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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009
Feasibility of Electronic Tap-Changing Stabilizers
for Medium Voltage Lines—Precedents and New
Configurations
Salvador Martínez García, Senior Member, IEEE, Juan Carlos Campo Rodríguez, Member, IEEE,
José Antonio Jardini, Fellow, IEEE, Joaquín Vaquero López, Alfonso Ibarzábal Segura, and
Pedro María Martínez Cid
Abstract—Thyristor-based onload tap-changing ac voltage stabilizers are cheap and robust. They have replaced most mechanical
tap-changers in low voltage applications from 300 VA to 300 kVA.
Nevertheless, this replacement hardily applies to tap-changers associated to transformers feeding medium-voltage lines (typically 69
kV primary, 34.5 kV line, 10 MVA) which need periodical maintenance of contacts and oil. The Electric Power Research Institute (EPRI) has studied the feasibility of this replacement. It detected economical problems derived from the need for series association of thyristors to manage the high voltages involved, and
from the current overload developed under line fault. The paper
reviews the configurations used in that field and proposes new solutions, using a compensating transformer in the main circuit and
multi-winding coils in the commutating circuit, with reduced overload effect and no series association of thyristors, drastically decreasing their number and rating. The stabilizer can be installed
at any point of the line and the electronic circuit can be fixed to
ground. Subsequent works study and synthesize several commutating circuits in detail.
Index Terms—AC voltage stabilizer, onload tap-changer
(OLTC), power conditioning, power quality, voltage control.
I. INTRODUCTION
C voltage stabilizers based on thyristor-commutated
onload tap-changers (OLTC) are reliable and have almost completely replaced mechanical switching equipment in
low-voltage applications, single- and three-phase, ranging from
A
Manuscript received May 02, 2008. Current version published June 24, 2009.
This work was supported by IBERDROLA S. A., Spain, I+D program, ESTRAP
Project: Feasibility of Electronic Medium Voltage Line Stabilizers, 1994–1997.
Paper no. TPWRD-00322-2008.
S. Martinez is with the Departamento de Ingeniería Eléctrica, Electrónica
y de Control (DIEEC) de la Universidad Nacional de Educación a Distancia
(UNED), Madrid 28040, Spain (e-mail: smartine@ieec.uned.es).
J. C. Campo is with the Departamento de Ingeniería Eléctrica, Electrónica,
Computación y Sistemas de la Universidad de Oviedo, Gijón 33205, Asturias,
Spain (e-mail: campo@ate.uniovi.es).
J. A. Jardini is with the Departamento de Energia e Automação Electricas da
Escola Politécinca da Universidade de São Paulo, São Paulo 05508-900, Brazil
(e-mail: jardini@pea.usp.br).
J. Vaquero is with the Departamento de Tecnología Electrónica de la Universidad Rey Juan Carlos, 28933 Madrid, Spain (e-mail: joaquin.vaquero@urjc.es).
A. Ibarzábal is is with the INCOESA, Bo Bidecoeche, Vedia 48390, Vizcaya,
Spain (e-mail: aibarzabal@incoesa.com).
P. M. Martínez is with IBERDROLA S.A., Dirección de Innovación, Calidad
y Medio Ambiente, Bilbao 48008, Vizcaya, Spain (e-mail: pedro.mcid@iberdrola.es).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPWRD.2009.2021032
300 VA to 300 kVA. This replacement does not apply to the
mechanical tap-changers associated to feeding transformers of
long medium-voltage lines (typically 69 kV primary, 34.5 kV
secondary, 10 MVA), usually housed in the transformer tank,
needing periodical renewal of switches and isolating oil [1].
The EPRI [2], [3] and other research institutes have evaluated
the feasibility of such a replacement, both in voltage stabilizers
and phase regulators, concluding (see II.F.1) that, although
technically possible, the equipment derived from the simple
substitution of mechanical switches by their semiconductor
counterparts is too expensive because of the amount of devices in series association needed to obtain medium-voltage
switches, and because of the high over-current under line faults.
They recommend looking for modified topologies needing less
semiconductors.
The long-standing positive results [4]–[12] on solid-state
tap-changers with commutating current limitation based on
multi-winding coils inspired us to explore their abilities for
medium-voltage lines. This work studies the feasibility of a direct structure (without compensating transformer) and a seriescompensated structure, both commutated by multi-winding
coil. The steady and commutating operations are analysed to
derive expressions for the electrical working conditions of
switches, transformers and coils, thus enabling their economical evaluation. The circuits exposed summarise the results
of an iterative analysis and synthesis cost comparative search
method applied in other research on power electronics [13],
[14], some of them bringing the tap-changers to the field of fast
voltage and phase regulators [15]–[17].
The direct configuration here analyzed in IV.A is similar to
those studied by EPRI. The results confirm its high cost and a
strong cost reduction for the new series-compensating solutions
here proposed, accomplished by the synergic effect of the compensating transformer, the multi-winding commutating coil and
a bypass fault switch.
II. ANTECEDENTS
A. Electromechanical Stabilizers
The early generalization of ac voltage in the electric lines was
an obvious encouragement to take advantage of tap-changers
associated to the transformers involved in energy distribution
to accomplish easy voltage stabilization. Taps were first selected manually and then electromechanically. Variants made
from iron toroid core and brushed over a one-layered coil are
0885-8977/$25.00 © 2009 IEEE
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MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
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Fig. 1. Fac simile of a mechanical tap-changer German patent 474 613, 1926,
awarded to B. Jansen. The incoming tap closes before the outcoming tap opens,
thus avoiding output voltage interruptions. The internal overcurrent during
overlap is limited by resistors, shorcircuited after the operation. By courtesy of
Maschinenfabrik Reinhausen (MR), Regensburg, Germany.
very common today in laboratories and industrial plants for
one-phase low-voltage regulators in the range of 100 VA to 15
kVA I . For voltage regulation and stabilization, from 30 kVA
to 300 kVA III , three-limb vertical core equipments with
brushes contolled by servomotor are still in use. Long-lasting
graphite materials used in the brushes give high reliability to
this classic, although inherently slow (about 1 s correcting
time), equipment.
B. Early Line-Voltage Correctors
Line breakers, load interrupters (both manual and automatic)
and other simple devices intended to help power management
have been used in medium-voltage lines since the twenties, located at any point from head to end of the lay-out. Remote control versions have been available since the seventies (Fig. 4,
(left)).
In Europe, most ac tap correctors are derived from patents
such us the German 474 613 one for mechanical resistive
changers, awarded to B. Jansen in 1926 (Fig. 1). This technology was acquired by the MR company and brought about
important improvements in 1973.
