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Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications www.ijera.com

ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255

www.ijera.com 248 | P a g e

Power Quality Improvement Using Hybrid Power Flow

Controller in Power System

Manidhar Thula1, Voraganti David2 ,Yellaiah Ponnam3

(Assistant Professors in Dept.of EEE, GNIT, Ibrahimapatnam Affiliated to JNTU Hyderabad, A.P) 1,2,3

Abstract This paper discusses the applicability of Hybrid Power Flow Controller (HPFC) as an alternative to Unified

Power Flow Controller (UPFC) for improvement of power system performance. UPFC is a flexible

AC transmission system (FACTS) device containing two switching converters, one in series and one in shunt.

To configure the HPFC, one of the switching converters of the UPFC is replaced by thyristor controlled

variable impedances, thus reducing the cost. In this paper, the HPFC has been configured by multilevel Voltage

Source Converter (VSC) used for the shunt compensation branches and a thyristor controlledvariable impedance used for series compensation. It is shown that with suitable c o n t r o l the HPFC

can inject a voltage of required magnitude in series with the line at any desired angle, just like

UPFC. This helps in providing compensation equivalent to UPFC and improving the steady state stability limits

of the power system.

Keywords — Flexible AC Transmission Systems, Unified Power Flow Controller, Hybrid Power Flow

Controller.

I. INTRODUCTIONThe demand for electrical power is rising

across the world. Setting up of new generating

facilities and building or upgrading the

transmission system is constrained by economic

and environmental factors. Flexible ACTransmission System (FACTS) provides an avenue

to utilize the existing system to its limits withoutendangering the stability of the system, thus

providing efficient utilization of the existing system.

FACTS devices can be broadly classified

into two types, namely (a) Variable Impedance

type devices, e.g. Static Var Compensator (SVC)

or Thyristor Controlled Series Capacitor (TCSC)and (b) Switching Converter type devices which

generally use Voltage Source Converters

(VSC‟s), e.g. Static Synchronous Compensator

(STATCOM) or Unified Power Flow Controller

(UPFC). The dynamic performance of VSC basedFACTS devices have been observed to be better

than that of the variable impedance type FACTS

devices [1]. Among the VSC based FACTS devices,

the UPFC [2, 3] is capable of controlling all the

parameters that effect power flow in a transmission

line either simultaneously or selectively. But themain constraint in the use of the UPFC is its cost.

The VSC especially for the transmission voltage

level comes at a very high cost. There are

reportedly very few installations of UPFC around

the world [4], as compared to the number of

installations of SVC and TCSC which are

comparatively cheaper.

In case it is imperative to install a UPFC in

a particular line in a given system, the idea of the

Hybrid Power Flow Controller (HPFC) proposed in

[5] can possibly be an alternative solution without

significant reduction in versatility. The HPFC is a

blend of switching converter based FACTS devicesalong with variable impedance type FACTS

devices. The motivation behind the proposal of the

HPFC is to provide possible alternative solutions to

the UPFC as far as economy is concerned, and to

improve the dynamic performance of the VariableImpedance type FACTS devices via coordination

with VSC based FACTS devices. In order to

conserve the properties of the UPFC, and to

configure the HPFC, the shunt converter in the

UPFC is replaced by two half sized shunt converterswith their DC links connected back to back, so that

the effective cost of the shunt converter remains

comparable. On the other hand, the series converterhas been replaced by a thyristor controlled variable

impedance type FACTS device which reduces the

cost of the series compensator considerably.The steady state analysis of the HPFC

has been presented in [5] with simplified models.

This paper focuses on the control structure and the

comparison of the steady state performance of the

HPFC with a model of the UPFC of equivalent

rating. In the configuration of the HPFC, the two

shunt VSC‟s are multilevel converters to suit the

higher voltage level. A fixed capacitor with

Thyristor Controlled Reactor (TCR) in parallel has

been used as the series compensator. A metal oxide

RESEARCH ARTICLE OPEN ACCESS

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varistor (MOV) is also connected in parallel to

provide protection against over voltages. Themodels of the HPFC and a UPFC of equivalent

rating have been connected in a single machine

infinite bus (SMIB) system one at a time and thesteady state performance have been compared. The

complete system has been simulated using

PSCAD/EMTDC.

