Download study the power flow control of a power system with unified power

STUDY THE POWER FLOW CONTROL OF A
POWER SYSTEM WITH UNIFIED
POWER FLOW CONTROLLER
SATYENDRA KUMAR*, ARVIND KUMAR SINGH** AND
UPENDRA PRASAD***
Abstract: Electrical power systems is a large interconnected network that
requires a careful design to maintain the system with continuous power flow
operation without any limitations. Flexible Alternating Current Transmission
System (FACTS) is an application of a power electronics device to control the
power flow and to improve the system stability of a power system. Unified Power
Flow Controller (UPFC) is a versatile device in the FACTS family of controllers
which has the ability to simultaneously control all the transmission parameters
of power systems i.e. voltage, impedance and phase angle which determines the
power flow of a transmission line.
1. INTRODUCTION
The technology of power system utilities around the world has rapidly
evolved with considerable changes in the technology along with
improvements in power system structures and operation. The ongoing
expansions and growth in the technology, demand a more optimal
and profitable operation of a power system with respect to generation,
transmission and distribution systems [1].
In the present scenario, most of the power systems in the developing
countries with large interconnected networks share the generation
reserves to increase the reliability of the power system. However, the
increasing complexities of large interconnected networks had
fluctuations in reliability of power supply, which resulted in system
*
**
***
Asst. Professor, EEE Dept., Gurunanadev Engg. College, Bidar, Karnataka 58540,
(E-mail: [email protected])
Elect. Dept., Nerist, Nirjuli, Itanagar, Arnachal Pradesh
Professor, Elect. Engg. Dept., Bit Sindri, Dhanbad, Jharkhand
IJPE, 4:1 (2012): 1-11
Research Science Press, New Delhi, India
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instability, difficult to control the power flow and security problems
that resulted large number blackouts in different parts of the world.
The reasons behind the above fault sequences may be due to the
systematical errors in planning and operation, weak interconnection
of the power system, lack of maintenance or due to overload of the
network [2].
In order to overcome these consequences and to provide the desired
power flow along with system stability and reliability, installations of
new transmission lines are required. However, installation of new
transmission lines with the large interconnected power system are
limited to some of the factors like economic cost, environment related
issues. These complexities in installing new transmission lines in a
power system challenges the power engineers to research on the ways
to increase the power flow with the existing transmission line without
reduction in system stability and security.
In this research process, in the late 1980’s the Electric Power
Research Institute (EPRI) introduced a concept of technology to
improve the power flow, improve the system stability and reliability
with the existing power systems. This technology of power electronic
devices is termed as Flexible Alternating Current Transmission Systems
(FACTS) technology. It provides the ability to increase the controllability
and to improve the transmission system operation in terms of power
flow, stability limits with advanced control technologies in the existing
power systems [3, 4].
The main objective to introduce FACTS Technology is as follows:
• To increase the power transfer capability of a transmission
network in a power system.
• To provide the direct control of power flow over designated
transmission routes.
• To provide secure loading of a transmission lines near the
thermal limits.
• To improve the damping of oscillations as this can threaten
security or limit usage line capacity [5].
FACTS technology is not a single power electronic device but a
collection of controllers that are applied individually or in
coordination with other devices to control one or more interrelated
power system parameters such as series impedance, shunt impedance,
current, voltage and damping of oscillations. These controllers were
designed based on the concept of FACTS technology known as FACTS
Controllers [5].
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FACTS controllers are advanced in relation to mechanical control
switched systems that are controlled with ease. They have the ability
to control the power flow and improve the performance of the power
system without changing the topology. Since 1980s, a number of
different FACTS controllers with advanced control techniques proposed
as per the demand of the power systems [5].
Unified Power Flow Controller (UPFC) is one among the different
FACTS controllers introduced to improve the power flow control with
stability and reliability. It is the most versatile device introduced in
early 1990s designed based on the concept of combined series-shunt
FACTS Controller. It has the ability to simultaneously control all the
transmission parameters affecting the power flow of a transmission
line i.e. voltage, line impedance and phase angle [2].
Aim of the Paper: In this Paper, I considered a case study network
of a power system with Unified Power Flow Controller (UPFC). The
power flow equations derived for the network solved using the
Newton-Raphson Algorithm and the simulations of the algorithm
carried out in MATLAB.
2. THE UNIFIED POWER FLOW CONTROLLER
Gyugyi in 1991 proposed the Unified Power Flow Controller. It is the
most versatile and complex power electronic device and member of
third generation FACTS Controller introduced to control the power
flow and voltage in the power systems. It is designed by combining
the features of second-generation FACTS controllers–Series
Synchronous Compensator (SSSC) and Static Synchronous
Compensator (STATCOM). It has the ability to control active and
reactive power flow of a transmission line simultaneously in addition
to controlling all the transmission parameters (voltage, impedance and
phase angle) affecting the power flow in a transmission line.
