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ISSN 2348–2370
Vol.07,Issue.08,
July-2015,
Pages:1380-1386
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SHE-PWM for Harmonic Elimination in VSC HVDC Transmission with
Fuel Cell System T. NARESH
1, DHANNANI SURESH
2, T. CHARAN SINGH
3
1PG Scholar, Swarna Bharathi Institute of Science and Technology Engineering College, Khammam, TS, India.
2Assistant Professor, Swarna Bharathi Institute of Science and Technology Engineering College, Khammam, TS, India.
3Associate Professor, Swarna Bharathi Institute of Science and Technology Engineering College, Khammam, TS, India.
Abstract: Control methods based on selective harmonic
elimination pulse-width modulation (SHE-PWM)
techniques with fuel cell system offer the lowest possible
number of switching transitions and improve the voltage
level in HVDC transmission system. This feature also
results in the lowest possible level of converter switching
losses. For this reason, they are very attractive techniques
for the voltage-source-converter-(VSC) based high-voltage
dc (HVDC) power transmission systems. The project
discusses optimized modulation patterns which offer
controlled harmonic elimination between the ac and dc
side. The application focuses on the conventional two-level
converter when its dc-link voltage contains a mix of low-
frequency harmonic components. Simulation results are
presented to confirm the validity of the proposed switching
patterns.
Keywords: Selective Harmonic Elimination Pulse-Width
Modulation (SHE-PWM), Current-Source Converter
(CSC), Insulated-Gate Bipolar Transistors (IGBTs).
I. INTRODUCTION The continuous growth of electricity demand and ever
increasing society awareness of climate change issues
directly affect the development of the electricity grid
infrastructure. The utility industry faces continuous
pressure to transform the way the electricity grid is
managed and operated. On one hand, the diversity of
supply aims to increase the energy mix and accommodate
more and various sustainable energy sources. On the other
hand, there is a clear need to improve the efficiency,
reliability, energy security, and quality of supply. With the
breadth of benefits that the smart grid can deliver, the
improvements in technology capabilities, and the reduction
in technology cost, investing in smart grid technologies has
become a serious focus for utilities. Advanced
technologies, such as flexible alternating current
transmission system (FACTS) and voltage-source
converter (VSC)-based high-voltage dc (HVDC) power
transmission systems, are essential for the restructuring of
the power systems into more automated, electronically
controlled smart grids. An overview of the recent advances
of HVDC based on VSC technologies is offered in. The
most important control and modeling methods of VSC-based
HVDC systems and the list of existing installations are also
available in. The first generation of utility power converters is
based on current-source converter (CSC) topologies. Today,
many projects still use CSCs due to their ultra-high power
capabilities. With the invention of fully controlled power
semiconductors, such as insulated-gate bipolar transistors
(IGBTs) and integrated gate-commutated thyristors (IGCTs)
the VSC topologies are more attractive due to their four-
quadrant power-flow characteristics. A typical configuration
of the VSC-based HVDC power transmission system is shown
in Fig.1.
The multilevel topologies for high-voltage high-power VSCs
are also briefly discussed in. Multilevel converters can be
more efficient but they are less reliable due to the higher
number of components and the complexity of their control and
construction. Increasing the number of levels above three is a
difficult task for the industry. The multilevel converters are
beyond the scope of this paper. This paper focuses on the
conventional three-phase two-level VSC topology (Fig.2) and
associated optimized modulation. In most cases, the voltage of
the dc side of the converter is assumed to be constant and the
ac network is assumed to be balanced. However, fluctuations
at various frequencies often occur on the dc side which
usually appear as harmonics of the ac-side operating
frequency.
Fig.1. Phase of the two-level VSC for the HVDC power
transmission system.
T. NARESH, DHANNANI SURESH, T. CHARAN SINGH
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1380-1386
Fig.2. Three-phase two-level VSC.