C. Present Electromechanical Line Voltage Correctors
Electromechanical changers are used today in connection
with transformers feeding long medium-voltage lines (11.9 kV
to 34.5 kV; 3.15 MVA to 40 MVA) from high-voltage lines (30
kV to 138 kV). The need for making the change before break
to avoid interruptions necessarily creates internal short circuits
requiring current limiting components, resistive or/and inductive. Improvements are made on transformers and commutating
circuits [18]–[22]. The switches are normally housed in the
transformer tank [Fig. 2, (upper left corner)].
In the U.S., the Cooper Power Systems Co. has combined the
inventions of T. A. Edison with the innovations of the McGraw
Co. (first tap-changing voltage correctors in 1959) and currently
makes more refined variants with inductive short-circuit limitations [Figs. 3 and 4 (right)].
Stabilizers with no connection to the line feeding transformer,
as showed in Figs. 3 and 4 on the right, are useful in long lines
with loads connected all along, correcting most voltage problems.
Mechanical changers have been diversified by other European, American, Australian and, more recently, Asian companies. There are variants with electronically-aided commutation
Fig. 2. Medium-voltage line transformer 132 kV/21.5 kV, 50 MVA with the
covering tank removed. It contains an electromechanical onload tap-changer
(OLTC, upper left) made by MR. Courtesy of INCOESA, Vedia, Spain.
Fig. 3. Single-phase, 32-step on-load voltage stabilizer with inductive limitation of internal short circuits. It operates as an autotransformer with a compensating winding in series with the load. The polarity of this winding can be
reversed by a switch and the turns changed by an 8-positions tap-changer associated to a bridging reactor. Courtesy of Cooper Power Systems [23], Waukesha,
WI.
Fig. 4. Outdoor equipment for medium-voltage line management. Left: Line
breaker. Right: McGraw-Edison-type voltage stabilizer (see Fig. 3). By courtesy
of IBERDROLA, Dirección de Innovación y Calidad, Spain, Madrid area.
[19]–[23] and some pure electronic variants [24]–[33]. The
later are not able to replace mechanical solutions in high and
medium-voltage lines yet.
Although the frequency of operation imposed by the system
to tap-changers is very low (typically several commutations per
hour) and the minimum time between changes is limited by the
control circuit (to no less that several minutes), the wearing of
switches and oil make periodical and expensive revisions necessary, which is the main problem with mechanical tap-changers.
The aim of the work conducted is to replace mechanical tapchangers by thyristor-based maintenance-free solutions.
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009
Fig. 5. (a) Direct configuration tap-changing stabilizer using a multi-winding
coil to limit commutation over-current with 50 Hz transformer equivalent power
of 5% of the nominal power, typically. Optional short-circuit winding L and
switch S are explained in III.C.4. (b) Current in the outgoing (up) and the
incoming triac (bottom) in a ten-tap, shared load, 1 kVA, 230 V 15% input,
2:2% output, at nominal load. Worst case current peaks. Courtesy of Electrónicas Boar, Madrid, Spain.
6
6
D. Electronic Tap-Changing Low-Voltage Stabilizers. Industry
and Office Applications
Low-cost plastic case thyristors and triacs up to 40 A, 800
V made it possible to replace most mechanical tap-changers by
their electronic counterparts in low-voltage applications under
10 kVA in the eighties. Both zero-crossing current commutation
and resistor- or inductor-aided commutation have been used to
reduce internal over-current at commutation [4]–[7], [24]–[33].
The multi-winding limiting coil with alternated corresponding
terminals [4]–[7] is especially useful in this respect, see Fig. 5.
It reduces the commutation over-current to a single impulse (8.3
ms or 10 ms) of about four times (seven times in the worst case)
the repetitive peak with nominal load, while pure resistive limiters reduce it to no less than fifteen times, especially in equipments over 10 kVA.
Since the nonrepetitive peak current allowed by triacs and alternistors is about thirteen times its repetitive peak (four to nine
times in an antiparallel thyristor pair) and assuming a security
coefficient of 0.333 in rms current [34], the multi-winding coil
in practice eliminates the need to oversize the semiconductors
because of commutation over-current. In fact, for the worst case
of the aforementioned numerical values (nonrepetitive peak current allowed by the semiconductor equal to four times its repetitive current peak), the switch can be able to withstand a nonrepetitive current peak of
%
%
(1)
of the repetitive peak under nominal load, while the
multi-winding coil demands no more than 700%. Since
the equivalent 50 Hz power of that coil (power of the 50 Hz
transformer feasible with the same iron and cooper [13]) is
about 3% to 11% of the nominal power of the stabilizer, the
global cost is not significantly increased.
The coil allows for two main steady-state operation modes:
shared load by two adjacent taps and nonshared load. The first
mode [4] (2nd and 3rd pat.), [5] yields in practice to very reliable equipments, as it performs rather well even with some
switches (not adjacent ones) out of operation. Stabilizers based
Fig. 6. Configurations for electronic low-voltage tap-changing stabilizers. (a)
Direct configuration, suitable between 1 and 4 kVA. (b) Compensating series
transformer—the single switch-comb configuration is suitable between 5 and
50 kVA. The switch current is typically one third of the value in the direct configuration. (c) Compensating series transformer—double switch-comb configuration, suitable over 50 kVA. Switch current is typically one sixth of direct
configuration value. The multi-winding coil shown in Fig. 5 can be used in all
configurations. In both compensating transformer configurations the taps can be
associated to the primary or to the secondary of the main transformer.
on multi-winding coil made by Electrónicas Boar in the 1980s
(later absorbed by Chloride) in the range of 0.3 to 300 kVA, sold
in more than 45 countries, drastically lowered failure frequency.
E. Advantages of the Series-Compensating Transformer
Another reason for the low-voltage electronic tap-changers
success is the series-compensating transformer configuration
[Fig. 6(b) and (c)], replacing direct configuration [Figs. 5(a)
and 6(a)]. In a typical stabilizer with
% input and
%
output deviation, the current in the switches is divided by three
[Fig. 6(b)] or six [Fig. 6(c)], compared with the current in the
switches of the direct configuration [Fig. 6(a)]. The switches
voltage is not increased, thus making it possible to reduce their
cost and to increase the power of practical equipment with
no parallel thyristor association. The power handed by the
compensating transformer is about 15% of the nominal power.
The series-compensating transformer and its economical
autotransformer variant [23] were used in very early electromechanical voltage stabilizers. The scheme in Fig. 3 shows
an example of it. It is also used in stabilizers not based on
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MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
tap-changing, such as ferroresonant and phase-controlled reactor stabilizers.
