional boards or distribution boards. Non-essential services are those that do not affect the safety of ship and personnel. For example, air conditioning compressors and fans are non-essential services as they don’t affect the safety of the ship or personnel.
The operating voltage is stepped down to 220 volts and this voltage is given to the 220 Volts auxiliaries’ panel. From this panel, the supply goes for various lighting systems and other sockets for accommodation and galley for domestic use.
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Distribution Boards/Group Start Panels
Shipboard machinery may not be connected to the main switchboard directly. There are small motors and other devices that consume very little power and thus they may be grouped together. Instead of providing cables for every such individual motor, a single cable is taken out from the main switch board, supplying the distribution board which has a small bus bar (equivalent to supply the motors). From this small bus bar, the supply is provided for various starters of these motors via the circuit breakers. In this way, the protection for the machinery is also precise, as a small fault in such small motors will not operate the protection devices on the main switch board.
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Emergency Switch Board
As the name indicates, it supplies to equipment and machinery that are essential during any emergency like fire, flooding, etc. One important aspect about the emergency switch boards is that it is located above the load water line or the upper most continuous deck, where as the main generators and main switchboards are located below the weather deck or the load water line. Thus this ensures that the emergency supply is maintained when the vessel is flooded with water to the weather deck.
During normal operation, power is supplied from the main generators to the main switch board. The emergency switch board gets it supply from the main switch board. When the main generators fail, the main switch board looses its supply and this causes the emergency generator to start automatically, which will supply the emergency switchboard that is connected to various ship board auxiliaries. Thus critical equipment and machinery (essential services) are supplied always to maintain the safety of the ship and personnel.
Marine Electricals By Janaka.
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