21

CHAPTER 3

DESIGN OF A PV-UPQC SYSTEM FOR VOLTAGE SAG AND

SWELL COMPENSATION

INTRODUCTION

The recent increase in the use of non-linear loads creates many

power quality problems such as voltage sag, swell and current

disturbances. Voltage sag is one of the major power quality problems,

which may cause equipment tripping, malfunction or shut down of

domestic and industrial equipments. These effects could be expensive for

both customers and utilities as discussed by Eldany et al (2001). Hirofumi

et al (1998) has proposed a UPQC system to mitigate the sag. Wong et al

(2004) uses a parallel active filter to compensate the imbalance, reactive

power, neutral current and harmonics of the source.

Photovoltaic energy has great potential to provide power supply

with minimum impact on the environment, since it is clean and pollution

free as discussed by Kuo Liang et al (2001). When the power supply crisis

started in the recent past, PV generation became a very popular alternative

for fossil fuel electrical power generation. A large number of solar cells

connected in series and parallel constitute solar arrays. One way of using

photovoltaic energy is, in a distributed energy system, as a peaking power

source.

There are many control strategies reported in the literature to

determine the reference values of the voltage and the current of UPQC. The

Fuzzy Logic Control (FLC) for the control of UPQC method has been

developed by Singh et al (1998). Bulent Irmak et al (2009) presented

Power Quality Distributed-Voltage (PQD-V) using ANN controller. The

drawback of the existing research work is that they address only current

22

and voltage harmonics problems. But, there is very meagre discussion of

other prominent power quality problems like deep sag, swell and

interruption.

This chapter presents the design and development of the proposed

PV-UPQC system. By using instantaneous p-q control theory techniques

long with PI and hysteresis band controller, the mitigation of voltage sag

and swell under different balance and unbalanced load conditions are

simulated. The use of a PV array for maintaining constant DC link voltage

is another distinguishing feature of the PV-UPQC system. With these

functions, the PV-UPQC is suitable for connecting at the PCC.

3.1 PV-UPQC POWER CIRCUIT AND ITS FUNCTIONS

The PV-UPQC power circuit structure is shown in figure 3.1. It

consists of major components such as shunt and series inverters, DC link

capacitor with PV array, the shunt and series inverter controllers, shunt and

series filters and series and shunt coupling transformers.

The basic function of the system is the compensation of voltage and

current disturbances. The function of the series inverter is to compensate

the voltage, when there are supply side disturbances, such as voltage sag,

swell and unbalance. The series part of the PV-UPQC consists of a series

inverter connected on the DC side to the energy storage capacitor with PV

array. On the AC side, it is connected in series with the feeder through the

series Low Pass Filter (LPF) and coupling transformers. The series LPF

prevents the switching frequency harmonics produced by the series VSI

entering the system. The coupling transformers connected in series provide

voltage matching and isolation between the network and the series inverter.

The series inverter injects compensation voltages in series with the

supply voltages, such that the load voltages are balanced and undistorted

and their magnitudes are maintained at the desired level.

23

Figure 3.1 Proposed power circuit diagram of PV-UPQC system

24

Based on the measured supply and load voltages, the control scheme

of the controller generates appropriate switching signals for the series

inverter switches. The series inverter is controlled in voltage-control mode,

using the PWM switching technique. In order to produce the injected

voltage of desired magnitude, phase shift and frequency, the desired signal

is compared with a triangular signal of higher frequency and appropriate

switching signals are generated. The DC link capacitor is alternately

connected to the inverter outputs with positive and negative polarity. The

switching harmonics present in the output voltages of the series inverter are

filtered out by the series low pass filter. The amplitude, phase shift,

frequency and harmonic contents of injected voltages are controllable.

The function of shunt inverter is to compensate load current

harmonics, reactive power and regulate the DC-link voltage between both

inverters. The shunt part of the PV-UPQC consists of a shunt inverter

connected to the common DC storage capacitor with PV array on the DC

side. On the AC side, it is connected in parallel with the load through the

shunt interface inductor and shunt coupling transformer. The shunt

interface inductors along with the shunt filter capacitor are used to filter out

the switching frequency harmonics produced by the shunt inverter. The

shunt coupling transformer is used for matching the shunt inverter voltage

and network voltage.

In order to achieve compensation objective, the shunt inverter filter

injects currents at the PCC so that the reactive and harmonic components

of the load currents are cancelled and the load current unbalance is

eliminated. The injection current is provided by the DC link capacitor.

Based on the measured current and voltage, the control scheme generates

the appropriate switching signals for the shunt inverter switches.

25

The particular currents and voltages to be measured depend on the

applied control strategy and the shunt inverter is controlled in current

control mode. The appropriate inverter switches are turned on and off at

certain time instances so that the currents injected by the shunt filter tracks

some reference currents within a fixed hysteresis band according to the

compensation objectives. This band refers to the constant bandwidth of the

upper and lower hysteresis current.

