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A Novel DFACTS Device for the Improvement of Power

Quality of the Supply

P. M. Meshram1 B.Y.Bagde2 R.N.Nagpure3

1,2 & 3 Sr. Lecturers, Dept. of Electrical Engg., Yeshwantrao Chavan College of Engg., Nagpur, INDIA,

1. praful_1087@yahoo.co.in, 2. by.bagde@rediffmail.com, 3. r.n.nagpure@ycce.edu

ABSTRACT

The proliferations of the non-linear devices cannot

be restricted at transmission and distribution level

because of their compactness and power handling

capacity but they also draw non-linear current and hencedegrade the power quality. The different non-linear loads

at the distribution side are adjustable speed drives,fluorescent lighting and personal computers (PCs),

television sets, refrigerators etc.

In this paper we propose the controller at the

distribution side, i.e., between the utility and thecustomer for the improvement of the quality of supply

and therefore it is called DFACTS device. The concept is

analyzed and completely simulated for different types of

loads i.e. linear balanced; linear unbalanced; non-linear

balanced and non-linear unbalanced.

Keywords: DFACTS, FACTS, power quality & Voltage

source converters (VSC)

1. INTRODUCTION

For a given transmission line, three key parameters

determine power flow: terminal bus voltages, lineimpedence, and the relative phase angle between the

sending and receiving end. To modify these parameters,

a variety of mechanically switched devices like shunt

connected capacitors and phase sifting transformers are

used but none responds quickly enough to changing

conditions to provide real-time flow control. Each ofthese conventional power controllers has a conceptual

equivalent based on power electronics. In addition, with

advanced thyristor technology, novel controllers that

have no single conventional analog have been developed

and are termed as FACTS devices [1]. There are several

FACTS controllers, namely, static var compensator

(SVC), thyristor controlled phase angle regulator,

thyristor controlled phase angle regulator (TCPAR),

static compensator (STATCOM), unified power flow

controller (UPFC), etc. [2-3].

The above FACTS devices are used for maintaining

i.e. either supporting or the preventing from rising the

voltage means supplying or absorbing the reactive power

but not exclusively for the improvement of quality of thesupply. This proposed DFACTS device is exclusively for

the improvement of the quality of the supply. The new

concept of DFACTS device uses two Voltage Source

Converters (VSCs) utilizing Pulse Width Modulation

(PWM) technique where inherent characteristics of

reducing the lower order harmonics is used for the

domestic, i.e., at the distribution side for improvement of

quality of the power supply.

The proposed concept has been simulated for

different types of loads and balanced as well as

unbalanced supply voltages.

2. PRINCIPLE OF OPERATION

The two- level, six-pulse voltage source converter

(VSC) is shown in Fig. 1.

Let

Es1 Fundamental component of the ac bus

voltage.

Ec1 Fundamental component of VSC s output

voltage.

Angle between Es1 and Ec2.

X Converter reactance.P Active powerQ Reactive power

dcVsbE

saE

scE

1sE 01cE

RL,

RL,

RL,

1I

2I

3I

1S 2S 3S

'S1

'S2

'S3

C

dcI

Fig. 1: Power Circuit of a Three Phase VSC.

X

EEP cs

sin11= (1)

X

EEEQ scs

)cos( 111 = (2)

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From (1), the active power flowing over the VSC is

primarily determined by . The flow of it is determined

by the relative position between Es1 and Ec1. When Ec1lags behind Es1, the VSC functions as a rectifier and

absorbs real power from the left ac network. When Ec1

leads Es1, the VSC works as an inverter and gives real

power to the left ac system. Similarly from (2) thereactive power is controlled by Ec1.

Three basic control modes for the VSC are (a)Constant dc voltage control, (b) Constant dc current

control, (c) Constant ac voltage control for the passive

load, mode (c) is used and one side of the system should

adopt constant dc control method.

3. SYSTEM DESCRIPTION

The DFACTS device consists of two controllers,

i.e., constant dc voltage controller and ac voltagecontroller. The constant dc voltage controller maintains

constant dc voltage while ac voltage controller maintains

the required rms value of the voltage at the load side.

The two-voltage source converters are shown connected

back-to-back in the Fig 2. Ps1 and Qs1 are the active and

reactive power supplied; Pc1 and Qc1 are the active and

reactive power absorbed by the VSC. Es1 and Es2 are

the peak-to-peak voltage of the supply and load side.

V S C V S CdcE

D C

C on tro ller

at

S tatio n 1

A C

C on tro ller

at

S tatio n 2

2Z

LZ

LI

02cE 22sE

1Z

11sE

I

01cE

A ctiv e

N etw ork

1sQ1sP 1cQ1cP

Fig.2: Back-to-Back VSCs

4. DCVOLTAGE CONTROLLER DESIGN

The complete mathematical analysis and block

diagram of the dc voltage controller is given [4]. The

block diagram of the same is realized in MATLAB 6.5

5. AC CONTROLLER DESIGN

For ac voltage control station, assume the dc voltageutilization ratio of the adopted PWM method is 1 and the

modulation index is .2M 10 2 M

2C2

2C2

dcE

02cE

2R 2L

22sE

loaQ

LZFilte

Fig. 3: Load Side of the Device.

