IMPROVED POWER QUALITY SOLUTIONS USING ADVANCED
SOLID-STATE SWITCHING AND STATIC COMPENSATION TECHNOLOGIES
Masatoshi Takeda, Ph.D.
Gregory F. Reed, Ph.D.
Member, IEEE
Shotaro Murakami
Mkubishi Electric Power Products, Inc.
Power Systems Division
Katsuhisa
Tokuhara
1. ADVANCED SSTS TECHNOLGOY
Utility distribution networks, sensitive industrial loads,
and critical commercial operations all suffer from
various types of outages and service interruptions which
can cost significant financial 10SSper incident based on
process down-time, lost production, idle work forces,
and other factors. The types of interruptions which are
experienced can generally be classified as power quality
related problems caused by voltage sags and swells,
lightning strikes, and other distribution system related
disturbances.
In many instances, the use of a SolidState Transfer Switch (SSTS) and/or a Distribution
Level Static Compensator (D-STATCOM) can be some
of the most cost-effective solutions for these types of
power quality problems.
The SSTS, which essentially consists of a pair of
thyristor switch devices, enables seamless transfer of
energy from a primary source to an alternate source in
order to avoid service interruption upon a deficiency in
power quality. The D-STATCOM, which consists of a
thyristor-based voltage source inverter, uses advanced
power electronics to provide voltage stabilization,
flicker suppression, power factor correction, harmonic
control, and a host of other power quality solutions for
and lMtUStIW
Aritsuka
Mitsubishi Electric Corporation
Power Electronics Systems Department
ABSTRACT
bOth Utlllty
Tomohiko
Isao Iyoda, Ph.D.
appllCiitlOIK.
Key Words – Solid-State Transfer Switch, SSTS,
Hybrid Switch, Thyristor, Static Voltage Compensation,
Voltage Source Inverter, D-STATCOM, Power Quality,
Custom Power
The successful operation of an SSTS scheme provides a
seamless transfer of electrical energy from a primary
supply to a secondary
supply without
service
interruption to even the most critical and sensitive loads.
As a result, power quality problems become transparent
to the critical or sensitive customer loads that the SSTS
protects. However, a thynstor is not a pure conductor
and raises some issues in terms of loss consumption and
cooling. In a conventional SSTS, line current flows in
the thynstors continuously, causing a great deal of loss
consumption
and element heating during normal
operation.
As a result, relatively
large cooling
equipment
is required
which imposes additional
operating costs on the user in order to maintain thyristor
cooling. It also results in reduced efficiency and lower
reliability in the device.
In order to solve the power quality related issues
previously discussed, the authors have developed an
advanced solid-state transfer switching scheme using a
novel hybrid switch device, resulting in negligible loss
consumption and eliminating the need for cooling
equipment.
The hybrid switch device essentially
consists of a pair of thyristors and a high-speed
mechanical parallel switch which has an opening time
capability of less than 1 millisecond.
During normal operation, the line current is by-passed
by the parallel switch and the thyristor does not conduct
the current. When an opening operation is required, the
parallel switch is opened and the thyristor is turned on,
is
Consequently,
the
current
simultaneously.
commutated to the thyristor immediately and blocked by
the thyristor at the first zero crossing of the current.
Thus, the resultant loss consumption of the devices is
negligible since the thyristor conducts only during the
few milliseconds of a transfer operation. The parallel
switch opening time of less than 1 millisecond secures
the same operational characteristics of a switching
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device consisting of thyristors alone. This characteristic
enables the hybrid switch device to be applied to a solidstate transfer switching
scheme.
Based on the
incorporation of the hybrid switch device and the
elimination
of cooling equipment,
an extremely
compact, lightweight, and highly reliable SSTS system
is realized.
The following breakthroughs were made during the
development of the hybrid switch device:
(1) Development of a driving scheme for the parallel
switch
(2) Realization of fast current commutation
parallel switch to the thyristors
During normal operation, line current is by-passed by
the parallel switch (l% 1). When the transfer operation is
required, PS 1 is opened and TS 1 is turned on,
simultaneously.
Consequently,
the
current
is
commutated to TS 1 immediately and blocked by the
TS 1 at the first zero crossing of the current. Immediately
after completing the blocking of current, the opposite
side thyristor switch (X32) behgs to conduct the current
to the load from the alternate source. The parallel switch
(PS2) is then closed and bypasses the current.