An added advantage of the compensating transformer is to reduce high over-currents in the switches as those imposed by line
faults. In this abnormal condition the stabilization function of
the equipment is not mandatory, the tap switches are opened and
the primary of the compensating transformer is short-circuited
(see Figs. 6
by means of an extra fault deviation switch
and 11). The short-circuit impedance of the compensating transformer appears between the mains and the load. Once the output
opens and the tapcurrent comes down to normal values,
switches are restored.
The series-compensating transformer and the switches can
be connected before or after the main transformer of the stabilizer. In the first case [Fig. 6(b) and (c)], the main transformer
sees almost constant voltage, which means a very economical
core design. In the second case, the switches are less exposed to
line over-voltages because of the isolation provided by the main
transformer.
F. Problems Found in Previous Studies and Solutions
1) EPRI Reports: Although modern monitoring and control
techniques improve the performance of both the mechanical and
the electronic versions [35]–[42], tap-changing ac voltage stabilizers and phase regulators show some common problems, when
applied to medium- and high-voltage lines, on account of their
discrete-regulation principle.
The 1988 and 1990 EPRI reports [2], [3] on improved
on-load-tap changers discussed the viability of electronic
versions to reduce the high-cost maintenance of mechanical
solutions. The main conclusions were as follows.
1) Electronic stabilizers derived by simple substitution of
switches in the mechanical equipment are not possible due
to the high number of semiconductors needed and their
cost.
2) New topologies minimizing the number of switches needed
to obtain a given number of voltage changes would help.
3) Even with less switches, the need to associate the semiconductors in series to support high steady-state and transient
voltage raises their number and complicates protection.
4) A no less important problem is the very high over-current
in medium-voltage lines developed as a consequence of
phase-to-phase and phase-to-ground faults. They reach up
to 1 200% the nominal current and last up to 100 ms, the
opening time of the ordinary line over-current protectors.
These over-currents are well supported by mechanical taps
providing that changes are inhibited in the meantime, but
in electronic versions they make oversizing the thyristors
necessary, as their over-current rate1 for 100 ms is about
200% of the nominal current, far from the 1 200% needed.
EPRI concludes that “major breakthroughs are required
before LTC equipment using solid-state devices can become
economically attractive for ordinary power transformers. Such
1High current thyristors allow during 100 ms about 200% of the nominal
current. Even taking into account the 0.333 current security coefficient mentioned in II-D [34] (and loosing it during faults), only 600% of the nominal load
could be supported with thyristors not oversized by this reason, thus still under
1 200%.
1493
breakthroughs could come by developing ways to avoid the
impact on device ratings of system short-circuit currents and
transient voltages”2.
2) Tap-Changer Association With the Line Transformer: The
need to reduce slow voltage variations is common in mediumvoltage (13 kV to 34.5 kV) very long and/or undersized (poor)
lines. The scalar line voltage drop reaches up to 10% of the
nominal voltage. Assuming an additional 9% scalar drop in a
typical 69 kV/34.5 kV, 10 MVA feeding transformer, the drop
rises to 19% from the high voltage feeding line to the end of
the medium-voltage line. Although the stability in the highvoltage side is usually good and the legal variation allowed in the
medium-voltage inlets is about %, the aforementioned transformer-plus-line drop is usually compensated by a tap-changer
associated with the line transformer [42], [43]. Since the actual
voltage at the end of the line is not known, a compound control
criteria (input voltage plus load current) is used to select the tap
[18], [42]–[44]. When the customers are connected all along the
medium-voltage line, it can be technically impossible to supply
a correct voltage to all of them by means of a single OLTC associated to the feeding transformer. In these cases one or more
additional OLTC are installed in selected intermediate points of
the line (see Fig. 4, (right): two single-phase OLTCs connected
in V-configuration serving a three-wire line [23]).
The following sections show that using an isolating seriescompensating transformer topology allows for the tap-changer
to be installed in any point of the line. If connected at the output
of the line-feeding transformer, it can replace the associated
OLTC, see Fig. 10. These options are not usually considered by
the studies on feasibility of electronic tap-changers, as [2], [3].
Besides the advantages in lowering the number and the current
rate of the switches, seen in Section II-E, this topology makes it
possible to fix them to ground-voltage level.
G. Antecedents on Live Line Installation and Maintenance
The stabilizer solution to be proposed in this paper can be
connected (and disconnected for maintenance purposes) to a live
medium-voltage line. Live working in overhead lines is a mature
and reliable activity today regulated by very precise protocols
from the electric companies [45] and governments [46]–[48].
III. ON CONFIGURATIONS, OPERATION
MODES AND COMMUTATION
A. Iterative Analysis and Synthesis Cost Comparative Search
The method applied to explore the power circuit configurations has been described in [13] and [14], when applied to the
active filters search. Applications to tap changers can be seen in
[8], [10], [11], [15]–[17]. It consists of the following steps: analysis of new circuits; synthesis of component rates as a function
of equipment specifications; study of circuit variants; derivation
of component cost by transformation to standard components;
technical and economical comparison of solutions.
This paper compares the advantages of the general configurations of stabilizers and introduces the commutation circuits.
Other solutions are studied in subsequent works. All of them are
based on natural zero-current commutation aided by the multi2EPRI
Perspective in the Technical Portofolio corresponding to [2].
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009
TABLE I
STATES FROM A GIVEN NUMBER OF TAPS. IN
BRACKETS TOTAL TAPS FOR THE EXAMPLE
winding coil seen in II-D and their variants [4]–[7] because of
the low overcurrent and overvoltage developed in the thyristors
during the changes and the good practical results in low-voltage
applications. They can be referred to as thyristor-based, slowcommutated tap-changers, acting once in a half-cycle or at a
lower rate (supercyclic commutation).
B. Configurations Comparison
Irrespective of the circuit configuration, the number of
voltage state changes needed by a line stabilizer is a function
,
of the input high-voltage deviation to be compensated (
positive and negative per unit deviation, respectively) and the
. The voltage drops
allowed output voltage deviations
and in the
with nominal load in the mains transformer
also have to be compensated. As most mechanical
line
tap-changing line stabilizers, the electronic counterpart must
offer the ability to add or detract an optional per-unit output
. By neglecting the voltage drops in the internal
deviation
transformers and coils and in the switches, the number of
changes needed is
(2)
Fig. 7. Shared load mode. Steady state with taps 1 and 2 activated. Ideal
switches and coil with perfect coupling. Load power factor 0.8 inductive.
is added or subtracted under the extreme condition. For the preof the nominal. This
vious numerical example
means that the maximum power managed by the compensating
transformer is 14.5% of the nominal power.
