Large Static Converters for Industry and Utility
Applications
HIROFUMI AKAGI, FELLOW, IEEE
Invited Paper
The emergence of high-power semiconductor devices such as
insulated-gate bipolar transistors (IGBTs) or injection-enhanced
gate transistors (IEGTs) and gate-commutated turn-off (GCT)
thyristors or integrated gate-commutated thyristors (IGCTs)
enables large static converters to expand into utility and industry
applications. For instance, a 680-kV 50-MW HVDC transmission
system based on a string of many IGBTs connected in series was
commissioned in 1999. This paper describes the present status of
large static converters, with focus on their applications to utility
and industry. It also describes their future prospects and directions
in the 21st century, including the personal views and expectations
of the author.
Keywords—Power electronics, power semiconductor devices,
static power converters.
I. INTRODUCTION
Since the late 1950s, power electronics has been developing by leaps and bounds without saturation into the key
technology essential to modern society and human life, as
well as to electrical engineering. With the help of microcomputers and digital signal processors (DSPs) brought by
microelectronics technology, the following great leaps have
been made in large static converters and its utility/industry
applications in the past ten years:
• installation of an 80-MVA static var generator (SVG)
or STATic synchronous COMpensator (STATCOM)
using gate turn-off (GTO) thyristors rated at 4.5 kV
and 2.5 kA for improvement of power system stability
in 1991 [1];
• commercial operation of a 400-MW adjustable speed
generator/motor controlled by a 72-MVA line-commutated three-phase 12-pulse cycloconverter for pumped
hydroelectric storage in 1993 [2];
Manuscript received August 24, 2000; revised December 18, 2000.
The author yis with the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8554, Japan
(e-mail: akagi@ee.titech.ac.jp).
Publisher Item Identifier S 0018-9219(01)04112-3.
• commission of a 160-MVA unified power flow controller (UPFC) using GTO thyristors rated at 4.5 kV
and 4 kA in 1998 [3], [4];
• commission of a 80-kV 50-MW HVDC transmission system based on series connection of many insulated-gate bipolar transistors (IGBTs) in 1999 [5];
• development of an 8-MVA three-level voltage-source
pulsewidth modulation (PWM) rectifier/inverter
system using 24 injection-enhanced gate transistors
(IEGTs) rated at 4.5 kV and 4 kA (maximum turn-off
current) for steel mill drives in 2000 [6];
• commission of a 250-kV 1.4-GW HVDC transmission system using 1920 light-triggered thyristors
(LTTs) rated at 8 kV and 3.5 kA (average current) in
2000 [7].
Generally, developments in power electronics yield needs
of power electronics, so that the needs induce the developments in turn, as if to constitute a positive feedback system
[8]. For instance, such a positive feedback effect promotes
a feasible study of the world’s largest self-commutated
back-to-back (BTB) system in a range of 800 MW to 1.5
GW in Japan. The BTB system is intended for asynchronous
link and/or frequency change between two adjacent power
systems with the same or different line frequency [11], [12].
Human beings have made a successful voyage from
“Mercury” to the “silicon compound planet,” thus reaching
and forming the power electronics world. As a result, power
electronics technology has brought high performance, high
efficiency, energy savings, high reliability, maintenance-free
operation and compactness to all electrical and electronic
equipment. Power semiconductor devices based on silicon
carbide (SiC) will emerge from the “silicon compound
planet” in the beginning of the 21st century. Silicon-carbide
devices are expected to have the following advantages
over silicon devices; low switching and conduction losses,
especially in high-voltage large static converters, and
high-temperature operation of 400 –500 . Hence,
silicon-carbide devices playing an essential role in reducing
0018–9219/01$10.00 ©2001 IEEE
976
PROCEEDINGS OF THE IEEE, VOL. 89, NO. 6, JUNE 2001
Table 1
Comparison in Converter Capacity per Power Semiconductor Device among a
Light-Triggered Thyristor (LTT), a Gate-Commutated Turn-Off (GCT) Thyristor, and an
Injection-Enhanced Gate Transistor (IEGT)
losses in power conversion systems will make large static
converters much more compact and efficient in the 21st
century.
