FACTS-GTO thyristor controlled series capacitor

1. 2004 IEEEIPES Transmission8 DistributionConference & Exposition: LatinAmerica
1
GCSC - Gate Controlled Series Capacitor: a
New Facts Device for Series Compensation of
- . . - .
'lransmissionLines
E. H. Watanabe, Senior Member, IEEE, L. F. W.de Souza, Member, IEEE, F. D. de Jesus,
J. E. R. Alves, Member, IEEE and A. Bianco,Member, IEEE
Abstract -Controllableseries compensationis a useful tech-
nique to increase tbc efficiency of operation of existingtransmis-
sion lines and improve overall power system stability. Up to date,
the TCSC is the most adopted solution whenever controllable
series compensationis required. This paper introduces the Gate
Controlled Series Capacitor (GCSC), a novel FACTS device for
series compensation. The principle of operation and some pro-
spective applications of the equipment nre presented. Special
attention is given to the duaIityof the GCSCwith the well-known
thyristor controlled renctor, used for sbunt compensation. It is
shown that the GCSC can be more attractive than the TCSC in
most situations. Simulation results illustrate the time response of
the equipment and its ability to control power flow in a transmis-
sion line. Finally, technology issues regarding high power self
commutatingvalves are discussed.
Ztidex TermsScries Compensation, TCSC, GCSC, FACTS.
I. INTRODUCTION
owadays, it is becoming increasingly difficult to build
new transmission lines, due to restrictions regarding en-
vironment and financial issues. Besides that, electrical
energy consumption continues to increase, leading to a situa-
tion where utilities and independent system operators have ta
operate existing transmission systems much more efficiently
and closer to their stability limits. One important benefit of
FACTS (Flexible AC Transmission Systems) technology is
that it makes it possible to improve the use of the existing
power transmission system and to postpone or avoid the con-
structionof new transmission facilities.
Among FACTS devices, those for series compensation
play an important role in a country as Brazil, where long
transmission lines connect remote hydro-generation plants to
Iarge urban areas. Conventional series compensation, provided
by f i e d capacitor bank, is a useful tool to improve the power
transfer capacity by neutralizing part of the seriesreactance of
transmission lines [I]. With the new controlled series compen-
sators, it is possible not only to control the power flow
through transmission lines, avoiding power flow loops, but
N
E. H. Watauabe andF.D.de Jesus anwith the Federal University of Rio
L. F.W. de Souzaand J. E.R.Alves are with Cepel, Rio de Janeiro, RJ.
A. Bianco is with Andrade e Cauellas Consulting, S a Paulo, SP,Brasil
de Jaaeiro, Rio de Janeiro,RJ, Brasil( watanahe@ufj.br;fabio@coe,uf?.br).
Brasil (Ifclipe@cepel.br; alves@cepel.br).
(andre.bianco@andradecanellas.com.br).
also to improve power system stability, through the fast ac-
tuation of its control loops after disturbances. Moreover, re-
cent changes in the power industry throughout the world in-
creased the interest in equipment capable of control power
flow through pre-determined paths, meeting transmission
contract requirements even in highly meshed systems.
Thyristor Controlled Series Compensators (TCSC) were
the first generation of series compensation FACTS devices.
Actually, TCSC may be credited as a cornerstone of FACTS
deveIopment, as the first equipment developed under the
FACTS concept. TCSC are made of a parallel connection of a
capacitor and a thyristor-controlled reactor [2]. In fact, the
TCSC is simply a static voltage controller (SVC) [3] con-
nected in series with a transmission line. The thyristor is its
switching device. Existing TCSC installation in the world and
in Brazil already proved the efficiency and robustness of the
equipment. Although the TCSC is capable of continuously
adjust its reactance, it has the disadvantage of presenting a
parallel resonance between the capacitor and the thyristor
controlled reactor at the fundamental frequency, for a given
firing angle of the thyristor. Also, the variation range of the
reactance presented by the TCSC is somewhat narrow.
This paper presents a novel equipment for controlled series
compensation: the Gate Controlled Series Capacitor (GCSC)
[4]. The GCSC, shown in Fig. 1, based on a concept first in-
troduced by Kurudy et al. [SI, is made simply of a capacitor
and a pair of self-cornmutated semiconductor switches in anti-
parallel, e.g., the GTO (Gate Tum-off Thyristor) or the IGCT
(Wegrated Gate Commutated Thyristor) [6]. It is capable of
continuously vary its reactance from zero to the maximum
compensation provided by the capacitor. The GCSC is simpler
Fig. I -TheGate ControolledSeries Capacitor4 C S C .