The inverter switches alternately connect the DC link capacitor to

the system, either in the positive or negative sequence. When the DC

capacitor voltage is connected in the positive sequence, it is added to the

supply voltage and the inverter current increases. When DC capacitor is

connected in the negative sequence, it is observed that the voltage is

opposite to the supply voltage which reduces the inverter current.

Alternately increasing and decreasing the current within the hysteresis band

results in generating the reference current. The DC side capacitor serves

two main purposes. Primarily, it maintains the DC voltage with a small

ripple in the steady state. Secondly, it serves as an energy storage element

to supply the difference between the active load and source power during

the transient period. The average voltage across the DC link capacitor is

maintained constant so that the shunt inverter filter can draw a leading

current. This voltage has to be higher than the peak of the supply voltage.

The proposed system can switch over to islanding mode operation during

interruption and supply the power to the load.

3.2 DESIGN OF PV-UPQC POWER CIRCUIT

The design of PV-UPQC power circuit includes the following three

main parameters:

Shunt interface inductors

DC link reference voltage

26

DC link capacitor

The design of the shunt interface inductor and the DC link reference

voltage are based on the following criteria. The first criterion is the limiting

of the high frequency components of the injected currents. The second

criterion is that the instantaneous rate of change of current generated by the

shunt filter should be greater than the rate of change of current of the

harmonic component of the load. This ensures the proper harmonic

cancellation. On the other hand, a higher value inductance is preferable for

a better harmonic cancellation and reactive power compensation. However,

very high value will result in slow dynamic response of the shunt

compensator. In this situation, an effective solution has to be explored.

A higher DC link reference voltage results in a higher rate of change

of the shunt compensator current, better dynamic response and reactive

power compensation performance. However, it increases the stress in the

inverter switching devices. Again, an efficient solution has to be adopted.

The DC link capacitor size is selected to restrict the DC voltage ripple

within reasonable limits. The DC voltage ripple is determined by both the

reactive power to be compensated and the active power supplied by DC

capacitor during the interruption.

3.2.1 Design of PV-UPQC Volt Ampere rating of Shunt and Series

Inverters

Volt Ampere (VA) rating of series and shunt inverters of PV-UPQC

determines the size of the PV-UPQC. The power loss is related to the VA

loading of the PV-UPQC system. Figure 3.1(a) shows the loading

calculation of shunt and series inverters of PV-UPQC with the presence of

PV at its DC link. The linear load is deigned based on fundamental

27

frequency.

Figure 3.1(a) VA rating phasor diagram of PV-UPQC

where,

Vs1 and Is1 = supply voltage and supply current

Vs2 and Is2= supply voltage and supply current during interruption

VL1and IL1 = load voltage and load current

VL2 and IL2 = load voltage and load current during interruption

IC1and IC2 = compensating current of reactive components of inverters

Io - output current, ZSHI –shunt inverter impedance

The load voltage is to be kept constant at Vo Per unit (pu), irrespective of

the supply voltage variation:

V =V =V =V =V pu (3.1)

The load current is assumed to be constant at the rated value:

I =I =I =I pu (3. 2)

Assuming that the PV-UPQC system to be lossless, the active power

demand in the load remains constant and is drawn from the source:

From phasor diagram 3.1(a)

V I =V I cos (3. 3)

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where

V I = Input power of inverter (Dc link power)

V I cos = output power of inverter (AC)

In case of an interruption:

V = (1-x)Vs1

= V (1-x) pu (3.4)

where, ‘x’ is per-unit sag.

To maintain constant active power under the voltage sag condition

I =( )

= pu (3.5)

Therefore series inverter VA (Sseinv) rating equals to

x-1)cos(xIV

IVS oos2injseinv pu (3.6)

Injected voltage through shunt inverter is V = V V

= V V (1 x) (3.7)

Injected current through shunt inverter is

I = I + I 2I I cos (3.8)

The injected current in terms of I is given by

I = I + ( ) 2I( )

= I +( )

2I( )

= I 1 +( )

2( )

29

= I ( ) ( )( )

= I ( ) { ( )}( )

(3.9)

The load voltage VL2 is given by V = V + V

= V (1-x) + V V (1 x)

= V (1-x) + V (V V x)

= V (1-x) + V (V 2V x + V x )

= V (1-x) + V 1 (1 x)

= V [(1-x) + 1 (1 x) ] (3.10)

Shunt inverter VA (S ) rating equals to

S = V I + PLOSS

= V I + I

Subsisting the values of VL2 and IC2 above equation and simplify

S = V [(1-x)+ 1 (1 x) ]× I0[ 1 x 2+cos2 1 2 1 x1 ]

+ I

=( )

{[(1-x)+ 1 – (1 x) ]

× [ (1 x) + cos {1 2(1 x)}] } +(I × )

Simplify the above equation

= ( )

{ 1 ×[(1-x)+ cos {1 2(1 x)]}+ (I × )

30

S =( )

(1 x) + cos {1 2(1 x)}

+ ZXx)-(1

)x1(21(cosx)1(2I SHI2

222o pu (3.11)

The proposed system highlights the efficient and imperative design

of power factor and VA loading of inverter. The VA loading of inverters is

calculated for the occurrence of supply voltage sag from 10 to 90% and

power factor variations from 0.6 lagging to unity. The range of supply

voltage sag has been chosen so that most practical cases can be evaluated

in this range.

