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Design, simulation and implementation of a UPFC (unified power flow
controller) for transmission line model
Conference Paper · April 2017
DOI: 10.1109/IPACT.2017.8245053
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International Conference on Innovations in Power and Advanced Computing Technologies [i-PACT2017]
Design, Simulation and Implementation of a UPFC
(Unified Power Flow Controller) for Transmission
Line Model
T.P.G.T.A. Priyankara, M.J.L.R. Fernando, I.A.K.Sandeepa, N.M.K.D Bandara, H.W.D.Hettiarachchi,
K.T.M.U.Hemapala.
Department of Electrical Engineering
University of Moratuwa
Katubadda, Sri Lanka.
Abstract—Generally the power system is frequently faced
various disturbances. These disturbances will cause to
create harmonics and oscillations in the system which lead
to operate protection relays. Further this will cause to create
a total system failure. Different techniques and devices are
used in order to control these power system oscillations and
improve the power quality. One of the techniques is use of
FACT (flexible alternating current transmission system)
devices. FACT devices improve controllability of power
system operation. The unified power flow controller (UPFC)
is one of the recently founded FACT device which is
constructed using modern power electronics technology to
control the active and reactive power of the power system.
Developing an UPFC for transmission line model and
analyzing impact on power quality on transmission line in
abnormal conditions would be presented in this paper.
Moreover, MATLAB/Simulink models for subsystems of the
UPFC are also developed to create a simulation model of
UPFC with transmission line model.
and other static equipment that provide control of one or
more AC transmission system and increase the capacity
of power transfer”
As Fig. 1 depicts, FACTS devices can be subdivided
in to two categories based on the power electronic
devices used in the control. They are Voltage Source
Converter (VSC) based FACTS and Thyristor valve base
FACTS. UPFC is modern FACTS device which was
developed based on VSC control technology. Different
FACTS controllers are used to provide corrective actions
for different steady-state operating problems in power
system. Table 1 shows some applications of the FACTS
devices.
Keywords—UPFC, FACTS, STATCOM, SSSC
I.INTRODUCTION
Dynamic nature of the power system causes to
frequent disturbances in the system which can drive the
system to an unstable state. This continues failures will
lower the power system reliability and power quality
index of the system. Power quality is a combination of
voltage quality and current quality. The UPFC is one of
the modern FACTS device that can be used to improve
power quality and reliability. UPFC also increases
transient and dynamic grid stability of the system by
controlling both active and reactive power flow through a
transmission line. With the increasing demand, the
existing transmission network should be used optimally
rather than constructing new transmission lines. UPFC
provide better utilization of transmission system assets
which already exist and provide environmental benefits.
FACTS technology concept was introduced by
Electric Power Research Institute (EPRI) in the late
1980’s. This concept is used in worldwide to improve the
system quality, reliability, stability and power flow with
the existing power systems [1]. FACTS technology is
defined by the IEEE as "a power electronic based system
Fig. 1.Classification of FACTS devices
TABLE I.
FACTS CONTROLLERS FOR STEADY- STATE
OPERATING PROBLMES
Operating problem
Corrective action
FACTS controller
Voltage limits:
Low voltage at
heavy load
High voltage at low
load
Supply reactive
power
Absorb reactive
power
SVC, STATCOM
SVC, STATCOM,
TCR
Thermal limits:
Transmission circuit
overload
Tripping of parallel
circuit
Parallel line load
sharing
Power flow direction
reversal
Reduce overload
TCSC,
SSSC,UPFC,PS
Limit circuit loading
TCSC,SSSC,UPFC,PS
Power flows:
Adjust series
reactance
Adjust phase angle
TCSC,SSSC,UPFC
TCSC, SSSC,PS
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978-1-5090-5682-8 /17/$31.00 ©2017 IEEE
International Conference on Innovations in Power and Advanced Computing Technologies [i-PACT2017]
The UPFC is the most versatile device which
designed based on the combination of both series and
shunt FACTS devices. Shunt FACTS device (Shunt
Converter) or STATCOM (Static Synchronous
Compensator) is used to control the reactive power flow
in the transmission line. Series FACTS device (Series
Converter) or SSSC (Static Synchronous Series
Compensator) is used to control the series injecting
voltage and phase angle which is injected to the
transmission line through series transformer. These two
converters are combined with a common dc link. This
capacitor has small energy storing capacity. So, active
power generated by the series converter should be equal
to the active power flow through the shunt converter [1].
Fig.2 shows schematic diagram of the UPFC.
II.FACTS DEVICES
Many research have been done in the field of FACTS.
Since UPFC is one of the modern FACTS device,
fewresearch have published with reference to UPFC. Lot
of them are simulation models and it is hard to find actual
harware design of a UPFC. Based on the control strategy
used, UPFC has the flexibility to control anyoperation
functions.D-Q control strategy is used in this research
among different control strategies. Operation of UPFC,
main two components of the UPFC, i.e. SSSC (static
synchronous series compensator strategy) and
STATCOM(static
synchronous
compensator)and
different control strategies would bediscussed here.
