Single Phase Unified Power Quality Conditioner with ... · Title: Single Phase Unified Power...

International Journal of Advancements in Research & Technology, Volume 3, Issue 1, January-2014

ISSN 2278-7763.

Copyright © 2014 SciResPub. IJOART

Single Phase Unified Power Quality Conditioner

with Minimum VA requirement B. Lakshmana Nayak

1, V. Vijaya Kumar

2 1M.TECH(APS), AMIE, Assoc.Prof., NMR Engineering College, Hyderabad, A.P, India,

2M.TECH, Assistant .Prof., NMR Engineering College, Hyderabad, A.P, India,

Abstract--This paper deals with “unified power

quality conditioners” (UPQCs) which aim to

integrate series active and shunt active filters. The

main purpose of a UPQC is to compensate for

voltage flicker/imbalance, reactive power, negative

sequence current and harmonics. It protects the

consumer at the load end from supply voltage sag,

and provides unity power factor condition at the

utility for different values of load power factor.

During the unbalanced voltage sag/swell at the

input side, the DVR maintains the rated voltage at

the load side. It regulates the load voltage with

minimum VA loading of the overall UPQC by an

optimum voltage angle injection. Some

experimental results of UPQC using MATLAB are

presented.

Index Terms— Active power filters, Dynamic

Voltage Restorer, optimized Unified Power Quality

Conditioner.

I. INTRODUCTION

The wide application of the nonlinear and

electronically switched devices in

distribution systems, the problems such as

voltage sag/swell, flicker, harmonics and

asymmetries of voltages have become

increasingly serious. Power electronics loads

inject harmonic currents in the ac system

and increase overall reactive power

demanded by the equivalent load. The other

development of the digital electronics

/communications and the process control

have increased the number of sensitive loads

that require ideal sinusoidal supply voltages

for the proper operation [1]. Spurious

tripping of voltage sensitive loads such as

PLCs or adjustable-speed drives due to the

voltage sags is a serious power quality

concern for industrial customers. In [2],the

adjustable-speed drives have been reported

to trip for voltage sag as small as 15% for 8

ms. IEEE Std. 1346-1998 defines voltage

sag as a decrease in rms voltage at the

power frequency for duration of 0.5 cycle to

1 minute, caused by faults in the electric

supply system and staring of large loads like

induction motors.

Customers describe tripping of equipment

due to disturbances in the supply voltage as

“bad power quality”. The increased use of

converter-driven equipment, such as

consumer electronics, up to adjustable-speed

drives has led to a large growth of voltage

disturbances. The main cause here is the

non-sinusoidal current of rectifiers and

inverters. In view of the proliferation of the

power electronic equipment connected to the

utility system, various national and

international agencies have been considering

limits on harmonic current injection to

maintain good power quality. As a

consequence, various standards and

guidelines have been established that

specifies limits on the magnitudes of

harmonic currents and harmonic voltage

distortion at various harmonic frequencies.

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Some of these are as follows:

EN 50 006, "The Limitation of Disturbance in

Electricity Supply Networks caused by

Domestic and Similar Appliances Equipped

with Electronic Devices", European Standard

prepared by Comité Européen Electrotechnique,

CENELEC;

IEC Norm 555-3, prepared by the International

Electrical Commission;

West German Standards VDE 0838 for

household appliances, VDE 0160 for

converters, and VDE 0712 for fluorescent lamp

ballasts;

IEEE Guide for Harmonic Control and

Reactive Compensation of Static Power

Converters, ANSI/IEEE Std. 519-1981, which

was revised in 1992 to 519-1992.

CENELEC, IEC, and VDE standards

specify the limits on the voltage (as a

percentage of the nominal voltage) at

various harmonic frequencies of the utility

frequency, when the equipment-generated

harmonic currents are injected into a

network whose impedances are specified.

The revised IEEE-519, which contains

recommended practices and requirements

for harmonic control in electric power

systems, specifies requirements on the user

as well as on the utility. Under this

circumstances, a new technology called

Custom Power emerged [3], which is

applicable to distribution systems for

enhancing the reliability and quality of

power supply. Custom power devices

include static shunt converter and static

series-converter. Static shunt-converters,

such as DSTATCOM, are mainly intended

for conditioning the current flowing from

the load into utility [4]. The series

converters, such as DVR, are used to

improve the quality of the voltage supplied

by the utility to the load [5]. Since the basic

compensation principles were proposed

around 1980, much research has been done

on active filters in the last years [6], [7], [8].