3.3 CONTROL STRATEGY OF THE PV-UPQC SYSTEM

There are many control strategies reported in the literature to

determine the reference values of the voltage and the current of UPQC. The

concept of instantaneous active power (p) and reactive power (q) and its

application in shunt filter reference current generation was introduced by

Akagi et al (1998).The p-q theory introduced by Tan Zhili et al (2006), has

been modified into a single-phase p-q theory by Khadkikar et al (2009).

The synchronous reference frame theory has been discussed by

Guorong et al (2007), symmetrical component transformation has been

presented by Ghosh et al (2004) and Unit Vector Template (UVT)

technique by Singh et al (2010). The Fuzzy Logic Control (FLC) for the

control of UPQC method has been developed by Singh et al (1998). Based

on the above discussion, p-q theory with hysteresis current control mode is

suitable for parallel mode operation of PV-UPQC system and p-q theory

with PWM voltage control mode is suitable for interruption mode

operation. The hysteresis control method is simple to implement and it has

enhanced system stability, increased reliability and mitigates power quality

problems.

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The proposed PV-UPQC consists of two main controllers as

follows:

sag and swell compensation using series inverter controller

Shunt inverter controller

Controller strategies are designed based on two operation modes,

interconnected (parallel mode) and islanding modes. In the interconnected

mode, the source and the PV array jointly supply power to the load. While

in the islanding mode, the PV arrays alone supply power to the load.

3.4 SAG AND SWELL COMPENSATION USING SERIES

INVERTER CONTROLLER

The series inverter control component of the controller injects the

appropriate voltage to the load during voltage sag and swell, such that the

load voltage becomes balanced, distortion free and have the desired

magnitude. Theoretically, the injected voltage can be of any arbitrary

magnitude and phase. However, the power flow and device rating are

important issues that have to be considered when determining the

magnitude and the phase of the injected voltage.

The PV-UPQC system ensures that the phase of the injected voltage

is maintained 90° in advance with respect to the supply current, so that the

series compensator consumes no active power in steady state. In the second

case, the PV-UPQC injected voltage is in phase with both the supply

voltage and current, so that the series compensator consumes only the

active power, which is delivered by the shunt compensator through the DC

link.

In the case of quadrature voltage injection, the series compensator

requires additional power capacity, since the shunt compensator Volt

Ampere (VA) rating is reduced as the active power consumption of the

32

series compensator is minimized. It compensates part of the reactive power

demand by the load. The series compensator does not compensate for any

part of the reactive power demand of the load and it has to be entirely

compensated by the shunt compensator. The shunt compensator must

provide the active power injected by the series compensator.

It can be concluded that PV-UPQC system is the optimum solution

for the active power supply, phase angle matching and supply of reactive

power. An approximate sub-optimal control strategy for the PV-UPQC

system to minimize the losses in operation has been proposed. This

approach of generating the reference voltage is based on the analysis of the

fundamental frequency. The load voltage (VL) is equal to the sum of the

source voltages ( SV ) and the voltage injected by the series filter (Vf).

jbaVVV fSL (3.12)

where,

a + j b - Vector of unity magnitude of the supply current

To compensate for supply voltage sag, swell and distortion, some

additional component has to be added. The reference voltage (Vref) value is

calculated using reference filter voltage (Vf*), which is obtained by

subtracting the positive sequence fundamental component from the

disturbed source voltage.

The function of the series inverter is to compensate the voltage

disturbances like sag and swell on the source side, which are due to the

fault in the distribution line. The series inverter control calculates the

reference voltage to be injected by the series inverter. This compares the

positive-sequence component with the disturbed source voltage. The

reference voltage for PWM switching of the series inverter is obtained

from the proportional control and the feed forward control. The control

equation needed to calculate the reference voltage is given in (3.13). Figure

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3.2 shows the configuration of the series inverter control, which is based on

this equation.

Figure 3.2 Block diagram of the series inverter control

Vref = [(Vs* -Vs) –Vf] x k + Vf* (3.13)

where

Vref - Reference voltage, Vs - Source voltage, Vf - Injected voltage

Vf* - Reference injected voltage Vs* - Positive-sequence voltage

In this case, when the PV- UPQC control strategy is applied, the

injected voltage is in phase with the supply voltage. Hence, the load

voltage is in phase with the supply voltage and there is no need for

calculating the angle of the reference load voltage. Thus, the reference load

voltage is determined by multiplying the reference magnitude (which is

constant) with the sinusoidal template phase-locked to the supply voltage.