II. THE CONCEPT OF THE UPFC &

THE HPFCA. Uni fi ed Power Fl ow Controll er

The UPFC is configured as shown in Fig.

1. It comprises two VSC‟s coupled through a

common dc terminal. VSC – 1 is connected in shuntwith the line through a coupling transformer and

VSC – 2 is inserted in series with the

transmission line through an interface transformer.The DC voltage for both converters is provided by a

common capacitor bank (C DC ). The series

converter is controlled to inject a voltage V pq in

series with the line, which can be varied between

0 and V pqmax. Moreover, the phase angle of the

phasor V pq can be varied independently

Figure 1. Basic Configuration of the UPFC.

between 0o

and 360o

. In this processthe series converter exchanges both real and

reactive power with the transmission line. While

the reactive power is internally enerated/absorbed by the series converter, the real power

generation/absorption is made feasible by the DC

capacitor. VSC – 1 is mainly used to supply the

real power demand of VSC – 2, which it derives

from the transmission line itself. The shunt

converter maintains the dc bus voltage constant.

Thus the net real power drawn from the ac system

is equal to the losses of the two converters and

their coupling transformers. In addition, the shunt

converter functions like a STATCOM and to

regulate the terminal voltage of the interconnected

bus independently, by generating/absorbingrequisite amount of reactive power.

B. Hybrid Power F low Controll er (H PFC)

The configuration of the HPFC followedin this paper is shown in Fig. 2. It comprises of

two VSC‟s coupled through a common DC circuit.

The VSC‟s are connected in shunt with thetransmission line through coupling transformers,

each on either side of the TCSC. Each VSC is half

the rated capacity of the shunt VSC in the UPFC.The proposed version of HPFC in [3] used Current

Sourced Converters (CSC) in shunt. However,

VSC has been chosen in this paper due to the fact

that VSC‟s offer better dynamic performance when

compared to CSC‟s and also VSC‟s use self

commutated converters which offer betterversatility when compared to the line commutated

converters used in CSC‟s. Also line commutated

converters have the risk of having a commutation

failure which does not occur in self commutatedconverters.

Just like the UPFC, the HPFC injects a

voltage in series with the transmission line voltage

and by varying the phase angle of this voltage

vector, offers control of the real and reactive power

flow through the line. The magnitude of the injected

series voltage can be varied by varying theimpedance of the series compensator through the

firing angle of the thyristors. The phase angle of the

injected series voltage can be controlled by

controlling the VAR outputs of the shunt

compensators. Actually the injected voltage is the

vector difference between the voltages V 1 and V 2.Therefore the angle of the injected voltage can be

Figure 2. Basic Configuration of the HPFC.

varied by varying the magnitudes of V 1and V 2. These magnitudes depend on the

reactive power output of the shunt connected

converters and hence can be controlled. This can

be explained using Fig. 3. Considering a constant

bus voltage V 2, and a particular value of the

magnitude of the injected voltage V c, angle of V c

will vary along a circular locus depending on the

magnitude of bus voltage V 1.

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Figure 3. Injection of Series Voltage by the

HPFC.

Figure 4. Multilevel Inverter (3-level)

Here V C max and V C min are determined

by the limits of the variable impedance of the

series compensator. The shunt compensators draw

a small amount of active power from the line in

order to maintain the DC bus voltage constant.

C. Voltage Source Converter (VSC)

A VSC is essentially a self commutated

DC to AC converter, generating balanced three phase voltages. The configuration shown in Fig. 4 is

a basic diode clamped multilevel inverter. The

switching device is Insulated Gate Bipolar-junction

Transistor (IGBT). Pulse Width Modulation (PWM)

switching technique is used to get an output voltage

closer to sinusoid. In this paper, multilevel inverter[6, 7] has used so that the voltage stress on each

switch is reduced. Also the use of multilevel

inverter reduces the harmonic content of the voltage

generated by the VSC.

