4. Ijeeer - Improved Power Quality Features Using Fuzzy Based Upqc - k. Dhilleswaramma
CHAPTER 6 FUZZY LOGIC BASED UPQC...
Transcript of CHAPTER 6 FUZZY LOGIC BASED UPQC...
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CHAPTER 6
FUZZY LOGIC BASED UPQC CONTROLLER
6.1 INTRODUCTION
This chapter presents a novel control strategy for the case of three
phase four wire Unified Power-Quality Conditioner (UPQC) based on the
concepts of fuzzy hysteresis band voltage and current control. Using fuzzy
hysteresis band voltage and current control, voltage sag and swell, and along
with current and voltage harmonics compensation, reactive power
compensation have been simulated and the results are analyzed. The
advantages of fuzzy control is that it does not depend on the precise
mathematical model of object to overcome the impact of nonlinear. Hence it
has a good dynamic response and strong robustness to the parameter-changes
of the regulating object. The operation and capability of the proposed system
was analyzed through simulations with MATLAB / SIMULINK.
6.2 FUZZY LOGIC BASED UNIFIED POWER QUALITY
CONDITIONER
UPQC is being used as a universal active power conditioning
device to mitigate both current as well as voltage harmonics at a distribution
end of power system network. The performance of UPQC mainly depends
upon how quickly and accurately compensation signals are derived. Also,
UPQC performances will depend on the design of power semiconductor
devices, on the modulation technique used to control the switches, on the
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design of coupling elements, on the method used to determine active filters
current and voltage references and on the dynamics and robustness of current
and voltage control loops. Control strategies related to fuzzy hysteresis band
voltage and current control methods, where the band is modulated with the
system parameters to maintain the modulation frequency nearly constant are
developed. The FLC-based compensation scheme eliminates voltage and
current magnitude of harmonics with good dynamic response.
Figure 6.1 Fuzzy logic based Unified Power Quality Conditioner
A UPQC is acombination of shunt and series active power filter
sharing a common dc link. It can compensate almost all power quality
problems such as voltage harmonics, voltage unbalance, voltage flickers,
voltage sags, voltage swells, current harmonics, current unbalance, reactive
current, etc. More attention is being paid on mitigation of voltage sags and
swells using UPQC recently. The aim is to maintain the load bus voltage
sinusoidal and at desired constant level in all operating conditions. One form
of UPQC structure, which is used in distribution systems, is shown in
Figure 6.1.
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The active power filtering has proven to be one of the best solutions
for mitigation of major power quality problems. The shunt APF is utilized to
overcome all current related problems, such as current harmonics, reactive
current and current unbalance. Whereas, all voltage related problems, such as
voltage harmonics, voltage sag and swell, and voltage unbalance, are handled
by using the series APF. The UPQC is controlled in such a way that the
voltage at load bus is always sinusoidal and has a desired magnitude.
Therefore the voltage injected by series APF must be equal to the difference
between the supply voltage and the ideal load voltage. Thus the series APF
acts as controlled voltage source. The function of shunt APF is to maintain the
dc link voltage at constant level. In addition to this the shunt APF provided
the var required by the load, such that the input power factor will be unity and
only fundamental active power will by supplied by the source.
The effectiveness of an active power filter depends basically on the
design characteristics of the current controller, the method implemented to
generate the reference template and the modulation technique used. The
control scheme of a shunt active power filter must calculate the current
reference waveform for each phase of the inverter, maintain the dc voltage
constant, and generate the inverter gating signals. Also the compensation
effectiveness of an active power filter depends on its ability to follow the
reference signal calculated to compensate the distorted load current with a
minimum error and time delay.
There are two control techniques including indirect control and
direct control, and the former is relatively common used. On the method, the
series inverter is controlled as a non-sinusoidal voltage source, whereas the
shunt inverter is controlled as a non-sinusoidal current source. It’s needed to
detect the voltage distortion and the fundamental wave deviation of power
grid. These quantities are used as voltage commands to control the series
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inverter outputting compensated voltages which are contrary with the
commands, in order to ensure the load voltage is the rated sinusoidal voltage.
It is also needed to detect the reactive current and harmonic current of loads.
These quantities are used as current commands to control the shunt inverter to
output compensated currents which are contrary with the commands, in order
to ensure the input current of power grid is the sinusoidal current and the
power factor is unity.
