International Journal of Advances in Engineering, 2015, 1(3), 276 – 285
ISSN: 2394-9260 (printed version); ISSN: 2394-9279 (online version); url:http://www.ijae.in
RESEARCH ARTICLE Multimode Power Controllers For Three-Phase Matrix Converters
Operating As Unified Power Flow Controllers (UPFC) N. Arun and Shanmugathai
Dept. Electrical and Electronics Engineering, Thiruvalluvar College of Engineering and Technology, India.
arunpyr@gmail.com
Received 26 February 2015 / Accepted 21 March 2015
Abstract: This project presents the design and compares the performance of linear, decoupled and direct power controllers (DPC)
for three-phase matrix converters operating as Unified power flow controllers (UPFC). A simplified steady-state model of the
matrix converter-based UPFC fitted with a modified Venturini high- frequency pulse width modulator is first used to design the
linear controllers for the transmission line active (P ) and reactive (Q) powers. In order to minimize the resulting cross coupling
between P and Q power controllers, decoupled linear controllers (DLC) are synthesized using inverse dynamics linearization.
DPC are then developed using sliding-mode control techniques, in order to guarantee both robustness and decoupled control.
The designed P and Q power controllers are compared using simulations and experimental results. Linear controllers show
acceptable steady- state behavior but still exhibit coupling between P and Q powers in transient operation. DLC are free from
cross coupling but are parameter sensitive. Results obtained by DPC show decoupled power control with zero error tracking and
faster responses with no overshoot and no steady-state error.
I.INTRODUCTION
The demand of efficient and high quality power is escalating in the world of electricity. Today’s power systems are
highly complex and require suitable design of new effective and reliable devices in deregulated electric power industry
for flexible power flow control. In the late 1980s, the Electric Power Research Institute (EPRI) introduces a new
approach to solve the problem of designing, controlling and operating power systems: the proposed concept is known
as Flexible AC Transmission Systems (FACTS). It is reckoned conceptually a target for long term development to offer
new opportunities for controlling power in addition to enhance the capacity of present as well as new lines in the
coming decades. Its main objectives are to increase power transmission capability, voltage control, voltage stability
enhancement and power system stability improvement. Since then different kind of FACTS controllers have been
recommended. FACTS controllers are based on voltage source converters and includes devices such as Static Var
Compensators (SVCs), static Synchronous Compensators (STATCOMs), Thyristor Controlled Series Compensators
(TCSCs), Static Synchronous Series Compensators (SSSCs) and Unified Power Flow Controllers (UPFCs).Among
them UPFC is the most versatile and efficient device which was introduced in 1991. In UPFC, the transmitted power
can be controlled by changing three parameters namely transmission magnitude voltage, impedance and phase angle.
Unified Power Flow Controller (UPFC) is the most promising version of FACTS devices as it serves to control
simultaneously all three parameters (voltage, impedance and phase angle) at the same time. Therefore it is chosen as
the focus of investigation. For the last few years, the focus of research in the FACTS area is mainly on UPFC. Many
researchers have proposed different approaches of installing UPFC in power systems. The concepts of characteristics
have been broadly reported in the literature. The UPFC has been researched broadly and many research articles dealing
with UPFC modeling, analysis, control and application have been published in the recent years. Mathematical models
of UPFC have been developed to study steady state characteristics using state space calculations without considering
the effects of converters and the dynamics of generator. The performance of UPFC has been reported by designing a
series converter with conventional controllers. Many power converter topologies have been proposed for the
implementation of FACTS devices such as multi pulse converter like 24 pulses and 48 pulses and multi level inverters.
The advantages and limitations of high power converters have been discussed. In the dynamic control of UPFC has
been analyzed with six pulse converter using switching level model. Their proposed technique aims at to control the
real and reactive power flow in the transmission lines, by effectively changing the firing angle of shunt converter and
modulation index of the series converter. Unified Power Flow Controllers (UPFC) are the most versatile and powerful
flexible ac transmission system allowing a fast dynamic performance of the power transmission network, while
providing reliable and precise control of both line active P and reactive Q power flow. The UPFC technology allows
the operation of power transmission networks near their maximum ratings by enforcing power flow through welldefined high voltage line corridors, without violating the transmission line thermal limits, and improving network
transient stability. The UPFC concept was proposed in the nineties by Gyugyi, consisting of two ac–dc converters, back
to back connected through a common dc link using large high-voltage dc storage capacitors. The ac side of each
converter is connected to the transmission line through a coupling transformer, with a shunt connection in one side and
series connection in the other, allowing bidirectional power flow control. The dc capacitor bank used in the UPFC
topology to link the two back-to-back converters increases the UPFC weight, cost, volume and introduces additional
losses. Replacing these two converters by a direct three-phase ac–ac matrix converter (MC) eliminates the dc-link
277 Int. J. Adv. Eng., 2015, 1(3), 276-285 capacitors, thus reducing costs, size, maintenance, while increasing reliability and lifetime. The ac–ac MC processes
electrical energy directly without large storage needs, allows bidirectional power flow, guarantees near sinusoidal input
and output currents, and controls the voltages amplitude and frequency at adjustable power factor. In the dependence
of the MC output voltage with the modulation coefficient was investigated, being the UPFC able to control an extended
range of power flow with lower power converters, usually sized for 10% of the line power .
