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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.

128

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.

133

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.

134

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

135

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.

136

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)

138

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

241

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.

140

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.

144

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

145

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

146

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

147

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

148

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

149

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.