0-7803-8775-9/041$20.00 02004 IEEE 981
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2. 2
Fig. 2-Typical voltage and currentwaveforms of theGCSC
than the TCSC, utilizes a smaller capacitor, does not need any
reactor and, differently from the TCSC, does not have an in-
trinsic internal resonance. For these reasons, the GCSC may
be a better solution in most situationswhere controlled series
compensation is required. One potentially interesting applica-
tion of the GCSC is in the retrofitting of existing fixed series
capacitors, making them FACTS devices. Another FACTS
devices developed for series compensation is the SSSC (Static
Synchronous Series Compensator) which is based on voltage
source converters .[2]. This device presents high flexibility
level but has a much higher cost involved due to the complex-
ity of the converters.
This paper presents the GCSC, its main components, prin-
ciple of operation, typical waveforms and main applications.
An important issue discussed in this paper is the duality of the
GCSC with the well-known TCR,largely used in static com-
pensation.Some rating comparisons with the TCSC are pre-
sented, showing that the GCSC may have several advantages
over the TCSC. Technological problems and possible trends
relating to the development of high-voltage and high-current
self-commutated valves ate also discussed. Results of ATP
digital simulations are presented, showing time-responses of
the GCSC and proving its effectiveness in controlling power
ffow through a meshed transmission system.
11. GATECONTROLLED SERIESCAPACITOR
A. Principle ofOperalion
From Fig. 1, one can see that if the self-cornmutated
switches turn off,the capacitor is inserted in the circuit, com-
pensating the line inductance. When the switches are turned
on, the capacitor is bypassed, canceling the compensation ef-
fect. The switches start to conduct only when their anode-
cathode voltage tends to become positive, exactly when the
capacitor voltage vc is zero. The line current i of the con-
trolled power line flows altemately through the switches and
the series capacitor.
The level of series compensation is given by the funda-
mental component of the capacitor voltage VC. This level may
be varied by controlling the blocking angle y of the semicon-
-I
90 100 110 120 130 140 150 160 170 180
Y (degrees)
Pig.3 -F u n k n t a limpedanceof the GCSC asa functionof theblocking
angle
ductor switches. This blocking angle y is measured from the
zero crossing of the line current. Fig. 2 shows typical current
and voltage waveforms for the GCSC of Fig. 1, for a given
blocking angle y. It is assumed that the transmissionline cur-
tent, i,is sinusoidal. In order to avoid dc voltage components
in the series capacitor, during normal operation, the blocking
angley should be greaterthan 90" and smaller than 180'.
Fig. 3 shows the relation between the fundamental imped-
ance of the GCSC and the blocking angley. A blocking angle
of 90" means that the capacitor is fully inserted in the circuit,
that is, the fundamental impedance is 1p.u and the switches
are turned off completely. On the other hand, ifthe blocking
angle is 180",the switches are on full conduction, bypassing
the capacitor, meaning a zero impedance. So, a continuous
variation of the equivalent series capacitance of the GCSC is
achievable in the range of 90"<y < 180".
Referring again to Fig. 2, one can see that the voltage
waveform in the capacitor is non-sinusoidal. Fig. 4 shows the
main harmonic components of the voltage waveforms as a
function of the blocking angle y. The voltages are in per-unit
values of the capacitor maximum voltage. As the voltage in
the GCSC is lower thanthe system voltage, depending on the
compensation level, the harmonics will be proportionally
lower, in percent values, when converted to the system basis.
B. Prospective Applications
The GCSC could be typically used in applicationswhere a
TCSC is used today, mainly in the control of power flow and
damping of power oscillations. The GCSC may operate with
an open Ioop configuration,where it would simply control its
reactance, or in closed loop, controllingpower flow or current
in the line, or maintaining a constant compensation voltage
[2]. Power Oscillation Damping schemes may also be easily
HannonlcsInlike GCSC0.2, I
o w ,U1
4 2
90 100 110 120 130 140 150 160 170 180
Y (&grreS)
Fig.4 - Harmonicvoltages in the GCSCas a functionof the blockingangley.