Figure.1 Transmission network with MC-UPFC.
Usually, UPFC use conventional control methods based on power systems linearized models, valid around an operating
point. These approaches do not guarantee robustness and may give origin to poor dynamic responses and/or undesired
instability while nonlinear approaches allow a better tuning of linear controller parameters. Nonlinear robust direct
power controllers (DPC) allow the dynamic response improvement based on sliding-mode control techniques to select
adequate state-space vectors to control active and reactive powers in real time. In this project a matrix converter-based
UPFC (MC-UPFC) for power transmission networks is used to control the P and Q power flow in a transmission line.
Three different types of controllers are designed and tested:
ƒ
ƒ
ƒ
Proportional integral (PI) linear controllers obtained from a linear P and Q steady-state power UPFC
linearized model, around an operating point, using a modified Venturing high frequency MC pulse width
modulator (PWM).
Decoupled linear controllers (DLC) designed by inverse dynamics linearization, allowing the elimination of
the cross coupling effect between P and Q power controllers.
DPC, which have been successfully used in power applications, owing to its simplicity and good performance.
This control method, based on sliding-mode control techniques allows real-time selection of adequate statespace vectors to control input and output variables. These controllers are insensitive to power system
nonlinearity, presenting robust behaviour to parameter variations and disturbances.
Mathematical Modeling of UPFC
In this model, we have considered the UPFC is placed at the centre of a 300km transmission line. This model was
derived with to study the relationship between electrical transmission system and UPFC in steady state conditions. The
basic scheme is shown in fig.3 [26]. A UPFC can be represented by two voltage sources representing fundamental
components of output voltage waveforms of the two converters and impedances being leakage reactance of the two
coupling transformers Based on the basic principle of UPFC and network theory, the active and reactive power flows in
the line, from bus-i to bus-j.
Figure.2 Equivalent circuit of UPFC
278 Int. J. Adv. Eng., 2015, 1(3), 276-285 Injection Model of UPFC: To obtain an injection model for UPFC, it is necessary to consider the series voltage source,
Figure1.3.
Figure.3 Representation of the series connected voltage source
The injection model is obtained by replacing the voltage source V se by a current source Iinj = Vjbse in parallel with
xs, Figure 3.
Figure.4 Transformed series voltage source
The apparent power supplied by the series voltage source converter is calculated from:
Figure.5 Injection model of the series part of the UPFC
Active and reactive power supplied by Converter 2 are distinguished as Afterwards, the series voltage source is
coupled with the shunt part of the UPFC, which can be modeled as a separate controllable shunt reactive source. Here
it is assumed that QCONV 1 = 0, but to allow for QCONV 1 6= 0 in the model is straight forward. Consequently, the
UPFC injection model is constructed from series connected voltage source model with the addition of power
equivalent to PCONV 1 + j0 to node i. The UPFC injection model is shown in Figure 6.
Figure.6 Injection model of the UPFC
Where r and q are the control variables of the UPFC. Besides the bus power injections, it is useful to have expressions
for power flows from both sides of the UPFC injection model. The UPFC injection model is thereby the constant series
branch susceptance, bs, which is included in the system bus admittance matrix, and the bus power injections Psi, Qsi,
Psj and Qsj. If there is a control objective to be achieved, the bus power injections are modified through changes of the
UPFC parameters r and Qconv1.
II.UNIFIED POWER FLOW CONTROLLER
A Unified Power Flow Controller (or UPFC) is an electrical device for providing fast-acting reactive power
compensation on high-voltage electricity transmission networks. The UPFC is a versatile controller which can be
used to control active and reactive power flows in a transmission line. The concept of UPFC makes it
possible to handle practically all power flow control and transmission line compensation problems, using solid
state controllers, which provide functional flexibility, generally not attainable by conventional thyristor controlled
systems.
