Electrical Distribution of Seagoing Vessel

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Introduction

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The ship's distribution system is design to safely and effectively convey the generated power to every electrical machines and equipment connected to be able to run the ship. The main switchboard supplies bulk power to motor starters and drive board, section board and distribution boards. Protection such as circuit breaker and fuses, strategically placed throughout the system to automatically disconnect a faulty circuits within the network. Transformers mediates between the high voltage supply side and the low voltage load side of the system.

In operating conditions, distribution main control center monitors the the status of power, voltage, currents and power factor in the system. Typically, conditions are usually indicated by the connected protective relays for overcurrent and earth-faults. Majority of ships are run through alternating current (ac) distribution system.

Main Distribution


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Usually, a.c. network is cheaper to install and operate than a d.c. system. In particular, a.c. offers higher power to weight ratio for generation, distribution and utilization of electricity. Transformer are available in a.c. system by which it is effective and efficient to step-up or step-down a.c. voltages where it is required.

Ship's electrical distribution scheme generally follows the shore practices. In this way, normal industrial equipment to be used onboard ship after being tested and configured to suit and withstand the rigorous sea-life, when necessary. It must withstand the vibration, humidity, high temperature, ozone, sea-water encountered in various location on board ship.

Three-phase a.c. is commonly utilize onboard ship due to its effectivity to convert rotational mechanical power from an engine shaft into electricity across the terminal of an a.c. synchronous generator. In addition, three-phase is used due to most of the electrical motors driven pumps needs three-phase voltage and current supply.

Majority of ships have 3-phase a.c., 3 wire, 440 V insulated-neutral system. Typically, the ships star-connected generators is not earthed to the ship's hull. Ships with very large connected loads have generators operating at high voltages (HV). Typically, ship's operates on 3.3 kV, 6.6 kV and even 11 kV depending on the ship's class.

Economically, high voltage (HV) are used to considerably reduced the current in the system resulting to a lower sizes of cables and conductors used in the ship's electrical system. Hence, it is impractical to have a large conductors installed on ships. This is not feasible due to the limited spaced available in ships. In addition, this could also add to more weight on the ship.

Operating at high voltage (HV) is becoming more common as ships size and complexity increases especially for large cruise liners today. Offshore and gas platforms operate at up to 13.8 kV, where equipment weight savings is at high importance due to limited space. Typically, distribution systems at these high voltages have their neutral points earthed by connecting a resistor or earthing transformer to the ship's hull.

The frequency of the ship's electrical power system operates at 50 hz or 60 Hz. The most adopted system frequency on board is 60 Hz. This higher frequency means the connected motors and generators are running on a higher speeds with a significant reduction in size in reference to is rate power.

In ships, lighting and low power single phase supplies usually operate at the lower voltage of 220 V a.c. although 110 V a.c. is also utilized. these voltage are derived by connecting the loads across a step-down transformer. Or, it could be possible that it is connected between the line-to-line terminal in a three-phase supply.

In the distribution system, busbar typically receives the bulk power from the main generators. This is then distributed using the feeder conductors to various motors, lighting, galley services, navigational lights, etc. which comprises as the ship's electrical loads.

The electricity is routed in the main switch board where you can locate the designated circuit breakers and switches for the different sections of electrical equipment. The electricity is distributed by means of cables to section and distribution boards to the final loads in the system.

The circuit breakers and switches are present in the distribution system as a means of interrupting the flow of current , and the fuses and relays to protect the distribution from the damage due to large fault currents during short-circuits and overloading.

The ship's distribution scheme is usually in radial or branching scheme. This is to ensure the reliability and security of service for both faulted and maintenance condition of the ship. This distribution system has a simple and logical structure where there is always pathway for current even though during maintenance of selected section in the power distribution system. Typically, all connected cables and protective devices are properly rated to suit the required operating conditions.

