NPV4I1031
International Journal Of Technical & Scientific Research -Vol.4, Issue .1
ISSN Online: 2319-9245
A NOVEL APPROACH FOR HARMONIC IMMUNITY IN VSC
HVDC TRANSMISSION USING SHE-PWM TECHNIQUE
#1
GUNDLA SHAILAJA, M.Tech Student,
VADLURI SRINIVAS, Associate Professor, Dept of EEE,
NIGAMA ENGINEERING COLLEGE, KARIMNAGAR, T.S., INDIA.
#2
Abstract: Recent advances in technology have realized the diode clamped topology to have a considerable reduction in
switching losses and the ability to control the harmonic content. Control methods based on selective harmonic elimination pulsewidth modulation (SHE-PWM) techniques offer the lowest possible number of switching transitions. This feature also results in
the lowest possible level of converter switching losses. For this reason, they are very attractive techniques for the voltage-sourceconverter-(VSC) based high-voltage dc (HVDC) power transmission systems. The paper discusses optimized modulation patterns
which offer controlled harmonic immunity 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 and experimental results
are presented to confirm the validity of the proposed switching patterns. Finally a seven level Multilevel converter topology is
applied for this application. The VSC based HVDC transmission system mainly consists of two converter stations connected by a
DC cable. This paper presents the performance analysis of VCS based HVDC transmission system. In this paper a 75kM long
VSC HVDC system is simulated for various faults on the AC side of the receiving station using MATLAB®/SIMULINK. The
data has been analyzed and a method is proposed to classify the faults by using back propagation algorithm.
Keywords: Amplitude modulation (AM), dc-ac power conversion, harmonic control, HVDC, insulated-gate bipolar transistor
(IGBT), power electronics, power transmission system, pulse-width modulation, voltage-source converter (VSC).
transmission system uses direct current for the bulk
I.INTRODUCTION
transmission of electrical power, in contrast with the more
Amplitude modulation (AM) is a modulation
common alternating current (AC) systems.[1] For longtechnique used in electronic communication, most
distance transmission, HVDC systems may be less
commonly for transmitting information via a radiocarrier
expensive and suffer lower electrical losses.
wave. AM works by varying the strength (amplitude) of the
For underwater power cables, HVDC avoids the heavy
transmitted signal in relation to the information being sent.
currents required to charge and discharge thecable
For example, changes in signal strength may be used to
capacitance each cycle. For shorter distances, the higher
specify the sounds to be reproduced by a loudspeaker, or the
cost of DC conversion equipment compared to an AC
light intensity of television pixels. This contrasts with
system may still be warranted, due to other benefits of direct
frequency modulation, in which the frequency of the carrier
current links.
signal is varied, and phase modulation, in which the phase is
HVDC allows power transmission between unsynchronized
varied, by the modulating signal.
AC transmission systems. Since the power flow through an
A power inverter, or inverter, is an electronic device or
HVDC link can be controlled independently of the phase
circuitry that changes direct current (DC) to alternating
angle between source and load, it can stabilize a network
current (AC).The input voltage, output voltage and
against disturbances due to rapid changes in power. HVDC
frequency, and overall power handling, are dependent on the
also allows transfer of power between grid systems running
design of the specific device or circuitry. A power inverter
at different frequencies, such as 50 Hz and 60 Hz. This
can be entirely electronic or may be a combination of
improves the stability and economy of each grid, by
mechanical effects (such as a rotary apparatus) and
allowing exchange of power between incompatible
electronic circuitry.Static inverters do not use moving parts
networks.
in the conversion process.
The insulated-gate bipolar transistor (IGBT) is a threeTypical applications for power inverters include:
terminal power semiconductor device primarily used as an
• Portable consumer devices that allow the user to connect a
electronic switch and in newer devices is noted for
battery, or set of batteries, to the device to produce AC
combining high efficiency and fast switching. It switches
power to run various electrical items such as lights,
electric power in many modern appliances: Variabletelevisions, kitchen appliances, and power tools.