Then, the reference series filter voltage is obtained using the expression

(3.13). Using the optimum of the different techniques for calculating the

34

reference voltage of the series compensator, the voltage rating of the series

compensator is considerably reduced.

3.5 SHUNT INVERTER CONTROLLER

The effectiveness of the shunt inverter basically depends on the design

characteristics of the current controller. The method implemented to

generate the reference current and reference voltages is the use of

instantaneous p-q control theory techniques. The control scheme of a shunt

inverter calculate the current reference waveform for each phase of the

inverter, maintain the DC link voltage constant and generate the inverter

gating signals. The current reference circuit generates the reference

currents required to compensate the load current harmonics and reactive

power.

The p-q control theory, firstly, transformation of the voltage and

currents from the abc to 0 coordinates. The Clarke transformation and its

inverse of three-phase generic voltages are given by

vv

0v=

2/32/12/1

2/302/112/12/1

32

cvbvav

(3.14)

cvbvav

=2/3

2/30

2/12/12/12/1

12/1

32

vv

0v

(3.15)

Similarly, three-phase generic instantaneous line currents, ia, ib and ic can

be transformed to the 0 axes.

One advantage of applying the 0 transformation is to separate

zero-sequence components from the abc-phase components. The and -

35

axes do not contribute to zero sequence components. No zero-sequence

current exists in a three-phase three-wire system, so that zero sequence

current (i0) can be eliminated from the above equations resulting in

simplification. If the three-phase voltages are balanced in a three-phase

three-wire system, no zero-sequence voltage is present, so that zero

sequence voltage (vo) can be eliminated. However, when zero-sequence

voltage and current components are present, the complete transformation

has to be considered.

The p-q theory defines three instantaneous powers in three phase

systems, with or without neutral conductor

qpp0

=v

v0

v0v000V

iii0

(3.16)

where,

po - instantaneous zero-sequence power,

p - instantaneous real power,

q - instantaneous imaginary power

There are no zero-sequence current components in three phase three-

wire systems, that is, io= 0. In this case, only the instantaneous powers

defined on the -axes exist, because the product voio is zero. Hence, in

three-phase three wire systems, the instantaneous real power p represents

the total energy flow per unit time, in terms of components. The

instantaneous active and reactive powers for a three-phase three-wire

system are defined as:

36

ii

vv-

vv

qp

(3.17)

where,

p –Active power, q -Reactive power

v and v -Transformation of voltages

The three-phase voltages are transformed from a-b-c to frame

and vice versa using the following transformation relations:

cvbvav

2/32/302/12/11

32

vv

(3.18)

cvbvav

=2/32/1

2/32/101

32

vv

(3.19)

The same transformation matrices are used for the transformation of

currents, from equation (3.19), the current i and i are expressed as:

ii

=2v2v

1vv-

vv

qp

(3.20)

where, i and i -transformation of currents

both active power( p) and reactive power (q) defined above are composed

of two components.

qq~qpp~p

(3.21)

37

where, and represent the oscillating and average parts active

power and represent the oscillating and average parts reactive power

The p-q theory has a prominent merit of allowing complete analysis

and real time calculation of various powers and respective currents

involved in a three-phase circuit. Further, knowing the values of

undesirable currents in a circuit in real time allows us to eliminate them.

For instance, if the oscillating powers were undesirable, by compensating

the currents ia and ia of the load and their corresponding currents in b

and c phases the compensated current drawn from the network would

become sinusoidal. It can be easily shown that, ia- (ia +ia ) produces a

purely sinusoidal waveform. This is one of the basic ideas of shunt inverter

filtering.

The shunt inverter of PV-UPQC can be controlled in two ways:

The shunt inverter reference current is tracked. The shunt inverter

current is used as feedback control variable and compared with the

load current and the shunt compensator reference current is

calculated from it. The reference current is determined by

calculating the active fundamental component of the load current

and subtracting it from the load current. This control technique

involves both the shunt inverter and load current measurements.

The shunt inverter can also be controlled by tracking the supply

current, when it is used as the feedback variable. In this case, the

shunt active filter ensures that the supply reference current is

tracked. Thus, the supply reference current is calculated rather than

the current injected by the shunt active filter. The supply current is

often required to be sinusoidal and in phase with the supply voltage.

Since the waveform and phase of the supply current is only known,

38

its amplitude needs to be determined. The hysteresis current control

technique involves only the supply current measurement. Therefore,

it has been used in the PV-UPQC simulation model.

Figure 3.3 shows the configuration of shunt inverter control, which

includes the current control for harmonic compensation and the output

voltage control in source voltage interruption mode. In normal operation,

the shunt control calculates the reference value of the compensating current

from the harmonic current and the reactive power, considering the power

loss (Ploss) due to the system and inverter operation. This loss should be

compensated to maintain the DC link voltage during operation of the series

inverter. The reference value of the compensating current is derived using

equation (3.22).

ii

*

*

=2v2v

1vv

v-v

qpp~ loos (3.22)

where,

V’ ,V’ - Transformed reference voltages

i* , i* - compensating currents

Here, the reference voltage is determined using (3.23) and (3.24).