As shown in Section II-E, in both structures the taps can be
in the primary or in the secondary of the main transformer. In
the direct structure with taps on the primary side, the semiconductors can be used to open the circuit in case of a line fault,
but they must be sized to support the fault current reflected to
the primary and the inrush current when connecting the transformer. Using autotransformers in the compensating structure
brings about an important reduction in their size and very economical equipments (Fig. 3). If the isolation transformers are
maintained the electronic switch combs [Fig. 6(b) and (c)] can
be fixed to ground, which simplifies the isolation components
and maintenance, as shown at the end of II-F-2.
C. Operation Modes Comparison. Tap Commutation
accounts for errors in voltage-sensing and
where
control management. As an example, for
(legal
and
, the result
maximum 0.07),
(eight in practice). Obviously, the number of
is
required voltage states is
(3)
The taps needed to obtain a given number of states depends
on the operation mode (shared, nonshared and mixed load, see
III-C) [5], [6] and the configuration (Fig. 6) resulting in expressions (4)–(9) in Table I.
Expressions (7)–(9) also apply to the direct configuration
with double-comb (not contemplated in Fig. 6), sometimes
used for low power with the second comb working as a voltage
vernier.
The configurations to be studied here belong to the direct
[Fig. 6(a)] and the compensating transformer [Fig. 6(b) and (c)]
structures. The secondary of this transformer adds or subtracts
to the line the voltage needed to keep the load voltage within
limits. In an optimal design, the same scalar value
(10)
In all solutions considered here, the multi-winding coil seen
in II-D, Fig. 5(a), is used to reduce the internal short-circuit current during commutation. This coil enables four steady state operation modes, each one associated to a tap commutating procedure. They shall be studied here with the minimum extension
needed to make a technical-economical comparison in part IV.
1) Shared Load:
a) Steady State Operation: The alternated corresponding
terminals of the multi-winding coil easily lend themselves to
sharing the load by two consecutive taps [4] (2nd pat.), [5]–[7].
Both operating windings make a 1:2 ratio autotransformer that
distributes the load in a 1:1 proportion between the selected taps
(taps 1 and 2 in Fig. 7). The output voltage must be the mean
value of both active taps voltage. A magnetizing current circulates in both equalizing windings
, where
is the voltage between consecutive taps and
the inducmodifies the scalar
tance of a single winding. In practice
and
to about 0.4 and 0.6 times the nominal
tap currents
current. The power rating of the stabilizer feasible with a given
semiconductor is then multiplied by (1/0.6).
b) Commutating Process: Fig. 8 shows the equivalent circuits and the relevant variables evolution for the change from
taps 1–2 to taps 2–3.
• Turning off the outcoming switch. Interval to .
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MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
Fig. 8. Commutating process for shared load. (a) Equivalent circuit before t :
from t to t , the outcoming switch S is de-excited but still conducting. (b)
Equivalent circuit from t on: the incoming switch S is turned on and shares the
load with the permanent switch S . (c) Relevant current and voltage waves. The
equivalent circuit from t to t , with S on and S ; S off, is not represented.
Note that i
is I
before t and goes towards -I
after t .
While the branches
and
, for instance, are
, see Fig. 8(a), the outturned on, each one producing
coming switch is de-excited at time . It is effectively turned
off after the first zero-crossing of its current
at time ’. The load is now fed by branch
until
is turned on at time . The equivalent circuit for ’ to is
not given in Fig. 8. This subinterval lasts less than half a cycle
is in practice 3 to 7 times its repetitive
and the peak value of
peak during steady state operation with nominal load.
(defined as
The equivalent total magnetizing current
flowing through a single winding, entering in dotted terminal)
until ’ and
from ’ to . The peak value
is
of
must not saturate the core to keep the limitation ability
during the next interval.
.
• Turning on the incoming switch
1495
is
At time , the switch of the incoming branch
remains conducting. The
turned on while branch
is chosen slightly over half a cycle to
time duration
ensure that
extinguishes before . A new equalizing current
operates whose magnetizing effect is opposite the effect
, due to the change of doted terminals. Nevertheless,
of
the process does not imply any step in the magnetizing current,
thus avoiding over-voltage in the load and switches.
reaches zero at time
In fact, when
the load current is
. Then, at time
this
current flowing through one winding produces the same magflowing through two windings. The
netizing effect that
magnetization sign is also maintained as deduced after the dot
of the involved terminals.
is closed at time
evolves in a few cycles
Once
from
to the new steady state value .
2) Nonshared Load:
a) Steady State Operation: The circuit in Figs. 7, 8 can
also operate with a single active branch in each voltage state
[6], [7]. One winding and one switch support the nominal load
current and the overloads, a less advantageous condition that in
shared load for the coil and switches. The number of voltage
states equals the number of taps, see (5), one state more than
for shared load, see (4). The voltage drop in the coil
(90 in advance with respect to
) can be up to four times the
drop in the switch, not appreciably disturbing the load voltage.
b) Commutating Process: The commutation to a next tap
has two steps. First, the switch of the incoming branch is turned
on and starts operating in shared load with the outgoing branch,
as explained in III.C.1. After a few milliseconds, the outgoing
switch is de-excited with no regard to its current. It is effectively
turned off in the first current zero crossing, as seen in III-C-1.b
for . No steps are forced in the magnetizing current, nor is
voltage transient generated. Since this operation in shared load
lasts no more than one or two half-cycles, a higher value for the
equalizing current than in shared load is permitted and a low coil
voltage drop in steady state is easily obtained. The equivalent
power of the coil is also reduced.
3) Mixed Load (Shared and Nonshared Load): By combining the above modes, a mixed operation is obtained. The
load can be fed in steady state either by one single branch or
by two consecutive branches [4] (1st and 3rd pat.), [6]. The
number of voltage states, see (6), is the sum of the states in
both modes. The switches and the coil windings must be rated
must be a compromise drawn from
for nonshared load.
the recommended values for shared and nonshared load. In a
change, the stabilizer passes from a given mode to the other
one. Only the following changes are possible:
1) Change from one operating branch, nonshared load, to two
branches (incoming the next or the precedent tap) shared
load.
2) Change from two branches, shared load, to a single branch
(outgoing: either the superior or the inferior tap), nonshared load.
In any case the commutation follows one of the processes
seen in III-C-1.b or III-C-2.b.
4) Nonshared Load and Mixed Load With Coil Short-Circuit
Winding: If the coil voltage drop in nonshared load is too high
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1496
(as in equipment with high values of the inter-tap voltage
which, in turn, happens when input and output voltage deviation
helps keep that
can be high), adding a short-circuit winding
drop low.
and
do not operate during commutation.
is shortened by means of an extra switch
After a change
[4] (1st and 3rd pat), Fig. 5(a). The inductive drop in the
tap winding is eliminated leaving the resistive drops and the
conducting drop.
closes one cycle
reflected value of the
after a tap change and opens one cycle before a new change.
is the same as in a
The power managed in winding
one-tap winding since both operate assembled as a transformer.
is reduced by the turns ratio of
with
The current in
respect to a main winding. If this ratio is chosen over ten, the
becomes negligible.
cost of
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009
Fig. 9. Solution 1. Three-phase stabilizer, direct configuration, taps in the secondary. Only phase R is detailed. Values for a typical medium voltage line.