Direct control strategy is to control the series inverter as a
sinusoidal current source and the shunt inverter as a sinusoidal voltage source.
In order to ensure the input current of power grid is sinusoidal current and the
power factor is unity, and the output load voltage is sinusoidal voltage. On
this method, the series inverter isolates the voltage disturbance of power grid
and loads, whereas the shunt inverters isolates reactive power current and
harmonic currents of loads and the neutral current into the grid. The other
advantage of this method is that UPQC is not needed to change the operation
modes when power grid is outage or recovered, because the shunt inverter is
controlled as a voltage sinusoidal source all the time.
PWM switched inverters provide superior performance to control
asymmetries and especially over currents during unbalanced faults. Three
single phase PWM VSIs are used in this control strategy . Use of single-phase
H-bridge PWM inverters in DVR power circuit makes possible the injection
of positive, negative and zero sequence voltages. The voltage control is
achieved by modulating the output voltage waveform within the inverter. The
main advantage of PWM inverter is including fast switching speed of the
power switches. PWM technique offers simplicity and good response.
Besides, high switching frequencies can be used to improve on the efficiency
of the converter, without incurring significant switching losses.
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6.3 GENERATING COMPENSATING CURRENT
The output, isp
is considered as magnitude of three phase reference
currents. Three phase unit current vectors (usa
, usb
and usc
) are derived in phase
with the three phase supply voltages (vsa
, vsb
and vsc
). These unit current
vectors (usa
, usb
and usc
) form the phases of three phase reference currents.
Multiplication of magnitude Isp
with phases (usa
, usb
and usc
) results in the
three-phase reference supply currents (isa
*, isb
* and isc
*). Subtraction of load
currents (ila, i
lband i
lc) from the reference supply currents (i
sa*, i
sb* and i
sc*)
results in three phase reference currents (isha
*, ishb
* and ishc
*) for the shunt
inverter.
These reference currents iref
(isha
*, ishb
* and ishc
*) are compared with
actual shunt compensating currents iact
(isha
, ishb
and ishc
) and the error signals
are then converted into switching pulses using PWM technique which are
further used to drive shunt inverter. In response to the PWM gating signals the
shunt inverter supplies harmonic currents required by load. In addition to this
it also supplies the reactive power demand of the load. In effect, the shunt bi-
directional converter that is connected through an inductor in parallel with the
load terminals accomplishes three functions simultaneously. It injects reactive
current to compensate current harmonics of the load. It provides reactive
power for the load and thereby improve power factor of the system. It also
draws the fundamental current to compensate the power loss of the system
and maintains the voltage of DC capacitor constant.
6.4 GENERATING COMPENSATING VOLTAGE
The series inverter, which is also operated in current control mode,
isolates the load from the supply by introducing a voltage source. This voltage
source compensates supply voltage deviations such as sag, swell, flicker and
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spikes. In closed loop control scheme of the series inverter, the three phase
load voltage (vla, v
lband v
lc) are subtracted from the three phase supply
voltage (vsa
, vsb
and vsc
), and are also compared with reference supply voltage
which results in three phase reference voltages (vla*, v
lb* and v
lc*).
These reference voltages are to be injected in series with the load.
By taking recourse to a suitable transformation, the three phase reference
currents (isea
*, iseb
* and isec
*) of the series inverter are obtained from the three
phase reference voltages (vla*, v
lb* and v
lc*). These reference currents
(isea
*, iseb
* and isec
*) are fed to a PWM current controller along with their
sensed counterparts (isea
, iseb
and isec
). The gating signals obtained from PWM
current controller ensure that the series inverter meets the demand of voltage
sag and swell, flicker etc. There by providing sinusoidal voltage to load.
Thus series inverter plays an important role to increase the reliability
of quality of supply voltage at the load, by injecting suitable voltage with the
supply, whenever the supply voltage undergoes sag. The series inverter acts as
a load to the common dc link between the two inverters. When sag occurs
series inverter exhausts the energy of the dc link. Thus, UPQC, unlike Dynamic
Voltage Restorer, does not need any external storage device or additional
converter (diode bridge rectifier) to supply the dc link voltage.