Frequency Drives(VFDs), electric cars, trains, variable
• Use in power generation systems such as electric utility
speed refrigerators, air-conditioners and even stereo systems
companies or solar generating systems to convert DC power
with switching amplifiers. Since it is designed to turn on
to AC power.
and off rapidly, amplifiers that use it often synthesize
• Use within any larger electronic system where an
complex waveforms with pulse width modulation and lowengineering need exists for deriving an AC source from a
pass filters. In switching applications modern devices boast
DC source.
pulse repetition rates well into the ultrasonic range—
A high-voltage, direct current (HVDC) electric power
frequencies which are at least ten times the highest audio
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International Journal Of Technical & Scientific Research -Vol.4, Issue .1
ISSN Online: 2319-9245
frequency handled by the device when used as an analog
audio amplifier.
Voltage-source converter
An HVDC converter converts electric power from
high voltage alternating current (AC) to high-voltage direct
current (HVDC), or vice-versa. HVDC is used as an
alternative to AC for transmitting electrical energy over
long distances or between AC power systems of different
frequencies.[1] HVDC converters capable of converting up
to two gigawatts (GW)[2] and with voltage ratings of up to
900 kilovolts (kV)[3] have been built, and even higher
ratings are technically feasible. A complete converter station
may contain several such converters in series and/or
parallel.
High voltage direct current (HVDC) transmission is an
economic way for long distance bulk power delivery and/or
interconnection of asynchronous system with different
frequency. HVDC system plays much more important role
in power grids due to their huge capacity and capability of
long distance transmission [1]. The development of power
semiconductors devices, especially IGBT’s has led to the
transmission of power based on Voltage source converters
(VSCs). The VSC based HVDC installation has several
advantages compared to conventional HVDC such as,
independent control of active and reactive power, dynamic
voltage support at the converter bus for enhancing stability
possibility to feed to weak AC systems or even passive
loads, reversal of power without changing the polarity of dc
voltage (advantageous in multi terminal dc system) and no
requirements or fast communication between the two
converter stations. HVDC light is also called voltage source
converter HVDC or VSC HVDC. HVDC light can control
both active and reactive power independently without
commutation failure in the inverters. Each converter station
is composed of a VSC. The amplitude and phase angle of
the converter AC output voltage can be controlled
simultaneously to achieve rapid, independent control of
active and reactive power is bi-directional and continuous
across the operating range. For active power balancing, one
of the converters operates on dc voltage control and other
converter on active power control. When dc line power is
zero, the two converters, can function as independent
STATCOMs. Each VSC has a minimum of three controllers
for regulating active power outputs of individual VSC. It
does not require reactive power compensator resulting much
smaller equipment size. HVDC light can be applied to the
voltage support in the receiver system. It provides interconnection between two asynchronous power systems, grid
connection of large wind farm, undersea power
transmission, bidirectional power flow etc., [2].
The basic function of a VSC is to connect the DC voltage of
the capacitor into AC voltage. The IGBT can be switched on
at any time by appropriate gate voltages. However, one
IGBT of a branch can be switched off before to prevent a
short circuit of storage capacitor. Reliable storage converter
inter lock function will preclude unwanted switching IGBT.
Alternating switching the IGBT’s of one phase module as
shown in Figure 1 successively connects the AC terminals
of the VSC to the positive tapping and negative tapping of
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the DC capacitor [3]. This results in a star stepped AC
voltage comprising two voltages levels +Vdc/2 and –Vdc/2.
Fig 1: Operational principle of VSC
The VSC based HVDC transmission system
mainly consists of two converter stations connected by a dc
cable. Usually the magnitude of AC output voltage of
converter is controlled by Pulse Width Modulation (PWM)
without changing the magnitude of DC voltage. Due to
switching frequency, that is considerably higher than the AC
system power frequency the wave shape of the converter
AC will be controlled to vary sinusoidal. This is achieved
by special Pulse Width Modulation (PWM). A three level
VSC provides significant better performance regarding the
Total Harmonic Distortion (THD).
