As the size of vessels continues to grow, so does their demand for power, which means that more powerful engines and other pieces of equipment will need to be installed. The larger size and increasing need for power necessitate the use of higher voltages on board, which enables the energy to be distributed throughout the vessel in a manner that is both efficient and effective.
In marine industry, voltages below 1000 Vac (1kV) are considered low voltages, while above that are considered high voltages. Usually, onboard vessels, the typical high voltage system is of 3.3 kV; 6.6 kV and 11 kV.
For example, on a modern container vessel with high reefer load, the power distribution system onboard vessel consists of a main 6.6kV switchboard, main 440V switchboard and the 440V emergency switchboard. The normal operating condition of the network is as follows:
- Both halves of the main 6.6kV switchboards will be linked, ie, the bus tie breakers will be closed, effectively forming one 6.6kV switchboard so that any of the diesel generators can feed the network.
- The main 440V LV switchboard is fed via the No.1, No.2 and No.3 High Voltage (HV) 6,600/450V transformers. Both halves of the switchboard are again connected via the bus tie for normal operation.
- The circuit-breakers in the 220V feeder panel will be closed, connecting the feeder panel to the 440/225V transformers supplying 220V to the ship’s mains.
The bus tie lines provide both redundancy and supply continuity in the event of any system failures and arc detectors are provided in the circuit-breaker, cable entry and bus compartments.
Generally, the HV main switchboards are of the air insulated type and the metal clad switchboard cubicles house withdrawal type vacuum circuit-breakers. Each cubicle is divided into various compartments for power equipment (circuit breaker, bus bar and feeder) and for auxiliaries (instrumentation). The circuit-breakers are of the vacuum type with automatic shutters, same as the bus tie breaker. The operating characteristics and specifications of both breakers are the same apart from the current rating. Circuit breakers are, usually, rated at lower current and bus tie breakers are rated at higher current (usually double). All incoming and outgoing sections have facilities for earthing and short-circuiting for maintenance purposes.
Normally, the bus bars in the HV and LV switchboards are arranged as follows:
The main 6.6kV switchboard consists of several sections mounted in the starboard switchboard room:
- Bow thruster ATR panel
- Bow thruster starter panel
- Feeders to 450V reefer transformers in case of containers vessels
- Main transformers’ feeder panels
- Main diesel generators’ panels
- Earthed voltage transformers’ (EVT) panels
- Bus tie and synchronising panel
The main 6.6kV generator panels are equipped with an ammeter, voltmeter, wattmeter and power factor meter.
The main 440V LV switchboard is mounted in the engine control room and consists of the following panels:
- 440V feeder panels
- 6,600/440V transformers’ incomer panels
- Bus tie panel
- 220V feeder panels
- Group starter panels
The main 440V switchboards have a 220V feeder section which is fed from the 440V switchboard via air circuit-breakers and transformers.
The 6,600V at the 6.6kV main switchboards is transformed down to 440V via main HV transformers to feed the 440V switchboards. The transformers are located in the transformers’ rooms on the engine room or in some cases, depending on vessel configuration in one of the cargo holds. The transformers are configured in such that one is working and other one or two are on standby.
High voltage circuits are potentially more dangerous than low or medium voltage circuits. This is not only due to the increased voltage, but also the explosion risk and because, under certain circumstances, high voltage circuits can retain a lethal charge even when switched off. In addition, dangerous potentials exist some distance from the actual live high voltage conductors, the distance being determined by the conductor voltage and the dielectric strength of the insulating materials (including air) surrounding the conductor.
It is therefore essential that all persons who may be required to work on or operate high voltage apparatus are fully aware of the hazards and how to avoid the associated danger. Personnel carrying out high voltage isolation, earthing, maintenance and inspection should have attended a recognised high voltage safety training course. High voltage apparatus is classified as any apparatus, equipment and conductors which are normally operated at a voltage exceeding 1,000 volts.
Interlocks are arranged to prevent configurations that are not allowed which may result in damage or a safety hazard. The key interlocking system allows for safe access to high voltage equipment for maintenance and repair. It ensures that the access to high voltage parts is prohibited in all cases, where the correct switch off/down and earthing procedure of the main breaker is not performed completely or in the wrong order. A specific step by step procedure is required to gain access to the keys for the converter cubicles and filter rooms.
Earthing of the 6.6kV main switchboard’s bus bars is carried out by means of a bus bar earthing switch located on each of the measuring panels. In order to prevent closing of the earthing switch while the bus bar is still live, a key interlock system is employed which ensures that the circuit-breakers for all incoming circuits that can supply power to the bus bar must be opened and withdrawn to the test position before the bus bar earthing switch on the measuring panel can be closed.
Each of these incoming circuits are controlled by a circuit-breaker and contain a fixed earthing switch. An electrical and mechanical interlock ensures that the circuit-breaker must be opened and withdrawn to the test position before the respective earthing switch can be closed. When the earthing switch is closed, an interlock prevents that particular breaker from being moved from the withdrawn to the inserted position.
The earthing switches are fitted with key interlocks which prevent the earthing switch from being used with the breaker in position.
The circuit-breakers are also provided with key interlocks. When the breakers are opened and withdrawn, they can be locked in order to prevent the breaker being inserted and the key can be removed. The keys for the different circuit breakers are not interchangeable.
Before commencing a working procedure for disconnecting and reconnecting of a high voltage system , you must ensure that proper tools are available for the job and a Permit to Work is available, issued and signed.
The correct working procedure of Earthing a Line and Draw Out of a HV circuit breaker is as follow:
- Open the Circuit Breaker and turn it out to isolated position. It is not advisable to open the front door when racking in or out the circuit breaker. It is wise to keep the door closed.
- Check for Earthing lever. Mark shows Earthing switch open. Only when circuit breaker is in isolated position it is possible to operated the Earthing switch
- To close the Earthing switch, the lever to be turned clock wise. See yellow mark on earth switch.
- The earth switch is closed and there is now a mechanical interlock, which prevents the circuit breaker to move back into service position.
- As an extra security a padlock can be used to shut the earthling slot.
- Behind the door to the LV compartment the safety key for interlock is located. With the Earthing Switch closed it is possible to push the plug down and remove the key. Before the Earthing Switch can be operated back to open position the key shall be back in position and the plug shall be lifted up.
- The key is now released, and the nominated person in control of work activity and others, can go to Transformer Room.
- With the circuit breaker in isolated/test position open the door to the circuit breaker. Take out the plug between the breaker and the switchboard.
- Move and adjust the trolley to correct position in line with breaker compartment The two locking pins on the trolley shall catch the corresponding holes in the HVS and the trolley will be blocked to the switchboard.
- Move the circuit breaker out on the the trolley.
- Unlock the trolley and remove the trolley and breaker from the HVS
In case that for any reason, the circuit breaker can’t be removed as described above, an emergency procedure must be available onboard vessel for removing the HV circuit breaker. In this case a HV protecting equipment must be worn by the person involved in the work.
After the job is finished the high voltage system shall be operated back to service position, the involved engineer should make sure that the zone / room / panel are clean and free from tools and any foreign object.
The following procedure can apply:
- Disconnect the portable earthing kit using the isolating rod.
- Start at the work location and move towards the earthing switch in the high voltage system.
- All signs used for work to be removed.
- Open the earth switch in the HV system panel.
- Depending of activity carried out, the isolating level of the HV system circuit shall be controlled.
- The circuit breaker to be drawn in to service position.
- The nominated person in control of work sign the permit and return it to the nominated person in control of the installation (Chief Engineer).
- Ready for service
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