III. CONTROL STRUCTURE OF THE

UPFC & HPFCA. The Shunt Compensator Control Strategy

Fig 5 shows the control structure of the

shunt converter [8 -

11]. The main objective of this control is tomaintain required voltage at the point of commoncoupling (PCC) and to control of the DC link

capacitor voltage simultaneously. These two control

actions take place in a decoupled manner by the use

of Parks transformation. A phase locked loop (PLL)

synchronizes the positive sequence component ofthe three-phase terminal voltage at PCC.

The outer loop of the PCC voltage

regulator compares the voltage reference ( E tref )

with the measured PCC voltage and the error is fed

to a PI controller which provides the reference

current for the quadrature axis, I qref . In the inner

loop, this I qref is compared with the measuredvalue of quadrature axis current ( I q) and the error

is fed to a second PI controller. As I q is in

quadrature with the terminal voltage, the reactive

power output of the converter (and in turn the

PCC voltage) is controlled through this part of the

controller.

The outer loop of the dc link voltage

regulator compares the preset dc link voltagereference (V DCref ) with the measured dc link

voltage and the error is fed to a PI controller which

provides the reference current for the direct axis,

I dref . In the inner loop, this I dref is comparedwith the measured value of direct axis current( I d ) and the error is fed to a second PI controller.

The direct axis current ( I d ) being in phase with the

terminal voltage helps to control the active

power so as to either increase or decrease the

DC link voltage (and to supply the active power

requirements of the series converter in the case of

the UPFC). The current regulators (inner loop)

generates signals E sd and E sq. These are then

transformed to a-b-c frame to get the referencewaves for the PWM. These signals are compared

with the carrier waves (which are triangular waves

whose peak to peak value is either equal to or

greater than the amplitude of the reference

waves) in order to generate the PWM switching

pulses for the inverter.

B. The Ser ies Compensator Contr ol Strategy

As mentioned in section I, the series

compensator of the HPFC consists of a fixed

capacitor shunted by a TCR. The controlstructure for this compensator [12] is shown in Fig.

6(b). The active power flow ( P ) through the line

containing the series compensator is taken as the

control variable. The measured value of P is

compared with the reference value of active power

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flow ( P ref ) and the error is fed to a P-I controller.

The output of the P-I controller is the firing angle

(α) of the thyristors of the TCR. This value of

firing angle (α) is limited between 1450 and 1800

to keep the net impedance of the compensatorwithin the capacitive operation zone (α). The

output of the limiter is supplied to the firing

circuit of the series compensator. In case of

UPFC, the series converter provides

simultaneous control of real and reactive powerflow in the transmission line. To do so, the series

converter injected voltage is decomposed into two

components. One component of the series injectedvoltage is in quadrature and the other in-phase with

the line current „i‟.

Figure 5. Control structure for the shunt converter for the UPFC as well as the HPFC.

Fig. 6. (a) Basic module of the series compensator. (b)

Control structure.

Fig. 7 Control structure for the series converter

for the UPFC.

The quadrature injected component

controls the transmission line real power flow. Thein-phase component controls the transmission line

reactive power flow. Fig. 7 shows the series

converter control system [8]. The transmission line

real power flow ( P line) is controlled by injecting a

component of the series voltage (V seq) inquadrature with the line current „i‟. The

Transmission line reactive power (Q2) is

controlled by modulating the bus voltage reference

„V 2‟. The voltage „V 2‟ is controlled by injecting a

component of the series voltage in- phase with the

line current „i‟.