Recent research effort have been made

towards utilizing a device called Unified

Power Quality conditioner (UPQC) to

solve almost all power quality problems [9],

[10],[11]. To put in nutshell, UPQC aims the

integration of series active and shunt active

power filters connected through a common

dc-link capacitor. This led to the

development of advanced control techniques

for UPQC. In [12] some works have been

done based on the quadrature voltage

injection by the series-converter of the

UPQC. This scheme, leads to purely reactive

power handling by the series-converter, does

not result in a minimum VA consumption of

the overall UPQC, cannot operate for supply

voltage swell, and is not optimized from

point of view of VA loading and efficiency.

The series voltage injection at an

optimized angle not only results in load

voltage regulation but also in overall

minimized VA loading and improved

efficiency. This paper presents an optimized

UPQC where the series voltage injection is

controlled at an optimized angle for

minimum VA loading of the overall UPQC.

To verify and validate the proposed Power

Conditioning equipment and the control

method, a laboratory prototype was

performed; the system is fully digital-

controlled using the fixed-point

TMS320F240 digital signal processor.

The proposed Unified Power Quality

Conditioner has the following goals and

characteristics:

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The overall UPQC VA loading is

maintained at a minimum value for the

whole range of operation;

Maintains the load voltage at the rated

voltage for supply voltage sag/swell;

the injection voltage assured by series-

converter is taken from the same dc-

link voltage controlled at a fixed level

(450VDC) by shunt-converter;

Compensates the load reactive power,

by controlling the current of shunt-

converter; in steady-state operation, at

the rated supply-voltage, no active

power is consumed by shunt-converter;

Assures an input current with very low

harmonic content, having in view the

regulations of the previous mentioned

standards.

In this paper, the analysis, design and

experimental results are presented to show

the feasibility of the proposed scheme.

II. BASIC CONFIGURATION OF THE UNIFIED POWER

QUALITY CONDITIONER

The configuration of the UPQC is

presented in Fig.1. The UPQC consists of

two single-phase PWM controlled

converters connected in cascade through a

common dc-link capacitor and a series low-

impedance transformer.

Fig 1. Single-phase Unified Power Quality Conditioner system.

The series-connected converter such as

DVR, inject/extract the necessary voltage to

compensate the supply-voltage sag/swell.

The shunt-converter, such as STATCOM

(STATic synchronous COMpensator),

connected in parallel with the load takes a

current from the supply to compensate the

reactive power requested by the load, to

reduce the harmonics and to control the dc-

link voltage at a desired value. Shunt-

converter acts not only as an active filter,

but assures the necessary active power for

series-converter, as well. The shunt-

converter can operate with hysteresis current

control mode to force the source current Ii,

in the same phase with Ui such the input

power factor is always maintained unity.

The series-inverter operates in unipolar

PWM switching mode. When voltage

sag/swell occurs, the DVR injects a voltage

in such a manner so that the load end

voltage is always maintained at the desired

magnitude. There are two main methods for

load voltage compensation. As [12]

presents, when voltage sag occurs, the DVR

injects a voltage in quadrature advance to

the supply voltage. This method cannot be

used for compensation of the voltage swell,

and the overall UPQC VA loading is not

minimized. Another method, as [9] presents,

is the controlling of the compensating

voltage in the same phase or opposite phase,

depending on the supply voltage event. In

this case the UPQC is capable of

compensating both voltage sag and swell,

being not optimized from point of view of

UPQC VA loading.

The main advantage of the shunt-series

controller is that it does not require any

energy storage. It is designed to mitigate any

supply-voltage variation of a certain

magnitude, independent of its duration.

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International Journal of Advancements in Research & Technology, Volume

ISSN 2278-7763.

Copyright © 2014 SciResPub.

A. PHASOR DIAGRAM

Fig. 2 Phasor representation of various currents and voltages in

UPQC

The authors [8] reported a control scheme

considering minimum energy requirement

only for the DVR. There is no report on the

overall UPQC optimization where the

combined operation of DVR and

STATCOM has been taken into account.