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3. 3
D Semiconductorswitchesin
L Seriesconnectedto transmis-
ISuppliedby a currentsource
ISwitches control amountof
parallelwitb a capacitor
sionlines
current in the caDacitor
attainable with the GCSC.
The typical configuration of the GCSC would be a system
composed of smaller devices connected in series, in a so-
called multi-module configuration. In this configuration, the
semiconductor valves have lower voltages and voltage har-
monic distortions are kept low.
The comparison between a GCSC and a TCSC will favor
the fmt equipment in most situations where controllable series
compensation is needed (see Section III). As research on the
GCSC is still under way, it is possible that a break-even MVA
rating is found, above which the TCSC will be more advanta-
geous due to possible valves and protection requirements of
the GCSC. The authors foreseen that the GCSC should also be
a very interesting alternative for retrofitting fixed series ca-
pacitor installations,making them FACTS devices.
S&condu& switches in
Shunt connectedto transmis-
Suppliedby avoltage source
Switches control amount of
serieswitha reactor
sion lines
voltagein the reactor
HI. DUALITYwml TZIETHYRISTOR CONTROLLEDREACTOR
One interesting feature of the GCSC is that its operation is
exactly the dual of the well-known thyristor Controlled reactor
( E R ) [2][3], used for shunt compensation, usually with a
fKed capacitor in parallel. In fact, one may easily observe that
the voltage waveform of the GCSC shown in Fig. 2 is similar
to current waveforms of the TCR (e.g., see [2] and [3]). Table
1shows a comparison between the dual characteristics of both
equipment. The duality can easily be extended to the valves
[7],making it easier to understand the requisites of a GCSC
valve. Considering that the TCR is the dual of the GCSC and
that the former is a longtime adopted solution for controlled
shunt compensation, one may conclude that the GCSC is the
naturalsolution for controlled seriescompensation.
TABLE1-DUALCHARACTERlSTlCSOF THEGCSC AND THE TCR
Gate Controlled Series CapacitorI ThyristorCothUed Reactor
e Voltage controlledby switches'
ISwitches 6re and block with
blocking angle
zerovoltage
Current controlledby switches'
Switchesfire and blockwitb
m gansle
zero current
A. Main Components
A simple comparison of rating of theGCSC and the widely
adopted TCSC is presented here. For this analysis, although
the TCSC may be designed to operate in the inductive region,
it is assumed that it normally operates only in the capacitive
region. Also, it is considered that the maximum compensation
capacity is equal for both devices: they should have the same
maximum capacitive impedance when compensating at their
maximum.
Fig. 5 shows a typical impedance curve for a TCSC, as a
/I- II
+
-2
ctmn 183
~ringalgleNdeim=)
Fig. 5- Typical impedancecharacteristicof theTCSC.
function of the firing angle a.The region where operation is
allowed is shaded. The resonance is also shown in this figure.
Z,, and Z,, are the maximum and minimum values of the
impedance of the TCSC operating in the capacitive region.
Z,, corresponds to the capacitive reactance oniy, that is, at
this point the thyristors do not conduct and the reactor is not
present. Z,, corresponds to the value of equivalent imped-
ance of the capacitor and the thyristor controlled reactor for
the minimum fuing angle ami,,.This angle is limited in order
to avoid the potentially dangerous operation near the parallel
resonance region.
For the GCSC, the minimum reactance is equal to zero.
The maximum reactance, which corresponds to the capacitor
reactance, should be equal to Zmxof the TCSC to obtain the
same maximum compensation level. The relationship between
capacitances of both devices is the following:
(1)
CCLX - 2"
CTCX zm,
Moreover, the same steady-state voltage is applied to both
the TCSC and GCSC capacitors. As for the current, it is al-
ways higher in the TCSC than in the GCSC [XI, as the paral-
lel-connected TCR needs to boost the capacitor current in or-
der to increase the capacitor voltage.
Besides needing a larger capacitor, the TCSC will also
need a reactor that should be rated for the same current of the
valve. As a general conclusion, the GCSC needs less passive
components, as its capacitor is much smaller, with lower cur-
rent rating, and it does not need any reactor at all.