II. POWER DEVICES AND LARGE STATIC CONVERTERS
The development of large static converters depends
strongly on both power semiconductor devices and
utility/industry applications.
Table 1 summarizes high-power semiconductor devices
that are now on the market, with focus on their ratings and
converter capacity per power semiconductor device [7],
[9], [10]. Note that the GCT thyristor is considered as an
advanced GTO thyristor, while the IEGT is regarded as
an advanced IGBT in terms of device structure. Both LTT
and GCT thyristor in Table 1 are fabricated on a silicon
wafer with a 6-in diameter, although the GCT thyristor
with turn-off capability is lower than the LTT in terms of
converter capacity per device. Note that converter capacity
per device depends on the applications of the high-power
converters using these devices. For example, extremely
high reliability is required for HVDC transmission systems
with a dc link voltage as high as 250 kV. This results in a
margin or redundancy of power devices, so that the converter
capacity per device is reduced somewhat, compared to that
in medium-power converters without series connection of
power devices.
Fig. 1 Circuit configuration of the
transmission system.
6500-kV 2.8-GW HVDC
Table 2
Specifications of the HVDC System
III. HVDC TRANSMISSION SYSTEMS
A. A 2.8-GW HVDC System Using LTTs
Fig. 1 shows a 2.8-GW HVDC transmission system using
light-triggered thyristors (LTTs) rated at 8 kV and 3.5 kA.
Each thyristor valve consists of a string of 40 LTTs. The
first stage of 1.4 GW was commissioned on June 22, 2000.
Table 2 summarizes the specifications of the HVDC system
in the first and second stages. The total number of thyristors
used in two converter stations is 1920 on the first stage [7].
The HVDC system can control a bidirectional power flow of
1.4 GW through a 51-km-long submarine transmission cable
and a 51-km-long overhead transmission line between the islands of Shikoku and Honshu in Japan. The HVDC system
has a plan of increasing the power rating to 2.8 GW at the
final stage. A main reason for introducing the HVDC system
into such a short-distance power transmission system is that
the 51-km-long submarine cable would impose restrictions
on the sending power capacity in an HVAC system because
a large amount of leading current would flow through nonnegligible parasitic capacitors in the submarine cable. The
HVDC system realizes the so-called “asynchronous link” between the two grids facing each other over the Kii-Channel
in the west of Japan. Another reason is not to constitute any
loop with an existing 500-kV ac transmission line.
B. A 50-MW HVDC System Using IGBTs
An HVDC system based on voltage-source PWM converters consisting of series-connected IGBTs is suitable for
AKAGI: LARGE STATIC CONVERTERS FOR INDUSTRY AND UTILITY APPLICATIONS
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Fig. 2
The 50-MW HVDC system using IGBTs.
a power range of 1–100 MW and for a dc voltage range
of 10–100 kV. This system may be referred to as “HVDC
light” by ABB Sweden. The world’s first HVDC transmission system using IGBTs started commercial operation on
the island of Gotland in the Baltic Sea in 1999 [5]. The dc
transmission power and voltage are rated at 50 MW and 80
kV, while the converter rating is 65 MVA.
Fig. 2 shows the system configuration of the HVDC
system, in which six IGBT valves form a three-phase
voltage-source PWM inverter. Each valve comprises many
IGBTs connected in series, so that it can withstand a dc link
voltage as high as 160 kV. Such an IGBT-based HVDC
transmission system makes it economical and feasible to
connect small-scale renewable power generators to a utility
grid of 70 kV.
IV. BTB SYSTEMS
A. A 100-MVA BTB Intertie Using IGCTs
In September 1996, a 100-MVA BTB intertie with a
dc link voltage of 10 kV was commissioned as a frequency changer between a three-phase grid of 50 Hz and a
single-phase 16 2/3-Hz railway grid in Germany.
Fig. 3 shows a simplified power circuit configuration of
the BTB system, eliminating auxiliary circuits such as harmonic power filters and shunt inductors at the 50-Hz grid,
and protection circuits from the actual power circuit [14].
This system is characterized by:
• the use of integrated gate commutated thyristors
(IGCTs) rated at 4.5 kV and 3.5 kA;
• a string of six IGCTs connected in series for the purpose of withstanding a 10-kV dc link voltage;
• a low-inductance high-power IGCT inverter system
without fuses.
