IEEE Electrification Magazine - December 2019 - 71

On the other hand, medium- and large-size ships
epitomize isolated power systems, where generation, distribution, and loads coexist in a bounded environment.
As presented, some vessel categories experienced a more
significant use of the electric energy onboard, pushing
designers and manufacturers to implement the all-electric ship concept. The proximity and the similarity of size
between loads and generators as well as the more significant presence of converters, make marine power systems extremely interesting. There is an opportunity to
migrate to the marine sector the methods and technologies developed from research activities related to smart
grids and microgrids.
The vessel's power plant comprises a set of synchronous generators, and diesel engines are the most common
prime mover. In addition to the classical arrangement of
the power station, recent seagoing projects have consolidated the utilization of BESSs, while fuel cells application is
gaining interest due to its low-carbon signature. Figure 2
depicts a modern, all-electric ship power system layout,
including the main structure of the control system architecture, in port operative condition.
The speed governor (GOV) of prime movers and the
automatic voltage regulator (AVR) of the synchronous
machines, fuel cell power control, and inverter-driven generation and storage control are shown to be GCs and SCs.
The power management system (PMS) is the CC of the SPS
and provides overall control of system's operations similar
to that of the MGCC, thus ensuring blackout prevention.
Finally, the shore connection arrives at a grid-connected
operating condition, striving to transfer the loads to the
distribution grid.

In a future vision, the PMS would be responsible for the
collaboration between the DSO and the ship operator. In
this case, the ship's power system may potentially provide
services to the main distribution network as a result of the
coordination between CCs and the DMS.

Microgrid Power Control
The microgrid control system must include islanding
and reconnection management, power balance, frequency, and voltage control. During connected operations, a
microgrid can provide control services to the grid,
however, while in islanded mode, only local sources are
available, and power balance must be maintained by
microgrid controllers.
Shipboard power plants of modern marine vessels and
islanded microgrids have concerns about control and protection. Diesel generators, storage systems, and inverterdriven loads require the presence of a control system
architecture, one that is capable of ensuring high performance levels, while preventing system black out.
The significant use of power converters for interfacing
with various energy sources in MGs requires the identification of proper control strategies. Inverters use the main
grid as a reference to obtain the signal with the correct
frequency and voltage. Distributed generators and energy
storage devices operating in this mode are referred to as
current sources, whose power flow can be controlled by
varying the current references. In islanding mode, the reference from the main grid is lost and the individual local
controllers must internally generate a new reference voltage. In this situation, the DG unit is typically identified as
a controlled-voltage source.

MV Distribution
Network
MGCC

MV/LV

DMS

PCC
MG
G

CL
GC

NCL

BESS
NCL
SC

Figure 1. A terrestrial microgrid concept architecture. CL: controllable load; NCL: noncontrollable load; LC: load controller; G: generator;
GC: generator controller; DMS: distribution management system.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 9

IEEE Electrification Magazine - December 2019

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