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International Research Journal of Computer Science (IRJCS) ISSN:
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SPACE VECTOR PULSEWIDTH MODULATION CONTROLLED MULTILEVEL
CURRENT SOURCE INVERTER B.ARULVANI Dr.V.PADMATHILAGAM, M.E.
(Power System) Asst. professor / Electrical Engg Annamalai
University Annamalai University
ABSTRACT: Space Vector Pulse Width Modulation (SVPWM) scheme is
chosen to control the multilevel current source inverter. Among
different modulation techniques, SVPWM is popularly accepted in
recent trend, as it is easier for digital realization. SVPWM
reduces the harmonics and the switching losses and has better dc
bus utilization. Multilevel inverter have capability to deliver
high output power with lower dv/dt (or) lower di/dt with less
distorted output waveforms resulting in reduction of harmonics
injected into the system. Multilevel current source inverter is one
of the effective solutions for better output power quality (ie)
less harmonic currents and less total harmonics distortion of the
output current. This paper focuses on step by step development of
Space Vector Pulse Width Modulation controlled multilevel current
source inverter to reduce the harmonics in the output in
MATLAB/SIMULINK platform. Results are presented and analysed.
KEYWORDS: Pulse Width Modulation (PWM), Space Vector Pulse Width
Modulation (SVPWM), Voltage Source Inverter(VSI), Current Source
Inverter(CSI),Total Harmonic Distortion(THD). 1. INTRODUCTION
Recent developments of high performance semiconductor power
switches, such as Metal Oxide Semiconductor Field Effect
Transistors (MOSFETs) and Insulated Gate Bipolar Transistors
(IGBTs) increases the research interest in high power converters
such as multilevel voltage source inverters and its dual multilevel
current source inverters. Multilevel inverters have the capability
to deliver high output power with lower harmonic distortion and
reduce the size of the filter. Multilevel inversion is a power
conversion strategy in which the output voltage is obtained in
step, thus bringing the output closer to a sine wave and reduces
the total harmonic distortion [1], [2]. There are many applications
for multilevel inverters, such as Flexible AC Transmission System
(FACTS), high voltage direct current lines and electrical drives.
Multilevel current source inverter has additional advantages like
short circuit production and over current production. Among several
modulation techniques like selective harmonic elimination, space
vector control, space vector pulse width modulation, space vector
pulse width modulation (SVPWM) techniques stand a step ahead. The
SVPWM is more sophisticated technique for generating sine wave that
provides a higher voltage with lower harmonics distortion [6]. 2.
WHY CURRENT SOURCE INVERTER
The pulse width modulated voltage source inverter is the most
common power converter topology for adjustable speed induction
motor drives. In high power areas, CSI using Gate Turn-off
Thyristors (GTOs) are of special interest. The power circuit of the
CSI is simpler and more robust than the VSI due to no freewheeling
diodes with unidirectional current flow. The CSI can provide higher
reliability related with a dc-link inductor than a capacitor for
the VSI and inherent over current production by current regulation
of the controlled rectifier. The CSI permits four quadrant
operations transferring the electric power in both directions using
the controlled rectifier with closed-loop current control [7]. In
addition, the CSI is more efficient because of the quasi-square
wave mode operation, which turns ON and OFF only once per cycle of
the output current. The multilevel CSI opens the possibility of
operation with high power levels as well as elimination of
harmonics generated by the converter. The higher the level lower
the harmonic content at the output current. An infinite number of
levels ensures zero harmonic distortion at the output current
waveform. In order to suppress the source of the harmonics (ie the
harmonic current on the dc side) the dc inductor is usually of
large value. So that the dc input current can be kept constant and
the output current waveform is purely determined by the PWM signals
[12]- [14]. 3. CURRENT SOURCE INVERTER
The main objective of there static power converters is to
produce ac output current waveforms from a dc source. For
sinusoidal ac outputs, its magnitude, frequency and phase should be
controllable. Due to the fact that the ac line currents feature
high di/dt, a capacitive filter should be connected at the ac
terminals in inductive load applications
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International Research Journal of Computer Science (IRJCS) ISSN:
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2014-15, IRJCS- All Rights Reserved Page -72
(such as adjustable speed drives).Thus, nearly sinusoidal load
voltages are generated that justifies the use of these topologies
in medium voltage industrial applications, where high quality
voltage waveforms are required [8] [11]. In a Current Source
Inverter (CSI), the current from the dc source is maintained at an
effectively constant level, irrespective of the load or inverter
conditions by inserting a large inductance in series with the dc
supply to enable changes of the inverter voltage to be accommodated
at low values of di/dt. The input to the CSI is obtained from a
fixed voltage ac source, battery, photo voltaic cell. 3.1. SINGLE
PHASE H-BRIDGE CURRENT SOURCE INVERTER The basic configuration of
an H-bridge CSI is shown in Fig.1. This inverter generates
three-level current waveform, i.e. level +I ,0 andI, as listed in
the Table.I.[2].
