A vessel may have a number of generating sets, a split bus bar and a variable load. Since the advent of UMS classification, automatic control has become common to ensure continuity of service and an efficient management of both generating and load equipment.
The main switchboard is fitted with a power management system. This system has various functions to ensure the continuous supply of the ship’s electrical systems. The system also automatically controls the diesel generators for efficient operation. Thus a modem PMS must not only ensure safety but also control fuel consumption and emissions.
Power management systems (PMS) have in the past been relay-based, but a programmable electronic system (PES) using a programmable logic controller (PLC) is now more commonly used. This can also be integrated into a distributed control system (DCS) for supplying information to the bridge. Automatic synchronising and load sharing is provided for the ship’s generator sets. Figure 1.7 depicts only one diesel generator; however there will be more than one such unit to so as to comply with SOLAS Regulations.
It is not necessary to have only a diesel-driven generator as the main source of power; there may be other types of prime movers too. The power management system automatically equalises the incoming generator’s frequency with the busbar frequency and energises the generator’s air circuit breaker to connect the two circuits at the moment when the parameters like voltage, frequency and phase sequence coincide. Automatic load sharing then ensures that each generator is equally loaded. A typical power management system also controls the following:
Load Demand Monitoring on ship power system
The total load is monitored and compared with the available on-line generating capacity
Generator Management on ship power system
Automatic blackout restart and connection of generators is ensured. The blocking of large motors until the number of running generators is sufficient to supply the starting current for motors and the ship’s power demand, is ensured. In response to varying load, the system will start a standby generator and initiate the synchronising sequence to bring it on-line, or off-load a generator, disconnect it from the bus and stop it when it is no longer required.
Load Sharing on ship power system
The load demand is shared between generating sets which are on-line. This can be done on a proportional basis according to the rated power and droop characteristics of each set.
Frequency Control on ship
The frequency of the bus is monitored and kept within a specified limit by means of raise / lower signals to each generator governor.
Load Inhibit on ship power system
Some systems operate a load inhibit circuit that prevents a sudden increase in load when the generating capacity is not available. Load shedding, to trip selected loads in a preset sequence if generating capacity is not sufficient, is often undertaken by a system separate from the PMS’s automatic control circuit.
Each of the earlier mentioned PMS functions will be defined within the system software. The necessary inputs are provided as on/off status inputs from circuit breakers and generator status signals, analog inputs of generator power and current, power of each load and voltage
and frequency of each bus section.
Outputs are given to raise / lower generator governor settings, close circuit breakers or initiate an auto synchronising system.
An operator interface will provide selection of duty and standby generating sets, and the ability to set levels at which generating sets are introduced or shut down. The operator will be provided with information such as power generated, power demands, running and standby capacity.
An interface to the ship’s main alarm system is likely, giving system alarms and indications of any faults within the PMS itself.
The main advantage of using a programmable electronic system / PLC for a PMS is the mathematical functions that it can perform.
For example, if the power input from the generator fails, generator power can be calculated using the current input and an assumed or actual power factor.
Overloads can also be calculated and hence the load reduced by a minimum amount, or an increase in load calculated in advance and the required generating capacity brought on.