The reference voltage (V*1) is expressed by the sum of the source voltage

(V s) and the filter current difference IPF calculated by the PI controller.

PFPFPF I*II (3.23)

pk*1V dtIIkI PFPF (3.24)

If the shunt inverter is assumed to generate the reference voltage for

each period of power frequency, the transfer function of the filter current

can be derived as in (3.25).

39

)/L(ks}L/R)k{(s)/L(ks)L/k(

II

PFIPFP2

PFIPFP*PF

PF (3.25)

where, IPF, I*PF - filter currents

kp, kI- proportional constant

The voltage reference determined by using positive-sequence

detector extracts the positive-sequence component from the disturbed

three-phase source voltage in figure3.4. This detector derives the

transformation of reference voltages V' , and V'S , based on the - -0

transformation. The measured source voltage passes through the Phase-

Locked Loop (PLL) and the sine wave generator to calculate the

fundamental component of the transformation currents i' = sin( 1t) and i'

= cos ( 1t).

The calculated active power ps' and reactive power qs' includes the

positive-sequence fundamental component of the source voltage VS. So,

the instantaneous value of the positive-sequence component is calculated as

given in equation (3.26).

'sq'sp

'i-'i

'i'i

'2i'2i

1'V

'V(3.26)

The two functions of the shunt inverter are to compensate the current

harmonics and to supply the active power to the load during voltage

interruption.

When the voltage interruption occurs, the operation mode is

changed from normal compensation mode to interruption mode. The PV

array provides the active power to maintain the load voltage constant. The

shunt inverter starts to perform the voltage and current control using the PI

controller.

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Figure 3.3 Block diagram of shunt inverter control

Figure 3.4 Block diagram of positive sequence voltage detector

3.6 AVERAGE DC LINK VOLTAGE REGULATION

The regulation of DC link voltage (Vdc) is one of the tasks in PV-

UPQC, since the injecting voltage (Vinj) depends on the regulated voltage

of DC link capacitor. The proposed PI controller regulates the DC link

voltage and reduces the harmonics in the inverter. The PI approach is used

to supply reference current determination and it is based on the fact that the

magnitude of the supply current depends on power balance between the

41

supply and the load. The DC link capacitor serves as energy storage

element. If the shunt active filter losses are neglected, in steady-state, the

power supplied by the system has to be equal to the real power demand of

the load and no real power flows into the DC link capacitor. The average

DC capacitor voltage is thus maintained at reference voltage level. If the

power unbalance caused by a load change occurs, the DC link capacitor

must supply the power difference between the supply and load that will

result in reducing the DC capacitor voltage.

To restore the average DC capacitor voltage to the reference level,

some active power has to be supplied to the DC capacitor. For this propose,

the supply current has to be increased. When the average DC capacitor

voltage increases, the magnitude of the supply current has to be decreased.

The amplitude of the supply current is automatically controlled by

controlling the average voltage across the DC link capacitor. The DC

voltage regulation is achieved by using a PI controller. The capacitor

voltage is compared with some reference value and a PI controller

processes the voltage error. The output of the PI controller is the magnitude

of the reference supply current and it is constant in steady-state. To get the

source reference current, a sinusoidal template that is in phase with the

supply voltage is multiplied by this magnitude. Applying this concept, the

numbers of current sensors are reduced resulting in the simplification of

the control circuit. Therefore, this control technique has been chosen to be

used in the PV-UPQC simulation model.

3.6.1 DC Link Voltage Regulation Using Fuzzy Logic Control

The voltage regulation in the UPQC DC link using fuzzy logic

control has been presented by Singh et al (1998). The structure of a

complete fuzzy control system is composed from the blocks, namely,

fuzzification, knowledge base, inference engine and defuzzification. The

fuzzification module converts the crisp values of the control inputs into

42

fuzzy values. Inputs to the fuzzy controller are categorized as various

linguistic variables with their corresponding membership values such as

low, medium and high, where each is defined by a gradually varying

membership function. In fuzzy set terminology, all the possible values that

a variable can assume are named universe of discourse and the fuzzy sets

cover whole universe of discourse. The shape of fuzzy sets can be

triangular and trapezoidal. A fuzzy controller converts a linguistic control

strategy into an automatic control strategy and fuzzy rules are constructed

by knowledge database. Initially, measured DC link voltage Vdc and the

input reference voltage Vdc-ref are the input variables of the fuzzy logic

controller. Then, the output variable of the fuzzy logic controller is

presented by the control current.

The control scheme consists of three phase sine wave generator for

reference current generation and generation of switching signals. The peak

value of reference currents are estimated by regulating the DC link voltage.