6.5 MODEL EQUATIONS OF THE UPQC
6.5.1 Computation of Control Quantities of Shunt Inverter
The amplitude of the supply voltage is computed from the three
phase sensed values as:
vsm
=[ 2/3(vsa
2+ v
sb
2+v
sc
2)]
1/2 (6.1)
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The three phase unit current vectors are computed as:
usa
= vsa
/vsm
; usb
= vsb
/vsm
; usc
= vsc
/vsm
(6.2)
Multiplication of three phase unit current vectors (usa
, usb
and usc
)
with the amplitude of the supply current (isp
) results in the three-phase
reference supply currents as:
isa
* = isp
.usa
; isb
* = isp
.usb
; isc
* = isp
.usc
(6.3)
To obtain reference currents, three phase load currents are
subtracted from three phase reference supply currents:
isha
* = isa
* - ila; i
shb* = i
sb* - i
lb
ishc
* = isc
* - ilc
(6.4)
These are the iref
for Direct current control technique of shunt
inverter. The iref
are compared with iact
in PWM current controller to obtain the
switching signals for the devices used in the shunt inverter.
6.5.2 Computation of Control Quantities of Series Inverter
The supply voltage and load voltage are sensed and there from, the
desired injected voltage is computed as follows:
vinj
= vs-v
l(6.5)
The magnitude of the injected voltage is expressed as:
vinj
= |vinj
| (6.6)
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whereas, the phase of injected voltage is given as:
inj= tan(Re[v
pq]/Im[v
pq]) (6.7)
For the purpose of compensation of harmonics in load voltage, the following
inequalities are followed:
a) vinj
< vinjmax
magnitude control;
b) 0 < inj
< 360° phase control;
Three phase reference values of the injected voltages are expressed
as:
vla* = 2v
injsin(wt+
inj)
vlb* = 2v
injsin(wt+2 /3+
inj)
vlc* = 2v
injsin(wt-2 /3+
inj) (6.8)
The three phase reference currents (iref
) of the series inverter are
computed as follows:
isea
* = vla*/z
se;
iseb
* = vlb*/z
se;
isec
* = vlc*/z
se; (6.9)
The impedance zse
includes the impedance of insertion transformer.
The currents (isea
*, iseb
* and isec
*) are ideal current to be maintained
through the secondary winding of insertion transformer in order to inject
voltages (vla, v
lband v
lc), thereby accomplishing the desired task of
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compensation of the voltage sag. The currents iref
(isea
*, iseb
* and isec
*) are
compared with iact
(isea
, iseb
and isec
) in PWM current controller, as a result six
switching signals are obtained for the IGBTs of the series inverter.
6.6 CONTROL OF DC VOLTAGE
In the UPQC the management of DC bus concerns the role of the
shunt active filter. This one determines the active power (current) necessary to
keep constant the DC voltage in steady state or transient conditions. There are
three principal factors that affect the voltage fluctuations of the DC capacitor.
The first is the alternating power of the load to be compensated, the second is
the active power imbalance during transients and the third is the active power
absorbed by the series active filter part for compensating network voltage sag.
If a power imbalance occurs, because of load changing or voltage dips, the
shunt active filter should consume or supply real power. To realize these
objectives, a fuzzy logic controller is considered.
Fuzzy logic is close in spirit to human thinking and natural
language than other logical systems. It provides an effective means of
capturing the approximate and inexact nature of systems. The fuzzy control is
basically a nonlinear and adaptive in nature, giving the robust performance in
the cases where in the effects of parameter variation of controller is present.
Fuzzy control is based on the principles of fuzzy logic. It is a non-linear
control method, which attempts to apply the expert knowledge of an
experienced user to the design of a controller. Fuzzy modeling provides the
ability to linguistically specify approximate relationships between the input
and desired output. The relationships are represented by a set of fuzzy If-then
rules in which the antecedent is an approximate representation of the state of
the system and the consequent provides a range of potential responses.
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In Fuzzy Logic Control, basic control action is determined by a set
of linguistic rules. These rules are determined by the system variables. Since
the numerical variables are converted into linguistic variables, mathematical
modeling of the system is not required in FLC. Fuzzy logic uses linguistic
variables instead of numeric variables. The process of converting a numeric
variable to a linguistic variable (fuzzy set) is called fuzzification. An arbitrary
membership function is assigned to each linguistic label. The database stores
the definition of the membership functions of the fuzzy system variables.