Table.I OPERATION MODES OF H BRIDGE CSI
Q1 Q2 Q3 Q4 OUTPUT CURRENT 1 0 1 0 +I 1 0 0 1 0 0 1 1 0 0 0 1 0
1 -I
Fig.1 THREE-LEVEL CURRENT SOURCE INVERTER
3.2 CASCADED H BRIDGE CURRENT SOURCE INVERTER The configuration
of the five-level CSI is obtained by connecting two H-bridge CSI in
parallel.
Fig.2 FIVE LEVEL CURRENT SOURCE INVERTER
Fig.2 shows the configuration of the chosen Five level current
source inverter composed of eight unidirectional power switches
Qc1-Qc8. Preferably IGBTs are used. The output current levels of
the five level CSI are +I, +2I, 0, I and 2I. In positive half cycle
four switches are ON and another four switches are OFF. In negative
half cycle the switching sequence is reversed. Therefore the output
is like a stepped waveform [2] [4]. 4.DC CURRENT GENERATION
CIRCUITS In the H-bridge based five level CSI, the dc current
source is obtained by employing an auxiliary circuit. The circuit
works as a regulated dc current source for the inverter. The
circuit simply consists of a controlled power switch (Q), a
smoothing inductor (L) as shown in fig.3. The switch regulates the
dc current flowing through the smoothing inductor and reduces the
smoothing inductor size owing to the high switching frequency
operation. Free wheeling diode is used to keep continuous current
flowing through the smoothing inductor.
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International Research Journal of Computer Science (IRJCS) ISSN:
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Fig.3 DC CURRENT MODULE
5.SPACE VECTOR PULSE WIDTH MODULATION
Space Vector Pulse Width Modulation(SVPWM) is a modulation
algorithm which translates phase voltage reference, coming from the
controller into time/duty cycles to be applied to PWM peripheral.
SVPWM is an advanced computation intensive PWM method and possibly
the best PWM technique for three phase inverters. SVPWM refers to a
special switching sequence of the upper three power switches of a
three phase inverter [1]. It is a type of modulation technique used
for changing the pulse width according to the given reference
vector. In this technique, all possible switching states are
represented as vectors in a two dimensional voltage space which are
obtained by transforming three phase dependent vectors to three
phase independent vectors The reference vector is assumed to be
constant for the corresponding sampling time. Ideally the sampling
frequency should be infinity but the sampling frequency is limited
by the turn-on and turn-off times of the device used. Therefore the
maximum possible sampling frequency is chosen for minimum total
harmonic distortion.
5.1 PRINCIPLE OF SVPWM The circuit of a typical three phase
voltage source inverter is shown in Fig.4. S1 to S6 are six power
switches that shape the output, which are controlled by the
switching variables x,x,y,y,z and z. When an upper switch is
switched ON , i.e., when x, y or z is 0, the corresponding lower
switch is switched OFF, i.e., the corresponding x y or z is 1.
Therefore, the ON and OFF states of the upper switches S1,S3 and S5
can be used to determine the output voltage [5].
Fig.4 THREE PHASE VOLTAGE SOURCE PWM INVERTER.
The operational principle of the PWM can be explained more
clearly with the space vector representation. Fig.5. displays the
relationship between abc and d-q reference frames. The voltage
vector can be expressed in the vector form as
Fig.5 RELATIONSHIP BETWEEN abc AND STATIONARY d-q REFERENCE
FRAMES
cbas VVVV 2 (1)
where
3
2exp j
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International Research Journal of Computer Science (IRJCS) ISSN:
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)120sin()120sin(
sin
tVVtVVtVV
mc
mb
ma
(2)
and Vm is the amplitude of the fundamental component. The
relationship between the switching variable vector [x, y, z]t and
the line-to-line voltage vector [Vab Vbc Vca] is given by as
follows
zyx
V101110
011
VVV
dc
ca
bc
ab
The relationship between the switching variable vector [x, y,
z]t and the phase voltage vector [Van Vbn Vcn] can be expressed
as
zyx
Vdc
211121112
3VVV
cn
bn
an
(3)
Fig.6 SECTORS AND SWITCHING VECTORS IN THE COMPLEX PLANE
Fig.6. shows the sectors and vectors in complex plane. The angle
between any adjacent two non-zero vectors is 60 degrees. Meanwhile,
two zero vectors (V0 and V7) are at the origin and apply zero
voltage to the load. The eight vectors are called the basic space
vectors and are denoted by V0, V1, V2, V3, V4, V5, V6 and V7. The
same transformation can be applied to the desired output voltage to
get the desired reference voltage vector Vref in the d-q plane.
There are eight possible combinations of ON and OFF states in the
three phase VSI configuration as shown in Fig.7 .The ON-OFF
sequences of the six switches should respect the following two
conditions [5]. Three of the six switches must be always ON and the
other three always OFF. The upper and lower switches of the same
leg are driven with two complementary pulse signals. In this way,
no
vertical short through could happen.
