International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
Optimal Power Flow Control on Power System
Transmission Network using UPFC
1
France O. Akpojedje, 2Abel O. Olomo, 3Emmanuel C. Mormah, 4Ese M. Okah
1
Department of Electrical/ Electronic Engineering Technology, National Institute of Construction Technology,
Uromi, Nigeria.
2
Jobel Green Energy Ltd, Lagos, Nigeria.
3
Department of Electrical/ Electronic Engineering, Delta State Polytechnic, Ogwashi Uku, Nigeria.
4
Department of Electrical/ Electronic Engineering, Delta State Polytechnic, Oghara, Nigeria.
Abstract: The paper e x a m i n e s the performance of
a unified power flow controller ( U P F C ) o n t h e
electric power transmission network.
The UPFC performance was investigated in controlling
the flow of power over the transmission lines. Voltage
source model was utilized to study the behaviour of the
UPFC in regulating the active power, reactive power,
voltage profile of the system; and the UPFC was used to
relief power congestion of the transmission system. The
model employed was used to combined the equations of
UPFC and the balance power equations of the
network into one set of non-linear algebraic equations.
Case studies were carried out on five standards bus
network and the load flow option of the powergui block
was used; the model was initialized with plant 1 and 2
generating 500MW and 1000MW respectively, with the
simulation done in Matlab environment. The results of
the network with and without UPFC were compared in
terms of active and reactive power flows in the line, and
the active and reactive power flows at the buses were
used to analyzed the performance of the UPFC.
Keywords: UPFC, Voltage Source Model, Series and
Shunt Converter, Active and Reactive Power flow and
Power Transmission Network.
1.0 Introduction
The Nigeria Complex transmission network and the
distribution system supply the vast needs of electrical
power to their citizenry [1]. Due to the tremendous
power requirement, we must constantly be concerned
with the efficient operation of our power transmission
network and the associated system [1].
The power-transfer capability of long transmission lines
is usually limited by large signals ability [2]. Economic
factors, such as the high cost of long lines and revenue
from the delivery of additional power, give strong
incentives to explore all economically and technically
feasible means of raising the stability limit [2]. On the
other hand, the development of effective ways to use
transmission systems at their maximum thermal
capability has caught much research attention in recent
years. FLEXIBLE AC transmission system is an
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evolving technology based on solutions to help electric
utilities to fully utilize their transmission assets. Its first
concept was introduced by N.G Hingorani [3], in April
19, 1988. Since then different kinds of FACTS devices
have been proposed [3]. Amongst them the UPFC is the
most versatile and effective device which was
introduced in 1991 [4, 5]. The fast progression in the
field of power electronics has already started to
influence the power industries. This is one direct
outcome of the concept of flexible AC transmission
systems (FACTS) aspects, which has become feasible
due to the improvement realized in power-electronic
devices. In principle, the FACTS devices could provide
fast control of active and reactive power through a
transmission line [6]. The unified power-flow controller
(UPFC) is a member of the FACTS family with very
attractive features. This device can independently
control many parameters, the UPFC is a combination of
the properties of a static synchronous compensator
(STATCOM) and static synchronous series compensator
(SSSC) [6]. These devices offer an alternative mean to
mitigate power system oscillations. The UPFC consist of
voltage source converters, one connected in series and
other in shunt and both are connected back to back
through a D.C capacitor [5]. In order to investigate the
impact of UPFC on power systems effectively, it is
essential to formulate their correct and appropriate
model. In the area of power flow analysis, models of the
UPFC which treat the UPFC either as one series voltage
source and one shunt current source model or both the
series and the shunt are represented by voltage sources
[7,8,9,10,11].
1.1 Basic Principle of Power Compensation on
Transmission System
Figure 1.0 (a) shows the simplified model of a
power transmission system. Two power grids are
connected by a transmission line which is assumed
lossless and represented by the reactance XL. 1 ∠δ 1v
and V2 ∠δ represent the voltage phasor of the two
power grid buses with angle δ= δ1- δ2 between the
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
two. The corresponding phasor diagram is shown in
Figure 1.0 (b).
controlling the voltages, phase angles and line
impedance of the transmission system. From the
power angle curve shown in Figure 2 (c), the
active power flow will reach the maximum when the
phase angle δ is 90º. In practice, a small angle is
used to keep the system stable from the transient and
dynamic oscillations [13]. Generally,
the
compensation of transmission systems can be
divided into two main groups: shunt and series
compensation.
Figure 1.0 Power transmission system: (a) simplified
model; (b) phase diagram [12].
1.2 Shunt Compensation
Shunt compensation especially shunts reactive
compensation has been widely used in transmission
system to regulate the voltage magnitude, improve the
voltage quality, and enhance the system stability
[12]. Shunt-connected reactors are used to reduce
the line over-voltages by consuming the reactive
power while shunt-connected capacitors are used to
maintain the voltage levels by compensating the
reactive power to the transmission line [15].
