IEEE Electrification Magazine - December 2019 - 72

In the master-slave control concept, one inverter (a
grid-forming unit) regulates the voltage and the resulting
current of this unit defines the current references of the
other units (grid-supporting units). The master acts as a
voltage source inverter (VSI) and the remaining units act
as current source inverters. This approach has high communication requirements and a supervisory control for
power distribution is necessary. For this reason, and to
avoid unnecessary wiring, measurements are taken locally at the generators and the droop control method is used
in a manner similar to that which is used by conventional
rotating machines in large systems. In this way, the system can be formed by paralleling VSIs and the supervisory
control, implemented in the MGCC and providing only the
parameter settings for each component, which encompass the reference values of frequency and voltage.
In today's all-electric ship applications, power plants
typically involve synchronous generators with diesel
engines prime movers due to their flexibility and versatility. Speed GOVs control the engines speed, i.e., the grid frequency, through the fuel intake regulation. Several types
of GOVs exist i.e., mechanical-hydraulic, direct mechanical, electrohydraulic, electronic, and microprocessor based.
As in islanded terrestrial microgrids, generators supply
a common load, and the load sharing among the generating units depends on the operating mode of the GCs. The
primary speed regulation can be set at isochronous or
droop mode. In isochronous mode, after a load perturbation, the speed returns to the reference value, while in
droop mode, the speed decreases by a defined percentage
after the load variation, and the nominal speed must be
restored by a secondary control loop.
The isochronous power-sharing scheme requires a
power-sharing line, which enables the control of isochronous generators working in parallel through a communication link among speed regulators. If the communication

link is interrupted, GOVs must switch to droop mode
without intervention from a higher-level controller. The
equivalent of the isochronous control for the voltage regulation is referred to as differential compensation or cross-current loop, where a reactive, power-sharing line is required.
When the droop control mode is employed, each generator operates with the same droop setting (typically
between 3 and 5%) and the load is equally divided among
the generating units without any communication between
the local speed controllers. The same practice can be also
adopted for voltage control and is referred to as reactive
droop compensation.
In summary, droop control is one the of the most reliable and flexible strategies for regulating frequency and
voltage within both terrestrial and marine microgrids,
where classical rotating units coexist with inverter-driven
generators, and, where the necessity to operate isolated
from a strong network represents an essential requirement.

CCs
In land microgrids, the MGCC oversees system management and the distribution utility interface. Local controllers of distributed generators, CLs, and storage systems
hold primary control (i.e., the first level of the consolidated
hierarchical control architecture), while a communication
link among local controllers and the MGCC allows for the
implementation of a higher-level control loop.
The main set of functionalities employed in the MGCC
includes sending set points and collecting data from local
controllers, load and generation forecasting, a black start
procedure, islanding, and reconnection management. The
secondary control loop applied in the MGCC has two different roles depending on the microgrid operating mode. During grid-connected mode, the main objective of the MGCC
is to regulate the total microgrid power import and export
according to a certain schedule determined by a previous

DMS

PMS

SC
BESS

GC
G

SC
Shore
Connection

BESS

MV
PL

MV
MV Distribution
Network

Figure 2. The marine microgrid concept architecture. PL: propulsion load; HL: hotel load (noncontrollable).

I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2019

IEEE Electrification Magazine - December 2019

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2019