II.PULSE WIDTH MODULATION (PWM)
A converter is interconnecting two electric
networks to transmit electric power from one network to
other, each network being coupled to a respective power
generator station. The converter, having an AC side and a
DC side, includes a bridge of semiconductor switches with
gate turn- off capability coupled to a control system to
produce a bridge voltage waveform having a fundamental
Fourier component at the frequency of the electric network
coupled to the AC side of the converter. The control system
includes three inputs for receiving reference signals
allowing controlling the frequency, the amplitude and the
phase angle of the fundamental Fourier component and the
alternating voltage of the network coupled to the DC side of
the converter [4]. The principle characteristic of VCSHVDC transmission is its ability to independently control
the reactive and real power flow at each of the AC systems
to which it is connected, at the Point of Common Coupling
(PCC). In constant to line commutated HVDC transmission,
the polarity of the DC link voltage remains the same with
the DC current being reversed to change the direction of
power flow.
III. ANALYSIS OF THE PWM CONVERTER
AND SHE-PWM
The
optimized
SHE-PWM
technique
is
investigated on a two level three-phase VSC topology with
IIGBT technology, shown in Fig. 2. A typical periodic two-
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level SHE-PWM waveform is shown in Fig. 3. The
waveforms of the line-to-neutral voltages can be expressed
as follows:
Where N+1 are the angles that need to be found. Using five
switching angles per quarter-wave in (N=4)SHE-PWM, k=
5, 7, 11, 13 to eliminate the 5th, 7th, 11th,and 13th
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
Fig. 3. Typical two-level PWM switching waveform with
five angles perquarter cycle.
Fig. 5. 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 (M=0.75). (c) Line-toneutral voltage. (d) Line-to-line voltage. (e) and (f) Positiveand negative-sequence line-to-line voltage spectra,
respectively.
Fig. 4. Solution trajectories. (a) Per-unit modulation index
over a complete periodic cycle. (b) Five angles in radians.
III. Cascaded H-Bridge Multilevel Converter
3.1 Full H-Bridge
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 [20] of
the waveform
Figure. 6 Full H-Bridge
(3)
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Switches Turn ON
Voltage Level
S1,S2
Vdc
S3,S4
-Vdc
S4,D2
0
Table 1. Switching table for H-Bridge
Fig.6 shows the Full H-Bridge Configuration. By using
single H-Bridge we can get 3 voltage levels. The number
output voltage levels of cascaded Full H-Bridge are given
by 2n+1 and voltage step of each level is given by Vdc/n.
Where n is number of H-bridges connected in cascaded. The
switching table is given in Table 1 and 2.
Figure. 7 Matlab/SImulink Model of CHB
Figure. 8 Carrier Signals of Phase Shifted Carrier PWM
Table 2. Switching table for Cascaded H-Bridge
Switches Turn On
S1, S2
S1,S2,S5,S6
S4,D2,S8,D6
S3,S4
S3,S4,S7,S8
Fig.8 shows the Phase shifted Carrier PWM wave form.
Here four carriers each are phase shifted by 90 degrees are
compared with sine wave.
Voltage Level
Vdc
2Vdc
0
-Vdc
-2Vdc
IV. SIMULATION RESULTS
4.1 Modeling of Cascaded H-Bridge Multilevel Converter
Fig.7 shows the Matlab/Simulink Model of five level
Cascaded H-Bridge multilevel converter. Each H-bridge DC
voltage is 50 V. In order to generate three phase output such
legs are connected in star/delta. Each llege gating pulses are
displaced by 120 degrees.
Figure. 9 Five Level output
Fig.9 shows the phase voltage of phase shifted carrier PWM
CHB inverter. Fig.10 shows the line voltage of phase shifted
carrier PWM CHB inverter. Here phase voltage has five
voltage levels where as line voltage has nine voltage levels.
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V.CONCLUSION
Increasing demand of electrical power and need for
bulk efficient electrical power transmission system lead to
the development of HVDC transmission system. HVDC
transmission system today become one of the best
alternative for transmitting bulk power over long distance
with very less losses. This paper provided a most efficient
method to reduce the harmonics contents in the HVDC
transmission system by improving the inverter topology
through Selective Harmonic Elimination Technique. A
sample system has been designed based on the SHE PWM
technique and the model is simulated to assess the
performance of the system. The fault data has been analyzed
using back propagation algorithm. The neural networks
system is used to identify the type of the fault on the AC
side of the inverter during various fault conditions. The
results presented indicate a better efficiency in identification
of type of fault.
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