Figure 1: Unified Power Flow Controller [26]
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2. 1. UPFC Circuit Description
The above Figure 1 taken from reference [26] gives a clear description
about how UPFC controller connected to a transmission line. It
consists of two back-to-back self-commutated voltage source
converters - one converter at the sending end is connected in shunt
as shunt converter and the other converter connected in between
sending and receiving end bus in series as series converter. One end
of the both the converters are connected to a power system through
an appropriate transformer and other end connected with a common
DC capacitor link [26].
2.2. Operation of UPFC
This arrangement of UPFC ideally works as a ideal ac to dc power
converter in which real power can freely flow in either direction
between ac terminals of the two converters and each converter can
independently generate or absorb reactive power at its own AC output
terminal. The main functionality of UPFC provided by shunt converter
by injecting an ac voltage considered as a synchronous ac voltage source
with controllable phase angle and magnitude in series with the line.
The transmission line current flowing through this voltage source
results in real and reactive power exchange between it and the AC
transmission system. The inverter converts the real power exchanged
at ac terminals into dc power which appears at the dc link as positive
or negative real power demand [3].
2.3. Operation of Two Converters
Series converter Operation: In the series converter, the voltage injected
can be determined in different modes of operation: direct voltage
injection mode, phase angle shift emulation mode, Line impedance
emulation mode and automatic power flow control mode. Although
there are different operating modes to obtain the voltage, usually the
series converter operates in automatic power flow control mode where
the reference input values of P and Q maintain on the transmission
line despite the system changes [3].
Shunt converter operation: The shunt converter operated in such a
way to demand the dc terminal power (positive or negative) from the
line keeping the voltage across the storage capacitor Vdc constant.
Shunt converter operates in two modes: VAR Control mode and
Automatic Voltage Control mode. Typically, Shunt converter in UPFC
operates in Automatic voltage control mode [3].
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2.4. Equivalent Circuit Operation of UPFC
As shown in Figure 2, the two-voltage source converters of UPFC can
modeled as two ideal voltage sources one connected in series and other
in shunt between the two buses. The output of series voltage magnitude
Vse controlled between the limits Vse max ≤ Vse ≤ Vse min and the angle θse
between the limits 0 ≤ θ se ≤ 2∏ respectively. The shunt voltage
magnitude Vsh controlled between the limits Vsh max ≤ Vsh ≤ Vsh min and the
angle between 0 ≤ θsh ≤ 2∏ respectively. Zse and Zsh are considered as
the impedances of the two transformers one connected in series and
other in shunt between the transmission line and the UPFC as shown
in the Figure 2 which is the UPFC equivalent circuit [11].
Figure 2: Equivalent Circuit of UPFC [28]
as
The ideal series and voltage source from the Figure 2 can written
Vse = V/ se (cos θse + j sin θse )
(1)
Vsh = Vsh (cos θsh + j sin θsh )
(2)
The magnitude and the angle of the converter output voltage used
to control the power flow mode and voltage at the nodes as follows:
(1) The bus voltage magnitude can be controlled by the injected a
series voltage Vse in phase or anti-phase.
(2) Power flow as a series reactive compensation controlled by
injecting a series voltage V′se in quadrature to the line current.
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(3) Power flow as phase shifter controlled by injecting a
series voltage of magnitude V″se in quadrature to node voltage
θm [28].
UPFC power Equations
Based on the equivalent circuit as shown in Figure 2, the active
and reactive power equations can be written as follows [27, 7]:
At node k:
Pk = V 2 k Gkk + Vk Vm (Gkm cos(θk − θm ) + Bkm sin(θk − θm ))
+Vk Vse(Gkm cos(θk − θ se ) + Bkm sin(θk − θ se ))
(3)
+Vk Vsh (Gsh cos(θk − θsh ) + Bsh sin(θk − θsh ))
Qk = −V 2 k Bkk + Vk Vm (Gkm sin(θk − θm ) − Bkm cos(θk − θm ))
+Vk Vse (Gkm sin(θk − θse ) − Bkm cos(θk − θse ))
(4)
+Vk Vsh (Gsh sin(θk − θsh ) − Bsh cos(θk − θsh ))
At node m:
Pm = V 2 m Gmm + VmVk (Gmk cos(θm − θk ) + Bmk sin(θm − θk ))
+VmVse (Gmm cos(θm − θse ) + Bmm sin(θm − θse ))
Qm = −V 2 m Bmm + VmVk (Gmk sin(θm − θk ) − Bmk cos(θm − θk ))
+VmVsh (Gmm sin(θm − θse ) − Bmm cos(θm − θse ))
(5)
(6)
Series converter:
Pse = V 2 se Gmm + VseVk (Gkm cos(θse − θk ) + Bkm sin(θ se −θk ))
+VseVm (Gmm cos(θse − θk ) + Bmm sin(θse − θm )
(7)
Qse = −V 2 se Bmm + VseVk (Gkm sin(θse − θk ) − Bkm cos(θse − θk ))
+VseVm (Gmm sin(θse − θm ) − Bmm cos(θse − θm ))
Shunt converter:
Psh = −V 2 sh Gsh + VshVk (Gsh cos(θ sh − θk ) + Bsh sin(θ sh − θk )
(8)
Qsh = V 2 sh Bsh + VshVk (Gsh sin(θ sh − θk ) − Bsh cos(θ sh − θk ))
(9)
Where
Ykk = Gkk + jBkk = Z −1 se + Z −1 sh
(10)
Ymm = Gmm + jBmm = Z −1se
(11)
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Ykm = Ymk = Gkm + jBkm = − Z −1se
(12)
Ysh = Gsh + jBsh = − Z −1 sh
(13)
Assuming a free converter loss operation, the active power supplied
to the shunt converter Psh equals to the active power demanded by the
series converter Pse [10].
