IEEE Electrification Magazine - December 2019 - 17

needs to be controlled for maintain-
ing system stability, whereas only
voltage needs to be controlled in the
dc grid. Nonlinear loads may cause
very high oscillations in the dc sys-
tem's voltage. Different voltage-
control methods using dc-to-dc
converters with voltage stabilizers
and dampers have been proposed in
the literature, and they look promis-
ing. Unlike large power systems, such
as terrestrial interconnected net-
works, shipboard power plants are
not stiff systems, meaning that sud-
den changes in the operating condi-
tions, whether due to abrupt loading
or the tripping of a generator, may
lead to instability. Frequency stabili-
ty is crucial to ac-islanded equip-
ment, and the governors should be
responsive enough to control and
maintain it. At the same time, the
AVRs must be fast and efficient
enough to stabilize the voltage. Dif-
ferent strategies are suggested in the
literature for better control and sta-
bility, for instance, a two-level

strategy where the master controller
provides voltage and frequency man-
agement. Other controllers follow the
grid-connected strategy, distributed
secondary control, strategic load-
shedding algorithms, and so on.
Various classification societies
recommend differing variation levels
for the voltage and frequency. DNV
GL's "Rules for Classification: Ships
(RU-SHIPS) part 4 chapter 8," the
American Bureau of Shipping's (ABS's)
"Rules for Building and Classing
Marine Vessels," part 4 chapter 8 sec-
tion 2, the Lloyd's Register's "Rules
and Regulations for the Classifica-
tion of Ships," and IEEE Standard
1662-2016 mention recommended
variations of the ac voltage and fre-
quency and dc voltage variation, as
presented in Table 3.

Harmonic Distortion
in ac Power Systems
The sinusoidal nature of ac current
and voltage is distorted by nonlinear
loads, which, in general, are power

converters, saturated magnetic devic-
es, arc-discharging devices, and so
on. The changed sinusoidal nature of
the ac waveform results in harmonic
distortion, and that is one of the
major challenges that power con-
verters create. Distortion in a diode
rectifier's line-current waveform
eventually injects the harmonic cur-
rent into the grid, causing voltage-
waveform distortion and harmonic
losses. It may also lead to a high
overvoltage and the overloading of
the circuit winding. In addition, har-
monic current can interfere with
communication circuits.
Line notching and voltage distor-
tion are other pertinent issues in
thyristor-based rectifiers. Phase-
controlled converters generally
result in a low power factor and
introduce large harmonics into
the system, essentially when the
average output dc voltage is sig-
nificantly less than the possible
maximum. Delays in the thyristor con-
trol-input response add complexity

TABLE 3. The recommended ac and dc system parameter variations for different class societies.
DNV GL

ABS

Lloyd's Register

IEEE Standard
1662-2016

Steady-state frequency

±5%

±5%

±5%

±5%

Transient frequency

±10%

±10%

±10%

±10%

Recovery time

-

5s

5s

5s

Steady-state voltage

±2.5%

−10 to 6%

−10 to 6%

−10 to 6%

Steady-state voltage

±2.5%

±2.5%

±2.5%

±2.5%

Transient voltage

−15 to 20%

±20%

±10%

±20%

Recovery time

-

1.5 s

1.5 s

1.5 s

Voltage tolerance
(continuous)

±10%

±10%

±10%

±10%

Voltage cycle deviation

5%

5%

5%

5%

Voltage ripple (ac root mean
square over steady dc)

10%

10%

10%

10%

Voltage tolerance when
equipment is connected to
batteries during charging

−25 to 30%

−25 to 30%

−25 to 30%

−25 to 30%

Voltage tolerance when
equipment is not connected
to batteries during charging

−25 to 20%

−25 to 20%

−25 to 20%

−25 to 20%

 
AC system

DC system

	

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IEEE Electrification Magazine - December 2019

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