IV. COMPARISON OF RESULTS OF

COMPENSATION WITH HPFC

AND UPFC The HPFC and the UPFC have been tested

in a Single Machine Infinite Bus (SMIB) system

shown in Fig. 8. The generator has been modeled as

a voltage source behind the transient reactance

(Classical model). Detailed data of the SMIB

system, the HPFC and the UPFC are given in the

Appendix (Table A1 and Table A2). At first, with

no compensator connected in the system, 73 MW

power flows through the transmission line from the

alternator to the infinite bus when the angle

between the generator voltage and infinite bus

voltage (δ) is kept equal to 22°. Now the HPFC is

connected as shown in Fig.2. The PCC voltages

for both the converters (V 1 and V 2) are

maintained at 230 kV and the angle δ ismaintained at 22°. This results in an increase in

the power flow through the line to 100 MW. A

plot of the steady state power in the uncompensatedand the compensated system is shown in Fig. 9.

This increase in power flow takes place because of

the voltage injection by the HPFC.

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Fig. 13. Phasor diagrams showing the injected seriesvoltage - cases 1, 2 and 3.

Fig. 14. Phasor diagrams showing the injected

series voltage - case 5.

TABLE I COMPARISON OF INJECTED SERIES

VOLTAGES

HPFC UPFCVoltage

across theseries

branch

Phase angle of

the injectedseries voltage

with respect to

Voltage

across theseries

branch

Phase angle of

the injectedseries voltage

with respect to

Case 1 18.05 KV -94.04740 19.12 KV -94.58800

Case 2 17.35 KV -80.31950 18.60 KV -81.85860 Case 3 19.55 KV -108.03560 21.05 KV -107.72250

Case 4 18.36 KV -75.65630 19.64 KV -77.59800

Case 5 16.32 KV -93.65220 17.64 KV -94.17210 Case 6 18.95 KV -111.76280 20.35 KV -111.46310

TABLE II OPERATING CONDITIONS OF HPFCAND UPFC: CASE 5

In all the cases, the synchronous machinehas been treated as a constant voltage source with

the sending end voltage at 230 KV, both the

UPFC and the HPFC try to maintain the powerflow through the line constant at 100 MW. Fig. 12

compares the steady state operating condition of the

HPFC and the UPFC for case 5. Fig 13 and 14 showthe phasor diagram of the injected series voltage of

the UPFC and the HPFC for cases 1 to 4 as above.

A comparison of the magnitude and phase angle of

HPFC with those for the UPFC is given in Table I.

It can be seen that the magnitude and angle of the

voltage injected by the HPFC for all the five caseare pretty close to those in case of compensation by

UPFC. Similarly, Table II shows a comparison of

active and reactive power of the series and shunt

branch and line power flow for compensation withHPFC and UPFC.

It is clearly understood from Figures 11,

12, and table I, that the HPFC behaves just like the

UPFC in its principle, in other words, the HPFC

injects a voltage source of controllable magnitude

and phase angle, in series with the transmission line,

thus controlling the real and reactive power flowthrough the line. Also Fig 10 shows that the

reactive power generated by the VSC‟s is found to

be almost the same. Hence the fact that two half

sized VSC‟s are used for the HPFC is justified.

V. CONCLUSIONIn this paper, the steady state performance

of the HPFC has been studied. The HPFC

configuration used here has two shunt connectedVSC‟s around a series connected variable

impedance type reactive compensator. The control

structure for the HPFC and the UPFC has been

presented. The HPFC and the UPFC have been

connected to an SMIB system. It has been shown

that the HPFC, similar to UPFC, can inject a voltage

source of controllable magnitude and phase angle in

series with the line. Also HPFC, with proper

control, is found to increase the power flow through

a line and reduce the value of the angle betweenthe voltages at the two ends of the line. Thus, the

performance characteristics of the HPFC are

similar to that of the UPFC without significant

reduction in versatility. Thus the HPFC can be

regarded as a cost effective alternative to the UPFC.