The most significant harmonic introduced to the dc-side
voltage spectrum by an unbalanced three-phase ac-network
is the 2nd harmonic. Inverters with 2nd harmonic on the dc
bus generate the third harmonic on the ac side. The
elimination of inverter low-order harmonics with
fluctuating input voltage is described in. The proposed M-
type modulation technique allows 33% reduction in the
switching transitions without lowering the order of the
predominant harmonic. The geometrical technique of
proposes a numerical calculation by modifying the
pulsewidth to cancel the harmonics produced by the dc-
side ripple voltage. It has lower total harmonic distortion
(THD) when compared with the conventional triangular
sinusoidal PWM in the case where the dc-link voltage also
fluctuates. However deal with sinusoidal-PWM techniques,
which require a relatively high number of transitions per
cycle to eliminate the low-order harmonics. Selective
harmonic elimination pulse-width modulation (SHE-PWM)
is the harmonic control with the lowest possible switching
to give tightly controlled voltage spectrum and increase the
bandwidth between the fundamental frequency and the first
significant harmonic.
II. FUEL CELL
A fuel cell is an electrochemical cell that converts a
source fuel into an electrical current. It generates electricity
inside a cell through reactions between a fuel and an
oxidant, triggered in the presence of an electrolyte. The
reactants flow into the cell, and the reaction products flow
out of it, while the electrolyte remains within it. Fuel cells
can operate continuously as long as the necessary reactant
and oxidant flows are maintained. Fuel cells are different
from conventional electrochemical cell batteries in that
they consume reactant from an external source, which must
be replenished a thermodynamically open system as shown
in Fig.3. By contrast, batteries store electrical energy
chemically and hence represent a thermodynamically
closed system. Many combinations of fuels and oxidants
are possible. A hydrogen fuel cell uses hydrogen as its fuel
and oxygen (usually from air) as its oxidant. Other fuels
include hydrocarbons and alcohols. Other oxidants include
chlorine and chlorine dioxide.
Fig.3. Demonstration model of a direct-methanol fuel cell.
The actual fuel cell stack is the layered cube shape in the
center of the image.
III. HIGH-VOLTAGE, DIRECT CURRENT (HVDC)
A high-voltage, direct current electric power transmission
system uses direct current for the bulk transmission of
electrical power, in contrast with the more common
alternating current (AC) systems. For long-distance
transmission, HVDC systems may be less expensive and
suffer lower electrical losses. For underwater power cables,
HVDC avoids the heavy currents required to charge and
discharge the cable capacitance each cycle as shown in Fig.4.
For shorter distances, the higher cost of DC conversion
equipment compared to an AC system may still be warranted,
due to other benefits of direct current links. HVDC allows
power transmission between unsynchronized AC transmission
systems. Since the power flow through an HVDC link can be
controlled independently of the phase angle between source
and load, it can stabilize a network against disturbances due to
rapid changes in power. HVDC also allows transfer of power
between grid systems running at different frequencies, such as
50 Hz and 60 Hz. This improves the stability and economy of
each grid, by allowing exchange of power between
incompatible networks.
Fig.4. HVDC in 1971: this 150 kV mercury-arc valve
converted AC hydropower voltage for transmission to
distant cities from Manitoba Hydro generators.
SHE-PWM for Harmonic Elimination in VSC HVDC Transmission with Fuel Cell System
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1380-1386
High voltage is used for electric power transmission to
reduce the energy lost in the resistance of the wires. For a
given quantity of power transmitted, doubling the voltage
will deliver the same power at only half the current. Since
the power lost as heat in the wires is proportional to the
square of the current for a given conductor size, but does
not depend on the voltage, doubling the voltage reduces the
line losses per unit of electrical power delivered by a factor
of 4. While power lost in transmission can also be reduced
by increasing the conductor size, larger conductors are
heavier and more expensive.
A. Voltage-Source Converters (VSC)
Widely used in motor drives since the 1980s, voltage-
source converters started to appear in HVDC in 1997 with
the experimental Hellsjön–Grängesberg project in Sweden.