A. SSSC
SSSC is one of the voltage source converter(VSC) based
FACTS device which is connected in series with
transmission line. SSSC provides advanced series
compensation. This has several advantages over a TCSC
(Thyristor Controlled Series Compensator). Some of the
advantages are listed below [2].
¾ SSSC doesn’t use bulky passive components
such as capacitors and reactors.
¾ Operation capability in both inductive and
capacitive mode.
¾ Improved technical characteristics.
¾ Real power exchange is possible between DC
side and AC network. i.e. energy source can be
connected on the DC side.
B.
STATCOM
STATCOM is also a VSC based FACTS device which is
shunt connected with transmission line. Current flowing
through converter is controlled to adjust the bus voltage,
i.e. reactive power flow of the transmission line.
STATCOM has many advantages over a SVC (Static Var
Compensator). Some of them are listed below. [2]
¾
¾
¾
Quick response.
Space requirement is less because STATCOM
doesn’t use bulky passive components such as
capacitors and reactors.
STATCOM can be interfaced with real power
sources.
UPFC is come with advantages of both SSSC and
STATCOM. And has technological edge from
controllability and flexibility aspect over the other
FACTS devices.
C.
D-Q axis control strategy
Three phase voltage and current is converted in to
two axes (D and Q) in this strategy for controlling
purposes. Here, measured voltage and current of the
transmission line is split into two components which are
in phase with D axis and Q axis using abc to dq0
transformation method. These two components provide
details about three phase voltage and current. According
to these readings, D and Q axis component of the
required injecting voltage can be calculated. It should be
added to the transmission line to maintain the system
stability. Then dq0 to abc transformation gives the
required three phase injecting voltage which should be
injected to the line through series transformer. Switching
signals for the power switching devices should be
generated according to that. In series converter, Real
power of the transmission line can be controlled using D
axis voltage by adjusting D axis current. Reactive power
of the transmission line can be controlled using Q axis
voltage by adjusting Q axis current component. [3-5]
In shunt converter, D-axis shunt current component is
related with DC capacitor voltage and Q-axis shunt
current component is related with reactive power.
D. Phase Shifter Strategy(PS)
In this strategy, injecting voltage is maintained in
quadrature with UPFC bus voltage which shunt converter
is connected. Phase angle of the transmission line sending
end voltage can be varied to obtain specific active power
flow. [7-9].
E. Operation of the UPFC
Fig. 2.Schematic diagram of the UPFC
The UPFC consist of mainly three parts. They are
converter circuit, inverter circuit and controller circuits.
Inverter circuit provide the main function of the UPFC.
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International Conference on Innovations in Power and Advanced Computing Technologies [i-PACT2017]
By using inverter circuit, controllable ac voltage with a
magnitude of ܸ௦ ሺͲ ൑ ܸ௦ ൑ ܸ௦ሺ୫ୟ୶ሻ ሻ and phase angle of
‫׎‬ሺͲ ൑ ‫ ׎‬൑ ͵͸Ͳሻcan be injectedto the transmission line
through series transformer. Fig.3 and Fig.4 shows the
single line diagram and voltage and current phasor
diagram of UPFC respectively.
The series injecting voltage which has magnitude of
ܸ௦ and phase angle of ‫ ׎‬can be controlled by controlling
the converter and inverter circuit operation. It implies that
ܸଶ and ߜ can be controlled. Active and reactive power
flow through transmission line can be calculated using
following equation.
‫ݓ݋݈݂ݎ݁ݓ݋݌݁ݒ݅ݐܿܣ‬ሺܲሻ ൌ
ܸଶ ܸଷ ܵ݅݊ሺߜሻ
ܺ
ܸଶ ሺܸଶ െ ܸଷ ‫ݏ݋ܥ‬ሺߜሻሻ
ܴ݁ܽܿ‫ݓ݋݈݂ݎ݁ݓ݋݌݁ݒ݅ݐ‬ሺܲሻ ൌ ܺ
According to that, by adjusting magnitude of
ܸ௦ and phase angle of ‫ ׎‬, both active and reactive power
flows through transmission line can be controlled.
The main function of converter is to absorb or
supply real power to inverter circuit through dc link. It
can also absorb generated adjustable reactive power if it
can provide bidirectional operation. By disconnecting
from DC link and connecting to the DC source, converter
and inverter can operate independently. (i.e. as
STATCOM and SSSC)
III.SIMULATION MODEL OF UPFC AND TRANSMISSION
LINE
In order to develop Simulink model of the UPFC,
Total system have been sub divided in to several sub
systems and develop Simulink models. Sub systems are
simulated separately and integrated these subsystems as a
one system.
A. Transmission line model
Transmission line model is designed to test the operation
of the UPFC. As Fig.5 UPFC is connected in between
transmission line 2 and 3. By using this configuration,
active power and reactive power flow through the
transmission line 3 can be controlled.
In this Simulink model, different loads are connected
to switch from the system at different times. UPFC is to
be connected between bus 3 and bus 4 as Fig. 6.