The valve currents in the TCSC are always higher for de-
vices where the relation between the maximum and minimum
impedance is greater than 2, what happens in most of the ex-
istent installations throughout the world [2][8]. On the other
hand, the GCSC valves should be rated for a voltage slightly
higher [XI.
B. Example ofcomparison ~ the BruziIiunNorth-South Inter-
connection
To illustrate the previous conclusions, a GCSC was rated to
prospectively substitute one of the TCSC already installed in
the Brazilian North-South Interconnection [SI.This transmis-
sion line needs a seriescontroller to damp out a low frequency
power oscillation between Brazilian North and South grids.
The existent TCSC has a reactive power rating of IDS Mvar
and is installed in a 550 kV transmission line with a rated CUT-
rent of 1500A. This equipment normally operates with a ca-
pacitive reactance of 15.92 0, when there is no need of
L
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4. 4
Fig. 6 -GCSCconnectedto acurrent source
damping power swings.
For rating purposes, it was assumed that the GCSC should
have the same maximum reactance and nominal Mvar of the
TCSC. Also, it was assumed that the GCSC operates at the
same continuous effective reactance of the TCSC. It should be
pointed out that, althoughboth devices have the same function
in the power system, they are quite different. For this reason,
other designing strategies are possible for the GCSC, but it is
beyond the scope of this paper to find an optimal designing
strategy. Table 2 Summarizes the basic characteristics of the
existing TCSC and a GCSCproposed to substituteit.
TABLE 2-EXISTENT TcsCA N D PROFQSED wscRATINGS FOR %AZIUAN
NORTH-SOUTHINTERCONNECTION
Parameter TCSC I ccsc j
CapacitorReactance 13.27i2 39.81R
Capacitance 200 p 66.6 pF
Max.Reactance 39.81 Q 39.81Q
DynamicControl Range 13.27-39.81Cl 0 -39.81 R
Max. Fundamental Voltage 59.7kV 59.7kV
Max.RMS Voltage 60.3kV 59.7kV
Max. RMS Capacitor Current 5025 A 1500A
Max. Valve Current(rms) 3735A 1500A
Max.Reactor Current (rms) 3735A no reactor
Mm.Voltage ofthe Valves 51.34I 59.73/
(rindpeak) 74.41kV 84.47kV
1.5
(kv)
1
0.5
0
4 . 5
-1
-1.5 0 0.1 0.2 0.3 0.4 t
Time ( 5 )
E I. [ FI20')-I200 FI 50'
rFig.7 -Timeresponseof a GCSC connected to a current source
17KlD1 , , , , I15WO
0 6 0 8 I 1.2 1 4 1 6 i a
time ( 5 )
Pig.8 -Openloopresponsesof the GCSC and TCSCwithlow levels of com-
pensation,varyingf"35% to45% at 800 ms,andback to35% at 1.3s
v. RE-SULTSOF DIGITAL fhdlLATIONS
A. Time Responses
The GCSC can rapidly vary its reactance, whenever its
blocking angle signal is varied. To demonstrate that, a simple
system was modeled in the ATP simulation package, consist-
ing of a GCSC fed by a current source, as shown in Fig. 6.
The GCSC has a maximum reactance of 26.5 R. Initially, the
self-commutated switches are operating with a blocking angle
of 120". At Zooms, the compensation level is decreased by
increasing the blocking angle to 150'. Then, at Moms, the
compensation level is returned to the initial value. The result
of the simulation is shown in Fig. 7.
The topology shown in Fig. 6, although very simple, is in-
teresting to analyze the dynamic behavior of the equipment, as
the only other element in the network is an ideal current
source. The same topology is used in the ATP to test both the
GCSC and TCSC of Table 2. The current source now has an
rms magnitude of 1500A. In the fust simulation, each equip-
ment is compensating at low level (35% of X-). Next, the
compensation increases to 45% and decreases again to 35%.
Fig. 8 shows the fundamental voltage response of each
equipment to this input. Both equipment have similar open
loop responses at this level of compensation.
