Ar4101248255

Yellaiah Ponnam et al Int. Journal of Engineering Research and Applications
ISSN : 2248-9622, Vol. 4, Issue 1( Version 1), January 2014, pp.248-255

RESEARCH ARTICLE

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OPEN ACCESS

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 controlled
variable 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.
comparatively cheaper.
In case it is imperative to install a UPFC in
I.
INTRODUCTION
a particular line in a given system, the idea of the
The demand for electrical power is rising
Hybrid Power Flow Controller (HPFC) proposed in
across the world. Setting up of new generating
[5] can possibly be an alternative solution without
facilities and building or upgrading the
significant reduction in versatility. The HPFC is a
transmission system is constrained by economic
blend of switching converter based FACTS devices
and
environmental factors.
Flexible
AC
along with variable impedance type FACTS
Transmission System (FACTS) provides an avenue
devices. The motivation behind the proposal of the
to utilize the existing system to its limits without
HPFC is to provide possible alternative solutions to
endangering the stability of the system, thus
the UPFC as far as economy is concerned, and to
providing efficient utilization of the existing system.
improve the dynamic performance of the Variable
FACTS devices can be broadly classified
Impedance type FACTS devices via coordination
into two types, namely (a) Variable Impedance
with VSC based FACTS devices. In order to
type devices, e.g. Static Var Compensator (SVC)
conserve the properties of the UPFC, and to
or Thyristor Controlled Series Capacitor (TCSC)
configure the HPFC, the shunt converter in the
and (b) Switching Converter type devices which
UPFC is replaced by two half sized shunt converters
generally
use Voltage Source Converters
with their DC links connected back to back, so that
(VSC‟s), e.g. Static Synchronous Compensator
the effective cost of the shunt converter remains
(STATCOM) or Unified Power Flow Controller
comparable. On the other hand, the series converter
(UPFC). The dynamic performance of VSC based
has been replaced by a thyristor controlled variable
FACTS devices have been observed to be better
impedance type FACTS device which reduces the
than that of the variable impedance type FACTS
cost of the series compensator considerably.
devices [1]. Among the VSC based FACTS devices,
The steady state analysis of the HPFC
the UPFC [2, 3] is capable of controlling all the
has been presented in [5] with simplified models.
parameters that effect power flow in a transmission
This paper focuses on the control structure and the
line either simultaneously or selectively. But the
comparison of the steady state performance of the
main constraint in the use of the UPFC is its cost.
HPFC with a model of the UPFC of equivalent
The VSC especially for the transmission voltage
rating. In the configuration of the HPFC, the two
level comes at a very high cost. There are
shunt VSC‟s are multilevel converters to suit the
reportedly very few installations of UPFC around
higher voltage level. A fixed capacitor with
the world [4], as compared to the number of
Thyristor Controlled Reactor (TCR) in parallel has
installations of SVC and TCSC which are
been used as the series compensator. A metal oxide
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varistor (MOV) is also connected in parallel to
provide protection against over voltages. The
models 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 the
steady state performance have been compared. The
complete system has been simulated using
PSCAD/EMTDC.

II.

THE CONCEPT OF THE UPFC &
THE HPFC

A. Unified Power Flow Controller
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 shunt
with 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 (CDC). The series
converter is controlled to inject a voltage Vpq in
series with the line, which can be varied between
0 and Vpqmax. Moreover, the phase angle of the
phasor Vpq can be varied independently

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B. Hybrid Power Flow Controller (HPFC)
The configuration of the HPFC followed
in 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 the
transmission 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 better
versatility 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 commutated
converters.
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 the
impedance 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 V1 and V2.
Therefore the angle of the injected voltage can be

Figure 1. Basic Configuration of the UPFC.
between 0o
and 360o. In this process
the 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/absorbing
requisite amount of reactive power.

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Figure 2. Basic Configuration of the HPFC.
varied by varying the magnitudes of V1
and
V2.
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 V2, and a particular value of the
magnitude of the injected voltage Vc, angle of Vc
will vary along a circular locus depending on the
magnitude of bus voltage V1.