The loading calculation of UPQC during the

voltage sag has been carried out on the basis

of a linear load for fundamental frequency at

rated load current ( ILRated). From the

phasor diagram of Fig. the presag source

voltage VS1 and load voltage ( V

the sag are assumed to be equal to the

healthy source voltage VS, which is 1 p.u.

Let the rated load current ( ILRated) a load

power factor angle be 1 p.u. with Ø

In case of a voltage sag, the

source voltage (VS2) is given as V

VS1(1-x) ,where x is per-unit sag. Now, to

maintain active power constant under the

voltage sag condition, the following

equation can be written as

12211 ).1( ssssss IXIIVIV −=⇒=

International Journal of Advancements in Research & Technology, Volume 3, Issue 1, January

Fig. 2 Phasor representation of various currents and voltages in

The authors [8] reported a control scheme

considering minimum energy requirement

only for the DVR. There is no report on the

overall UPQC optimization where the

operation of DVR and

STATCOM has been taken into account.

The loading calculation of UPQC during the

voltage sag has been carried out on the basis

of a linear load for fundamental frequency at

Rated). From the

the presag source

and load voltage ( VL2) during

the sag are assumed to be equal to the

, which is 1 p.u.

Rated) a load

power factor angle be 1 p.u. with Ø1.

In case of a voltage sag, the postsag

) is given as VS2 =

unit sag. Now, to

maintain active power constant under the

voltage sag condition, the following

2s

Since the STATCOM is locally supplyi

the reactive current component of the load

and VS1 is in phase with the PCC voltage

(VPCC) the source current I

component of the load current. Therefore,

and (1) can be rewritten as Therefore, I

Ø = IS1 and (1) can be rewritten as

)1/()cos(.2 xII Ls −Φ=

III. VA REQUIREMENT OF THE

OPTIMUM UPQC

Because the shunt-converter has the

main role to assure an input current with a

very low THDi, this is connected and

controlled in a such way to compensate the

distortion component of the load

(IdisL=IdisSh).The THDSh (Total Harmonic

Distortion of shunt-converter current,

one of the main factors, which affects the

VA ratings of the UPQC. It is very

important to find a relationship with

(Total Harmonic Distortion of the load

current IL).

Series VA loading

1/()cos(... 2 IVIV Linjsinj −Φ=

Shunt VA loading = IC2VL2

IVtotalVA Linj /()cos(..)( Φ=

Equation (1) shows that at rated load current

( IL rated = 1 p.u), the total VA loading of

the UPQC is the function of three quantities,

viz. x, ø and θ, and sag x and p.f. angle

independent quantities. θ is the angle by

which leads the postsag voltage . The

voltage is injected by the DVR at an angle L

w.r.t postsag as shown in Fig.2. By applying

any standard method for function

minimization with x and ø as two

January-2014

IJOART

Since the STATCOM is locally supplying

the reactive current component of the load

is in phase with the PCC voltage

(VPCC) the source current IS1 is the real

component of the load current. Therefore,

and (1) can be rewritten as Therefore, IL cos

and (1) can be rewritten as

REQUIREMENT OF THE

converter has the

main role to assure an input current with a

, this is connected and

controlled in a such way to compensate the

distortion component of the load current

(Total Harmonic

converter current, IC) is

one of the main factors, which affects the

VA ratings of the UPQC. It is very

important to find a relationship with THDL

(Total Harmonic Distortion of the load

Series VA loading

)x−

L2+ IC22

ZSLC P.U

uPZIVIx slcCLc ...)1/(2

222 ++−

-----------(1)

rated load current

rated = 1 p.u), the total VA loading of

the UPQC is the function of three quantities,

, and sag x and p.f. angle ø are

θ is the angle by

which leads the postsag voltage . The

ed by the DVR at an angle L

w.r.t postsag as shown in Fig.2. By applying

any standard method for function

minimization with x and ø as two

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constraints and as a variable varying from 0

to 90 , an optimum angle θ can be found out,

that results in minimum total VA loading.

IV. SIMULATION RESULTS

The results presented in this paper are for

a diode bridge rectifier with the non-linear

load at dc side - an inductance L in series

with a parallel RC load.

In Fig.3.a and Fig.3.b there are shown

from top to bottom supply-voltage, load

current, input current and compensating

current-shunt-converter, for two different

loads.