The fuzzy control algorithm consists of a set of fuzzy control rules
which reflects the experience gained from the plant operation. The rules are
combined by using the implication and the compositional inference.
Figure 6.2 DC voltage control using Fuzzy Logic
The FLC comprises of three parts: Fuzzification, Interference
engine and Defuzzification. The FLC 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. The
knowledge bases are designed in order to obtain a good dynamic response
under uncertainty in process parameters and external disturbances.
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DC voltage control using Fuzzy Logic is shown in Figure 6.2. The
membership functions are triangular shaped with 50% overlap for a soft and
progressive control adjustment .In our application, the fuzzy controller is
based on processing the voltage error and its derivation. Figure 6.3 shows the
membership functions of the input and the output linguistic variables.
Triangle shaped membership function has the advantages of simplicity and
easier implementation and is chosen in this application.
In the fuzzification stage numerical values of the variables are
converted into linguistic variables. Seven linguistic variables namely NB
(negative big), NM (negative medium), NS (negative small), ZE (zero), PS
(positive small), PM (positive medium), and PB (positive big) are assigned
for each of the input variables and output variable. Normalized values are
used for fuzzy implementation. As there are seven variables for inputs and
output there are 7 × 7 = 49 input output possibilities as tabulated in Table 6.1.
A membership function value between zero and one will be assigned to each
of the numerical values in the membership function graph. In this chapter, we
applied max-min inference method to get implied fuzzy set of the turning
rules.
Figure 6.3 Membership functions for input and output variables
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Table 6.1 Fuzzy set rules of inference for the DC voltage
/e NL NM NS EZ PM PS PL
NL NL NL NL NL NM NS EZ
NM NL NL NL NM NS EZ PS
NS NL NL NM NS EZ PS PM
EZ NL NM NS EZ PS PM PL
PM NM NS EZ PS PM PL PL
PS NS EZ PS PM PL PL PL
PL EZ PS PM PL PL PL PL
6.7 FUZZY ADAPTIVE HYSTERESIS CURRENT CONTROLER
The core of active filter is the control section that must be able to
derive the reference current waveform matching the harmonic content of the
line current and to drive the inverter producing a filtering current faithfully
tracking the reference one. The objective is to get sinusoidal line currents in
phase with the supply voltages at the common coupling point.
The current control strategies can be classified as hysteresis current
control, the ramp comparison control methods associated with linear
controller and the predicted current control. Hysteresis current control method
is very simple and easy to implement, but has the disadvantage of an
uncontrollable high switching frequency. This high frequency produces a
great stress for the power transistors and induces switching losses. The second
and third methods allow operating at a fixed switching frequency and are
usually performed by software using the system parameters. In this case, the
operating conditions must be known to meet sufficient and accurate control.
Consequently, a fuzzy hysteresis band circuit control for a sinusoidal input
current is involved for our application.
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The fixed hysteresis band method has the drawbacks of variable
switching frequency, heavy interference between the phases in case of three
phase active filter with isolated neutral and irregularity of the modulation
pulse position. These problems result in high current ripples, acoustic noise
and difficulty in designing input filter. To overcome these difficulties, this
chapter presents an adaptive hysteresis band current control technique in
which the hysteresis bandwidth is determined by the fuzzy logic controller.
Adaptive fuzzy hysteresis band current control technique can be
programmed as a function of the active filter and supply parameters to
minimize the influence of current distortions on modulated waveform. The
Hysteresis band (HB) can be modulated at different points of fundamental
frequency of the cycle to control the PWM switching pattern of the inverter.
Figure 6.4 shows Hysteresis band current control.
Figure 6.4 Hysteresis band current control
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Figure 6.5 Single phase voltage- source inverter
Fuzzy logic controller is used to determine the hysteresis band
width according to the supply voltage and the rate of change of filter current.
The principle of operation of the proposed technique is explained by
considering the equivalent circuit of a single phase voltage source inverter as
shown in Figure.6.5, where Lf and Rf are the smoothing inductance and
resistance of the filter and VS is the supply voltage.
