ISSN 2319-8885
Vol.04,Issue.06,
March-2015,
Pages:1157-1163
www.ijsetr.com
Simulation Analysis of Nine Level Cascaded H-Bridge Inverter as Series
Active Filter for Power Quality Enhancement
U.PRASAD1, K.SATISH KUMAR2
1
2
PG Scholar, Dept of EEE, Brahmaiah College of Engineering, AP, India.
Assistant Professor, Dept of EEE, Brahmaiah College of Engineering, AP, India.
Abstract: This paper investigates mitigation of current harmonics using different configuration of cascaded multilevel inverter
based shunt hybrid active power filter (SHAPF) and to improve power quality of the system. This paper presents an innovative
synchronous reference frame (SRF) controlled nine-level cascaded multilevel inverter based shunt active filter improve the
power quality in the distribution system. Cascaded multilevel configuration of the inverter has the advantage of its simplicity and
modularity over the configurations of the diode-clamped and flying capacitor multilevel inverters. Synchronous reference frame
(SRF) is used to control reactive power and harmonics compensation due to non-linear loads. The cascaded multilevel inverter is
controlled by the triangular wave produced by the SRF. Fuzzy logic controller (FLC) is used to maintain almost constant under
transient and steady state condition. The proposed new model is simulated using MATLAB/SIMULINK and simulation results
are demonstrated.
Keywords: Active Power Filter (APF), Adjustable Speed Drives (ASDs), Point of Common Coupling (POCC).
I. INTRODUCTION
Recently, power quality has been given attention due to
wide application of power converters for controlling and
converting ac power to feed electrical loads. These
converters are used at different power levels, ranging from
large adjustable speed drives (ASDs) to low power
household appliances, office equipment and computer. The
large amount of power pollution produced from these power
converters or non-linear load will causes a low power factor
efficiency of the power system, implies to voltage
distortion, and increases losses in the transmission and
distribution line (Singh et al., 1998). The conventional
compensation, approaches is done using passive LC filters
to eliminate line current harmonics and improve the system
power factor. These passive filters have the disadvantage of
large size, resonance and fixed compensation (Singh et al.,
1999). In the last couple of decades, the concept of active
power filter has been introduced and many publications
have appeared on this subject. Harmonic distortion is also
an important power quality problem. Nonlinear loads such
as high power diode/thyristor rectifiers, cyclo converters,
arc furnaces and low-power diode rectifier behave as current
source, injecting harmonic current into the supply network.
This constitutes the problems of power system harmonics.
Therefore compensation of harmonics is becoming a
significant concern.
Generally, the quality of electrical power refers to
conditions in term of constant voltage level, low voltage
harmonic distortion and low occurrence of disturbing
transient events .This can be achieved by installing extra
equipment to mitigate the possible cause of low power
quality. The passive filter is often used to improve the
power quality for its simple configuration. Bulk passive
elements, fixed compensation characteristics and series and
parallel resonance are the main drawbacks of using passive
filters. The active filters which are custom power devices
based on inverter topology are developed to improve the
power quality. Active power filter (APF) is capable of
handling harmonics ranging between fluctuating frequencies
and simultaneously maintaining a good compensative
performance These harmonics induce malfunctions in
sensitive equipment, overvoltage by resonance, increased
heating in the conductors and harmonic voltage drop across
the network impedance that affects power factor.
Traditionally passive filters have been used to compensate
harmonics and reactive power; but passive filters are large
in size; aging and tuning problems exist and can resonate
with the supply impedance. Recently active power line
conditioners (APLC) or active power filters (APF) are
designed for compensating the current-harmonics and
reactive power simultaneously.
Cascaded multilevel configuration of the inverter has the
advantage of its simplicity and modularity over the
configurations of the diode-clamped and flying capacitor
multilevel inverters. In this paper fuzzy logic based nine
level cascaded multilevel inverter as shunt hybrid active
power filter by employing indirect current control algorithm
technique with phase disposition Pulse Width Modulation
were presented. The proposed work attempts to analyze
compensation the characteristics of cascaded multilevel
Copyright @ 2015 IJSETR. All rights reserved.
U.PRASAD, K.SATISH KUMAR
inverter based shunt hybrid active power filter by employing
to nonlinear loads. However, the cost of shunt active filters
indirect current control algorithm with a view to improve
is relatively high and they are not preferable for a largepower quality is analyzed in this paper. The paper also
scale system since the power capacity of the filter should be
discusses a comparative analysis of different configuration
directly proportional to the load current to be compensated.
of cascaded multilevel inverter based shunt hybrid active
In addition, their compensating performance is better in the
power filter with respect to source current and its harmonic
current- type harmonic source than in the voltage-type
spectrum are to be analyzed and compared. The simulation
harmonic source. To overcome these difficulties, the series
is carried out in MATLAB/SIMULINK environment.
active filter was introduced at the end which is considered
as one of the practical solution for the ever growing
problems of power quality.