The actual capacitor voltage is compared with a set reference value. The

error signal is then processed through a fuzzy controller, which contributes

to zero steady error in tracking the reference current signal.

3.7 VOLTAGE CONTROL IN DC LINK CAPACITOR

In voltage source inverter, the DC voltage has to be maintained at a

certain level to ensure the DC to AC power transfer. Because of the

switching and other power losses inside PV-UPQC system, the voltage

level of the DC capacitor will be reduced if it is system is interrupted.

Thus, the DC link voltage control unit is intended to keep the average DC

bus voltage constant and equal to a given reference value. The DC link

voltage control is achieved by adjusting the real power supply by the PV

array. This real power is adjusted by changing the amplitude of the

fundamental component of the reference current. The PV array source

43

provides some active current to recharge the DC capacitor. Thus, in

addition to supplying the reference current, the shunt active filter has to

supply some amount of active current as compensating current. This active

compensating current flowing through the shunt active filter regulates the

DC capacitor voltage.

3.7.1 Voltage Control in DC Link Capacitor using ANN Controller

The DC link capacitor voltage control by using ANN controller in

Power Quality Distributed-Voltage (PQD-V) is discussed by Bulent Irmak

et al (2009). The detection of the disturbance signal with the reference

signal of the controller is the prime requirements for the desired

compensation in case of PQD-V. The ANN is made up of interconnecting

artificial neurons. It is essentially a cluster of suitably interconnected

nonlinear elements of very simple form that possess the ability to learn and

adapt. It resembles the human brain in two aspects:

Knowledge is acquired by the network through the learning process

Interneuron connection strengths are used to store the knowledge.

These networks are characterized by their topology, the way in which they

communicate with their environment, the manner in which they are trained

and their ability to process information. ANN is being used to solve AI

problems without necessarily creating a model of a real dynamic system.

For improving the performance of a PQD-V, a multilayer feed forward-

type ANN-based controller is designed. This network is designed with

three layers, the input layer with 2, the hidden layer with 21 and the output

layer with 1 neuron, respectively. The large data of the DC-link current

and intervals from the conventional method are collected. These data are

used for training the NN. The activation functions chosen are tan sigmoid

for input and hidden layers and pure linear in the output layer,

respectively. This multilayer feed forward-type NN works as a

44

compensation signal generator. The ANN is shown in figure 3.5, the

training algorithm used is Levenberg–Marquardt back propagation

(LMBP).

Figure 3.5 Exploded diagram of the artificial neural network.

In order to sustain the constant frequency in the utility, Singh et al

(1998) have utilized the Fuzzy Logic Controller based constant frequency

UPFC. A Constant Frequency (CF) UPQC is composed of a UPQC and a

matrix converter based frequency changer. UPQC is a combination of

series and shunt active filters. The series and shunt active filters have been

employed to compensate the voltage, current imbalance and harmonics.

The Frequency Converter (matrix converter) has been used to control the

supply frequency when it exceeds the power quality limit.

To overcome above limitation, a PI controller is used for

determining the magnitude of this compensating current from the error

between the average voltage across the DC capacitor and the reference

voltage. The PI controller has a simple structure and fast response. A

simple linear control technique is applied to calculate the DC capacitor

average voltage error by using proportional gain control and the

proportional coefficient. Here, the expression used for calculation of the

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proportional coefficient is obtained through integration of a first-order

differential equation. However, the formula derivation for the proportional

coefficient is not that simple for a three-phase PV-UPQC, a residual

steady-state error occurs with a proportional controller only.

3.8 DEVELOPED PV-UPQC SYSTEM SIMULATION MODELS

This section discusses the proposed system model, which includes

the following:

• Series inverter and shunt inverter models

• Series inverter and shunt inverter control system models

• PV array with control model

Figure 3.6 shows the overall PV-UPQC models implemented using

Matlab /Simulink software. They consist of a series inverter and a shunt

inverter models and its control models and the PV array with control

model. The system is connected in distribution system.

In PV-UPQC model, three phase inverters are connected to the

common DC link capacitors with photovoltaic array. This voltage source is

an external source supplying DC voltage to the inverter for AC voltage

generation. The PV-UPQC is installed in the distribution system, as shown

in figure 3.6. The design considered for the development of PV-UPQC

system Simulink models include circuit topology, conversion efficiency,

maximum load power and power quality.

46

47

3.9 SIMULATION PARAMETERS

The following circuit parameters used in the simulation is given in

Table 3.1

Table 3.1 PV-UPQC simulation parameters

Source Voltage 230Vrms(325Vpeak) 50Hz

Impedance R=0.04 , L=0.4mH

DC-Link Capacitor C1=2000µf

Reference Voltage 400V

Shunt Inverter Filter L, C 1.245 mH,20µF

Switching Frequency. 10kHz

Series Inverter Switching Frequency. 10kHz

Filter L, C 1.245 mH, 140µF

PV Array Power 10kW

3.10 RESULTS AND DISCUSSION

The performance of PV-UPQC is evaluated in terms of load

balancing, unbalancing and mitigation of voltage sag and swell under

different load conditions. The simulation results of the Simulink model

(from figure 3.6) are represented in figures 3.7 to 3.11, which indicate the

performance of the proposed system for seven different cases.