Essential and Non-essential Loads

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Ship's main electrical loads are divided into two groups: essential and non-essential loads. Essential services are those required for the safety of personnel's on board. This also include safe navigation and propulsion. Typically, essential loads should be supplied with power without failure. This include supplies to navigational equipment, communications, machinery spaces, control stations and steering gear.

The essential load may be supplied directly or through distribution boards. Emergency supplies are necessary for loads which required to handle potentially dangerous situations. To maintain generator operation in an event of overload, a preferential load shedding arrangement is in placed. This is to ensure that the emergency generator will supply the essential loads until such time the main service generators is online.

Preferential tripping is achieved by connecting a special overload relay on the system that will energize the preferential section boards. If a generator overload develops, the preferential trip relay sets an alarm and trips-off the selected non-essential load. In this manner, the load connected is reduced so that only essential loads are connected.

Each generator is connected with a normal overcurrent relay to trip its own circuit breaker which is typically high set at 150% of its rated current. In addition, the preferential trip is set in a much lower percentage at 110% current instantaneous operation. To ensure that at the initial overload condition of the generator, the preferential trip will activate before the the generator's overcurrent relay activates. Usually, the generator's OCR is set at a 20 second time delay.

In event of overload in the main generator set, the preferential trip activates and sequentially turns of the non-essential loads connected in the system. The time delay will deactivate the air conditioning and ventilation within the 5 seconds after activation of the preferential trip. Refrigerated cargo plant at 10 seconds and deck equipment within the next 5 seconds. The order of tripping varies with the ship type. When sufficient non-essential load has been disconnected, the preferential overload trip resets and no loads are connected.

Preferential trip is also activated by a detected low system frequency or low speed in the generator prime-mover. In many case, preferential trip relays are interconnected with an electronic relay to monitor the presence of overcurrent and reverse power in the generator due to interval dropping of loads.

Emergency Condition

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An emergency electrical power service is incorporated in the ship's electrical distribution system. In the event of failure of the main generator, emergency generators will automatically be on-the-line. This will supply emergency lighting, alarms, communications, watertight doors and other services necessary to safety and to permit safe evacuation of the ship, as arises.

Emergency supplies are either emergency generator or a battery bank. The emergency supply must not be dependent upon any other machinery in the engine room. It should have all the necessary auxiliaries to run on its own in the event of main power failure. In relation to the earlier statement, emergency generated must have its own internal combustion engine as prime mover and a fuel tank which is separate from the main generator set. It also has a switchboard and starting equipment in a close vicinity.

The emergency power sources must be activated immediately after the main power fails. Emergency batteries can be arranged to be switched into service immediately following a main power failure. On the other hand, emergency generators can be hand cranked, but usually automatically started by a compressed air or a battery to ensure immediate run-up without delay.

Emergency battery is permitted by regulation to be the sole emergency power source. In reality, battery banks are quite bulky and large which is impractical with the limited space on board ship. In addition, generators ahs its own small starting battery connected or start air supply, which is installed locally. Thus, it is must suitable to have an emergency generator as to an emergency battery supply.

On passenger ships, emergency power supply is provided to last at least 36 hours by a diesel-driven generator. In case main supply can not be restored, there is sufficient power for evacuation. For non-passenger vessels, 18 hours are permitted. Transitional battery supply is also connected to provide and maintain vital services such a lighting for at least 3 hours. This is to ensure that total blackout cannot occur during transitional periods. In addition, it will give an ample time to start the manually-started emergency generators.

The emergency supply must be ready and available all the times. To ensure reliability in operation, emergency sources a re tested and maintained at a regular interval.

References

  1. Reasons for Using High Voltage Systems On board Ships

  2. How to Get the Most Out of Your Ship’s Electrical System

  3. Preferential Trips on Ship : Construction and Working

  4. Marine Electrical Knowledge

  5. Practical Marine Electrical Knowledge

  6. How is Power Generated and Supplied on a Ship?



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