IEEE Electrification Magazine - June 2015 - 57

the generators to compensate the voltage decrease caused by
the droop action, recovering the rated voltage on the switchboard. This recovery is set rather slow when compared to the
AVR control cycle, occurring in about 10 s. Since the main
switchboard is usually divided into two separable busbars, two
MAVRs are required, one for each section. When the two busbars are connected by means of the interconnection switch,
one of the two MAVRs goes into standby, leaving the control to
the other, but remaining ready to return online if the busbars
are separated again or if the other MAVR fails.
Despite being a useful feature, in some cases, it is not
adopted for reasons both economic and technical. For the
ship owners, it is an additional cost, requiring an additional
device, sensors, and cables plus AVRs capable of being interfaced with the MAVR against a voltage drop under load that
is of low entity. Nevertheless, the use of Master AVRs allows
for increasing the power quality of the system and makes it
easier to comply with the marine classification rules limits
for the voltage deviations from the rated value, recovering
the rated voltage in the steady state.

Multiple-Input, Multiple-Output Control (Wire)
As previously seen, the complexity of an IPS is rather high and
the electric variables of the system are tightly coupled. Trying to
control them separately is the simplest but a less effective way
to approach the problem. In fact, the common practice is to
control a multiple-input, multiple-output (MIMO) system with
an array of separated, and noninteracting, single-input, singleoutput (SISO) controllers. It is evident that this approach could
lead to the system's unexpected behavior in some conditions.
Indeed, both the literature and ship owners' experiences report
frequent power quality issues, from abnormal changes in voltage to blackouts. Even in the most modern and automated
ships, these power quality issues are not unusual.
The reasons behind these unforeseen behaviors are the
following:
xx
The poor information sharing between controllers
(AVRs, MAVR, and SGs) makes it difficult to manage
the dynamic interactions between them.
xx
No information is shared between the voltage controllers
and the shipboard power management system (PMS).
xx
The shipboard automatic reactive power management is
rather poor: for example, in some ships, harmonics filters are still manually operated.
xx
The droop mode is affected by voltage measurement
errors, which determine the reactive power recirculation between alternators.
xx
No dynamic decoupling between different reactive
power loops is assured by actual droop mode regulation.
An innovative approach to the ship power system's
control could be the adoption of a voltage and VAR integrated regulator, substituting the standard SISO controllers with a MIMO controller that is capable of regulating
the entire power system in a coordinated way.
This MIMO controller could acquire voltages and currents
from generators and busbars and process these multiple

inputs to calculate, for every point of interest, the active and
reactive power. Such information, along with every generator
capability curve, allows the regulator to fix the reactive power
to be produced by each alternator to reach the reference voltage on the busbar. The dynamic interactions between the
reactive control loops can be taken into account in a MIMO
regulator, so the multiple outputs, consisting in the voltage
references for the single generator's AVRs, can be dynamically decoupled. An example of a possible realization of this
MIMO controller is the voltage and VAR integrated regulator
reported in Figure 10, derived from land power systems.
An important advantage of the MIMO solutions is their
ability to implement additional functionality. Full-digital
over- and under-excitation limiter functions prevent alternator damage, thus allowing the transitory trespassing for
transient network VAR demands. The full inverse time/current characteristic performs better than the simple threshold clipping used in the standard AVRs, improving the voltage quality. In addition, other functions recommended by
the state-of-the-art standards can be easily implemented.
Moreover, the controller can be realized with a fully redundant architecture and with automatic data loggers to
improve the system's reliability and provide the tools for an
understanding of the transient or fault situations that may
occur during the life cycle of the ship.

Critical Issues and Advancements
Soft Start and Propulsion Motor
Transformers Inrush Currents
As stated previously, power quality is a relevant issue in an
IPS since the system is islanded from a stiff utility source.
Some loads, such as propulsion motors, thrusters, or air conditioning compressors, have a rated power comparable with
that of the generators, so their operation has a strong effect
on the system. As an example, the transient measured during a sequential startup of two 2.2-MW thrusters on an IPS
that has a total generator power of 88 MW can be seen in
Figure 11. The voltage shows the typical shape of an asynchronous motor startup, presenting relevant sag at the motor
connection, followed by a voltage peak. The former is caused
by the high current absorption when the rotor accelerates
from stationary condition, while the latter is caused by the
drop in the motor current when the rated speed is reached.
Similar transients also occur for ac compressors startup and,
generally, for every high-power asynchronous machine
installed on board. It is relevant to notice that, at the present
moment, all of these motors are directly connected to the
main switchboard and operated at a fixed speed and without
a soft-start apparatus.
Although there are several systems that are capable of
starting electrical machines at low power, thus reducing
the starting procedure impact on the network, they are not
commonly used. The simple star-delta starting method is
avoided in these power systems because of the large disturbances that occur during the star-delta commutation.
IEEE Electrific ation Magazine / j une 2 0 1 5

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2015