In case 1, the system is in normal operation, three-phase voltages

and currents are sinusoidal and balanced. In case 2, three-phase balanced

voltage sag results in 30% decrease from the nominal value.

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While in case 3, single-phase sag results in 30% decrease from

nominal value. Sag event occurs in phase A. As the other phases are not

affected, both phases B and C keep their nominal value.

In case 4, the three-phase unbalanced sag in phase A is 30% and it is

20% in phase C resulting in the decrease from their nominal values. As the

phase B is left in normal operation, nominal value of the phase B remains

unchanged. In case 5, three-phase balanced voltage swell results in 15%

increase from nominal value. In case 6, single-phase swell occurs resulting

in 15 % increase from nominal value. While swell event occurs in phase A,

other phases are not affected and both phase B and C keep their nominal

value. In case 7, with the unbalanced swell mode, phase A voltage

increases by 15% and phase C voltage increases by 10% from their

nominal values. As the phase B is left in normal operation, nominal value

of the phase B remains change.

Case 1: Normal Operation Balanced Supply with Linear Loads

In case 1, normal operation mode is simulated. Under normal

conditions, three-phase voltages are sinusoidal under balanced condition

and the simulation result is illustrated. The peak load voltage measured is

325V and it is depicted in figure 3.7(a) and the load current is shown in

figure 3.7(b).

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Figure 3.7 (a) Load voltage during normal operation

Figure 3.7 (b) Load current during normal operation

Case 2: Balanced Voltage Sag

In case 2, the simulation is conducted considering balanced voltage

sag with and without PV-UPQC. The occurrence of a three-phase fault

results in 30% of sag and the voltage decreases from its nominal value

between the period 0.1s and 0.2s in all phases. The simulation is illustrated

through figures 3.8(a) to 3.8 (d). The output load voltage is kept constant

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(325V) by using PV-UPQC. Figure 3.8 (e) load voltage sag compensation

with PQD-V using ANN controller.

Figure 3.8 (a) Source voltage with sag

Figure 3.8 (b) Load voltage sag compensation with PV-UPQC using PI

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Figure 3.8 (c) source current with sag

Figure 3.8 (d) Load current with PV-UPQC

Figure 3.8 (e) Load voltage sag compensation with PQD-V using ANN

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Figure 3.8 (f) Load voltage sag compensation with UPQC using FLC

Case 3: Single Phase Voltage Sag

In case 3, the single-phase voltage sag is evaluated with and without

PV-UPQC. The occurrence of a single-phase fault results in 30% of sag

and the voltage decreases from its nominal value. Sag event occurs only in

phase A. Consequently, as other phases are not affected, both phase B and

C keep their nominal value, as show in figures 3.9 (a) to 3.9 (d). The output

load voltage is kept constant (325V) with PV-UPQC.

Figure 3.9 (a) Single-phase source voltage with sag

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Figure 3.9 (b) load voltage with PV-UPQC

Figure 3.9 (c) Single-phase source current with sag

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Figure 3.9 (d) Load current with PV-UPQC

Case 4: Unbalanced Voltage Sag

In case 4, the simulation is conducted considering three phases

unbalanced sag with and without PV-UPQC. In this case, the three phase

unbalanced sag in phase A is 30% and it is 20% in phase C resulting in the

decrease from their nominal values between the period 0.1s and 0.2s as

shown in figures 3.10 (a) to 3.10 (d).

Figure 3.10 (a) unbalanced sag source voltage

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Figure 3.10 (b) Load voltages with PV-UPQC

Figure 3.10 (c) Source current with unbalanced Sag

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Figure 3.10 (d) Load current with PV-UPQC

Case 5: Balanced Voltage Swells

In case 5, the simulation is conducted considering balanced voltage

swell with and without PV-UPQC. The occurrence of a three-phase fault

results in 15% of swell and the voltage increases from its nominal value

between the period 0.1s and 0.2s in all the phases. The simulation is

illustrated through figures 3.11 (a) to 3.11 (d). The output load voltage is

kept constant.

Figure 3.11 (a) Source voltage with swell

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Figure 3.11 (b) Load voltages with PV-UPQC

Figure 3.11 (c) Source current with swell

Figure 3.11 (d) Load current with PV-UPQC

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Case 6: Single Phase Voltage Swells

In case 6, the single-phase voltage swell is evaluated with and

without PV-UPQC. The occurrence of a single-phase fault results in 15%

of swell and the voltage increases from its nominal value. Swell event

occurs only in phase A. Consequently, as other phases are not affected,

both phase B and C keep their nominal value, as show in the figures

3.12(a) to 3.12(d). The output load voltage is kept constant.