By the end of 2011, this technology had captured a
significant proportion of the HVDC market. The
development of higher rated insulated-gate bipolar
transistors (IGBTs), gate turn-off thyristors (GTOs) and
integrated gate-commutated thyristors (IGCTs), has made
smaller HVDC systems economical. The manufacturer
ABB Group calls this concept HVDC Light, while Siemens
calls a similar concept HVDC PLUS (Power Link
Universal System) and Alstom call their product based
upon this technology HVDC MaxSine. They have extended
the use of HVDC down to blocks as small as a few tens of
megawatts and lines as short as a few score kilometres of
overhead line. There are several different variants of VSC
technology: most installations built until 2012 use pulse
width modulation in a circuit that is effectively an ultra-
high-voltage motor drive. Current installations, including
HVDC PLUS and HVDC MaxSine, are based on variants
of a converter called a Modular Multi-Level Converter
(MMC). Multilevel converters have the advantage that they
allow harmonic filtering equipment to be reduced or
eliminated altogether. By way of comparison, AC
harmonic filters of typical line-commutated converter
stations cover nearly half of the converter station area.
With time, voltage-source converter systems will probably
replace all installed simple thyristor-based systems,
including the highest DC power transmission applications.
B. Amplitude Modulation
Amplitude modulation (AM) is a modulation technique
used in electronic communication, most commonly for
transmitting information via a radio carrier wave. In
amplitude modulation, the amplitude (signal strength) of
the carrier wave is varied in proportion to the waveform
being transmitted. That waveform may, for instance,
correspond to the sounds to be reproduced by a
loudspeaker, or the light intensity of television pixels. This
technique contrasts with frequency modulation, in which
the frequency of the carrier signal is varied, and phase
modulation, in which its phase is varied as shown in Fig.5.
AM was the earliest modulation method used to transmit
voice by radio. It was developed during the first two
decades of the 20th century beginning with Roberto
Landell De Moura and Reginald Fessenden's
radiotelephone experiments in 1900. It remains in use today in
many forms of communication; for example it is used in
portable two way radios, VHF aircraft radio and in computer
modems. "AM" is often used to refer to medium wave AM
radio broadcasting.
Fig.5. An audio signal (top) may be carried by a carrier
frequency using AM or FM methods.
One disadvantage of all amplitude modulation techniques
(not only standard AM) is that the receiver amplifies and
detects noise and electromagnetic interference in equal
proportion to the signal. Increasing the received signal to
noise ratio, say, by a factor of 10 (a 10 decibel improvement),
thus would require increasing the transmitter power by a
factor of 10. This is in contrast to frequency modulation (FM)
and digital radio where the effect of such noise following
demodulation is strongly reduced so long as the received
signal is well above the threshold for reception. For this
reason AM broadcast is not favored for music and high
fidelity broadcasting, but rather for voice communications and
broadcasts (sports, news, talk radio etc.).
IV. ANALYSIS OF THE PWM CONVERTER AND
SHE-PWM
The optimized SHE-PWM technique is investigated on a
two level three-phase VSC topology with IGBT technology,
shown in Fig.6. A typical periodic two-level SHE-PWM
waveform is shown in Fig.7.
Fig.6. Three-phase two-level VSC.
The waveforms of the line-to-neutral voltages can be
expressed as follows:
T. NARESH, DHANNANI SURESH, T. CHARAN SINGH
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1380-1386
(1)
when is the operating frequency of the ac, and is the dc-
link voltage.
Fig.7. Typical two-level PWM switching waveform with
five angles per quarter cycle.
Fig.8. Solution trajectories. (a) Per-unit modulation
index over a complete periodic cycle. (b) Five angles in
radians.