II. CONFIGURATION OF SERIES ACTIVE FILTER
(SAF)
The series active filter works as a kind of harmonic
The indiscriminate use of non-linear loads has given rise
isolator
rather than a harmonic voltage generator, since it
to investigation into new compensation equipment based on
provides
high impedance for the harmonics while providing
power electronics. The aim of this equipment is the
zero impedance for the fundamental. Also, the series active
elimination of harmonics in the system and reduction in
power filter can regulate the Point of Common Coupling
reactive power flow. Depending on application type, series
(POCC) voltage at a desired value by controlling the
or parallel configurations or combination of active and
inverter output so as to compensate for abnormal utility
passive filters are used. Active power filters can be used in
voltage. The series converter ensures a balanced, sinusoidal,
conjunction with passive filters improving compensation
and regulated voltage. In most cases the three-phase system
characteristics of the passive filter and to avoid the possible
voltages and currents are unbalanced and contain higher
occurrence of the generation of series or parallel resonance
order frequency components (harmonics) and a series active
as shown in Fig.1. The term active filter is a generic one and
filter unit is used to compensate for unbalanced conditions
is applied to a group of power electronic circuits consisting
and regulate the voltages. The imbalance or distortion of a
of power switching devices and passive energy storage
three phase system may consist of positive, negative, and
elements such as capacitors and inductors.
zero sequence fundamental and harmonic components.
The system (utility) voltage can be expressed as
(1)
(2)
(3)
(4)
Where subscripts +, - and 0 refer to the positive, negative,
and zero-sequence fundamental components, respectively.
Fig.1. Configuration of series active filter (SAF).
The power switching devices are driven with specific
control strategy to produce a three-phase current that are
able to compensate for harmonic current poor power factor
and load current imbalance. The main purpose of the active
filter installed by individual consumer is to compensate for
current harmonics and/or current imbalance of their own
harmonic-producing loads. On the other hand, the primary
purpose of active filter installed by utilities is to compensate
for voltage harmonics and/or voltage imbalance, or to
provide ―harmonic damping‖ throughout the power
distribution systems. In addition, active filters have the
function of harmonic isolation at the utility-consumer point
of common coupling in power distribution systems. In the
past, power quality problems are focused mainly on the
harmonic contamination that can be solved by the help of
shunt active power filters, which are regarded as a kind of
current source, compensating for the harmonic current due
(5)
Usually, the voltage at POCC is expected to be sinusoidal
with a fixed amplitude VL:
(6)
Hence the series converter will need to compensate for
the following components of voltage:
(7)
(8)
The control system should automatically control the
series converter so that its generated voltage at its output
terminals is Vc(t) and matched with equation (8). The
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.06, March-2015, Pages: 1157-1163
Simulation Analysis of Nine Level Cascaded H-Bridge Inverter as Series Active Filter for Power Quality Enhancement
performance of series active filter will also depend
different cells. Each single-phase full-bridge inverter
essentially on the type of modulation, adopted voltage
generates three voltages at the output: +Vdc, 0, and –Vdc.
controller, the design of the output switching ripple filter
The resulting output ac voltage swings from -2Vdc to +2Vdc
and on the performances, limitations of the methods used to
with five levels and the staircase waveform is nearly
identify the disturbances. In this paper series active power
sinusoidal. The structure of three phase cascaded H-bridge
filter has been realized using three Cascaded H-Bridge nine
nine level inverter is shown in Fig. 2. The ac outputs of the
level inverter.
converters are connected in series to the power system
through transformers, necessarily involves a series injection
of the compensating voltage source.
III. CASCADED H-BRIDGE MULTILEVEL
INVERTER
Though various topologies may be used to realize the
IV. PROPOSED FUZZY LOGIC BASED CONTROL
VSC, multilevel inverter technology has emerged recently
SCHEME
as a very important alternative in the area of high-power
To obtain a low distortion nearly sinusoidal output
medium-voltage energy control. It is hard to connect a
voltage, a triggering signal should be generated to control
single power semiconductor switch directly to medium
the switching frequency of each power semiconductor
voltage grids (2.3, 3.3, 4.16, or 6.9 kV). Multilevel
switch. Multicarrier Phase-shifted sinusoidal pulse width
converters can realize high power and high voltage using
modulation (PS-SPWM) switching scheme is proposed to
semiconductor switches of relative small ratings while
operate the switches in the system. Optimum harmonic
avoiding the voltage and current sharing problems
cancellation is achieved by phase shifting each carrier by:
associated with series and parallel connection of switches
(9)
commonly employed in two-level converter realization.
th
Multilevel converters can synthesize the output voltage with
Where k is the k inverter, n is the number of seriessmaller steps and reduced harmonic content, while
connected single phase inverters
potentially
resulting
in
smaller d v/dt thus lower
electromagnetic interference (EMI). Compared with diode
(10)
clamped multilevel topology, the H-bridge cascaded
Where
L
is
the
number
of
switched
DC
levels
that
can
be
structure can avoid unequal device rating and unbalanced dc
achieved
in
each
phase
leg.