D. Coil Magnetic Evolution. Equivalent 50 Hz Transformer
Power
If a commutation starts with no heed paid to the magnetic
state of the coil core, the induction can in practice reach a transient value up to 2.8 times the steady state value, [5], [6]. Then,
typically a steady induction of 0.5 T must be chosen to guarantee
no more than 1.4 T in transients which, in turn, results in a high
core section. The resulting worst half-cycle peak current during
commutation found in equipment designed under this criteria,
as mentioned in II.D, is less than 4 times the steady repetitive
peak current with nominal load for shared load, less than 6 times
for nonshared load and less than 7 times for mixed load (when
operating in single tap).
For stabilizers over 100 kVA it is economically advisable to
limit the maximum induction (thus reducing the security factor
from 2.8 to 1.3 or less) by choosing a convenient instant to start
commutation. This requires a delay of one cycle or less from the
tap change order of the control circuit. The delay has no practical
effects in slow acting (supercyclic) stabilizers. The equivalent
50 Hz transformer power of the coil is reduced in the same proportion as the maximum induction. The commutation start time
can be chosen by monitoring the core induction, by voltage and
current phase criteria or by a mixed procedure.
IV. INITIAL ANALYSIS AN SYNTHESIS OF CONFIGURATIONS
Here the direct and the compensating transformer configurations (see III-B) shall be briefly analyzed and synthesized to
compare their pros and cons independently of the operation
mode (shared, nonshared and mixed load, see III-C) applied.
As this study will show, the direct configuration (Solution 1)
is at a clear disadvantage, both technically and economically.
It is similar to the solutions studied by EPRI [2], [3]. The stabilizers derived from the compensating configuration here proposed (Solutions 2 to 5) can be installed in any point of the line
with no association to the line transformer, thanks to an auxiliary feeding shunt transformer.
A. Stabilizers With Direct Configuration (Solution 1)
The electronic switches are linked to the secondary of the
HV/MV transformer that feeds the medium voltage line. They
manage the complete line power and voltage. In many mechanical stabilizers the switches are in the primary to reduce the cur-
Fig. 10. Stabilizer, with series compensating transformer CT, connected at the
output of the line feeding transformer. Circuit variant with independent shunt
transformer ST feeding the tap changer. To calculate ST power, the single comb
is assumed. Only one phase is detailed.
Fig. 11. Single-phase circuit of the stabilizer for solutions with compensating
transformer CT and fault deviation switch S . The tap topology chosen in the
figure is a single switch-comb of five taps (Fig. 6(b)). The shunt transformer ST
feeds CT through de tap comb and the limiting multiple winding coil L
L.
ST also adds a fixed voltage to the line to accomplish balanced operation of CT.
An optional medium voltage disconnecting switch MDS makes live installation
and removal possible.
0
rent rate. In electronic equipments this advantage is cancelled
by the need for more series-connected semiconductors.
Fig. 9 shows the general circuit with only one phase in detail
for the sake of simplicity. Three-phase equipment is normally
made with three wye-connected single-phase circuits, although
other connections are sometimes preferred for specific applications and control strategies [22].
1) Thyristor Rating: The steady estate current in the switches
(nonshared load; mixed load in the nonshared condition)
is the nominal line current . The maximum overload typiduring 1 h every 8
cally previewed by line operators is 0.2
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MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
h and, under fault condition, 11.5
during 0.1 s. Normally,
all semiconductors are chosen so as to withstand the voltage of
the upper tap. The worst case voltage (maximum line voltage
condition) is derived taking into account that the transforming
ratio between the secondary upper tap and the primary must
guarantee the minimum allowed load voltage plus the optional
, while compensating for
output deviation
the voltage drops
and , under minimum input voltage
. Then (for the numerical limits given in the example in III-B, for the line of 34.5 kV and 10 MVA in Fig. 9,
and for the said 11.5 overload rating), each switch withstands
MVA
kV
A rms during
A rms during
kV
repetitive peak
s
(11)
(12)
kV
(13)
kV rms
kV
Taking into account the usual security coefficients [34] such
as
for current; 0.83 supplementary for parallel association;
for voltage; 0.7 supplementary for series association;
the ratings for the series-parallel ensemble in each switch are
A
rms during h
A
A repetitive peak during
s
kV
kV repetitive peak
(14)
(15)
(16)
An economical and easy option (March 2007), which is not
in the limit of the available current and voltage, could be the
thyristor Westcode K1947ZC450 (330 /unit) whose characteristics are
A RMS
C and
for an antiparallel pair
A
A rms
A av
A
C and
for an antiparallel pair
A
A rms
A repetitive peak during
A nonrepetitive peak
V repetitive peak
1497
) are needed for the equipment. Thyristors for the coil shortseen in III-C-4, if any, have been ignored
circuit switches
because of their low rating. This confirms the EPRI conclusion
seen in II-F-1: the electronic upgrade of mechanical stabilizers
by simple substitution of switches in the direct configuration is
possible but expensive.
2) Mains Transformer Power: The effective power of the
mains transformer exceeds the nominal power
[10 MVA in
the example] due to the extra winding necessary to feed the extreme tap operating with minimum input voltage and maximum
load. The extra secondary voltage needed to compensate for the
negative input deviation and the line drop , in per unit of
, so that the power
the nominal voltage, is
of the transformer rises in that proportion.
In transformers cooled by oil, the typical transient line overload condition (20% during 1 h every 8 h, as shown in IV-A-1)
for this equipment makes it necessary to overrate the power
following the average quadratic rule. Afterwards, the effective
power of the transformer for this configuration and the values
of the example is
MVA
MVA
The per unit extra secondary voltage
has been calculated for nonshared load for the sake of simplicity.
For other operation modes similar values are obtained. The average quadratic factor 1.027 also would apply for a line transformer without taps.
3) Equivalent 50 Hz Transformer Power of the Multi-Winding
Coil: The mixed load mode is considered here because it gives
the best economical results. Every winding withstand the nominal current in nonshared load, although in shared load the current is lower. The voltage handled by any winding is equal to
the output voltage change which, in turn, must not exceed the
. This value
allowed step of the output voltage
can be affected in practice by 0.9 due to voltage drop in the conducting thyristors. Pressumably, a 0.8 coefficient acounts for the
under rate of the windings section due to nonsimultaneous operation. Thus, the equivalent power becomes
MVA
ms
ms
Subsequently, a series association of
kV
kV
steps is needed. Parallel association is not required. Thus,
thyristors (59 400 )
for the three phases
per tap in one phase are needed. For the best case, mixed load
[five taps per phase, see expression (6)], 900 thyristors (297 000
(17)
(18)
where 6 is the number of windings (five main windings and one
short-circuit winding, see Sections III-B and III-C.4).