PrimarySource
AkerrmteSource
from the
ceder 2
Feede
(3) Compact design using 12 kV, 1.5 kA thyristors
A circuit diagram of the hybrid switch device is shown
in F&ure 1. During normal operation, line current is bypassed through the parallel switch (PS) and the thyristor
(TH) does not conduct the current. When an opening
operation is required, PS is opened and TH is turned on,
current
is
the
simultaneously.
Consequently,
commutated to TH immediately and blocked by TH at
the first zero crossing of the current.
I Sensitive Load
I
Figure 2 - Principal Configuration of SSTS Using
the Hybrid Switch Device
The overall ratings of the SSTS are shown below in
Table 1 for the 15 kV design and 600 A and 1200 A
continuous current ratings.
Table 1 - Ratings of SSTS
Rated Voltage
Rated Current
Interrupting Current
BIL
PS: Parallel Switch
TH 1, TH2 Thyristor Switch
AR: Zkc Oxide Surge Amester
Cooling Method
Overall Efficiency
I
I
1
15 kv
600 A/l,200A
12.5 kAl 25 kA
95 kV
Natural Cooling
Greater than 99.99%
Figure] - Circuit Diagram of Hybrid Switch
Figure 2 shows a one-line circuit diagram of the SSTS
system incorporating the hybrid switches. A pair of
hybrid switch devices are utilized for the SSTS scheme.
An external outline view of the SSTS system for a 15
kV, 1200 rating is shown in Figure 3. Figure 4 shows an
example of the test results. Upon sensing the voltage
sag, the transfer operation is made within % cycle, or
less than 4 milliseconds.
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3. ADVANCED D-STATCOMTECHNOLGOY
The
development
efforts
of
advanced
static
compensation technology at the distribution level have
resulted in a new Compact D-STATCOM device which
exhibits
high speed control
of reactive
power
compensation in order to provide voltage stabilization,
flicker suppression, and other types of system control.
The D-STATCOM incorporates an innovative design
essentially consisting of a GTO-or GCT-based voltage
source inverter set connected to the power system via a
multi-winding inverter transformer.
Figure 3- External View of SSTS (15 k~ 1200 A)
Figure 5a shows a basic configuration
.
equivalent
Iw,
Ntcmw
SOlua
voltage
Rc(WCC
.,
IIKv,
M*
V*
,m“,
Akmaie
S’Ymx euncnt
sOuraLMsmalt
.
‘+
1~
k“~
.
.
1
D-STATCOM.
The
Vs
OTO’2+
PowerSystem
Invener
.’
Ed~
k
Tmsf.OmrAm
Figure 4 - Test Results of Load Transfer
Operation with SSTS
d
A novel switching device using a hybrid system of a
parallel switch and a thyristor switch, and the
of this switching
device
to the SSTS system
application
are briefly presented. The operating characteristics of
the device are exhibited in the test results.
hybrid switching device and its use in the
SSTS system promise to be effective tools in solving
power quality issues for power distribution networks.
m
K VoItsge
Source
Transformer
4
I
(a) Basic Configuration of Compact D-STATCOM
VI
2. SSTS SUMMARY
The advanced
of the Compact
and
i“
,0+
m,
circuit
diagram
D-STATCOM mainly consists of DC voltage source
behind self-commutated inverters using GTO or GCT
thyristors, and a transformer. The GTO inverter with a
DC voltage source can be modeled as a variable voltage
source, as shown in the equivalent circuit of F@ure 5b.
Volwc
Rcralc.i
The compact design has resulted in a size ratio
improvement of nearly 1/3 the area and 1/5 the volume
of a conventional D-STATCOM device. This enables
greater flexibility in terms of installation possibilities,
and also provides a means to easily relocate the device
at various locations within the power system.
Inverter
output
Vokage
Tr Irnpedanee
I
System
Voltage
(b) Equivalent Circuit of Compact D-STATCOIW
Figure 5 - Basic Configuration and Equivalent
Circuit of the Compact D-STATCOM
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The distribution power system can also be modeled as a
voltage source. The two voltage sources are connected
by a reactor representing the leakage reactance of the
The phase of the output voltage of the
transformer.
thyristor-based inverter, Vi, is controlled in the same
way as the system voltage, Vs. F@re 6 shows the
principal operation modes of the D-STATCOM output
current, I, which varies depending upon Vi.