APPENDIX

TABLE A1 PARAMETERS OF THE HPFC

HPFC UPFC Active power of the

shunt branch VSC – 1 -1.1891 MW

0.5990 MW VSC – 2 -0.9768 MW

Active power of theseries branch 0.0315 MW -2.1685 MW

Reactive power ofthe shunt branch

VSC – 1 17.3178MVAR 31.6142

MVAR VSC – 2 17.3190

MVAR Reactive power ofthe series branch 11.8244 MVAR 13.1143

MVAR Voltage across the

series branch 16.32 KV 17.64 KV

Power flow throughthe line 100 MW 100 MW

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TABLE A2

PARAMETERS OF THE UPFC

Shunt Com ensator Parameters

Transformer 11/230 KV, Y/Δ, 60 MVA.Reactance = 0.1 pu (With respect to

transformer rating).

Filter Inductance Lf = 0.0001 H, R f = 0.003 Ω

Filter Ca acitor 400 F DC link Ca acitance 3 mFRated DC bus 22 KV

Series Compensator Parameters

Transformer

Number of 1 Phase Units = 3

Primary side rated voltage = 11 KV

Secondary side rated voltage = 33 KV

Primary side connection = Δ

Rated capacity of each unit = 8 MVA

Reactance = 0.1 pu (With respect to therating of the individual unit)

Filter Inductance Lf = 0.0001 H, R f = 0.003 Ω

Filter Capacitor 400 μF

DC link Capacitance 3 mF

DC bus voltage 22 KV

REFERENCES[1]. Narain. G. Hingorani, Laszlo Gyugyi,

“Understanding FACTS.” IEEE Press,

First Indian Edition, Standard Publishers

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[6]. Giuseppe Carrara, Simone Gardella, Mario

Marchesoni, Raffaele Salutari, Giuseppe

Sciutto, “A New Multilevel PWMMethod: A Theoretical Analysis.”, IEEE

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[7]. Jih-Sheng Lai, Fang Zheng, “MultilevelConverters - A New Breed of Power

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[8]. S. Kannan, S. Jayaram, M. M. A.

Salama, “Real and Reactive Power

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[9]. M. S. El-Moursi, A. M. Sharaf, “ Novel

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STATCOM and SSSC for VoltageRegulation and Reactive Power

Compensation.”, IEEE Transactions on

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April 2007.

Series Compensator Capacitance 41.1 μF. Inductance 0.05 H.

Shunt Compensator Parameters VSC-1 VSC-2

Transformer details

11/230 KV, Y/Δ, 30MVA. Reactance =

0.1 pu (With

respect toTransformer rating).

11/230 KV, Y/Δ, 30MVA. Reactance =

0.1 pu (With

respect toTransformer rating).

Filter Inductance Lf = 0.0001

H R f =

Lf = 0.0001

H R f =

Filter Capacitor 400 μF 400 μF DC link Capacitance 3 mF

Rated DC bus voltage 22 KV

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ABOUT AUTHORS

Manidhar. Thula, Asst.Professor

Received B.Tech degree in

Electrical and Electronics

Engineering from the University of

JNTUH, M.E in Industrial Drives &

Control from College of

Engineering, Osmania University,

Hyderabad. He is currently working

as Asst. Professor in EEE

Department of Gurunanak

Institutions, Hyderabad, His

currently research interests Power

electronics & Drives, Application

of Power electronics in Powersystems and Power quality.

Voraganti David Asst.Professor

Received B.Tech degree in Electrical

and Electronics Engineering from the

University of JNTUH, M.Tech in

Power Electronics from the

University of JNTU-Hyderabad. He is

currently Asst. Professor in EEE

Department of Guru Nanak Institute

of Technology, Hyderabad. His

currently research interests include,

Power electronics & Drives,

Application of Power electronics in

Power systems.

Yellaiah. Ponnam, Asst.Professor

Received M.Tech degree in Control

Systems in Dept. of Electrical and

Electronics Engineering, JNTU

Hyderabad. He is currently working

as Asst. Professor in EEE

Department of Guru Nanak Institute

of Technology ,Hyderabad, His is

doing currently research in Real time

application in control systems, Fuzzy

logic controller, Power electronic

drives and FACTS

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