Harmonic
Mitigation In Ship Power Systems Using Passive Harmonic Filters
Abstract — For the safe,
reliable and efficient operation of a
ship, a high level of power quality has to be maintained. With the increasing
usage of nonlinear devices like variable frequency drives (VFDs) made the ship
power systems vulnerable. These are the main cause of harmonics on board and
require an immediate attention to comply with new rule in emission regulation
of the maritime industry. The subject of this paper is to analyze and mitigate
the harmonics generated on board ship using passive filters using MATLAB/
Simulink.
Index Terms— Harmonic distortion, passive filters, power factor, propulsion
I. Introduction
Shipboard power systems are
similar to land based systems and harmonic distortion is prevalent in both the
systems. Because of the modern
electronic variable speed drive (VSD) technology, uninterruptible power
supplies (UPSs), converters and switch mode power supplies (SMPSs) used the
harmonic distortion calculation and equipment rating acceptability has been a
challenge [4]. The standard that can be followed for power quality problems of
a ship are IEEE-519 in addition to the STANAG 1008 which is a dedicated
standard for ships and warships in particular. The Mil Std-1399 allows total
harmonic distortion of 5%. In recent years harmonics have become an area of
interest & concern as the total harmonic distortion acceptance level has
grown to between 5% and 10% [1,3]. Few
more potential factors for low power quality as shown in figure 1 is the
repeated pulse loads due to high power weaponry and multiple rectifiers and
invertors for AC/DC conversion and propulsion motor speed control [2].
Most of the ships still have the traditional DC
propulsion and often the motor-drive load is nonlinear and would involve commutation process within the converter. The rectification introduces the
harmonic content and this harmonic distortion propagates into other service and
sensitive loads. [6]. In addition to the drive load, ships do contain lower power capacity- sensitive loads, such as radars, navigation, computers, lighting loads etc. Though the total capacity of these sensitive loads is
much smaller
than that of main load, there can be mal-operations because of the distortions. The loads are
also sensitive to short-duration disturbances, in the form of voltage sags or swells. The disturbances can be due to faults or most likely, the fluctuating load cycles of the main drives.
Fig. 1. Shipboard power
system
In this the
paper the effect of harmonics onboard ship are discussed and appropriate
filters are designed to improve the power quality. It is often required that
the harmonic distortion is compensated at the rectification. It is also
desirable to compensate the fundamental frequency reactive power consumption of
the rectifier system to reduce the generator VAR requirement. These filters
reduce the harmonic content present and also improve the power factor of the system.
II. Rectification
Rectification
can be implemented in several ways. Typically they can be classified as
6-pulse, 12-pulse, 24-pulse, and even more, depending on the ratio of the
DC-link voltage ripple frequency to the fundamental frequency. The rectifying
device can be either diode or thyristor. Thyristors are used when dc-link
voltage control is desired. A simple 6-pulse thyristor rectifier schematic is
shown in figure 2. There’s a commutation reactance for each phase of the source
to control the derivative of phase current (a.k.a. di/dt). There is generally
also a reactance on the DC side to reduce the voltage ripple of DC voltage. In
the case of systems with line-frequency propulsion transformers, this reactance
is achieved through proper specification of the leakage reactance on the
transformer with or without additional separate inductors.
A 12-pulse rectifier is simply a
combination of two 6-pulse rectifiers. A phase shift transformer is needed in
this topology to reduce the harmonic current flowing in the generator. A 24-pulse
rectifier is a combination of four 6-pulse rectifiers. It further reduces the
harmonic current. There are rectifiers that have more than 24 pulses to reduce
the harmonic current and lower voltage of rating of the rectifying devices.
Fig. 2. A simple six pulse rectifier schematic
Other
than 6-pulse rectifiers, all other rectifiers need transformers to phase shift
the source voltage. Although they have better harmonic performance,
transformers are often undesirable for ship propulsion due to the weight and
volume limitations, particularly in combat vessels. This makes 6-pulse
rectifier very attractive where the electrical isolation is not mandatory. The
price is paid by the need of additional harmonic filters at the point of
coupling.
III. Simulation
The ship
power system is simulated using MATLAB/Simulink. The various loads including propulsion load
and service loads are considered. The effect of sensitive loads due to the
rectifiers is studied.
The circuit
consists of a three phase AC power supply. In order to get close to a real
circuit, inductive and resistive loads are added. To generate harmonics
non-linear loads are required to be added to the circuit for which a three
phase transformer and a universal bridge rectifier is added. The rectifier
receives pulses from a synchronized pulse generator. A resistive and inductive
load is added to the DC output of the rectifier. Two units are added to measure
the voltage and current in the circuit.
The circuit
consists of a three phase AC power supply. In order to get close to a real
circuit, inductive and resistive loads are added. To generate harmonics
non-linear loads are required to be added to the circuit for which a three
phase transformer and a universal bridge rectifier is added. The rectifier
receives pulses from a synchronized pulse generator. A resistive and inductive
load is added to the DC output of the rectifier. Two units are added to measure
the voltage and current in the circuit.
IV. RESULTS
Without filters the system is analyzed
and the current and waveforms are as shown in figure 4. The power factor and
Total harmonic distortion is as shown in figure 5.
Fig. 3. Voltage and current waveform without filter
Fig. 4. Power factor and
frequency analysis without filter
Without filters the Total harmonic distortion is
found to be 19.2% and the power factor is 0.62. Both the THD and power factor
exceeds the permissible limits.
To reduce the harmonics in the power systems high pass filters are
placed as shown in figure 3. The voltage and current waveforms are shown in
figure 6 and the power factor and THD are as shown in figure 7. The THD content
has reduced to 0.62% and the power factor has improved to 0.93.
Fig. 5. Voltage and current waveform with filter
Fig. 6. Power factor and
frequency analysis with filter
V. Discussions
The
shipboard power system contains significant amount of fluctuating nonlinear
load and a high level of harmonic distortions is observed. This paper presented
a few considerations in the reducing the harmonics and increasing the power
factor. The future scope of this project is to analyze the active and reactive
power versus ship speed.
References
[1] Moni Islam, Guy hardwick, Jeff Gamble “Design practive challenges
for electric ships and floating facilities” 2009 proceedings, Naval
Engineers.org
[2] Philip Crapse, Jingiang Wang, Yong-JuneShin
“Analysis and comparison of electric ship Integrated power system architecture
via harmonic meter”
[3] Ian C Evans, Tony Hoevenaavs, “Homing in on Harmonics”,
Offshore Engineers
[4] B.Hare and G.Atkinson Hope “Harmonic
Measurements on ships and cold Ironing”,
[5] Deniss T.Hall, “ Practical Marine Electrical
Knowledge”, 2nd ed, Witherby.
[6] J. C. Das, “Passive filter—Potentialities and
limitations,” IEEE Trans. Ind. Appl., vol. 40, no. 1, pp. 232–241, Jan. 2004.
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