Thus, the line-to-line voltages are given by
(2)
The SHE-PWM method offers numerical solutions which
are calculated through the Fourier series expansion of the
waveform
(3)
where are the angles that need to be found. Using five
switching angles per quarter-wave in SHE-PWM, 5, ,7, 11,
13 to eliminate the 5th
, 7th
, 11th, and 13
th harmonics. During
the case of a balanced load, the third and all other harmonics
that are multiples of three are cancelled, due to the 120
symmetry of the switching function of the three-phase
converter. The even harmonics are cancelled due to the half-
wave quarter-wave symmetry of the angles, being constrained
by
(4)
A. Ripple Repositioning Technique
In this section, the technique to reposition the low-order
harmonics produced by the dc-link ripple voltage of a VSC is
described. The switching angles are pre calculated for every
available modulation index to obtain the trajectories for the
SHE-PWM, as shown in Fig.8. The complete sets of results
are presented in. The intersections of the trajectories shown in
Fig. 8 with any horizontal straight line, called the modulating
signal (i.e., an imaginary line of 0.75 p.u.), give the switching
angles of the specific modulation index. Those switching
angles are identical to the solution of the conventional SHE-
PWM method, so when the dc bus voltage is constant, all
harmonics before the 17th
one are eliminated. However, when
the dc bus voltage is fluctuating, other harmonics are
introduced. When the dc link has a ripple voltage of constant
frequency and amplitude times the dc-side voltage, the line-to-
neutral voltage is represented as
(5)
Therefore, the modified line-to-line voltage of (2) becomes
(6)
The method is used in the same way as in (5) to derive the
other two line-to-line voltages of the three-phase converter:
and . As was already mentioned, unbalance on the ac network
can cause the 2nd harmonic on the dc-side voltage. Hence, ,
and by substituting in (5), the lower order harmonics are given
by
(7)
(8)
The negative-sequence fundamental component and the
positive- sequence 3rd
harmonic are created on the ac side
since it is proven in (6) and (7), respectively. For a constant
dc-bus voltage, the modulating signal is a straight line of
magnitude equal to the modulation index. For the fluctuating
dc-bus voltage, the modulating signal is divided by , which is
the sum of the average per-unit value of the dc link and the
ripple voltage in order to satisfy the repositioning technique.
So when the magnitude of the dc-link voltage is
instantaneously increased by a certain amount, the modulating
signal’s amplitude is reduced by using the switching angles of
a lower modulation index. Therefore, by using the higher
modulation index at the instants that the voltage is reduced
SHE-PWM for Harmonic Elimination in VSC HVDC Transmission with Fuel Cell System
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1380-1386
and lower modulation index at the instants that the voltage
is increased, the amount of ripple is reversed. According to
Fourier transform properties, multiplication in one domain
corresponds to convolution in the other domain. So even if
one frequency is removed from the modulated signal, it is
expected to appear as sidebands of the switching
frequency. SW is the switching function of the
conventional SHE-PWM and the new switching function is
represented by
(9)
Therefore, the relevant line-to-neutral voltage is given by
(10)
.The new switching function has the property of nullifying
the low-order harmonics of the ac side, produced by the
ripple of the dc-side voltage. This new switching function
is generated from the respective intersections of the
modified modulating signal and the trajectories of
harmonic elimination solutions.
V. MODELING SIMULATION RESULTS
The MATLAB software is used to demonstrate the dc-
link ripple-voltage repositioning technique. Key results are
presented in Figs.9 to 18.
Fig.9. Simulation results for SHE-PWM eliminating
5th, 7th, 11th, and 13th
harmonics. (a) DC-link voltage.
(b) Solution trajectories to eliminate harmonics and
intersection points with the modulating signal 0.75). (c)
Line-to-neutral voltage. (d) Line-to-line voltage. (e) and
(f) Positive- and negative-sequence line-to-line voltage
spectra, respectively.
Fig.10. Simulation results for conventional SHE-PWM
with 10% ripple of 2nd harmonic at the dc bus (without
the repositioning technique). (a) DC-link voltage with 10%
ripple. (b) Solution trajectories to eliminate harmonics
and intersection points with the modulating signal 0.75).
(c) Line-to-neutral voltage. (d) Line-to-line voltage. (e) and
(f) Positive- and negative-sequence line-to-line voltage
spectra, respectively.