(14)
Pse + Psh = 0
Furthermore if the coupling transformers are assumed to contain
no resistance then the active power at bus k matches the active power
at bus m; that is,
Psh + Pse = Pk + Pm = 0
(15)
The UPFC power equations linearised and combined with the
equations of the AC transmission network. For the cases when the UPFC
controls the following parameters:
(1) voltage magnitude at the shunt converter terminal
(2) active power flow from bus m to bus k and
(3) reactive power injected at bus m, and taking bus m to be PQ bus.
3. NEWTON RAPHSON ALGORITHM AND FLOW CHART
From the mathematical modeling point of view, the set of nonlinear,
algebraic equations that describe the electrical power network under
the steady state conditions are solved for the power flow solutions.
Over the years, several approaches have been put forward to solve for
the power flow equations. Early approaches were based on the loop
equations and methods using Gauss-type solutions. This method was
laborious because the network loops has to be specified by hand by
the systems engineer. The drawback of these algorithms is that they
exhibit poor convergence characteristics when applied to the solution
of the networks. To overcome such limitations, the Newton-Raphson
method and derived formulations were developed in the early 1970s
and since then it became firmly established throughout the power
system industry [7].
In this Paper a Newton Raphson power flow algorithm is used to
solve for the power flow problem in a transmission line with UPFC as
shown in the flow chart in Figure 3 [18].
3.1. Steps to Solve the Newton-Raphson Algorithm
Step 1: Read the input of the system data that includes the data needed
for conventional power flow calculation i.e. the number and types of
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buses, transmission line data, generation, load data and location of
UPFC and the control variables of UPFC i.e. the magnitude and angles
of output voltage series and shunt converters.
Step 2: Formation of admittance matrix Ybus of the transmission line
between the bus i and j.
Step 3: Combining the UPFC power equations with network
equation, we get the conventional power flow equation:
n
Pi + jQi =
∑VV Y ∠(θ
i
j =1
j
ij
ij
− δi + δ j ) + P'i + jQ'i
(16)
Where
P′i + Q’i = active and reactive power flow due to UPFC between the
two buses.
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Figure 3: Flow Chart for load flow by Newton Raphson with UPFC [18]
P′i + jQ’i Active and reactive power flow at the ith bus.
Vi ∠ δi Voltage and angle of ith bus
Vj ∠ δj = Voltage and angle at ith bus
Step 5: The conventional jacobian matrix are formed (P ki and Qki)
due to the inclusion of UPFC. The inclusion of these variables increases
the dimensions of the jacobian matrix.
Step 6: In this step, the jacobian matrix is modified and power
equations are mismatched (∆Pki , ∆Qki for i = 2, 3,…, m and ∆Pki i , ∆Qki i ).
Step 7: The busbar voltages are updated at each iteration and
convergence is checked.
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If convergence is not achieved in the next step the algorithm goes
back to the step 6 and the jacobian matrix is modified and the power
equations are mismatched until convergence is attained.
Step 8: If the convergence achieved in Step 7, the output load flow
is calculated for PQ bus that includes the Busbar voltages, generation,
transmission line flow and losses.
REFERENCES
[1] R. Billinton, L. Salvaderi, J. D. McCalley, H. Chao, Th. Seitz, R.N. Allan, J.
Odom, C. Fallon, “ Reliability Issues In Today’s Electric Power Utility
Environment”, IEEE Transactions on Power Systems, Vol. 12, No. 4, November
1997.
[2] Jinfu Chen, Xinghua Wang, Xianzhong Duan, Daguang Wang, Ronglin Zhang,
“Application of FACTS Devices for the Interconnected Line Between Fujian
Network and Huadong Network”, IEEE.