The multilevel concept using 12 single-phase transformers
performs filterless operation at the 16 2/3-Hz grid, because
the ac terminal voltage is a 25-step waveform.
B. An 800-MW BTB System Using GCT Thyristors
Fig. 4 shows a feasible circuit configuration of a parallel-redundant 800-MW BTB system for asynchronous link
and/or frequency change between two utility grids with the
same or different line frequency [12]. The BTB system is
different from an HVDC transmission system in terms of dc
link voltage and dc link cable/line length.
The BTB system consists of eight BTB units numbered
1–8, which are connected in parallel between the two utility
grids via two step-down transformers with a voltage rating
978
Fig. 3 The 100-MW BTB system using IGCTs.
Fig. 4
The parallel-redundant 800-MW BTB system.
of 500 kV/33 kV. Each BTB unit, rated at 100 MW, consists
of two sets of 16 three-phase inverter cells, two sets of 16
three-phase transformers, and a common dc capacitor. Each
inverter cell rated at 6.25 MW consists of six gate-commutated turn-off (GCT) thyristors and diodes, forming a
voltage-source inverter. The primary terminals of each transformer are connected in series with each other. This results
in a significant reduction of voltage/current harmonics, even
though the switching frequency of the GCT thyristors is
equal to the line frequency. The GCT thyristor fabricated on
a silicon wafer with a 6-in diameter is rated at 6 kV and 6
kA (maximum turn-off current), and, therefore, the dc link
voltage across the common dc capacitor is designed to be
3 kV [11]. This makes insulation, isolation, and protection
much easier than a dc link voltage as high as, or higher than,
250 kV in a conventional HVDC transmission system. The
total number of the GCT thyristors used in the 800-MW
.
BTB system reaches
V. UNIFIED POWER FLOW CONTROLLERS
The installation of a unified power flow controller (UPFC)
has been completed and a series of commissioning tests has
been conducted at the Inez Substation of American Electric
Power (AEP) in eastern Kentucky. This installation of a
UPFC consisting of two 160-MVA voltage-source GTO
thyristor-based inverters is the first large-scale practical
demonstration of the UPFC concept, and its completion is a
significant milestone on the progress of power electronics
technology for flexible ac transmission systems (FACTS).
The main reason for putting the UPFC into practical use is
the critical need to increase power transfer capability and
provide voltage support in the Inez area.
Fig. 5 shows a one-line diagram of the 160-MVA UPFC
that was installed at Inez in 1998. Each inverter is of the
PROCEEDINGS OF THE IEEE, VOL. 89, NO. 6, JUNE 2001
Fig. 5
Inez.
One-line diagram of the 160-MVA UPFC installation at
Fig. 7 System configuration.
VI. FLYWHEEL ENERGY STORAGE SYSTEM
A 200-MJ/20-MW flywheel energy storage system has
been installed in the vicinity of an arc furnace, and was commissioned in August 1996 at the Chujowan substation of
the Okinawa Electric Power Company in Japan [21]. Fig. 6
shows a cutaway view of a generator-motor equipped with a
flywheel. This is not a trial application but is in commercial
use for frequency regulation on a 132-kV bus.
A. System Configuration
Fig. 6
Cutaway view.
so-called three-level type, comprising multiple “pole” structures, each carrying four GTO-thyristor valves. Each valve in
turn comprises a string of eight or nine 4.5-kV 3-kA GTOthyristors connected in series, where the dc link voltage is 24
kV. The UPFC has the ability to independently control the
active and reactive power flows on a 138-kV transmission
line, as well as to regulate the local bus voltage [3], [4].