279 Int. J.. Adv. Eng., 20015, 1(3), 2766-285 The UPFC iss a combinatiion of a static synchronouss compensatorr (STATCOM)) and a staticc synchronous series
compensator (SSSC)
(
coupleed via a comm
mon DC voltage link. It is capable off controlling simultaneouslly or
line. The parrameters usuallly are
selectively, all
a the param
meters affectingg the power flow in a transmission
t
voltage, impeddance and phasse angle.
Figure.7 Blo
ock diagram off Unified Power flow controlllers.
This topology offers three degrees
d
of freedom or more precisely - foour degrees oof freedom (ttwo associatedd with
each VSC) wiith one constraaint (active pow
wers of the VSCs must match). Therrefore, a fundaamentally diff
fferent
range of conttrol options iss available com
mpared to ST
TATCOM or SSSC.
S
The UP
PFC can be used
u
to controol the
flow of activee and reactivee power througgh the line annd to control the amount of reactive pow
wer supplied tto the
line at the pooint of installaation. While opperating both iinverters as a UPFC,
U
the excchanged powerr at the terminals of
each inverter can
c be imaginaary as well as reeal.
FC
Figure.8 Sttructure of UPF
ng both inverteers as a UPFC, the exchangeed power at thee terminals of each inverter can
c be imaginaary as
While operatin
of the last genneration of FA
well as real. Representative
R
ACTS devices is the Unifiedd Power Flow Controller (UP
PFC).
The UPFC is a device whicch can controll simultaneoussly all three parameters of lline power flow (line impeddance,
p
angle).T
The UPFC com
mbines togetheer the featuures of the Staatic Synchronnous Compennsator
voltage and phase
(STATCOM)aand the Static Synchronous Series Compensator (SSSC
C). In practice, tthese two deviices are two Vooltage
Source Invertters (VSI’s) co
onnected respeectively in shuunt with the transmission lline through a shunt transfoormer
and in series with the traansmission lin
ne through a sseries transforrmer, connected to each othher by a common dc
link including
g a storage cappacitor. The shunt inverter is used forr voltage reguulation at the point
p
of conneection
Injecting an opportune reeactive power flow into the
t line and to
t balance thhe real poweer flow exchaanged
between the series inverteer and the trransmission liine. The series inverter cann be used to control
c
the reaal and
reactive line power flow inserting an opportune
o
volttage with con
ntrollable maggnitude and phase
p
in seriess with
the transmissioon line Thereby, the UPFC can fulfill functtions of reactivve shunt compeensation, activee and reactive series
compensation and phase sh
hifting. Besides, the UPFC allows a secondary but impportant functioon such as staability
control to supppress power syystem oscillatioons improving the transient stability
s
of pow
wer system. Th
he basic compoonents
of the UPFC are
a two voltagee source invertters (VSI's) ssharing a com
mmon dc storaage capacitor, and connecteed to
the system th
hrough coupling transformerss. One VSI is connected in shunt to the transmission
t
s
system
via a shunt
transformer, while
w
the othher one is coonnected in series through
h a series trannsformer. Thee series invertter is
controlled to inject a symm
metrical three phase
p
voltage ssystem of conttrollable magniitude and phaase angle in series
with the line to control acttive and reacttive power floows on the traansmission linne. So, this inv
verter will exchhange
active and reaactive power with
w the line. The
T reactive poower is electro
onically providded by the seriies inverter, annd the
active power is
i transmitted to
t the dc termiinals. The shuunt inverter is operated in such a way as to demandd this
280 Int. J. Adv. Eng., 2015, 1(3), 276-285 dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc
constant.
III.OPERATING PRINCIPLES OF UPFC
Figure.9 Control scheme for the proposed methods
The UPFC is the most versatile and complex of the FACTS devices, combining the features of the STATCOM and the
SSSC. The main reasons behind the wide spreads of UPFC are: its ability to pass the real power flow bi-directionally,
maintaining well regulated DC voltage, workability in the wide range of operating conditions etc .The basic
components of the UPFC are two voltage source inverters (VSIs) sharing a common dc storage capacitor, and
connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission
system via a shunt transformer, while the other one is connected in series through a series transformer. The DC
terminals of the two VSCs are coupled and this creates a path for active power exchange between the converters. Thus
the active supplied to the line by the series converter can be supplied by the shunt converter as shown in figure.