A simplified model of a transmission system with
shunt compensation is shown in Figure 2 (a). The
voltage magnitudes of the two buses are assumed
equal as V, and the phase angle between them is δ.
The transmission line is assumed lossless and
represented by the reactance XL. At the midpoint of
the transmission line, a controlled capacitor C is
shunt- connected. The voltage magnitude at the
connection point is maintained as V [15].
The magnitude of the current in the transmission line is
given by:
(1)
The active and reactive components of the current flow
at bus 1 are given by:
(2)
The active power and reactive power at bus 1 are given
by:
(3)
Similarly, the active and reactive components of the
current flow at bus 2 is given by:
(4)
The active power and reactive power at bus 2 are given
by:
Figure 2 : Transmission system with shunt
compensation: (a) simplified model; (b) phase
diagram; (c) power-angle curve [14].
(5)
Equations (1) through (5) indicate that the active and
reactive power/current flow can be regulated by
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As discussed previously, the active powers at bus 1 and
bus 2 are equal [15].
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
(6)
T h e i njected reactive power by the capacitor to
regulate the voltage at the mid-point of the
transmission line is calculated as:
(7)
From the power angle curve shown in Figure 2 (c),
the transmitted power can be significantly increased,
and the peak point shifts from δ=90º to δ=180º. The
operation margin and the system stability are
increased by the shunt compensation. The voltage
support function of the midpoint compensation can
easily be extended to the voltage support at the end of
the radial transmission, which will be proven by the
system simplification analysis in a later section of this
paper. The reactive power compensation at the end of
the radial line is especially effective in enhancing
voltage stability.
1.3
Series Compensation
Series compensation aims to directly control
the overall series line impedance of the transmission
line [15]. Tracking back to Equations (1) through (5),
the AC power transmission is primarily limited by
the series reactive impedance of the transmission
line [15]. A series-connected can add a voltage in
opposition to the transmission line voltage drop,
thereby reducing the series line impedance [15].
A simplified model of a transmission system with
series compensation is shown in Figure 3(a) [15]. The
voltage magnitudes of the two buses are assumed
equal as V, and the phase angle between them is δ
[15]. The transmission line is assumed lossless and
represented by the reactance XL.
A controlled
capacitor is series-connected in the transmission line
with voltage addition Vinj. The phase diagram is
shown in Figure 3(b) [15].
Figure 3: Transmission system with series
compensation: (a) simplified model; (b) phase
diagram; (c) power-angle curve [14]
Defining the capacitance C as a portion of the line
reactance,
X C = kX L
(8)
The overall series inductance of the transmission line is,
(9)
The active power transmitted is,
(10)
The reactive power supplied by the capacitor is
calculated as:
(11)
Figure 3(c) shows the power angle curve from
which it can be seen that the transmitted active
power increases with k.
1.4
The Operating Principle of UPFC
A simplified scheme of a UPFC connected to an infinite
bus via a transmission line is shown in Figure 4. UPFC
consists of parallel and series branches, each one
containing a transformer, power-electric converter with
turn-off capable semiconductor devices and DC circuit.
Inverter 2 is connected in series with the transmission
line by series transformer. The real and reactive power
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
in the transmission line can be quickly regulated by
changing the magnitude and phase angle of the injected
voltage produced by inverter 2. The basic function of
inverter 1 is to supply the real power demanded by
inverter 2 through the common DC link. Inverter 1 can
also generate or absorb controllable power [12, 4]. An
attempt is made in this paper to simulate and implement
UPFC system.
(14)
Where
and
The element of transfer admittance matrix can be put as:
(15)
Figure 4: UPFC Installed in a Transmission Line[16]
1.6
Mathematical Modeling of UPFC
Figure 5: Voltage Source Model of UPFC [17].
The two ideal voltage sources of the UPFC can be
mathematically modeled as:
The UPFC converters are assumed lossless in this
voltage sources model. This implies that there is no
absorption or generation of active power by the two
converters for its losses and the active power demanded
by the series converter at its output is supplied from the
AC Power system by the shunt converters via the
common D.C link. The DC link capacitor voltage Vdc
remains constant. Hence the active power supplied to the
shunt converter Psh must be equal to the active power
demanded by the series converter Pse at the DC link.
Then the following equality constraint has to be
guaranteed.
(16)
From Figure 5 and by equation (11), (12), (13) for the
series and shunt sources the power equations of UPFC
can be written
(12)
(13)
UPFC is connected between two buses k and m in the
power system. Applying the Kirchhoff‟s current and
voltage laws for the network in Figure 4 gives:
(17)
(18)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
1.5
Test System and Implementation
Figure 6: A Single-line diagram of the modeled
electric power transmission network [16]
The UPFC is used to control the power flow on a
500 kV /230 kV transmission network or system.