Fig. 7 shows the circuit configuration of the
200-MJ/20-MW energy storage system. It consists of a
26.5-MVA doubly fed generator-motor coupled with a
flywheel of 74 000 kg with a 4-m diameter, a three-phase
12-pulse line-commutated cycloconverter, an excitation
transformer, and a controller. Table 3 summarizes the system
ratings. This flywheel energy storage system could be
referred to as ROTES (ROTary energy storage) [21]. This
newly developed generator-motor has a three-phase distributed wound rotor of 50 000 kg, equipped with collector
rings. The rotor windings of the 12-pole generator-motor
are excited with three-phase low-frequency ac currents,
which are supplied via the collector rings by a 6.55-MVA
cycloconverter using light-triggered thyristors. The stator
terminals rated at 6.6 kV are connected to the 66-kV utility
grid through a step-up transformer. The output frequency
of the cycloconverter is controlled within 0.25–9.0 Hz, and
the line frequency is 60 Hz. This enables the cycloconverter
to operate in a circulating current-free mode. The generator-motor with a synchronous speed of 600 rpm is adjusted
in a speed range from 15% (510 rpm) to 15% (690 rpm).
AKAGI: LARGE STATIC CONVERTERS FOR INDUSTRY AND UTILITY APPLICATIONS
979
ventional constant-speed generator system at the Ohkawachi
pumped hydroelectric storage plant, owned and operated by
the Kansai Electric Power Company of Japan [2]. The field
windings of a 20-pole generator rated at 400 MW are excited
with three-phase low-frequency ac currents, which are supplied via slip rings by a 72-MVA three-phase 12-pulse linecommutated cycloconverter. The armature terminals, rated
at 18 kV, are connected to a 500-kV utility grid through a
step-up transformer. The output frequency of the cycloconverter is controlled within 5 Hz, and the line frequency is
60 Hz. This enables the cycloconverter to operate in a circulating current-free mode in the adjustable speed system,
which has a synchronous speed of 360 rpm with a speed
range from 8.3% (330 rpm) to 8.3% (390 rpm). Speed
control provides the capability to control the input active
power in a range of 80 MW in pump mode.
The 400-MW adjustable-speed system has the following
potential advantages over the conventional constant-speed
system:
Table 3
System Ratings
The electric energy is stored as the rotating kinetic energy
in the rotor and flywheel of the steel structure. Adjusting
the rotor speed makes the generator-motor either release
the energy to the power system or absorb it from the power
system. The 26.5-MVA generator-motor coupled with the
flywheel has a thrust bearing capable of withstanding its
adjustable-speed operation. Note that the flywheel effect of
710 t-m in Table 3 includes the rotor inertia.
B. Dynamic Performance
The 200-MJ/20-MW flywheel energy storage system is
the world’s first commercial operation of such a large-capacity flywheel energy storage system. The dynamic performance of the energy storage system has been confirmed by
actual measurements. When the ROTES was disconnected
from the 66-kV bus, frequency fluctuation over 0.4 Hz
often appeared in the line frequency, resulting from sudden
and frequent load changes as large as 30 MW in a dc arc furnace. When the ROTES was connected, the frequency fluctuation was suppressed to within 0.3 Hz, thus meeting the
goal of installation. The ROTES has been operating properly
for more than four years, showing great promise as a FACTS
device which has the capability of repetitively releasing or
absorbing electric power with a response time as fast as less
than 100 ms.
VII. ADJUSTABLE-SPEED GENERATOR SYSTEMS
PUMPED HYDROELECTRIC STORAGE PLANTS
FOR
The total capacity of pumped hydroelectric storage plants
is 14 GW in the United States, while it is 24 GW in Japan. Recent progress in power electronic technology has made it possible to achieve adjustable-speed operation of 300–400-MW
generators in pumped hydroelectric storage plants.
A. A Line-Commutated Cycloconverter-Based System
In December 1993, a 400-MW adjustable-speed generator system was commissioned along with a 400-MW con980
• high-speed active-power control can be achieved by the
combination of the cycloconverter and the inertia effect of rotating parts. For instance, the actual input active power follows its reference with the ramp response
spending 20 s from 256 MW to 400 MW in pump mode.
This makes a significant contribution to improvement
of frequency control, especially in pump mode at night;
• the total operational efficiency of the system increases
by 3%;
• the stability of the adjustable-speed system under line
fault and reclosing conditions is far superior to that of
the conventional constant-speed system.
B. A GTO Rectifier/Inverter-Based System
In June 1996, an adjustable-speed generator system rated
at 300 MW was commissioned at the Okukuyotu II pumped
hydroelectric storage plant, owned and operated by the Electric Power Development Co., Ltd. in Japan [22]. A voltagesource PWM rectifier/inverter using GTO thyristors rated at
4.5 kV and 3 kA is connected between the stator and rotor
winding terminals of the generator, instead of a line-commutated cycloconverter.