Another simulation was performed, with higher levels of
=z%?ow40000
0 6 0 8 1 1 2 1 4 1 6 I B
tune (SI
Fig 9 - Open loop responsesof the GCSC and TCSCw~thhgh levefsof
compensation,varymg from 80% to90% at BOO ms,andback to 80% at 13 s
I
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5. 5
' Fig,10-Equivalent 500 kV and765 LV Sy&m ofthe South of Brazil.
series compensation. Now the blocking angle is varied fiom
80% to 90% and back to 80%,in a pattern similar to that of
the previous simulation. The results are in Fig. 9. It is clear
that the TCSC is much slower thanthe GCSC at high levels of
compensation, i. e., with high currents af capacitor and reac-
tor. On the other hand, the open loop response of the GCSC
does not differ too much from that shown in Fig. 8, with low
level of series compensation.
B. Power Flow Scheduling in a Meshed Network
Zn order to show the capability of the GCSC to control
power flows, an ATP simulation of a meshed transmission
system was performed. The system, shown in Fig. 10, is an
equivalent ofpart of the 500 kV and 750 kV South-Southeast
Brazilian Network. Transmission lines 1 and 2, both in
500 kV, form a loop-flow: PIin Line 1 is 50% higher than the
P2 in Line 2. A GCSC, capable of compensate up to 80% of
seriesreactance, is operating with about halfof its capacity.
The GCSC increases its compensation to the maximum,
thus boosting the power flow through line 2 and establishing
the balancebetween the usageof both lines. Fig. 11showsthe
2000 I
I
j
. . ..I
F1400
E
5 l2O0
g 1000
::I , - ;400
0 3 0 4 0 5 0 6 0 7
time 6)
Fig I 1 -Power flows throughLines I and 2. aftercampensailonof Line 2
increases from 43%to 80% of the senesreactance
power flows in both lines before and &er the increment of
compensation by the GCSC. It is clear that the device could
quickly establish power flow equilibrium between both lines.
This test shows, in fact, that the GCSC can be used to control
power flow at different levels, which can be chosen by the
system operator. Fig. 12 shows the voltage in the GCSC be-
fore and after the step in the compensation of Line 2.
VI. HIGH POWER SELF-COMMLJTATEDVALVES: SOME
,"OLOGlCAL T"Ds
The design of a reliable high power self-commutated
switch is of paramount importance for the development and
manufacturing of a GCSC for an EHV transmission line. A
typical GCSC would he a multi-module equipment. Each
module might be designed to be a small GCSC cell, compris-
ing a relatively low voltage switch valve or even a single pair
of high power switches. Several GCSC cells could be con-
nected in series to form larger multi-module GCSC. The self-
commutated switch could be the GTO or, most likely, a more
modern semiconductor device, like the IGCT. The switch has
to be of the symmetrical type, in order to block reverse volt-
l_._._.l_l_-..
50
g o
-50
1 d
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6. ages.
Even with small GCSC cells, it may be necessary to con-
nect a number of semiconductor switches in series. The series
connection of GTO in hard-commutated converters to form
high power adjustable speed drive systems (ASD) has been a
technical challenge for years. This is not the case of the
GCSC, as it is a zero voltage switching equipment, what
makes the series connection of self-commutated switches
much easier [9]. Care must be taken with stray inductances
due to leads and cables that should be considered in the proj-
ect of snubber circuits. Another important issue to consider is
the Wdt limit of the semiconductor switches [lo].
VII. CONCLUSIONS
This paper presented a novel equipment for controllable se-
ries compensation of transmission lines: Gate Controlled Se-
ries Capacitor (GCSC). Some of the basic concepts behind the
equipment were reviewed. Emphasis was given to the fact that
the GCSC is the dual device of the Thyristor Controlled Re-
actor (TCR). This special characteristic not only helps to un-
derstand the GCSC principle of operation, but also makes the
analysis and possibly the equipment design rather easier. Due
to this duality, the authors believe that the GCSC may be a
more natural solution for series compensation than the TCSC,
and may be as widely adopted for series compensation as the
TCR is for shunt Compensation.
Comparison with the TCSC has shown that the GCSC is
more compact, with lesser passive components: it does not
need reactors and its capacitor bank is much smaller. Also, the
switches and capacitor currents are smaller in the GCSC. Be-
sides that, the semiconductor of the GCSC should be rated to a
slightly higher voltage than the SCR valves of the TCSC.
Some important issues regarding the development of high-
power valves are discussed. The main focus is the need of
development of a high-power valve comprising series con-
nected self-commutated switches capable of blocking reverse
voltage. Attention should also be given to the rate of rise of
current in the valves.