Figure 3.12 (a) Single-phase source voltages with swell

Figure 3.12 (b) Load voltages with PV-UPQC

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Figure 3.12 (c) Single-phase source current with swell

Figure 3.12 (d) Load current with PV-UPQC

Case 7: Unbalanced voltages swell

In case 7, the simulation is conducted considering three phases

unbalanced swell with and without PV-UPQC. In this case, the three phase

unbalanced swell in phase A is 15% and it is 10% in phase C resulting in

the decrease from their nominal values between the period 0.1s and 0.2s as

shown in figures 3.13(a) to 3.13 (d). The output load voltage is kept

constant.

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Figure 3.13 (a) unbalanced source voltage with swell

Figure 3.13 (b) Load voltages with PV-UPQC

Figure 3.13 (c) Unbalanced source current with swell

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Figure 3.13 (d) Load current with PV-UPQC

Table 3.2 Comparison of percentage load voltage compensation for

Power Quality Distributed-Voltage (PQD-V) by Bulent Irmak et al

(2009) and proposed PV-UPQC.

Case

NoEvents

Percentage of load

voltage

compensation for

PQD-V with ANN

Percentage of load

voltage

compensation for

PV-UPQC with PI

1 30% sag 3 phase

with balanced

96.5 100

2 30% sag 3 phase

with unbalanced

96.5 100

3 15% swell 3 phase

with balanced

96.5 100

4 15% swell 3 phase

with un balanced

96.5 100

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For comparative analysis, 30% sag and 15% swell are considered

for both PQD-V with ANN system and PV-UPQC system. As shown in

table 3.2, the maximum percentage load voltage compensation achieved by

the PV-UPQC system is 3.5% higher than that of the PQD-V with ANN

system. It has been shown that the proposed system has better performance

in comparison with the PQD-V. The comparisons are also shown in figures

3.14 and 3.15.

Figure 3.14 Percentage load voltage compensation of PQD-V and PV-

UPQC with sag

Figure 3.15 Percentage of load voltage compensation of PQD-V and

PV-UPQC with swell

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Table 3.3 Comparison of source voltages and load voltages with and

without PV-UPQC

CaseNo

Events

With out PV-UPQC

source voltages [V rms](volts)

With PV-UPQC Load voltages [V rms](volts)

Phase A Phase B Phase C PhA

PhB

PhC

PhA

PhB

PhC

1 Normal Normal Normal 230 230 230 230 230 230 2 30% sag 30%sag 30% sag 161 161 161 230 230 230 3 30% sag Normal Normal 161 230 230 230 230 230 4 30% sag Normal 20% sag 161 230 181 230 230 230 5 15%swell 15%swell 15%swell 266 266 266 230 230 230 6 15%swell Normal Normal 266 230 230 230 230 230 7 15%swell Normal 10%swell 266 230 264 230 230 230

Table 3.3 shows voltage sag and swells with balanced and

unbalanced supply with and without PV-UPQC. The system restores the

load voltage to normal values. A comparison of the evaluation conducted

considering voltage sag and swells is shown with and without PV-UPQC.

In addition, figures 3.16 and 3.17 highlight the accomplished results of

enhancement of power quality by the proposed system.

Figure 3.16 Load voltages with and without PV-UPQC with sag

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Figure 3.17 Load voltages with and without PV-UPQC with swell

On the basis of the research work presented in this chapter, a paper

entitled ‘Improved Power Quality Using Photovoltaic Unified static

Compensation Techniques’ has been published in the Asian Power

Electronics Journal. Vol.4 No.2, pp. 59-63. Aug 2010.

On the basis of the research work presented in this chapter, a paper

entitled “Implementation of UPQC for Voltage Sag Mitigation”, -

International Journal of computer communication and information

system, Vol.2, No.1, pp.180-184, Dec 2010.

3.11 CONCLUSION

In this chapter, the power circuit structure, design considerations

and the controller design is discussed. It can be observed that the average

DC voltage regulation method turned out to be the most feasible control

solution that accomplishes two tasks simultaneously. It controls the voltage

across the dc link capacitor and also determines the amplitude of the supply

current. Among the all control techniques, the hysteresis current control

technique is the most preferable for the shunt inverter controller. The

hysteresis control method leads to enhanced system stability, compared to

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ANN and FLC controllers The PV-UPQC design helps in the development

of the simulation model and controllers. With this controller, the DC link

voltage will be maintained stiff by controlling PV array DC-DC converter.

The PV-UPQC system has been simulated in different sag and

swells conditions. The results obtained show that the proposed system has

the ability to compensate voltage sag and voltage swell. The effectiveness

of the simulation results has been verified under balanced and unbalanced

load conditions.

By comparing PQD-V system with respect to IEEE and IEC

standards the proposed PV-UPQC system gives much improved

performance. The simulation results show that the proposed system gives

more efficiency and better dynamic response. The main advantage of the

proposed system is that the control strategy enables it to produce balanced

voltage waveforms.