Table.II OUTPUT VOLTAGES FOR THE VARIOUS SWITCHING PATTERNS
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International Research Journal of Computer Science (IRJCS) ISSN:
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Fig.7 THE INVERTER VOLTAGE VECTORS (V0 TO V7)
Space voltage vector and corresponding states SVPWM refers to a
special switching sequence of the upper three power transistors of
a three phase power inverter. 5.2 SVPWM ALGORITHM Five steps can be
identified to implement the SVPWM technique: Step1 : Definition of
the possible switching vectors in the output voltage space. Step 2
: Identification of the separation planes between the sectors in
the output voltage space. Step 3 : Identification of the boundary
planes in the output voltage space. Step 4 : Obtaining
decomposition matrices. Step 5 : Definition of the switching
sequence 6. SIMULATION AND RESULT DISCUSSION The Matlab Simulink
software is used to simulate five level current source inverter,
where all parameters and blocks are modeled based on basic concepts
explained in above sections. One of the key feature is that it
allows the user to simulate the design over a specified period of
time. 6.1 SIMULATION OF FIVE-LEVEL CASCADE CURRENT SOURCE INVERTER
Five level current source inverter is modeled based on the
theoretical concept as explained. The modulating signal is
generated by space vector algorithm. Among chosen four carrier
signals, two carrier signals are applied across the positive half
cycle of the modulating signal and remaining two are applied across
the negative half cycle of the modulating signal. Gate signals are
generated when the carrier signals intersect with the reference
signal. SVPWM generated gate pulses are applied to the switches.
Here in CSI each switch is paired with other switch so there are
eight switches in CSI but four pulse generations is sufficient
because of its paired nature. These pulses are given to one
subsystem in Simulink model from where all switches of gate
terminal are connected. Based on the concepts explained in
modulation techniques, four pulses are generated. Remaining four
pulses are generated by shifting the phase angle. PARAMETERS: No of
switches - IGBTs : 8 Diodes : 8 Number of ideal switches : 2 DC
input voltage : 160 V Input inductance : 500H Load resistance :
2
Fig.8 Represents the Space Vector Pulse Width Modulation for
Five Level Current Source Inverter Where Space Vector is Compared
to Triangular Carrier Wave.
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International Research Journal of Computer Science (IRJCS) ISSN:
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Fig.8 SIMULATED SPACE VECTOR PULSE WIDTH MODULATION FOR FIVE
LEVEL CURRENT SOURCE INVERTER.
SIMULATION RESULTS: The simulated output current and voltage
waveform of multilevel current source inverter is displayed in
Fig.8 and Fig.9. respectively. This current flow through the load
and the five level voltage waveform is obtained across the load.
Fig .11.(a) and Fig.11.(b) shows the FFT spectrum of output current
generated by unoptimized switching pulse controlled and SVPWM
controlled five level current source inverter respectively. Fig
.12.(a) and Fig.12.(b) shows the FFT spectrum of output voltage
generated by unoptimized switching pulse controlled and SVPWM
controlled five level current source inverter respectively.
Fig.9 SIMULATED OUTPUT CURRENT OF SVPWM CONTROLLED CHOSEN FIVE
LEVEL CURRENT SOURCE INVERTER WITH R=2 LOAD.
Fig.10 SIMULATED OUTPUT VOLTAGE OF SVPWM CONTROLLED CHOSEN FIVE
LEVEL CURRENT SOURCE INVERTER
WITH R=2 LOAD.
Table.III THD OF OUTPUT CURRENT AND VOLTAGE UNOPTIMIZED
SWITCHING PULSE CONTROLLED AND SVPWM CONTROLLED CHOSEN FIVE LEVEL
CSI.
Parameters
%THD Unoptimized switching Pulse
signal SVPWM switching signal
Current 45.35 35.77 Voltage 43.00 20.83
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International Research Journal of Computer Science (IRJCS) ISSN:
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Table.III displays the THD of output current and voltage
unoptimized switching pulse controlled and SVPWM controlled chosen
five level CSI.
Fig.11 (a) FFT SPECTRUM OF OUTPUT CURRENT OF Fig.11 (b) FFT
SPECTRUM OF OUTPUT CURRENT OF UNOPTIMIZED SWITCHING PULSE
CONTROLLED SVPWM CONTROLLED CHOSEN FIVE LEVEL CSI CHOSEN FIVE LEVEL
INVERTER
Fig.12 (a) FFT SPECTRUM OF OUTPUT VOLTAGE Fig.12 (b) FFT
SPECTRUM OF OUTPUT VOLTAGE OF UNOPTIMIZED SWITCHING PULSE
CONTROLLED CONTROLLED CHOSEN FIVE LEVEL CSI CHOSEN FIVE LEVEL
INVERTER From the Table.III, Fig.10 and Fig.11 The total harmonic
distortion of output current and Voltage of SVPWM controlled chosen
five level Inverter is lesser than the unoptimized switching pulse
Controlled five level inverter. CONCLUSION:
Space Vector Pulse Width Modulation (SVPWM) is a more
sophisticated technique for generating sine wave that provides a
higher voltage with lower harmonic distortion and advanced
computation intensive PWM method. SVPWM technique controlled five
level CSI is simulated in simulink block. The output voltage
waveform and current waveform and FFT spectrum of current and
voltage is presented and analyzed. The THD of output current and
voltage in SVPWM controlled CSI compared to the un optimized
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International Research Journal of Computer Science (IRJCS) ISSN:
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