1.2 is the core section overrating if the commutating instant
is optimized (see III-D).
B. Stabilizers With Series Compensating Transformer and
Fault Current Deviation (Solutions 2 to 5)
They show the advantages listed in Sections II-E and III-B as
follows.
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1498
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009
• The power managed electronically is, typically, 15% of the
nominal.
• The series association of thyristors is replaced by parallel
association, which is more reliable and economical.
• Overload in the switches due to line faults is practically
eliminated.
• The switches achieve galvanic isolation so that the assembly can be separated from the transformers and fixed
to ground voltage. Both thyristor isolation and live maintenance become easier.
• The stabilizer can be settled at any point of the line, with
no connection to the feeding line transformer.
The general circuit for these solutions is given in Fig. 10.
As shown in Fig. 3, and in Section II-E, Fig. 6(b), (c), the secondary of the series compensating transformer CT adds or subtracts the voltage required by the line at the settling site chosen.
The voltage needed to feed the primary is obtained by means
of a shunt transformer ST and associated taps. The power managed by CT is proportional to the voltage added-subtracted to
the line voltage, typically 15% of the nominal power as advised
in Section II.E. This figure also shows the power managed by
the switches.
The optimum design (minimum managed power) seeks equal
voltages added and subtracted by the secondary of CT in the extreme conditions. If the combination of required line input and
output voltage limits, MT drop and line drop do not result in an
overall symmetrical voltage addition and subtraction, an extra
balance winding in the ST primary, in series with CT primary,
is needed to attain balanced operation of CT. The extra winding
works as an autotransformer with the main primary winding and
adds half the difference between the system adding and subtracting voltages. For the example in Section III-B, the extreme
conditions are:
• Extreme condition 1: Minimum input voltage, nominal
load. The maximum phase-neutral voltage to be added by
the stabilizer is (numerical values for the example given
in III-B)
kV
(19)
• Extreme condition 2: Maximum input voltage, no load.
The maximum phase-neutral voltage to be subtracted is
kV
(20)
The phase-neutral voltage added by the balance winding becomes
and the voltage added by the CT secondary in extreme condition
1 (equal to the voltage subtracted in extreme condition 2) is
kV
(22)
This is a very economical configuration if the voltage given
. The operation and
by expression (22) is lower than 0.3
commutation modes seen in Section III-C also apply to this
group of solutions.
Besides the stated reduction of the power managed by the
in the example, see (22)), CT
switches (reduced to 0.145
helps to drastically cut down their overcurrent due to line faults
connected
by means of an extra fault deviation switch
across the primary, see Fig. 11. Once the fault is detected, the
conducting main switches are inhibited turning off at the first
current zero crossing. One half-cycle later (to avoid internal
is turned on absorbing the line over-current
short-circuit)
reflected into the CT primary until the ordinary line protection
switch opens (normally after 100 ms maximum). In this way
the maximum line current (typically, 12.5 times the nominal)
reflected to CT primary is supported by the main switches
during no more than
during one or two half cycles, and by
are usually open, they do not
100 ms. As the thyristors of
need cooling heatsinks. Under fault condition, the stabilizing
operation is inhibited and the equipment is seen by the line as
a short-circuited CT in series with the balance winding of ST.
The rising effect of ST in the line fault current is compensated
approximately by the short-circuit impedance of CT.
1) Single Switch-Comb Circuit: The single-comb configuration, Fig. 11, shall be first studied in detail for the sake of conceptual and circuit simplicity.
a) Thyristor Rating: Tap thyristors: The line current reflected in the CT primary is the steady state current in any conducting tap switch for the nonshared load, and the maximum
value of this current for mixed load. For single-comb, Figs. 6(b)
and 11, the voltage seen by the extreme tap switches is the secondary voltage of ST, which must be chosen to take maximum
advantage of available medium voltage thyristors without series association. For using thyristors of 4 500 V repetitive peak
, for instance, the secondary rms voltage of ST
V. Following the example
must be 4 500 V
in Section III-B, the balanced configuration in Figs. 10, 11, and
supposing, for the sake of simplicity, that the common tap is in
the middle of ST secondary, the sec./prim. ratio of CT (ignoring
voltage drop in switches and ohmic drop in coils), according to
(22), is
V
V
kV
(21)
(23)
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MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
where
is the voltage drop in CT for nominal load
in per unit of its own voltage.
Extending to every tap switch the voltage requirements for
the extreme taps, each switch withstands
MVA
kV
A rms during
(24)
A nonrepetitive ms peak
V repetitive peak
(25)
(26)
Introducing the security coefficients seen in Section IV-A-1,
the nominal ratings for every tap semiconductor ensemble become
A
A rms during h
A
A nonrep
ms peak
V
V repetitive peak as ab initio
(27)
(28)
(29)
By using the thyristor Westcode K1974ZC450 as in the direct configuration in Section IV-A-1, two units in parallel are
needed. After the ratings related in Section IV-A-1 for a single
SCR, the ratings for the tap ensemble (four SCR: two paralleled
antiparallel pairs) become
A
change with respect to a standard nontap transformer of the
nominal power (10 MVA in the example).
c) Effective Power of the Compensating Transformer CT:
To guarantee independent voltage stabilization of the phases,
three single-phase compensating transformers are needed. The
power managed by one CT in the extreme condition 1 (see IV-B)
is the product of the added phase voltage, expressed in (22), by
the nominal line current . Taking into account the 1 h overload
as in (17), the CT effective power becomes
MVA
(33)
d) Effective Power of the Shunt Transformer ST: In extreme condition 1, the single phase power given by the balance
winding is the product of the voltage expressed in (21) by the
nominal line current . The single-phase power given by the
upper half of the secondary to CT is given by (33) plus CT
losses. The lower half manages similar power in the extreme
condition 2. A 1.4 factor is applied as the secondary halves
do not operate simultaneously. Approaching the sum of unitary
losses in CT, the coils and the switches to four times the CT
, the equivalent
voltage unitary drop under load
50 Hz power of ST becomes
MVA
C permanent
(30)
A
A nonrepetitive
V repetitive peak
1499
ms peak
(31)
(32)
that surpasses the requirements given by (27) and (28).