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UPFC & HPFC

Figure 3. Injection of Series Voltage by the
HPFC.

Figure 4. Multilevel Inverter (3-level)
Here VCmax and VCmin 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

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A. The Shunt Compensator Control Strategy
Fig 5 shows the control structure of the
shunt converter [8 11]. The main objective of this control is to
maintain required voltage at the point of common
coupling (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 of
the three-phase terminal voltage at PCC.
The outer loop of the PCC voltage
regulator compares the voltage reference (Etref)
with the measured PCC voltage and the error is fed
to a PI controller which provides the reference
current for the quadrature axis, Iqref. In the inner
loop, this Iqref is compared with the measured
value of quadrature axis current (Iq) and the error
is fed to a second PI controller. As Iq 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 voltage
reference (VDCref) with the measured dc link
voltage and the error is fed to a PI controller which
provides the reference current for the direct axis,
Idref. In the inner loop, this Idref is compared
with the measured value of direct axis current
(Id) and the error is fed to a second PI controller.
The direct axis current (Id) 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 Esd and Esq. These are then
transformed to a-b-c frame to get the reference
waves 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 Series Compensator Control Strategy
As mentioned in section I, the series
compensator of the HPFC consists of a fixed
capacitor shunted by a TCR. The control
structure 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 (Pref) 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 compensator
within the capacitive operation zone (α). The
output of the limiter is supplied to the firing
circuit of the series compensator. In case of

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UPFC, the series
converter
provides
simultaneous control of real and reactive power
flow in the transmission line. To do so, the series
converter injected voltage is decomposed into two
components. One component of the series injected
voltage 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.
Transmission line reactive power (Q2) is
controlled by modulating the bus voltage reference
„V2‟. The voltage „V2‟ is controlled by injecting a
component of the series voltage in- phase with the
line current „i‟.

IV.
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. The
in-phase component controls the transmission line
reactive power flow. Fig. 7 shows the series
converter control system [8]. The transmission line
real power flow (Pline) is controlled by injecting a
component of the series voltage (Vseq) in
quadrature with the line current „i‟. The
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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 (V1
and V2) are

maintained at 230 kV and the angle δ is
maintained 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 uncompensated
and 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. 8 Schematic diagram of the SMIB system
The corresponding phasor diagram of V1,
V2 and injected voltage Vc is shown in Fig. 10.
Next, the power flow through the line is maintained
at 100 MW and the angle δ is allowed to vary. The
steady state values of δ for the uncompensated
system and the system compensated by HPFC are
plotted in Fig. 11. It can be seen that the value of δ
for the uncompensated system is 0.54 radian
which comes down to 0.39 radian when HPFC is
connected to the system. A decreased value of δ
means more power can be transmitted through the
line. Now if the UPFC is connected to the system,
the value of δ becomes 0.35 radian which is also
shown in Fig. 11. Thus all these results indicate
that compensation with HPFC increases the power
carrying capacity of a transmission line and the
effect of HPFC and UPFC in this regard are
comparable.

Fig. 11 Value of δ uncompensated and
for
compensated system

Fig. 12. Comparison of the steady state reactive
power generation: Case5.

Fig. 9 Power flow for uncompensated and HPFC
compensated system

Fig. 10 PCC voltages of the shunt converters of
the HPFC and the series voltage injected by the
HPFC
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Simulations have been performed for the
UPFC and the HPFC in order to prove that, just like
the UPFC, the HPFC injects a voltage source of
controllable magnitude and phase angle, in series
with the transmission line. In order to fulfill this
particular objective, the following cases have been
considered where the reference variables for the
UPFC and the HPFC have been adjusted in such a
manner that the bus voltages „V1‟ and „V2‟ are
maintained at:
Case 1: V1 = V2 = 230 KV (Line to line). Case 2:
V1 = 237 KV, V2 = 230 KV.
Case 3: V1 = 222 KV, V2 = 230 KV. Case 4: V1
= 235 KV, V2 = 225 KV Case 5: V1 = V2 = 237
KV.
Case 6: V1 = 225 KV, V2 = 235 KV

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

Fig. 14. Phasor diagrams showing the injected
series voltage - case 5.

