IEEE Electrification Magazine - September 2017 - 71

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Peak shaving: The generator-set power supply should

Power Smoothing

be bounded between a lower and a higher limit, and
the generator-set load variation should not exceed a
predefined magnitude. This operation is important in
cases where engines might automatically start and
stop, such as during DP operations, leading to reduced
efficiency in case of excessive engine running. Peakshaving reduces fuel consumption and emissions and
improves safety.
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Spinning reserve: Recent developments in class rules
and governmental regulations allow an ESD, with certain requirements, to act as a spinning reserve. Hence,
for redundancy purposes, fewer generators need to be
connected to the bus at any point in time. This can be
used to move the load per generator toward the optimal working condition and thereby reduce the fuel
consumption and emissions.
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Strategic loading: By charging and discharging the ESD,
it is possible to strategically load the generator.
Through high/low engine load cycles, it is possible to
lower the average fuel consumption and emissions
compared to a system without strategic loading. The
viability of strategic loading is directly related to the
engine fuel consumption curve characteristics. Strategic loading viability requires a study for each individual vessel.
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Zero-emissions operation: By shutting down the generators and using ESDs only, it is possible to operate
without any emissions. A large ESD is required to
supply the power demand from the vessel. This
operation is interesting and may become a requirement in the future for operations in ports and harbors. It is also the sole operational mode on fully
electric vessels.

The use of batteries for energy storage is increasing for
marine power plants. These batteries may be tens of kilowatt hours used for short-term storage, or they may be
multiple megawatt hours used as the primary energy
source for a voyage. One of the use cases for smaller batteries is power smoothing. The batteries are designed to
manage load variations, while diesel engines take the
main load. However, heat will be produced in the batteries if they are used extensively. Battery ageing is accelerated when the batteries are hot, and batteries may need
to disconnect when they are too warm. We therefore suggest removing the most challenging load fluctuations
when the load fluctuations are large, while a broader
spectrum of load fluctuations are removed when the fluctuations are small. This ensures that as much load fluctuation is removed as possible without overheating the
batteries. This is done by using a statistical model to predict the expected heat generation in the batteries in combination with an MPC to optimize the power smoothing.
The MPC gives a configuration of the power smoothing
controller that controls the power smoothing done by the
battery (Figure 4). An MPC can also include the statistical
model to optimize the configuration, which means that
the MPC can predict how different configurations will
affect the temperature of the battery and therefore manage the system proactively instead of reactively.

MPC
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DC Grid
During the last decade, dc grids have become an important alternative to ac grids for marine vessels. One of the
reasons is the ability to run a diesel-generator set with
variable frequency. This makes it possible to increase the
efficiency of the diesel engines by decreasing the engine
speed at partial loads as the friction losses decrease. In
general, the engine speed should be as low as possible to
increase the efficiency of the diesel engines. However, the
speed may be increased such that the rotational mass can
be used as an emergency ESD in case of sudden trip of a
generator set. The suggested model predictive controller
(MPC) (Figure 3) automatically sets the optimal speed of
the diesel engine, such that sufficient energy storage is
available in the case of sudden disconnection of a generator set. The MPC is suited for this task as it includes a
model of the system. The MPC optimizes the speed on the
basis of fuel consumption, torque variations, and speed
variations. It also makes it possible to include the safety
constraint (i.e., minimum frequency after a sudden disconnection of a generator).

Gov

tref

Gov

tref

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=

+
υdc

Clink

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Figure 3. A block diagram of an MPC. Gov: governor; DE: diesel engine;
G: generator.

IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 1 7

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2017