The instantaneous inverter output voltage (uo) has a rectangular
waveform of amplitude Vdc with a period T as shown in Figure 6.4. The load
current i satisfies the equation
sff VdtdiLiRu0 (6.10)
If i* is the reference current, the instantaneous current error can be
defined as
*ii (6.11)
and the reference voltage as
sffo VdtdiLiRu
** (6.12)
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By considering negligible resistance, Equation (6.12) becomes
sfo VdtdiLu
** (6.13)
In Figure 6.4,the current i tends to cross the lower hysteresis band
at point1 hence the switch S1 is closed. The linearly rising current (i+) then
touches the upper band at point 2, where the switch S2 is closed. The
following equations can be written in the respective switching intervals t1 and
t2 from Figure 6.4.
)(1sdc
f
VVLdt
di (6.14)
)(1sdc
f
VVLdt
di (6.15)
HBtdtdit
dtdi 21
*
1 (6.16)
HBtdtdit
dtdi 22
*
2 (6.17)
sc f
Ttt 121 (6.18)
where sf is the switching frequency.
Adding Equation (6.16) and Equation (6.17) and substituting
Equation (6.18), it can be written as
01 *
21 dtdi
fdtdit
dtdit
s
(6.19)
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Subtracting Equation (6.17) from Equation (6.16), it gives
dtditt
dtdit
dtditHB
*
2121 )(4 (6.20)
Substituting Equation (6.15) in Equation (6.20), gives
dtditt
dtdittHB
*
2121 )()(2 (6.21)
Substituting Equation (6.15) in Equation (6.19), simplifying
)/(/*
21 dtdifdtditt
s
(6.22)
Substituting Equation (6.22) in Equation (6.21)
2*
2
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125.0dtdi
LV
VL
LfV
HBf
s
dc
f
fs
dc (6.23)
Equation.6.23 shows the hysteresis bandwidth as a function of
modulation frequency, supply voltage, dc capacitor voltage and slope of the
reference current wave. Hysteresis band can be modulated as a function of Vs
anddt
di* . Hence, these variables are taken as input to the fuzzy controller, and
the hysteresis band width (HB) is the output. In a hysteresis controller the
reference compensation current is compared with the actual current that is
being injected by the compensation circuit. A positive pulse is produced if the
actual current tends to decrease below the lower hysteresis limit, while a
negative pulse is produced if the current exceeds the upper hysteresis limit.
Thus, in a hysteresis current controller the actual compensation current is
forced to stay within a particular hysteresis band.
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Figure. 6.6 Simplified model for fuzzy hysteresis current control
Figure 6.7 Membership functions for the input variables (a) )(tvs , (b)
dtdi* and (c) output variable HB
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Figure. 6.6 shows the block diagram of the adaptive hysteresis band
current control. In order to establish a fuzzy logic controller, input and output
variables must be treated. The presented fuzzy current controller has two
inputs and one output for each of the three converter phases. The supply
voltage wave VS and mains current reference slope are selected as input
variables and HB as output variable. The following step is to determine the set
of linguistic values associated to each variable.Each input variables is
transformed into linguistic size with five fuzzy subsets, PL: positive large,
PM: positive medium, PS: positive small, EZ: zero, NL: negative large, NM:
negative medium, NS: negative small and for the output variables are: PVS:
positive very small, PS: positive small, PM: medium positive, PL: positive
large, PVL: positive very large. Then, Figure 6.7 shows the membership
functions of the input and the output variables. The resulting rules of
inference for Fuzzy hysteresis current control is presented in Table 6.2. In this
approach, the switching frequency is kept constant and the current error is
appreciably reduced ensuring better stability and insensitivity to parameter
variation.
Table 6.2 Rules of inference for Fuzzy hysteresis current control
dis*/dt/ vs(t) NL NM EZ PM PL
NL PS PS PM PS PS
NM NL PM PL PM PS
EZ PVS PM PVL PM PVS
PM NL PM PL PM PS
PL PS PS PM PS PS
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6.8 SERIES ACTIVE FILTER OUTPUT VOLTAGE REGULATION
The main function of a series active power filter is the protection of
sensitive loads from supply voltage perturbations such as sags or voltage
harmonics. The control part of the series active filter must be able to derive
the reference voltage waveform with matching the harmonic content of the
line voltage. A fuzzy hysteresis band control is adopted allowing to operate at
nearly fixed frequency. The adaptive hysteresis band is given by,
2
2
2
49
16 dt
tdVCRtV
VCR
CRfVHB ref
fsfs
sl
dc
fsfs
fsfsc
dc (6.24)
This Equation 6.24 shows that the hysteresis band can vary while
keeping the switching frequency nearly constant. In order to establish a fuzzy
logic controller, the input and the output variables are again treated. The
voltage reference slope and it derivation are selected as input variables and
HB as output variable. PWM technique is used to generate switching patterns
for the VSI. The partition of fuzzy subsets and the shape of membership
function adapt the shape up to appropriate system. The value of input error E
and change in error C 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.