From Fig. 3, we have

EdcM

EC2

2

2 = ( ) (3)10 2 M

2

2

2 C

L

L

S EZZ

E+

= Z (4)

From (3) and (4) we get

EdcZZ

ZME

L

L

S+

=2

2

22

( ) (5)10 2 M

02

1

22

2;Edc

ZZ

Z

M

E

L

LS

+=

(6)

where:

2SE load voltage.

2SE VSC output voltage at the load side.

2M Modulation index at the load side.

LZ load impedance.

dcE dc voltage

2Z Impedance between VSC inverter and load.

Eq (6) shows that there is a linear relationship between

and , so pure PI controller is adequate to meet

the control equipment.

2SE 2M

G en erating

F irin g

P u lse

P I

F requ en cy

S ign al

F irin g

P u lse

C a lcu latio n

o f

F irin g

P u lse

L o ad

S id e

V o ltag es

Fig.4: AC Controller

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6. SIMULATION RESULTS

Load

Voltage

Load

Voltage

Load

Voltage

Load

Voltage

Fig.5: For Linear Balanced and Unbalanced, Non-

Linear Balanced & Unbalanced Loads.

Supply

Voltage

Supply

Voltage

Fig.6: For Linear Balanced and Unbalanced Loads

Supply

Current

Supply

Current

Fig.7: For Linear Balanced and Unbalanced Loads

Load

Current

Load

Current

Load

Current

Load

Current

Fig.8: For Linear Balanced and Unbalanced, Non-

Linear Balanced and Unbalanced Loads.

Supply

Voltage

SupplyVoltage

Fig.9: For Non-Linear Balanced and Unbalanced Loads

Supply

Current

Supply

Current

Fig.9: For Non-Linear Balanced and Unbalanced Loads

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The rated parameters of the system are as follows:

Es1=11kV (peak to peak); R1=0.002ohm, L1=20 H,

The DC voltage is 500V. The rated capacity of each

VSC is 500kW and the switching frequency is 2 kHz.

Increasing the modulation index with increasing DC

voltage could vary the load voltage magnitude. The

following different loads are considered.

(1) Linear balanced load

La =100 kW, 40 kVAR;

Lb = 100 kW, 40 kVAR;

Lc = 100 kW, 40 kVAR

(2) Linear unbalanced load

La = 50 kW, 30 kVAR;Lb = 100 kW, 30 kVAR;

Lc = 50 kW, 50 kVAR;

(3) Non-linear balanced load

La = Lb = Lc = 500 kW.

(4) Non-linear unbalanced loadLa = 167 kW;

Lb = 167 kW;Lc = 100 kW.

where La, Lb and Lc are the loads connected to a, b and

c phases.

Simulation results for the different loads as well asbalanced as well as unbalanced supply voltages are

shown in the Fig 5 and Fig 6.

Following conclusions can be made from the

Table 1 and Table 2: (a) Supply side voltage and current

distortions are well within the limits though the loads are

made deliberately distorted for the balanced supplyvoltages. (b) Though supply is made unbalanced and the

loads are of different types, supply side voltage and

current distortions are well within the limits. (c) The fullload capacity is 500kW and it is fully applied in the case

of non-linear balanced load.

Table 1: Total Harmonic Distortion (THD) in the Load

and Supply for Balanced Supply Voltages.

%

THDin

Linear

Balancedload

Linear

unbalancedload

Non-

linearbalanced

load

Non-linear

unbalancedload

Loadvoltage 1.87 3.44 9.26 32.53

Load

current1.86 2.97 24.24 55.19

Supplyvoltage

0.6 0.37 0.48 0.60

Supply

current0.61 2.72 0.49 0.38

Table 2: Total Harmonic Distortion (THD) in the Load

and Supply for Unbalanced Supply Voltages.

%

THD

in

Linear

Balanced

load

Linear

unbalanced

load

Non-

linear

balanced

load

Non-linear

unbalanced

load

Load

voltage7.67 3.44 9.13 32.25

Loadcurrent

7.23 2.97 24.67 58.15

Supply

voltage0.6 0.37 0.54 0.49

Supplycurrent

0.92 2.72 0.49 0.40

7. CONCLUSIONS

The power quality at the supply side is maintainedthough loads are deliberately made distorted and follows

the IEEE 519 1992 standards. Therefore this DFACTS

device could be used at the distribution side i.e. between

utility and the customer. This device could be called

series-series controller. In the case 3 i.e. for non linear

balanced load the full capacity of load was applied and itstill shows the distortions on the supply side are well

within the standards. Therefore whatever the impurities

on the load side are not transferred on the supply side

irrespective loads natures.

The characteristics of the power qualityimprovement of the proposed DFACTS device is also

hold good for the balanced as well as un-balanced supply

voltages. And there is no rigmarole method of

identifying the harmonics to reduce those and

subsequently reducing the distortions i.e. for

improvement of quality of the supply.

8. REFERENCES

[1] Karl E. Stahkopf and Mark R. Wilhem TighterControls for Busier SystemsIEEE Spectrum, April

1997 pp48-52.

[2] L. Gyugui, Unified power flow for flexible AC

transmission systemIEE Proceeding, PartC, Vol.

139, No. 4 July 1992,pp323-332.

[3] N. G. Hingorani and and L.Gyugui, UnderstandingFACTS, IEEE press, New York,1999.

[4] Blasko.V. and Agirman I, Modellinng and Control

of Three-Phase Regenerative AC-DC Converters

IEEE2001 pp2235-2240.