The final result is four individual thyristors per tap, which, for
three-phase equipment, means
12 thyristors (3 960 ) per
tap in one phase. For the best case (mixed load) of the proposed
example (5 taps per phase), this gives 60 thyristors (19 800 )
for the equipment. Coil short-circuit switches are ignored as in
IV-A-1.
withstands the fault
Fault deviation thyristors: Switch
line current reflected to the primary of CT and, for the singlecomb circuit (Fig. 11), supports half the voltage seen by the
tap switches, as one side of the CT primary is connected to the
common tap. After the security coefficients seen in IV-A-1, the
is expressed in (25) exrepetitive peak current rating for
tended to 100 ms. The voltage is half the value expressed in
can be made, for unification, with 12
(26). Three switches
units K1947ZD450 (two paralleled antiparallel pairs per phase)
used for the taps, although underused in voltage. This represents
an additional 3 960 to the cost of the equipment.
b) Mains Transformer Power: The equipment is independent of the mains feeding transformer which does not need to
(34)
e) Equivalent 50 Hz Transformer Power of the
Multi-Winding Coil: As for the direct configuration, the
mixed load mode is considered. The value given in (18) is
also valid for the compensating configurations as, although the
, its current is multiplied
winding voltage is now divided by
by the same ratio.
2) Double Switch-Comb Circuit: In the double-comb configuration [Fig. 6(c)]—more suitable than the single-comb for high
power, as in the example (10 MVA)—we take (as in IV-B-1-a) 1
273 V again for the complete ST secondary. The voltage stress
ratio and the current
of the switches does not change. The
in the taps are divided by two, as the voltage in CT primary for
the extreme conditions is twice the value for single-comb. For
the example, shared load, 6 taps per comb result, see (9). Following the calculus procedure of IV-B-1 for the single-comb,
the results are:
a) Thyristor Rating: Tap thyristors: The current demand
per tap is given by (27) and (28) divided by two. The repetitive
peak voltage is given by (29). By using the same K1974ZC450,
a single antiparallel pair per tap (no parallel or series associataps per phase) 36
tion needed) results. For mixed load (
thyristors (11 880 ) for the equipment are needed.
In both the single and the double-comb circuit, the cost of the
(see III-C-4) to shorcircuit the coil, if any, has
small switch
not been considered.
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1500
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009
Fault deviation thyristors: Switch
withstands twice the
voltage (4 500 V repetitive peak) and half the current (expression (25) divided by two: 6 774 A repetitive peak during 100
ms) needed with the single-comb. For the example, six units
(one antiparallel pair per phase) of the aforementioned thyristor
are needed, adding 1 980 to the cost of the equipment.
Comparison of the thyristor rating with direct configuration:
The results in IV-B-1.a and IV-B-2.a for the assembly of tap
and fault deviation thyristors show that the compensating transformer configuration improves the poor expectations of early
economical studies carried out excluding this configuration [2],
[3]. Although both the single and the double switch-comb drastically reduce the number of thyristors, the double-comb has a
clear advantage.
b) Mains Transformer and Compensating Transformer
Power: The same considerations than for single-comb apply
to the mains transformer. The compensating transformer has
the same secondary than in the single comb. The primary has
half the current and twice the voltage so that the same effective
power (33) applies. The ratio
’ is half the value given in
(20).
c) Effective Power of the Shunt Transformer ST: The
working conditions of the balance winding are the same as that
for single-comb. Only one secondary is needed with 1 273 V
rms to have the same nominal repetitive peak voltage rate in
is
the switches than for single-comb (4 500 V). The ratio
half the value given in (23). Equation (34) for the equivalent 50
Hz power of ST with a single-comb, becomes the following for
the double-comb:
MVA
(35)
d) Equivalent 50 Hz Transformer Power of the
Multi-Winding Coils: For the proposed example, mixed
load, double comb, two multi-winding coils per phase with
three tap coils (plus one short circuit coil) are needed. The
voltage winding is twice and the current is half the respective
values for single comb. By modifying expression (18), the
equivalent power of each coil becomes
MVA
(36)
TABLE II
THYRISTORS AND MAGNETIC COMPONENTS IN DIRECT AND COMPENSATING
CONFIGURATIONS FOR THE 10 MVA STABILIZER EXAMPLE INTRODUCED IN
=/UNIT, MARCH,
III-B (CHOOSEN THYRISTOR: WESTCODE K1947ZC450 (330 C
2007) COMPARED TO THE CLASSICAL ELECTROMECHANICAL SOLUTION
magnetic components have to be considered for a more complete comparison. In the first column, the three-phase direct mechanical configuration (as appears in Fig. 6(a) for single-phase)
is given. The thyristor’s cost of the compensating transformer,
double-comb, configuration is lower than the cost of the electromechanical switch assembly (Fig. 2, upper, left).
V. LIVE WORKING INSTALLATION AND REPAIR
As a consequence of their independence from the line feeding
transformer, the stabilizers derived from the compensating configuration proposed in IV-B can be connected to any point of the
line to regulate the downstream voltage or to adjust the voltage
of a derived line.
The stabilizer (in the single-phase equivalent circuit) is seen
by the line as a dipole (see Fig. 11) similar to the medium voltage
disconnecting switches used to isolate parts of a complex net
[49], [50]. Subsequently, the stabilizer can be connected and
removed by using the same live working installation procedure
[45]–[47] developed by the electric companies for such switches
without service interruption, see Fig. 12 and Table III.
Since the power-electronic assembly is at ground potential,
some maintenance operations can be performed in live status.
VI. AUTOREMOVAL
In case of stabilizer internal malfunction, the tap switches and
associated circuits can be functionally eliminated by following
the removal sequence in Table III up to point 3, keeping in coordination the OLTC control and the MDS control. The line can
remain operative during the stabilizer reparation in situ or the removal and reparation in a workshop, in this case following the
complete removal sequence.
C. Quick Economical Comparison of Direct and Compensating
Transformer Configurations
VII. CONCLUSION
Table II compares the thyristors and the magnetic components
needed in the direct and the compensating configurations, mixed
load, to implement the 10 MVA stabilizer of the example introduced in III-B. The compensating configuration (both with
single and double-comb) adds extra advantages as a free connection to any point of the line (see Section V). The costs of the
The feasibility of voltage stabilizers for medium voltage lines
based on electronic tap changers has been reviewed drawing
from the advantages of using a multi-winding commutating coil,
largely proved in power distribution applications. This coil reduces the commutation stress of the thyristors so that overrating
is not necessary.
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MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
1501
TABLE III
LIVE WORKING INSTALLATION STEPS PLANNED FOR THE COMPENSATING
TRANSFORMER CONFIGURATION STABILIZER PROPOSED IN FIGS. 10–12
The configuration proposed needs additional electromagnetic components (multi-winding commutating coil, shunt
transformer and compensating transformer) whose power is
keep under reasonable limits (about 15% of the nominal power).
