Page 26 - Industrial Plants

satisfy the need of improving the efficiency and of
getting a faster power response.

Hybrid propulsion assisted by
energy storage system

The basic schematic of the electric system of an hybrid
tug is shown in fig.2. The solution provides two shaft
lines, driving the thrusters. Each shaft is driven by an
ICE and a PEM is mounted on the same shaft or on the
gearbox. The PEM can drive the propeller by itself or in
parallel to the main diesel. For each shaft line an
electric Power Unit is provided. The Power Unit
Fig. 1- Simplified load example can be taken and modified accordingly. Another performs the following tasks: i) controlling the charge
cycle for an harbor important feature to take into account is the reaction and discharge cycles of the battery, ii) connecting and
tug. time of the propulsion equipment when a sudden controlling the power given by the auxiliary generator,
increase in power is requested for fast accelerations, in iii) providing energy at fixed voltage and frequency to
order to get the best maneuverability. The hybrid the auxiliary onboard equipment, iv) driving the PEM in
solution assisted by an energy storage equipment can two modes, motor or generator according to the power
flow required. This power unit can be seen as a
This power unit can be seen as a “4-power-port”equipment, with the capability of adding
th
4-power-port equipment, with the a 5 power port for sharing its internal dc bus with the
capability of adding a 5th power port for other power unit in order to offer redundancy.
Fig. 2- Electric system sharing its internal dc bus with the other
for an hybrid harbor power unit in order to offer redundancy
tug. The power unit
The design of the power unit is based on an HW
solution that comes from the industrial world and
known as a “common DC bus”, multi-
converter system. The simplified schematic is shown in
fig.3. The internal structure shows a DC bus that
connects four power converters, each one with a
specific function and serving a dedicated power
port. Each power converter is equipped with a
dedicated control system. The DC/AC power inverter
of port A is used for driving the PEM at variable voltage
and frequency. It’s a bidirectional three-phase inverter,
with a vector motor control algorithm, that can run the
PEM either in motor mode or in regenerating
mode. The DC/AC power inverter of port B is used
for controlling the power generated by the auxiliary
generator. Here the active power flow is unidirectional.
The inverter can provide reactive power to the auxiliary
generator and it allows the usage of an induction
generator instead of a synchronous rotating machine,
with the advantage of a less expensive and simpler
solution. The DC/DC power converter of port C connects
the battery to the internal dc bus and can manage the
power flow in both direction. It controls the current
either in charging mode (step down) or in discharging
mode (step up). The DC/AC power inverter of port D
provides a fixed voltage and frequency to theonboard
grid that feeds the auxiliary equipment. This inverter is
reversible. When the vessel is at the quay, the shore
connection can also be used in order to get energy

IndustrIal Plants - May 2017
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