Several composition methods such as Max–Min and Max-Dot have
been proposed in the literature. In this fuzzy hysteresis band control, Min
method is used. The output membership function of each rule is given by the
minimum operator and maximum operator. 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 UPQC, the
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active power, reactive power, terminal voltage of the line and capacitor
voltage are required to be maintained. In order to control these parameters,
they are sensed and compared with the reference values. The set of FC rules
are derived from Equation (6.25).
u= -[ E + (1- )* C ] (6.25)
where is self-adjustable factor which can regulate the whole operation. E is
the error of the system, C is the change in error and u is the control variable.
A large value of error E indicates that given system is not in the balanced
state. If the system is unbalanced, the controller should enlarge its control
variables to balance the system as early as possible. On the other hand, small
value of the error E indicates that the system is near to balanced state. During
the process, it is assumed that neither the UPQC absorbs active power nor it
supplies active power during normal conditions. So the active power flowing
through the UPQC is assumed to be constant. The Fuzzy inference rules are
the same that presented in Table 6.2.
6.9 SIMULATION RESULTS
Computer simulation has become an indispensable part of the
power electronics design process. UPQC is a complex power electronics
device and the analysis of its behaviour, which leads to improved
understanding, would be very difficult without computer simulations. The
overall design process can be shortened through the use of computer
simulations, since it is usually easier to study the influence of a parameter on
the system behaviour in simulation. The results obtained from the simulation
shows better performance of UPQC when fuzzy logic controller used in terms
of harmonic compensation and dc capacitor voltage balancing at load
terminals in switching as well as unbalanced conditions. Under this condition
the dynamic response of fuzzy logic controller proved to be faster.
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This section presents the details of the MATLAB simulation carriedout to demonstrate the effectiveness of the proposed control strategy for theactive filter for harmonic current filtering, reactive power compensation, loadcurrent balancing and neutral current elimination. All the compensators areimplemented using equivalent discrete blocks. To observe the performance ofshunt filter for voltage correction the shunt is switched on first, and then theseries filter is switched on. Source impedance is considered almost negligiblewith R
sand L
svalues being 0.1ohms and 0.1mH respectively. Both series and
shunt inverters are modeled using universal bridges with IGBT/diodes. Thepower circuit is modeled as a three phase four wire system with a non-linearload that is composed of a three phase diode-bridge rectifier with RL load.The system circuit parameters adopted are presented in Table 6.3.
Table 6.3 System parameters of UPQC
System parameters SpecificationsSystem frequency 50 HzDc link capacitance C1=4400 F,C2=4400 FDc-link voltage 600VNon-Linear Load R =20 ohms,
L=15 mH,2.6 KVA
Shunt Inverter Filter L=5.5 mH ,C=12 µF
Series Inverter Filter L=5.5 mH ,C=12 µF
Switching Frequency 9730 HzPWM Control Fuzzy Hysteresis controlCoupling transformer 3.3 KVA
6.9.1 Compensation of Load Voltage
Figure 6.8(a) shows Three-phase load voltages before compensation.The series APF starts compensating for the voltage harmonics immediately byinjecting out of the phase harmonic voltage, making the load voltage distortion
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free. The voltage injected by the series is shown in Figure 6.8 (b). Three-phaseload voltages after compensation is shown in Figure 6.8 (c).
The THD of the distorted three-phase load voltages are 47.5%,44.78% and 43.19% respectively. The THD of load voltages in phase A, Band C has reduced to 4.36%, 4.18% and 3.99% respectively. These results ofsimulations show us that the application of fuzzy logic in the control loopsmakes it possible to fulfil the desired requirements even under the mostunfavourable conditions.