Fig 3.(a) - Rectifier bridge load - L=3mH, R=4Ω, C=1,500µF

THDL=0.37; THDSh=0.82; THDi=0.001.

Fig.3.(b) - Rectifier bridge load - L=5mH, R=4Ω, C=20µF

THDL=0.18; THDSh=0.94; THDi=0.001.

Figure.3. Distortion compensation and power factor correction.

1 - Supply-voltage (320V/div); 2 - Load current;

3 - Input current; 4 - Compensating current; (80A/div; time -

5ms/div)

Figure. 4. Load Voltage compensation.

Ui sag from 220V to 154V; γ = 410;UTr = 79.5V. (200V/div,

20ms/div).1-Supply-voltage; 2-Peak supply-voltage; 3-Injected

voltage; 4-Load voltage

V. CONCLUSION

Voltage compensation method both for

supply voltage sag and swell also shows a

very good performance. The power factor is

improved at unity by compensation. The

load voltage is maintained at its reference

value. UPQC operates in the minimum VA

optimization mode; the optimized operation

can result not only in reduced overall size,

but also in the increased efficiency.

From the experimental results, one can say

that the proposed control methods have good

compensation characteristics and the

proposed UPQC system can have an

important role for power quality

improvement.

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REFERENCES

1. D. Sabin and A. Sundaram, "Quality enhances reliability," IEEE Spectrum, vol. 33, no. 2, Feb. 1996, pp. 34-41.

2. M.H.J. Bollen, "Characterization voltage sags experienced by the three phase adjustable-speed drives," IEEE Transaction on Power

Delivery,vol. 12, no. 4, Oct. 1997, pp. 1166-1171.

3. N. G. Hingorani, "Introducing Custom Power," IEEE Spectrum, vol. 32, no.6, June 1995, pp. 41-48.

4. P.W. Lehn and M.R. Iravani, " Experimental Evaluation of STATCOM closed loop dynamics," IEEE Transaction on Power

Delivery, vol. 13. no. 4, Oct. 1998, pp. 1378-1384.

5. T. Wunderlin, O. Amhof, P. Dahler and H. Guning, "Power supply quality improvement with a dynamic voltage restorer (DVR)," in Proceedings of EMPD'98 - Energy Management

and Power Delivery,1998, vol.2, March 3-5, 1998, pp. 518-525.

6. H. Akagi, "Trends in Active Power Line Conditioning," IEEE Transaction on Power

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7. T.Thomas, K. Hadded, G. Joss and A. Jaafari, "Design and performance of active filters," IEEE

Industry Application Magazine,vol. 4, issue 5, Sept.-Oct. 1998, pp. 38-46.

8. F. Barrero, S. Martinez, P.M. Yeves and P.M. Martinez, "Active Power Filters for Line Conditioning; A critical Evaluation," IEEE

Transaction on Power Delivery, vol. 15, no. 1, Jan. 2000, pp. 319-325.

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AUTHORS

First Mr. B. Lakshmana

Nayak, Born on 15th June 1980, Anigandlapadu, Andhra Pradesh, India. He received the B.Tech. Degree in Electrical and Electronic Engineering

from Sri Sarathi Institute of Engineering and Technology, Nuzvid, Andhra Pradesh, and M.Tech. Degree in Advanced Power System from the Jawaharlal Nehru technological university, Kakinada, Andhra Pradesh.

Presently he has been working as a Associate Professor in the Electrical and Electronic Engineering Department in Nalla Malla Reddy Engineering College, Hyderabad, Andhra Pradesh, India. He has 8 years of experience. His main research area power systems. Email: [email protected]

Second Mr. V. Vijaya

Kumar, Born on 8th May 1983, Shad Nagar, Andhra Pradesh, India. He received the B.Tech. Degree in Electrical and Electronic Engineering

from Sri Saijothi Engineering college, Mahabubnagar, Andhra Pradesh, and M.Tech. Degree in Electrical Power System from J. B. I. E. T, Andhra Pradesh.

Presently he has been working as a Assistant Professor in the Electrical and Electronic Engineering Department in Nalla Malla Reddy Engineering College, Hyderabad, Andhra Pradesh, India. He has 7 years of experience. His main research area power systems. Email: [email protected]

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