Marine electrical power distribution

Marine Electrical Power Distribution

A ship can be considered as a mobile power plant. Most of the machinery is driven electrically, which thus makes for a huge electrical power demand. This makes a shipboard electrical power distribution system very important. This article is about the layout of the main switch board and the bus bar.

Introduction

Electricity has to be distributed withminimal 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.
The Main Bus Bar
The mainbus 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 machineryrequired 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 generalcargo 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 ofthe 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 isthe 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 electricallyisolated 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. Referto 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. Thefeeders 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...