Page 27 - Industrial Plants

Fig. 3- Schematic of
the power unit. A dc
bus links three
inverters and one dc/
dc converter for the
battery.

from the shore and switching off the on-board auxiliary
generators. In such a case the inverter of port D
takes the energy from the shore and feeds the internal
dc bus, allowing a recharge of the battery from the
shore. The last power port E is used for connecting the
dc buses of the two systems. A disconnecting switch
is also provided in between.

Electric motor

The PEM can be sized as a fraction of the main
propulsion diesel with two scopes. First of all it can
work as an helper in parallel to the main diesel at full
power, for giving an extra power boost and for
making a faster transient response either to sudden
load increase or to sudden torque request from
the lever. Second, the motor can drive the vessel at low
speed without the need of keeping the main diesel
running. This feature is desirable especially in transit, fig. 1 for calculation, the first step is to identify when the Fig. 4- Power and
when the speed is less than 40-50% of the max battery is requested to provide energy to the loads and Torque as a function
speed and the power demand can go down to 20-30% when the battery can be recharged. The main load for of speed for the PEM.
of the max power. In such a case the main diesel would the battery is the PEM. In transit mode the PEM can
work in an non optimal way and its efficiency would be be used for the electric propulsion, while in working
lowered. Using a PEM, whose rating is about 20-30% mode it can be used as helper to the main diesel. In
of the max power demand, the efficiency of the system between, the recharging time must be identified. In
is improved. Due to the intrinsic characteristic of an order to provide the proper energy balance during the
electric machine driven by an inverter, the PEM can be full cycle, the energy taken or given to the battery vs.
designed for a base speed of 50% of max speed. It will
run in constant torque mode up to the base speed and
in constant power mode over the base speed and up The capacity of the battery
to 100% (See fig. 4). Of course, the PMS can also is determined by the duty required
provide a command for running the PEM as a generator during the full cycle
in order to recharge the battery and providing power to
the LV network. If so, the mechanical power taken from
the main diesel is converted into electrical power by the time must be plotted over the power diagram of fig. 1,
PEM. as shown in fig. 5. The simple diagram shows that the
vessel can be started in electric propulsion only, up
to approx. half speed. Then, before arriving to the
Battery system working zone the main diesel is switched on and it
The capacity of the battery is determined by the duty provides power to the propeller as well as to the
requiredduring the full cycle. Assuming patterns as in battery in order to restore the full charge of the battery.

IndustrIal Plants - May 2017
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