The advanatages of the proposed stabilizer are as follows.
• Isolation of the electronic ensemble with respect to the line.
It can be connected to ground potential.
• Series association of the thyristor is not needed. For power
of more than 10 MVA, an easy parallel association would
be needed.
• The fault line current is derived from the tap thyristors to a
specific, not expensive, fault deviation thyristor switch.
• Connection to any point in the line without any connection
to the head of the line feeding the transformer.
• The configuration that is chosen facilitates eventual live
working installation and repair.
REFERENCES
Fig. 12. (a) Single-phase conceptual scheme for the line connection of the compensating configuration stabilizer proposed in Section IV-B. In parallel with the
stabilizer there is a classic mechanical disconnecting switch (MDS). (b) Stabilizer inhibited, MDS open, S closed. (c) Equivalent circuit for (b) status. Only
the short circuit impedance of CT remains in series with the line. (d) Possible
ground installation at a generic line point allowing a live working connection
and removal. Single-phase representation after Fig. 1 of [49]. Aerial installation
as in Fig. 4, right, is also possible.
The study confirms the high costs of thyristors previewed
[2], [3] if the classic direct configuration used in most mechanical tap changers is considered. Nevertheless, it also demonstrates drastic reductions and the possibility of adding technical advantages by using compensating transformer configurations, well-known in low- and medium-voltage low power
applications.
This paper shows the feasibility of a 69 kV/34.5 kV, 10 MVA
stabilizer for long medium voltage lines by using 42 standard
thyristors rated 1 847 A, 4 500 V, with no need of series, nor
parallel, association.
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[3] “Study of improved load-tap-changing for transformers and
phase-angle regulators,” Apr. 1990, p. 108, EPRI Staff, EPRI,
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Portugal, Jun. 1–2, 1999.
[10] J. Vaquero, “AC on-load tap-changing fast voltage stabilizers for
power quality improvement,” Ph.D. dissertation, Universidad a Distancia, Madrid, Spain, Apr. 2000.
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Spain, Oct. 2000.
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1502
[12] J. C. Campo, J. Vaquero, M. A. Perez, and S. Martinez, “Fast twotaps changing AC stabilizer with seminatural switching,” Elect. Power
Quality Utilization, vol. 6, no. 1, pp. 111–114, Dec. 2000.
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[15] J. C. Campo, J. Vaquero, M. A. Perez, and S. Martinez, “Dual-tap chopping stabilizer with mixed seminatural switching. Analysis and synthesis,” IEEE Trans. Power Del., vol. 20, no. 3, pp. 2315–2326, Jul.
2005.
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Salvador Martínez García (M’88–SM’90) was
born in Spain in 1942. He received the M.Sc. and
Ph.D. degrees in electrical engineering from the
Polytechnic University of Madrid, Madrid, Spain, in
1966 and 1969, respectively.
He was Associate Professor at the Polytechnic
University of Madrid from 1975 to 1979 and at
the National Distance University of Spain, Madrid,
from 1979 to 1982, where he has been Full Professor since 1982. He was a Design Engineer of
power-electronics equipment in several companies
for eight years. His research interests are in integrated magnetics and power-line
conditioners.
Prof. García was a member of the IEEE Industry Applications Society Industrial Static Converter Committee EWG from 1990 to 1997.
Authorized licensed use limited to: UNIVERSIDADE DE SAO PAULO. Downloaded on March 10,2010 at 11:28:43 EST from IEEE Xplore. Restrictions apply.
MARTÍNEZ GARCÍA et al.: FEASIBILITY OF ELECTRONIC TAP-CHANGING STABILIZERS FOR MV LINES
1503
Juan Carlos Campo Rodríguez (M’97) born in
Spain in 1970. He received the M.Sc. degree in
electrical engineering from the Oviedo University,
Asturias, Spain, in 1995 and the Ph.D. degree in
electrical engineering in 2000.
Currently, he is an Associate Professor in the Department of Electrical and Electronic Engineering at
Oviedo University, where he has been since 1995. He
has conducted research on a fast-tap changers for ac
voltage control. His current research interests include
power quality, line conditioners, and tap changers.
Alfonso Ibarzábal Segura was born in Spain in
1945. He received the M.Sc. degree in electrical
engineering from the Polytechnic University of
Bilbao, Bilbao, Spain, in 1974.
He is the Technical Director of INCOESA, dedicated to the development and manufacture of distribution power transformers, medium-voltage transformers, and onload tap changers.
Mr. Segura is a member of the Standards
Technical Committee of AENOR CTN207/GT 14,
“Power Transformers.”
José Antonio Jardini (M’66–SM’78–F’90) was
born in Brazil in 1941. He received the M.Sc. and
Ph.D. degrees from the Polytechnic School of São
Paulo University, EPUSP, São Paulo, Brazil, in 1971
and 1973, respectively.
He was with Themag Eng. Ltd. for 25 years,
where he conducted numerous studies on power systems and participated in many important Brazilian
projects, including Itaipu. Currently, he is a Professor in the Departamento de Energia e Automação
Electricas da Escola Politécinca da Universidade de
São Paulo, São Paulo, Brazil, teaching power analysis and digital automation.
He has conducted research work in power systems for many Brazilian utilities.
Prof. Jardini is a member of the CIGRE Working Group on HVDC transmission.
Pedro María Martínez Cid was born in Spain in
1956. He received the M.Sc. degree in electronic and
control engineering from the National Distance University of Spain, Madrid, in 1989.
Since 1995, he has been a Lecturer with the
National Distance University of Spain at the Associated Centre of Bilbao. In 1979, he joined the
electrical company IBERDROLA S.A. where he has
conducted research-and-development projects on
flexible ac transmission systems (FACTS), including
the ESTRAP Project among them. He is with the
Division for Innovation, Power Quality, and Ambient Studies.
Mr. Cid is a member of several international committees for the promotion
of FACTS.
Joaquín Vaquero López was born in Spain in 1968.
He received the M.Sc. degree in electrical engineering from the Polytechnic University of Madrid,
Madrid, Spain, in 1994 and the Ph.D. degree from
the National Distance University of Spain, Madrid,
in 2000.
In 1995, he joined the Department of Electrical,
Electronic, and Control Engineering as Assistant Professor at the Universidad Rey Juan Carlos. He was
a Design Engineer of power-electronics equipment
with SEPSA from 2000 to 2007. He joined the Electronics Technology Department at Rey Juan Carlos University of Madrid as an
Associate Professor with research interests in fast multi-tap changers and equipment to improve power quality.
Authorized licensed use limited to: UNIVERSIDADE DE SAO PAULO. Downloaded on March 10,2010 at 11:28:43 EST from IEEE Xplore. Restrictions apply.