The GTO thyristor-based rectifier/inverter system is
rated at 40 MVA. Each of four three-phase rectifiers is
composed of three single-phase H-bridge voltage-source
rectifiers. One pair of three-phase rectifiers is connected to
one excitation transformer, and the other pair of three-phase
rectifiers is connected to the other excitation transformer.
Thus the three-phase rectifier system has positive and negative terminals and a neutral terminal on the dc side. Each
of three three-phase voltage-source inverters connected in
parallel through ac reactors is a three-level voltage-source
PWM inverter. This forms the 40-MVA inverter system, the
ac terminals of which are connected to the rotor terminals
via three slip rings without transformers. The switching
frequency of the GTO thyristors is 500 Hz [22].
PROCEEDINGS OF THE IEEE, VOL. 89, NO. 6, JUNE 2001
Fig. 8 The 48-MVA shunt active filter installed for power conditioning in the Shintakatsuki
substation.
VIII. ACTIVE FILTERS FOR POWER CONDITIONING
A. The State of the Art of Active Filters
Practical applications of shunt active filters are expanding
not only into industry and electric power utilities but also into
office buildings, hospitals, water supply utilities and rolling
stock. At present, voltage-source PWM inverters using IGBT
modules are usually employed as the power circuits of active
filters in a range from 10 kVA to 2 MVA, and dc capacitors
are used as the energy storage components.
Since a combined system of a series active filter and a
shunt passive filter was proposed in 1988 [16], a great deal
of research has been done on hybrid active filters and their
practical applications. The reason is that hybrid active filters are attractive from both practical and economical points
of view, in particular, for high-power applications. A hybrid
active filter for harmonic damping has been installed at the
Yamanashi test line for high-speed magnet-levitation trains
[17]. The hybrid filter consists of a combination of a 5-MVA
series active filter and a 25-MVA shunt passive filter. The series active filter makes a great contribution to damping of harmonic resonance between a supply inductance and the shunt
passive filter.
B. A 48-MVA Shunt Active Filter
Fig. 8 shows a power system delivering electric power
to the Japanese “bullet trains” on the Tokaido Shinkansen.
Three shunt active filters for compensation of fluctuating reactive current/negative-sequence current have been installed
in the Shintakatsuki substation by the Central Japan Railway
Company [18]. The shunt active filters consist of voltage-fed
PWM inverters using GTO thyristors, each of which is rated
at 16 MVA. A high-speed train with maximum output power
of 12 MW draws imbalanced varying active and reactive
powers from the Scott transformer, the primary of which is
connected to the 154-kV utility grid. More than 20 highspeed trains pass per hour during the daytime. This causes
voltage impact drop, variation and imbalance at the terminals of the 154-kV utility system, accompanied by a serious
deterioration in the power quality of other consumers connected to the same power system. The purpose of the shunt
active filters with a total rating of 48 MVA is to compensate
for voltage impact drop, voltage variation, and imbalance at
the terminals of the 154-kV power system, and to improve
the power quality. The concept of the instantaneous power
theory or the so-called “p-q theory” [19] in the time-domain,
has been applied to the control strategy for the shunt active
filter.
Fig. 9 shows voltage waveforms on the 154-kV bus and
the voltage imbalance factor before and after compensation,
measured at 14:20–14:30 on July 27, 1994. The shunt active
filters are effective not only in compensating for the voltage
impact drop and variation, but also in reducing the voltage
imbalance factor from 3.6% to 1%. Here, the voltage imbal-
AKAGI: LARGE STATIC CONVERTERS FOR INDUSTRY AND UTILITY APPLICATIONS
981
Fig. 9 Compensation effect on the impact drop, variation, and imbalance of voltage.
Fig. 10
The 8-MVA IEGT-based rectifier/inverter circuit.
Fig. 11 Detailed circuit configuration per leg.
ance factor is the ratio of the negative to positive-sequence
component in the three-phase voltages on the 154-kV bus. At
present, several shunt active filters in a range of 40 MVA to
60 MVA have been installed in substations along the Tokaido
Shinkansen [20].