[3] S. Tara Kalyani, G. Tulasiram Das, “Simulation of Real and Reactive Power
Flow Control With UPFC connected to a Transmission Line”, Journal of
Theoretical and Applied Information Technology, 2008.
[4] S. Y. Ge, T S Chung, “ Optimal Active Power Flow Incorporating Power Flow
Control Needs In Flexible AC Transmission Systems”, IEEE Transactions on
Power Systems, Vol. 14, No. 2, 2009.
[5] K. R. Padiyar, A. M. Kulkarni, “Flexible AC Transmission Systems: A Status
Review”, Sadhana, Vol. 22, Part 6, pp. 781-796, December 1997.
[6] John J. Paserba, “How FACTS Controllers Benefit AC Transmission Systems”,
IEEE.
[7] N. G. Hingorani G. Gyugyi Lazlo, “Understanding FACTS: Concepts &
Technology of Flexible AC Transmission Systems” ISBN 0-7803-3455-8.
[8] Nadarajah Muthulananthan, Arthit Sode-yome, Mr. Naresh Acharya,
“Application of FACTS Controllers in Thailand Power Systems”, Asian
Institute of Technology, Jan 2005.
[9] Jody Verboomen, Dirk Van Hertem, Pieter H. Schavemaker, Wil L. Kling,
Ronnie Belmans, “ Phase Shifting Transfomers: Princples and Application”,
IEEE.
[10] M. P. Bahrman, P.E., “ HVDC Transmission Overview”, IEEE.
[11] W. Breuer, D. Povh, D. Retxmann, E. Teltsch, “ Trends for Future HVDC
Applications”, 16th Conference of Electric Power Supply Industry, November
2006.
[12] Rajiv K. Varma, “Introduction to FACTS Controllers”, Member, IEEE.
[13] M. Noroozian, C. W. Taylor, “Benefits of SVC and STATCOM for Electric
Utility Application”.
[14] Mark Ndubuka NWOHU, “ Voltage Stability Improvement using Static
Var Compensator in Power Systems”, Leonardo Journal of Sciences, Issue 14,
p. 167-172, January-June 2009.
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[15] Sidhartha Panda and N. P. Padhy, “Thyristor Controlled Series Compensatorbased Controller Design Employing Genetic Algorithm: A Comparative
Study”, International Journal of Electronics, Vol. 1.
[16] Mazilah Binti A Rahman, “Overview of Thyristor Controlled Series Capacitor
(TCSC) In Power Transmission System”.
[17] Laszlo Gyugyi, Colin D. Schauder, Kalyan K. Sen, “ Static Synchronous Series
Compensator: A Solid-State Approach To The Series Compensation of
Transmission Lines”, IEEE Transaction on Power Delivery, Vol.12, January
1997.
[18] Nitus Voraphonpiput, Teratam Bunyagul and Somchai Chatratana, “ Power
Flow Control with Static Synchronous Series Compensator (SSSC)”.
[19] M. Noroozian, C. W. Taylor, “Benefits of SVC and STATCOM for Electric
Utility Application”.
[20] Kalyan K. Sen, “STATCOM – Static Synchronous Compensator: Theory,
Modelling and Applications”.
[21] Rusejla Sadikovic, “Power flow Control with UPFC”.
[22] X.P. Zhang, “Robust Modeling of the Interline Power Flow Controller and
the Generalized Unified Power Flow Controller with Small Impedances in
Power Flow Analysis”, Electrical Engineering, Vol. 89, pp. 1-9, 2006.
[23] Yankui Zhang, Yan Zhang and Chen Chen, “A Novel Power Injection Model
of IPFC for Power Flow Analysis Inclusive of Practical Constraints”, IEEE
Transactions on Power Systems, Vol. 21, November 2006.
[24] Bhanu Chennapragada Venkata Krishna, Kotamarti S. B. Sankar, Pindiprolu.
V. Haranath, “Power System Operation and Control Using FACT Devices”,
17th International Conference on Electricity Distribution, 12-15 May 2003.
[25] Bhanu Chennapragada Venkata Krishna, Kotamarti S. B. Sankar, Pindiprolu.
V. Haranath, “ Power System Operation and Control Using FACT Devices”,
17th International Conference on Electricity Distribution, 12-15 May 2003.
[26] C. Bulac, M. Eremaia, R. Balaurescu and V. Stefanescu, “Load Flow
Management in the Interconnected Power Systems Using UPFC Devices” ,
2003 IEEE Bologna Power Tech Conference, Bologna, Italy, June 23th-26 th.
[27] Samina Elyas Mubeen, R. K. Nema and Gayatri Agnihotri, “Power Flow
Control with UPFC in Power Transmission System”, World Academy of
Science, 2008.
[28] Felix F. Wu, “Technical Considerations For Power Grid Interconnection in
Northeast Asia”.