In
this
paper,
to
obtain
a
five
link voltage problems. The H-bridge cascaded multilevel
level output, four carriers of triangular in nature having
converter has less storage capacitors and requires simpler
same frequency, amplitude and phase shifted by 90º are
control. Modularity nature of the H-bridge cascaded
used.
multilevel converter makes an easier realization.
In FLC, basic control action is determined by a set of
linguistic rules. These rules are determined by the system.
Since the numerical variables are converted into linguistic
variables, mathematical modeling of the system is not
required in FC. The FLC comprises of three parts:
fuzzification, interference engine and defuzzification. The
FC is characterized as; i. Seven fuzzy sets for each input and
output. ii. Triangular membership functions for simplicity.
iii. Fuzzification using continuous universe of discourse. iv.
Implication using Mamdani‟s „min‟ operator. v.
Defuzzification using the „height‟ method.
Fig.2. Nine level cascaded H-bridge MLI.
In this paper, power circuit for one phase leg of a ninelevel inverter consists of three cells in each phase with
separate dc sources. The resulting phase voltage is
synthesized by the addition of the voltages generated by the
A. Fuzzification
Membership function values are assigned to the
linguistic variables, using seven fuzzy subsets: NB
(Negative Big), NM (Negative Medium), NS (Negative
Small), ZE (Zero), PS (Positive Small), PM (Positive
Medium) and PB (Positive Big). The partition of fuzzy
subsets and the shape of membership function adapt the
shape up to appropriate system. The value of input error
E(k) and change in error CE(k) are normalized by an input
scaling factor. In this system the input scaling factor has
been designed such that input values are between -1 and +1.
The triangular shape of the membership function of this
arrangement presumes that for any particular input there is
only one dominant fuzzy subset.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.06, March-2015, Pages: 1157-1163
U.PRASAD, K.SATISH KUMAR
and transformed back to abc frame. Transformed voltages
B. Inference Method
Several composition methods such as Max–Min and
are fed to a multicarrier phase shifted PWM signal generator
Max-Dot have been proposed in the literature. In this paper
to generate final switching signals fed to the active filter.
Min method is used. The output membership function of
Therefore, block shown in Fig. 3 works as a controller for
each rule is given by the minimum operator and maximum
series active filter. If θ is the transformation angle, then the
operator.
voltage transformation from abc to d-q-0 frame is defined as
C. Defuzzification
As a plant usually requires a non-fuzzy value of control,
a defuzzification stage is needed. To compute the output of
the FLC, „height‟ method is used and the FLC output
modifies the control output. Further, the output of FLC
controls the switch in the inverter.. In order to control these
parameters, they are sensed and compared with the
reference values. In the present work, for fuzzification, nonuniform fuzzifier has been used. If the exact values of error
and change in error are small, they are divided conversely
and if the values are large, they are divided coarsely.
V. SCHEME FOR REFERNCE VOLTAGE
GENERATION
Synchronous reference Frame theory is based on the
transformation of the stationary reference frame three phase
variables (a,b,c) to synchronous reference frame variables
(d,q,0) whose direct (d) and quadrature (q) axes rotate in
space at the synchronous speed ωe. ωe is the angular
electrical speed of the rotating magnetic field of the three
phase supply, given by ωe =2Пfs, where fs is the frequency
of the supply. The series compensator is implemented by
using the voltage-control scheme that receives three-phase
input and output voltage and supplies a reference. Sensed
inputs Vsa, Vsb, Vsc and VLa, VLb, VLcare fed to the controller.
(11)
(12)
The accuracy of the reference signal generation
determines the performance of the SAF.
V. SIMULATION RESULTS
To prove the capabilities of the aforementioned control
methods,
the
test
system
is
modeled
with
MATLAB/Simulink Sim Power System block set. Total
Harmonic Distortion (THD) is calculated to verify the
efficiency and well-performance of the designed control
method. The power circuit is a three phase system supplied
by a sinusoidal balanced three phase voltage of 415 V with a
source inductance of 16.58mH and source resistance of
0.8929 Ohms. The MLI consists of IGBT based two Hbridges in each phase. An inductance has been connected in
series with the MLI to eliminate the high frequency
components at the output of the inverter. Owing to the
advantage of MLI, the series active filter is connected to the
power system through an isolation transformer. Fig.4 shows
the Matlab/Simulink Model of proposed Compensator using
Phase Shifted Modulation Technique based CHB APF.