IX. STEEL MILL DRIVES
The steel industry around the world has been actively introducing new technologies in power electronics and motor
drives since the beginning of the 1960s.
Fig. 10 shows a newly developed three-level voltagesource PWM rectifier/inverter 1 circuit using IEGTs rated at
1This
kind of three-level voltage-source inverter was originally presented
in [13], where it is referred to as a “neutral-point-clamped voltage-source
inverter.”
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4.5 kV and 4 kA. The 8-MVA rectifier/inverter system can
drive a 3.3-kV ac motor, although the output power rating
of the motor depends on the overload capacity required to
a steel mill motor.
Fig. 11 shows a detailed circuit configuration per leg in
Fig. 10. In a conventional GTO thyristor-based system, a
turn-on snubber consisting of an anode inductor was connected in series to each GTO thyristor in order to reduce
of the anode current. In addition, a turn-off snubber
consisting of a capacitor, a diode and a resistor was connected
of the
across each GTO thyristor in order to reduce
anode-to-cathode voltage. In the IEGT-based system, neither
turn-on nor turn-off snubber is required for each IEGT. However, each IEGT leg needs two simple and efficient clamp circuits that are connected as close as possible between the positive or negative bus and the neutral point. Table 4 summarizes
the comparison between the GTO thyristor and IEGT-based
systems. The IEGT-based system is superior in efficiency
by 2% to the GTO thyristor-based system. This implies that
the total loss in the IEGT-based system is reduced by 160
kW [6]. Hence, the IEGT-based system not only leads to energy savings, but also makes the system more reliable and
compact.
X. CONCLUSION
This paper has presented the state of the art of large static
converters in the world, with emphasis on their practical applications to utility and industry. Human beings would like
PROCEEDINGS OF THE IEEE, VOL. 89, NO. 6, JUNE 2001
Table 4
Comparison between the GTO Thyristor-Based and the
IEGT-Based Rectifier/Inverter System Rated at 8 MVA
for Steel Mill Drives
to expect a soft landing from the “silicon compound planet”
to the “silicon-carbide compound planet” in the 21st century.
This will open a new world of power electronics being different in technology and performance from the power electronics world in the 20th century.
ACKNOWLEDGMENT
The author would like to thank S. Saito and H. Yamamoto
of Toshiba Corporation, and R. Uchida and Dr. M. Koyama
of Mitsubishi Electric Corporation for their helpful and valuable information.
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Hirofumi Akagi (Fellow, IEEE) was born in
1951. He received the Ph.D. degree in electrical
engineering from the Tokyo Institute of Technology in 1979.
In 1979, he joined the Nagaoka University
of Technology as an Assistant and then Associate Professor in the department of electrical
engineering. In 1987, he was a Visiting Scientist
at the Massachusetts Institute of Technology
for ten months. From 1991 to 1999, he was
a Professor in the department of electrical
engineering at Okayama University. From March to August of 1996, he
was a Visiting Professor at the University of Wisconsin-Madison and
then the Massachusetts Institute of Technology. Since 2000, he has been
a Professor in the Department of Electrical and Electronic Engineering
at the Tokyo Institute of Technology. His research interests include ac
motor drives, high-frequency resonant-inverters for induction heating and
corona discharge treatment processes, and utility applications of power
electronics such as active filters, self-commutated BTB systems, and
FACTS devices. He has published about 120 peer-reviewed journal papers,
including 44 IEEE Transactions papers. He holds nine patents. He has made
presentations many times as a keynote or invited speaker internationally. He
was invited as a keynote speaker to the 1995 European Power Electronics
Conference (1995 EPE).
Prof. Akagi has received nine IEEE prize paper awards, including the
IEEE IAS Transactions prize paper award for 1991 and the IEEE PELS
Transactions prize paper award for 1998. He was elected as a Distinguished
Lecturer of the IEEE IAS and PELS for 1998–1999. He won the IEEE
William E. Newell Power Electronics Award in 2001.
AKAGI: LARGE STATIC CONVERTERS FOR INDUSTRY AND UTILITY APPLICATIONS
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