Figure.10 Basic circuit arrangement of UPFC
Figure.11 Phasor Diagram of UPFC
281 Int. J. Adv. Eng., 2015, 1(3), 276-285 Figure.12 Equivalent circuit of UPFC
Therefore, a different range of control options is available compared to STATCOM or SSSC. The UPFC can be used to
control the flow of active and reactive power through the transmission line and to control the amount of reactive power
supplied to the transmission line at the point of installation.
Matrix Converter :Basically, a matrix converter (MC) is composed by 9 bidirectional switches, as shown in Fig. 3.11,
where each dot of the grid represents a connection between the output and the input terminals. The converter is usually
fed at the input side by a three phase voltage source and it is connected to an inductive load at the output side. The
schematic circuit of a matrix converter feeding a passive load is shown in Fig. 3.12. The system is composed by the
voltage supply, an L-C input filter, and the MC and load impedance.
Figure.13 Basic scheme of matrix converters.
Input Filter: The input filter is generally needed to smooth the input currents and to satisfy the EMI requirements. A
reactive current flows through the input filter capacitor, leading to a reduction of the power factor, especially at low
output power. As a consequence, the capacitor is chosen in order to ensure at least a power factor of 0.8 with 10% of
the rated output power. After the selection of the capacitor, the input filter inductance of the matrix converter can be
chosen in order to satisfy the IEEE Recommended Practices and Requirements for Harmonic Control in Electrical
Power Systems (IEEE Std. 519-1992).
IV. SIMULATION RESULT
282 Int. J. Adv. Eng., 2015, 1(3), 276-285 The performance of the proposed control methods was evaluated first using a detailed simulation model representing
the Three-phase MC, series and shunt transformers, sources, trans-mission lines, loads, and additional blocks to
simulate the control system using MATLAB/Simulink SimPowerSystems tool-ox. MC semiconductors were simulated
as Sim Power System switches with appropriate snubbers.
Figure.14 Matrix converter switch Model
Figure.15 Matrix converter switch subsystem
Figure.16 Matrix converter 3 phase o/p voltage
Figure.17 Matrix converter 3 o/p current
283 Figure.18 Matrix converter 3phase o/p current
Int. J. Adv. Eng., 2015, 1(3), 276-285 Figure.19 Matrix converter active (P) and Reactive Power(Q)
Figure.20 Matrix converter Based unified power flow control
Figure.21 UPFC Data Acquisition control
284 Int. J. Adv. Eng., 2015, 1(3), 276-285 Figure.21 STATCOM wave model
Figure.21 UPFC wave model
CONCLUSION
This paper designs and compares three different control methods for active and reactive power flow using MCs
connected to power transmission lines as UPFC: PI linear controllers, DLC, and sliding-mode-based nonlinear DPC.
MC-UPFCs need almost no energy storage, which is a clear advantage in the converter sizing and design, and the
proposed controllers and high frequency filter placement were chosen to obtain control parameters almost independent
of load and filter characteristics. Simulation and experimental results show that P and Q power flow can be effectively
controlled using the MC-UPFC and one of the three controllers. The nonlinear DPC methodology show no steady-state
errors, no cross coupling, insensitivity to non-modeled dynamics, fast response times, and low THD, whereas the
simpler PI linear P and Q power controllers using a modified Venturini high-frequency PWM show a small cross
coupling and slower response times. DLC, although dependent on system parameters, show no cross coupling, fast
response, and small ripples in steady state, but higher THD.
ACKNOWLEDGEMENT
At the outset, I express my gratitude and thanks to our principal, Dr. S.Arunachalam, M.E., Ph.D., and management of
Thiruvalluvar College of Engineering for extending all the instrumentation and facilities. I wish to express my sincere
thanks to our Head of the Department Prof. V. Balu, M.E., Ph.D., for his guidance, useful comments and excellent
encouragement in course of this work. I wish to thank with deep sense of acknowledge to my supervisor
Miss.Shanmugathai, M.E., for his valuable mentorship, timely suggestions and encouragement during all phases of this
work. I take immense pleasure in conveying my thanks to our project co-coordinator Mr.P. Suresh, M.E. for his
valuable support for completion of this project. I would like to thank our department faculty members are Asst. Prof.
Mr.A.Manivannan, M.E., Mr.C.Anandraj, M.E.,Mrs.R.Divyabharathi,M.E.,cMiss. K.Shanmugathai,M.E,Mr.S.K.
Ezhilarasu ,M.E and others. I would also like to thank all my classmates for sharing their views on the project issues.
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