IEEE Electrification Magazine - June 2015 - 58

BVR

VB ref

q Level

BVR

AVR1

GEN1
Main Busbar

..
..
..

Matrix of
Dynamic
Reactive
Diagonal Matrix of
Decoupling of
Power
Generator
Reference Reactive Power Reactive Power
Vector
Control Loops
Regulators
[Qref]+
1 h .. h
h 1 .. h
Σ
GRPR

h h .. 1

AVRn

GENn

Q
[Qlim+/
Qlim-]

P
V

Diagonal Matrix of
Reactive Power Limit
Calculation
VB

Three-Phase
Input
Processing

Reactive
Power Limit
Vector

Three-Phase Busbar Voltages
and Generator Currents

GRPR: Generator Reactive Power Regulators
BVR: Busbar Voltage Regulator

Figure 10. The possible architecture of an MIMO voltage and a reactive integrated regulator.

Also, autotransformers are rarely used because of the
additional space and weight that they need compared
with benefits, which are considered limited by the owners.
A smart choice, which is presently not adopted, could be
the use of the so-called "soft-starters." These are powerelectronic devices that are capable of modifying the voltage module value at their output, applying at the motor a
voltage ramp during the startup process instead of the
step caused by the closing of a breaker.
The current advancements in power electronics have
led to the appearance on the market of high-power motor
starters that are capable of also starting high-power asynchronous machines (up to 2 MW at this time). Their introduction in an IPS could boost the power quality with a low
economic impact and occupying little space on board. The
last low-impact starting method is the adoption of a variable-frequency drive (VFD), in particular, one using pulsewidth modulation (PWM). These drives are currently used
for thruster motors when dynamic positioning is required
but are also starting to be considered useful in standard
applications. Extending this solution to all the high-power
motors could be an effective way to gain better power
quality because of the transient management improvement they provide. With a VFD, it is possible to strictly
control both the voltage and the current absorbed by the
electric machine.
Another advantage of PWM drives is energy savings
with respect to motors directly connected to the grid,
gained by operating propellers, fans, and pumps, in variable
speed. The PWM solution seems to be the best from a voltage quality perspective, but it collides with the harmonic
disturbances that such devices cause, which must be

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2015

carefully evaluated. The transient response improvement
must not be achieved at the price of insertion of big harmonic filters on the network because the capacitors included in them will cause the rise of short-circuit currents.
Regarding the propulsion motors, a soft start is already
provided by the propulsion drive, which limits the impact of
the propulsion power variations on the main busbar voltage.
Nevertheless, the propulsion drive has a sensible impact on
the IPS, mostly in a condition that is normally not considered: the no-load connection of the propulsion transformers
to the busbar. In fact, when the propulsion is off, because it is
not needed, the entire propulsion drive is disconnected from
the network. In this way, the reactive power absorbed by the
high-power propulsion transformers, which are mandatory
to realize multipulse (12+) converters, is removed from the
power balance, freeing the alternators from their generation.
If the propulsion has to be turned on, these transformers
must be reconnected to feed the propulsion drives. In this
situation, their magnetic circuit needs to be re-energized,
behaving essentially as a large inductance. Then, until the
complete magnetization of the transformer's iron core, a
large inrush current is drawn from the IPS, causing a sensible
voltage sag (Figure 12).
To avoid this undesirable behavior, in certain cases, an
auxiliary low-power transformer (called a magnetization
transformer) is implemented. This transformer is connected
in parallel with the main one and is controlled by the ship's
automation. The magnetization transformer is connected
before the connection of the main one and disconnected
right after. Because of its low power output, it slowly energizes the main transformer iron, thus reducing the voltage sag
on the main switchboard.

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