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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.

VII. REFERENCES

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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.