Case 1

TABLE I COMPARISON OF INJECTED SERIES
VOLTAGES
HPFC
UPFC
Voltage Phase angle of Voltage Phase angle of
across the the injected across the the injected
series
series voltage
series
series voltage
branch with respect to
branch with respect to
bus „V
bus „V
18.05 KV -94.04740 2‟
19.12 KV -94.588002‟

Case 2
Case 3

17.35 KV -80.31950
19.55 KV -108.03560

18.60 KV
21.05 KV

-81.85860
-107.72250

Case 4

18.36 KV -75.65630
16.32 KV -93.65220
18.95 KV -111.76280

19.64 KV

-77.59800
-94.17210

Case 5
Case 6

17.64 KV
20.35 KV

-111.46310

TABLE II OPERATING CONDITIONS OF HPFC
AND UPFC: CASE 5
Active power of the
shunt branch

VSC – 1

HPFC
-1.1891 MW

VSC – 2

-0.9768 MW

Active power of the
series branch
Reactive power of
the shunt branch
Reactive power of
the series branch
Voltage across the
series branch
Power flow through
the line

0.0315 MW
17.3178
MVAR
17.3190
MVAR

VSC – 1
VSC – 2

UPFC
0.5990 MW
-2.1685 MW
31.6142
MVAR

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In all the cases, the synchronous machine
has 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 power
flow 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 show
the 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 case
are 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 with
HPFC 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 flow
through 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.

CONCLUSION

In this paper, the steady state performance
of the HPFC has been studied. The HPFC
configuration used here has two shunt connected
VSC‟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 between
the 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

11.8244 MVAR

13.1143
MVAR

16.32 KV

17.64 KV

100 MW

100 MW

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Capacitance
41.1 μF.
Inductance
0.05 H.
Shunt Compensator Parameters
VSC-1
VSC-2
11/230 KV, Y/Δ, 30
11/230 KV, Y/Δ, 30
MVA. Reactance =
MVA. Reactance =
Transformer details
0.1 pu (With
0.1 pu (With
respect to
respect to
Transformer rating).
Transformer rating).

Series Compensator

Lf = 0.0001
H Rf =
0.003 ΩμF
400

Filter Inductance
Filter Capacitor
DC link Capacitance
Rated DC bus voltage

Lf = 0.0001
H Rf =
0.003 Ω
400 μF
3 mF
22 KV

TABLE A2
PARAMETERS OF THE UPFC
Shunt Compensator Parameters
11/230 KV, Y/Δ, 60 MVA.
Transformer
Reactance = 0.1 pu (With respect to
transformer rating).
Filter Inductance
Lf = 0.0001 H, Rf
= 0.003 Ω
Filter Capacitor
400 μF
DC link Capacitance
3 mF
Rated DC bus
22 KV
voltage
Series Compensator Parameters
Number of 1 Phase Units = 3
Primary side rated voltage = 11 KV
Secondary side rated voltage = 33 KV
Transformer
Primary side connection = Δ
Rated capacity of each unit = 8 MVA
Reactance = 0.1 pu (With respect to the
rating of the individual unit)
Filter Inductance

Lf = 0.0001 H, Rf

Filter Capacitor
DC link Capacitance
DC bus voltage

= 0.003 Ω

400 μF
3 mF
22 KV

REFERENCES
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[2].

[3].

[4].

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Schauder, E. Stacey, L. Kovalsky, L.
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[6]. Giuseppe Carrara, Simone Gardella, Mario
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ABOUT AUTHORS
Manidhar. Thula, Asst.Professor
Yellaiah. Ponnam, 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 Power
systems and Power quality.

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

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.

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