Simulation results demonstrate the operating principles of
the GCSC. Its open-loop dynamical response is faster than
that of the TCSC, specially at higher compensation levels.
Also, an example proved the capability of the GCSC to con-
trol power flow in transmission lines.
As a final remark, the authors believe that this new device
may be an excellent solution for transmission line controlled
series compensation. In the near future, the authors expect to
prove this technology by developing a full-scale GCSC pro-
totype to operate in an HV transmission system.
VIII. R E " C E . 3
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[4] A. A. Edris,Tower Electronic-BasedT&D ControllersAt Technologi-
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[6]
{TJ
[8]
191
E.BIOGRAPHIES
Edson EIimkamWntannbe(M'76, SM'02) was born in Rio dc JaneiroState,
Brazil, on November07, 1952.He received the B.Sc. in Electronic Engineer-
ing and MSc. in ElectricalEngineering in 1975 and 1976, respectively,h m
the Federal University of Rio de Janeiro. In 1981 he got the D.Eng. degree
&om Tokyo Institute of Tecbnology,Japan. In 1981 he becamean Associate
Professor and in 1993 a Professor at COPPEiFederalUniversity of Rio de
Janeiro. where he teaches Power Eleclronics. His main fields ofinterests arc
convertersanalysis,mcdelingand desim activefilters andFACTS technolo-
gies.Dr. Watanabe is a member of the IEEJapan, The Brazilian Society for
AutomaticControlandTheBrazilianPowerElecbnics Society.
Luiz Felipe W ~ W Ide Son= (S'94, A'98, M W )was bom in Niterbi, Rio
de Janeiro State. B d . on Januaty IO. 1972. He received the B.Sc. degree
from Flumineme Federal University,Rio de Janeiro State, in 1994 a d the
U&.degree in Electrical Engineering from Federal University of Rio de
Janeiro in 1998.He is cwenlly w o r m towards his doctoratedegree at Fed-
eral University of Rio de Janeiro. From 1994 to 1996 he worked at Fumas
Centrais Eltirifas W A as a hydro power plant maintenance engineer. Since
1996 he works at CEPEL as a researchengineer.His main fields of interests
arepower quality andFACTS.
Fhbio Dominguesde Jesw was bomin Bnrretos,S b Paul0 State,B m l , on
May 12, 1971. He reoeived the E. S. degree in Electrical Engineeringfiom
FederalInstitutionof High Education of SHO Jo%Odel Rei, Brazil in 2000 and
the USc. degree at Elecbical Engineering Deprbmni in Federal University
of Juiz de Fora,Brazil in 2002. He is pursing his D.Sc.degree at Electrical
Engineering Department from COPPE-Federal University of Rio de Jnneim,
Bmil Hispresent research interestsincludethe high-power electronics,mdy-
sis andmni~olinFACTS.
JX.kAlvm Jr. (M'92F was h m in Juiz de Fora,B d , on November 30,
1963. He receivedthe B.Sc.,M.Sc. andD.Sc.degreesin electricalengineer-
ing, in 1986,1991 and 1999,respectivety. fiom the Federal Universityof Rio
de Janeiro.Since.1995hehasbeen wormat CEPEL, theBrazilianEleclrical
Energy ResearchCenter. Heis currentlyProjwtManager.Dr.Aives' research
interests are in the analysis of HVdc "ission systems, FACTS devices,
PowerElectroniccontrollers,DistributionSystemsand Metering.He became
a Member of the Institute of Electrical and Elecbnics Engineers (EEE) in
1992.He is currently a Member of the IEEE Power Engineering Society and
Sectetary of IEEERio deJaneiroSection.
Andd Bmnm "99) was born in Nova Igmqy Rio de Janeiro, Brazil, on
Junc 27, 1967. He received ttte B.Sc. and M.Sc. degrees in electrical enpi-
neering,in 1990 and 1994, respectivelyfiom the GamaFilho University and
from the Catholic University of Rio de Janeiro. From 1990 to 2003 he was
with =EL, initially as a graduafd student and then as a research engineer
with inkrest in the transienddynamic analysis of power systems includmg
HVdctransmission and FACTS devices.In 2004, Mr.Biancojoined Andrade
& Canellas Consulting, where he is the head of the elechical and energetic
studiesgroup.
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