Figure 6.8 (a) Three-phase load voltages before compensation
Figure 6.8 (b) UPQC compensator Voltage
Figure 6.8 (c) Three-phase load voltages after compensation
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6.9.2 Compensation of Voltage Interruption
Figure 6.9 shows the simulation results when the source has avoltage interruption for 0.06 s from 0.06 to 0.12s. Figure 6.9 (a) shows three-phase load voltages with voltage interruption .The load voltage maintains aconstant value by the support of the shunt inverter voltage. During the voltageinterruption, the shunt inverter only provides power to the load. The voltageof DC bus maintains a constant value by the support of FLC during thevoltage interruption. Figure 6.9 (b) shows Three-phase Load voltages aftervoltage interruption compensation. Thus, it shows the stability and thereliability of the proposed system.
Figure 6.9(a) Three-phase Load voltages with voltage interruption
Figure 6.9(b) Three-phase Load voltages after voltage interruption
compensation
6.9.3 Compensation of current harmonics and unbalanced currents
An ideal three-phase sinusoidal supply voltage is applied to the
non-linear load (Thyristor rectifier feeding an RL load) injecting current
harmonics into the system. Figure 6.10 shows the effectiveness of the
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proposed system for compensation of current harmonics and unbalanced
currents. It shows the simulation results when the shunt inverter of UPQC
operates as an active power filter.
Figure 6.10(a) shows Three-phase Load current before
compensation. Load current can be compensated by the shunt-inverter current
to make the source current sinusoidal. There is no drop in the capacitor
voltage when it feeds shunt inverter, because shunt inverter draws only
reactive power to compensate the load current harmonics The performance of
the proposed control algorithm of the active power filter is found to be
excellent and the source current is practically sinusoidal. Three-phase source
current after compensation is shown in Figure 6.10(b).The THD of the
distorted three-phase line currents (Ia, Ib & Ic ) are 39.47%, 34.95% and
36.67% respectively. The THD of current in phase A, B and C has reduced to
4.54%,4.58% and 4.42% respectively.
Figure 6.10 (a) Three-phase Load current before compensation
Figure 6.10 (b) Three-phase source current after compensation
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6.9.4 Compensation of Voltage Sag
Figure 6.11 shows the simulation results when the source has
almost 30% of three-phase voltage sag. Figure 6.11 (a) shows the Three-phase
source voltage with sag. The load voltage maintains a constant value as
expected. During the sag interval, the reverse-flow source power is reduced
and the series inverter covers this reduced amount to maintain the load power
constant. Three-phase Load voltages after voltage sag compensation is shown
in Figure 6.11 (b). Results show that UPQC is maintaining the load voltage
sinusoidal and at desired constant level even during the sag. While series
active filter is providing the required real power to the load, the shunt active
filter is maintaining the DC link voltage at constant level and the source
delivered more current.
Figure 6.11 (a) Three-phase source voltage with sag
Figure 6.11 (b) Three-phase Load voltages after sag compensation
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6.9.5 Comparison of Different Control Strategies
The proposed scheme is a simple scheme for achieving effectivecompensation for current harmonics, reactive power compensation, voltageinterruption, and voltage harmonic mitigation under distorted and unbalancedinput/utility voltages. Comparison of different control strategies is presentedin Table 6.4
Table 6.4 Comparison of THD Level of different control strategies
THD Level of Source current THD Level of Load voltageControlsstrategies Before
compensationAfter
compensationBefore
compensationAfter
compensationPhase A 37.35 5.12 41.51 5.98Phase B 28.12 4.66 35.17 5.17ISCTPhase C 33.74 4.98 40.36 5.86Phase A 36.47 4.23 51.87 5.23Phase B 31.42 4.97 48.25 5.11IRPTPhase C 38.27 4.56 49.56 5.05Phase A 39.47 4.54 47.5 4.36Phase B 34.95 4.58 44.78 4.18FHBCPhase C 36.67 4.42 43.19 3.99
6.10 CONCLUSION
This chapter demonstrates the validation of simpler controlapproach for the unified power quality conditioner based on the fuzzy logic.The UPQC can compensate the reactive power, harmonic current, voltage sagand swell, and voltage imbalance. The current and voltage bands can be easilyimplemented with fuzzy logic to maintain the modulation frequency nearlyconstant for each control. Simulation results confirms the viability of theproposed approach and proves that the UPQC, allows to improve powerquality by maintaining the load voltage at desired level even duringunbalanced, distorted or supply voltage sag conditions. Therefore, theproposed control can easily be adapted to others more severe constraints.