Fig.5 shows the Simulink model for Reference signal
current Generation. Fig.6. shows the Simulink model of
PWM generation using multicarrier phase shifted technique.
The performance of designed controller has been verified
under various conditions such as: a) Three phase fault and
b) Dynamic load condition.
A. Three phase Fault
Fig.3. Simulink
Generation.
diagram
for
Reference
voltage
The three- phase source voltages (Vsa, Vsb and Vsc) are
applied to three-phase Phase Locked Loop (PLL) to
synchronize the three-phase voltages at the converter output
with the zero crossings of the fundamental component of the
supply phase voltages. Vsa, Vsb, Vsc and VLa, VLb, VLc are
transformed to d–q frame, where these signals are filtered
Fig.4. Simulink model of proposed fuzzy logic based nine
level CHB APF.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.06, March-2015, Pages: 1157-1163
Simulation Analysis of Nine Level Cascaded H-Bridge Inverter as Series Active Filter for Power Quality Enhancement
Fig.7. Source voltage before and after compensation
during three phase faulty condition.
Fig.5. Simulink model for Reference signal current
Generation.
Fig.8. Harmonic spectrum of source voltage before
compensation under three phase fault condition.
Fig.6. Simulink model of PWM generation using
multicarrier phase shifted technique.
For a three phase short circuit in the system during a
fault, all three phase sag will be of equal in magnitude. A
three phase fault is introduced between t1=0.2s to t2=0.4s by
grounding the three phases by fault resistors Rf =0.001.
Voltages of all three phases reduce in magnitude from 415
V to 250 V before compensation and the active filter which
acts as a sag compensator is triggered between t1=0.2s to
t2=0.4s to restore the source voltage back to its original
value. Fig.7 depicts the performance of series active filter
under three phase faulty condition. Fig.8 shows the %THD
when the system is uncompensated. The 5th and 7th are
0.23 % and 0.17 % of fundamental frequency respectively.
The compensated harmonic spectrum is shown in Fig.9.
Fig.9. Harmonic spectrum of source voltage after
compensation under three phase fault condition.
B. Dynamic Loading
During startup an induction motor takes a larger current
than normal, typically five to six times large. This current
remains high until the motor reaches its nominal speed
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.06, March-2015, Pages: 1157-1163
U.PRASAD, K.SATISH KUMAR
typically between several seconds and one minute. Because
of this larger current, there occurs voltage sag. As stated
above, an induction motor of 5.4 HP is suddenly connected
between t1=0.2s to t2=0.4s. Because of sudden inclusion of
induction motor, the source voltage falls from 415 V to 315
V. Fig.10 shows the performance of series active filter for
source voltage and source current before and after
compensation for dynamic load condition. The 5 th and 7th
are 0.21% and 0.16% of fundamental frequency
respectively.
Fig.12. Harmonic Spectrum of source voltage after
compensation under dynamic load condition.
The performance analysis of phase shifted multilevel
inverter as series active filter under various conditions have
been carried out and the results are tabulated for easy
reference as follows:
TABLE I: Harmonic Analysis under Three Phase Fault
Condition
TABLE II: Harmonic Analysis under Dynamic Load
Condition
Fig.10. Performance of SAF controlled by using SRF
Theory under dynamic load condition.
During closed loop operation, the controller senses the
reduction in voltage and injects the compensation voltage so
that the source voltage is maintained at its normal value
during the induction motor starting. The source voltage
compensation and its harmonic spectrum are given in Fig.11
and Fig.12.
Fig.11. Harmonic Spectrum of source voltage before
compensation under dynamic load condition.
TABLE III: Simulation Parameters
VI. CONCLUSION
The paper has investigated the FLC integrated nine level
cascaded multilevel inverter based shunt hybrid active
power filter for compensation of current compensation are
presented. The test results bring out the advantage of
cascaded multilevel inverter based shunt hybrid active
power filter for power quality enhancement. The total
harmonic distortion of source current has been reduced from
a high value to an allowable limit and to meet the IEEE 519
standard. The simulation results clearly shows that reduction
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.06, March-2015, Pages: 1157-1163
Simulation Analysis of Nine Level Cascaded H-Bridge Inverter as Series Active Filter for Power Quality Enhancement
in THD is better in nine level inverter based shunt hybrid
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Volume.04, IssueNo.06, March-2015, Pages: 1157-1163