Fig.11. Simulation results for 10% ripple of the 2nd
harmonic at the dc bus by using the repositioning
technique. (a) DC-link voltage with 10% ripple. (b)
Modified modulating function and its intersection with the
solution trajectories. (c) Line-to-neutral voltage. (d) Line-
to-line voltage. (e) and (f) Positive- and negative-sequence
line-to-line voltage spectra, respectively.
T. NARESH, DHANNANI SURESH, T. CHARAN SINGH
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1380-1386
Fig.12. Simulation diagram of fuel cell based hvdc
converter.
Fig.13. Simulation results fuel cell based for 10% ripple
of the 2nd harmonic at the dc bus by using the
repositioning technique. (a) DC-link voltage with 10%
ripple. (b) Modified modulating function and its
intersection with the solution trajectories. (c) Line-to-
neutral voltage.
Fig.14. Magnitudes of the significant harmonics with
respect to the fundamental component while the
percentage of ripple on increases.
Fig.15. Per-unit values of the low-order harmonics up to
the 19th for the dc bus with a ripple of 10% 2nd harmonic.
Fig.16. Per-unit values of the low-order harmonics up to
the 19th for a dc bus with a ripple of 10% 6th harmonic.
Fig.17. Per-unit values of the low-order harmonics up to
the 19th for a dc bus with a ripple of 7.5% 2nd and 7.5%
6th harmonics.
SHE-PWM for Harmonic Elimination in VSC HVDC Transmission with Fuel Cell System
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.08, July-2015, Pages: 1380-1386
Fig.18. Per-unit values of the low-order harmonics up
to the 19th for a dc bus with a ripple of 25% 2nd
harmonic.
VI. CONCLUSION
An optimized SHE-PWM technique, which offers
immunity between the ac and dc side in a two-level three-
phase VSC, is discussed in this paper. The technique is
highly significant in HVDCs due to the elimination of
every low-order harmonic of the ac side produced by the
dc-link ripple voltage. The dc-link ripple repositioning
technique regulates the magnitude of the fundamental
component and eliminates the low-order harmonics of the
ac side even when the dc bus voltage fluctuates. This is an
online method which can be applied for eliminating any
low-order harmonic frequency regardless of amplitude or
phase shift of the ripple. There are some limitations related
to the maximum modulation index available for SHE-
PWM angles. The repositioning technique also causes a
reflection with respect to the midpoint between the
fundamental component and the first significant harmonic.
There are cases where the technique is not beneficial. On
the other hand, it eliminates all low-order ac-side
harmonics for every dc-bus ripple voltage of frequency
below the midpoint harmonic.
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Author’s Profile:
T.NARESH, received B.Tech degree in
Electrical and Electronics Engineering
from Mohammadiya Institute of
Technology Khammam, Telangana,
India. And currently pursuing M.Tech in
Electrical Power Electronics at Swarana
Bharathi Institute of Sciences &
Technological, Khammam, Telangana. My areas of interest
are Power Electronics, Electrical Machines.
Mr. Dhannani Suresh, presently working
as Assistant professor in Swarna bharathi
Insititute of Science and Technology,
Engineering College, Khammam,
Telangana, India. He did his B.Tech degree
in Electrical & Electronics Engineering
from Dr.Paul raj Emgineering college,
Bhadrachalam, Khammam. And then completed his P.G in
Power Systems at Dr.Paul Raj Engineering college,
Bharachalam .He has a teaching experience of 5 years. His
area of interest in Power Systems
T. Charan Singh, he obtained his B.Tech
in Electrical and Electronics Engineering
from JNTU, Hyderabad in 2001. He
obtained M.Tech in Energy Systems in
2006 from JNTU college engineering
Hyderabad and pursuing Ph.D at JNTU,
Hyderabad. He has teaching experience of
11 years and guided more than 20 UG projects and 7 PG
projects. He has six International Journal publications to his
credit. Present he working as, Assoc. Professor & H.O.D of
Dept of EEE SBIT, Khammam Telangana India. 507002And
his areas of interest include 6-Phase system, Power Systems
stability, FACTS and Smart Grid.