What you need to know about electrical preferential tripping and sequential restarting

Onboard vessels, modern power management systems make it less likely for power to go out for no reason, but engineers must be familiar with the specific procedures, where to find the instructions and procedures, and ready to act in case of automation failure.

Example of Power Management System

In case of a problem with power main diesel generators or in case of overcurrent, non-essential loads are interrupted automatically, in order to prevent the ship’s power failure. Preferential tripping has been fitted to reduce the possibility of the loss of essential vessel supplies if an event as described above will occur. It has been arranged that load reduction will be achieved by an ordered disconnection of non-essential supplies to prevent a blackout that may put the vessel’s safety at risk.

Example of preferential trip alarm

Depending on the vessel electrical layout and design, there are different stages (two or three) of preferential tripping. For example:

      • if the current on a running generator exceeds 100% of the generator rating for a period exceeding 5 seconds, the PMS will initiate the release of the 1st stage preferential tripping (PT1), thereby providing protection against the overcurrent which would otherwise trip the circuit-breaker – non critical ship systems.
      • if the current on a running generator exceeds 100% of the generator rating for a further 5 seconds, the PMS will initiate the release of the 2nd stage preferential tripping (PT2) – cargo hold vent fans and packaged air conditioning units.
      • in some vessels if the current on a running generator exceeds 100% of the generator rating for a further 5 seconds, the PMS will initiate the release of the 3rd stage preferential tripping (PT3).

Example of 1st and 2nd stage preferential trips alarm

on some other systems the preferential trips can be triggered in three ways regardless of the generator being under PMS or manual control:

      • an overcurrent condition in a main generator for a preset time will initiate first stage trips – if the current on a running generator reaches 120 % of the rated full load current for a period of 10 s, the breaker overcurrent protection circuit will initiate the release of the first stage of preferential tripping. If the 120% overload condition should continue for 40 seconds the generator VCB itself would trip, initiating the 2nd stage trip.

Example of DG overcurrent alarm leading to preferential trip alarms

When normal conditions resume, the tripped breakers must be manually reset.

      • abnormal trip of the generator VCB – second stage tripping. This abnormal trip of the generator VCB can be caused by any of the following:
          • low lubricating oil pressure
          • high fresh water cooling temperature
          • overspeed of generator engine
          • short-circuit current
          • nuisance trip

Should such trip occur then the remaining preferential trips will operate simultaneously.

      • manual tripping – the bus tie panel carries emergency stop contacts and preferential trips.

To trip the specific consumers, their circuit breakers are fitted with undervoltage (UV) trips. The supply to these UV trips is interrupted by the preferential tripping relays.

Example of UVT trip device

The identifications labels of the consumers configured for preferential tripping are coloured yellow with either PT1, PT2 or PT3 engraved into them.

Preferential tripping stages are accompanied by alarms into the alarm and monitoring system.

The vessel’s automatic control system will automatically restart the required machinery to restore power to the vessel, but to fulfil this requirement, at least one diesel generator must be left in the automatic standby mode. The essential machinery is started automatically according to the example sequence shown below.

Example of sequential restart list

The sequence is started when power is restored to the 440V main switchboard. The restart sequence is usually left enabled, however, the operator may disable the sequence, and if the sequence is disabled, an ‘auto start sequence disabled’ alarm is raised. The sequence is automatically halted in the case of another blackout.

When normal power is restored after a blackout, all essential service machinery in service before the blackout will be started automatically when the main switchboard has regained power. Motors that were selected for duty before the blackout will be automatically returned to duty when power is restored. Similarly, motors selected for standby will automatically return to standby. If the machinery designated for duty does not restore normal system conditions, such as pressure, within a preset time, the standby motor will cut in automatically. If power is only restored to the emergency switchboard, motors whose supply is from the emergency switchboard will start irrespective of any previous selection.
In the event of a blackout all transformers in the HVS will switch off. After the first main generator is connected to the HV busbar the transformer outlets will automatically switch in sequentially in two steps, depending upon the part of the switchboard concerned.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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