B. Inverter & Converter circuits
Inverter circuit controls the magnitudeሺܸ௦ ሻand phase
angle ሺ‫׎‬ሻof the injecting voltage. By controlling the gate
signals of the IGBT, the series injecting voltage can be
controlled.
Converter circuit control the DC link voltage input to
the inverter circuit. Active power flow through converterinverter circuit can be controlled by controlling the gate
signals of the IGBTs.
Bus 2
Bus 1
Bus 3
Bus 4
Bus 5
Fig. 5. Demonstration model of UPFC
Fig.3.Single line diagram of the UPFC
Fig.4.Phasor diagram
Fig.6.Simulink Model of the Transmission Line Model
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International Conference on Innovations in Power and Advanced Computing Technologies [i-PACT2017]
B. Series controller
IV.CONTROLLING OF UPFC
In this model, it calculates the required voltage
magnitude and the phase angle of the injecting voltage
which need to maintain desired condition of the system.
D-Q control strategy is used to calculate this injecting
voltage.
A. Shunt controller
In here, Voltage at DC link (Vdc) and bus 3 voltage
(V3) are taken as inputs to shunt controller. In that
controller, first three phase voltage is converted to d-q
axis values. Vdc and V3 are used to calculate reference daxis current (Id) and q- axis current (Iq) values. Then
current regulator is used to calculate required voltage and
required phase angle. Fig .7 shows the Simulink model.
Voltage at bus 3 & 4(V3 & V4) and line current
flowing through the bus 4 (I4) are used as inputs in series
controller. Here also d-q axis control method is used. By
taking reference active and reactive power as inputs (Pref
and Qref), it calculates the reference d-axis and q-axis
current component (Idref and Iqref) which should be
maintained in the system.
PI controller is used to get required voltage
which should inject in series with line to maintain the
stability and quality of the power flow. Fig .8 shows the
Simulink model for the series controller with necessary
control blocks.
Fig 7: Simulink model of the shunt controller
Fig.8. Simulink model of series controller
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International Conference on Innovations in Power and Advanced Computing Technologies [i-PACT2017]
V.SIMULATION RESULTS AND ANALYSIS
Validity of the simulation is tested using the designed
transmission model under several circumstances to ensure
the reliability and better performance of the UPFC over
other power electronic devices for voltage compensation.
As shown in Fig.9, at t=0.2 s large reactive load is
parallelly connected to the transmission line before the
UPFC and followed by the another load connected at
t=0.4s after the UPFC while those are disconnected at
t=0.6s and t=0.7s respectively. Transmission line voltage is
observed under two scenarios where UPFC connected and
SSSC is connected. Same type of analysis has been done
using a comparative study of UPFC and shunt device
STATCOM which is shown in Fig.10.
In both cases, it is clearly seen that voltage suppression is
better represented by the UPFC device under the
perturbation of the loads connection and disconnection.
With the statical data shown in Table 2. this is further
proved as the standard deviation of the voltage get reduced
and the mean voltage gets enhanced with the replacement
of UPFC.
Voltage Variation
TABLE II.
STAT ICAL DATA OF SIMULATION
STANDARD
DEVIATION(KV)
MEAN VOLTAGE(KV)
UPFC
0.486
2.85
STATCOM
0.624
2.76
SSSC
0.711
2.66
Reactive Power Reference
Fig.11. Comparative analysis of reactive power reference of
UPFC and SSSC
Fig.11 depicts the variation of reactive power reference
signal generation of the control system of UPFC and
SSSC. When the reactive power reference of the
transmission line is set up to 0.5 at t=0.5 s UPFC controller
responds to it quickly where it shows the rising time of
0.1s only.
Fig.9. Comparative analysis of UPFC and SSSC
Voltage Variation
Fig.10. Comparative analysis of UPFC and STATCOM
In case of a large reactive load(30 Mvar) connected
transmission line, behavior of the system voltage with and
without UPFC is studied in Fig.12. At t=0.1 s UPFC is
bypassed where it shows serious voltage unitability of the
system and then at t-0.3s it is reconnected to the line,
where it shows responsive stability of voltage despite of
load switching at t=0.6s.
Voltage Variation
Fig.12. Disconnection and reconnection of UPFC
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International Conference on Innovations in Power and Advanced Computing Technologies [i-PACT2017]
VI.CONCLUSION
UPFC is one of the modern technology use in the world
to maintain the power quality. The concept is still in
developing stage and most of research are still in
simulation level. It uses modern power electronic
concepts and power system theories to achieve its
optimum operation in the system.
MATLAB/Simulink was used for simulation purpose
in this research. This paper provides a detailed discussion
about UPFC and its operation.
Addition to that,
importance of FACTS devices to increase the power
quality and reliability of power transmission is also
included here. Furthermore, the paper discussed about
control strategies of UPFC, transmission line model
design, series and shunt controller designs.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the support provided
by the Senate Research Council, University of Moratuwa
(SRC/LT/2015/04).
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