The system connected in a loop configuration,
consists of essentially five buses (B1 to B5)
interconnected through transmission lines (L1, L2,
L3) and two 500 kV/230kV transformer banks T1
and T2 [16]. Two power plants located on the 230kV
system generate a total of 1500MW which is
transmitted to a 500kV, 15000MVA equivalent and
to a 200MW load connected at bus B3 [16]. The plant
models include a speed regulator, an excitation
system as well as a power system stabilizer (PSS). In
normal operation, most of the 1200MW generation
capacity of power plant 2 is exported to the 500kV
equivalent through three 400MVA transformers
connected between buses B4 and B5 [16]. For this
study we are considering a contingency case where
only two transformers out of three are available (T2=
2*400MVA = 800 MVA). Figure 7 show the
Matlab/simulink model of the test system.
Using the load flow option of the powergui block, the
model is initialized with plants 1 and 2 generating
500MW and 1000MW respectively and the UPFC
out of service. The resulting power flow obtained at
buses B1 to B5 is indicated by red numbers on the
circuit diagram in figure 7 below. The load flow
shows that most of the power generated by plant 2 is
transmitted through the 800MVA transformer bank
(899MW out of 1000MW), the rest (101MW),
circulating in the loop. Transformer T2 is therefore
overloaded by 99 MVA. The study illustrates how the
UPFC can relieve this power congestion.
Figure 7: The Matlab/Simulink Model of the Test System
The UPFC located at the right end of line L2 is used
to control the active and reactive powers at the
500kV bus B3, as well as the voltage at bus
B_UPFC. It consists of a phasor model of two
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100MVA, IGBT-based, converters (one connected in
shunt and one connected in series and both
interconnected through a DC bus on the DC side and
to the AC power system, through coupling reactors
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
and transformers). The series converter can inject a
maximum of 10% of nominal line-to-ground voltage
(28.87 kV) in series with line L2. The blue numbers
on the diagram show the power flow with the UPFC
in service and controlling the B3 active and reactive
powers respectively at 687MW and -27Mvar.
4.0
Results Analysis
The UPFC reference active and reactive power are
set in the modeled blocks Pref (pu) and Qref (pu).
Initially the Bypass breaker is closed and the
resulting natural power flow at bus B3 is 587MW and
-27Mvar. The Pref block is programmed with an
initial active power of 5.87pu corresponding to the
natural flow. Then, at t = 10s, Pref is increased by 1pu
(100MW), from 5.87pu to 6.87pu, while Qref is kept
constant at -0.27pu as shown in Figure 8 and Figure 9
respectively. The P and Q measured at bus B3 follow
the reference values as shown in Figure 13 and
Figure 14. At t = 5s, when the Bypass breaker is
opened the natural power is diverted from the Bypass
breaker to the UPFC series branch without noticeable
transient. At t =10s, the power increases at a rate of 1
pu/s. It takes one second for the power to increase to
687MW as seen in Figure 8. The 100MW increase of
active power at bus B3 is achieved by injecting a
series voltage of 0.089pu with an angle of 94 degrees,
this is shown in Figure 10 and Figure 11 respectively.
This resulted in an approximate 100MW decrease in
the active power flowing through T2 (from 899MW
to 796MW), which now carries an acceptable load.
See the variations of active powers at buses B1 to B5
on the VPQ Lines scope, Figure 12 show the Bus
voltages. The UPFC P and Q signals vary according
to the changing phase of the injected voltage. The
trajectory of the UPFC reactive power as a function
of its active power, measured at bus B3 is shown in
Figure 15. The area located inside represents the
UPFC controllable region.
TABLE I: Bus Voltages and Power With and
Without UPFC
Bus
No.
1
2
3
4
5
Bus
Voltage
Without
UPFC
Voltages
( pu)
Bus
Voltage
With
UPFC
Voltages
( pu)
Bus
Power
Without
UPFC
Bus Power
With UPFC
P
(MW)
0.9966
0.9993
0.9996
0.9926
0.9978
0.9967
1.0020
1.0010
0.9942
0.9978
95
589
587
899
1279
P
(M
W)
197
690
687
796
127
7
Q
(Mv
ar)
-16
-64
-27
27
106
Figure 8: Pref Signal
Figure 9: Qref Signal
Figure 10: Injected Series Voltage
Q
(Mv
ar)
-30
-94
-27
16
-89
Figure 11: Injected Series Voltage Phase Angle
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International Journal of Engineering Trends and Technology (IJETT) – Volume 33 Number 3- March 2016
Q (pu)
Figure 12: Bus Bar Voltages
Figure 15: UPFC Controllable Region
Figure 13: Bus bar Active Power
5.0
Conclusion
The simulation study carried out in matlab/simulink
P (pu)
environment was used to simulate the model
of
UPFC connected to a 3 phase system. This study
presents the control and performance of the UPFC
used for optimal power flow in a power transmission
network and voltage compensation was done using
UPFC as a studied as well. The real and reactive
power increase with the increase in angle of injection.
The simulation results shows the effectiveness of
UPFC to control the real and reactive power in the
system. It is found that there is an improvement in the
real and reactive power through the transmission
lines when UPFC is introduced. The UPFC system
has the advantages to reduced maintenance and the
ability to control real and reactive power in the
network.
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Figure 14:Bus Bar Reactive Power
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