●
F@re 7 below shows the main circuit configuration of a
20 MVAr Compact D-STATCOM which consists of a
GTO inverter and multiplex transformer.
Resulrsnt Voltoge
No Load mode (Vs=Vi)
*Unit Inverter Voltage
and Current
&
●
(a)Vs=Vi
%
Capacitive mode (Vi>Vs)
vs. .<
Figure 7-
Vi
‘)------q-j
1j
vi
*
/
.
Reactive mode (VieVs)
‘s~’
Figure 6-
,1. ~i
Main Circuit Configuration for
20 MVAr Compact D-STATCOM
,,N
VS
-..
1
1
●
I
Multiple Transformer
am,,, ,
Principal Operation Modes of the
Compact D-STATCOM
As shown in F@e
6, if Vi is equal to Vs, then no
If Vi is
reactive power is delivered to the system.
greater than Vs, the phase angle of Ii is leading with
respect to the phase angle of Vs by 90 degrees. Thus, a
leading reactive power flows in the Capacitive Mode of
the D-STATCOM.
If Vi is lower than Vs, the phase
angle of Ii is lagging with respect to the phase angle of
Vs by 90 degrees. Thus, a lagging reactive power flows
in the Inductive Mode of the D-STATCOM.
The
quantity of the reactive power flow is proportional to the
difference between Vs and Vi.
A typical control circuit of the Compact D-STATCOM
is shown below in F@re 8. The three-phase load
currents to be compensated (iLa, iLb, and iLc) are
measured from the system and transformed to two-phase
orthogonal
components
(ip and iq) on rotating
coordinates synchronized with the line voltage.
T
i
1
Figure 8-
I
Control Circuit Configuration of
Compact D-STATCOM
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The outputs of the filter circuit are inversely
transformed to three-phase components (isa, isb, and
isc). The output current of the Compact D-STATCOM
is controlled by three-phase current feedback control
using isa, isb, and isc as reference signals for each
phase. The output signals of the current control added
by a sensed system voltage signal becomes the voltage
reference signal of the PWM control. The PWM control
circuit generates the firing signal of the GTO by
comparing triangular wave carrier signals to the voltage
reference signal.
The ratings of the GTO-based inverter for the 20 MVAr
unit are shown below in Table 2.
Table 2 - Ratings of GTO Inverterfor
20 iWAr Compact D-STATCOM
Rated Capacity
20 MVA
DC Voltage
2500 V
Output Voltage
1250 V
Output Current
1500A
Switching Frequency
5 Pulse PWM
(300 Hz for 60 Hz System)
Flicker Compensation
Application:
The amount of
fllcker generated by arc furnaces tends to increase with
the increased size and higher efficiencies of arc
For weak distribution systems where the
furnaces.
operation of arc furnaces causes a significant power
quality problem, the high-perfomnance of a flicker
compensation device is necessary. As a solution to this
particular power quality need, the D-STATCOM has
been applied for a number of applications and has
provided excellent performance for arc furnace flicker
suppression. F@re 9 shows the system configuration
for a flicker compensation installation.
The fllcker caused by the arc furnace operation was
measured by use of an approved flicker meter, The
output of the meter was A lO, and it was used as an
indicating factor of voltage flicker. The voltage
deviation of the meter from the reference value is
calculated for each cycle. It is then filtered by a human
eye sensitivity curve and integrated for one minute to
output A1O.
Distribution
system f
R
x
Rs+x
M;! Transformer
Ijis
i,
Transformer
of Arc Furmce
,
.
-r
D-STATCOM
Arc Furnace
Figure 9 - System Configuration for Arc Furnace
Flicker Compensation Application
Table 3 shows the fourth maximum values and the
improvement ratio for operation of the D-STATCOM to
compensate the flicker. In this application, the flicker
realized was 58~o on average with
suppression
utilization of the D-STATCOM.
In this case, the
capacity of the D-STATCOM was 2170 of the maximum
reactive power generated from the arc furnace.
The
measured results clearly indicate the high-performance
achieved by the D-STATCOM for flicker suppression.
Table 3 - Evaluation of D-STATCOM Arc Furnace
Flicker Compensation Installation
No. of 4’
A1O
A1O
Improvement
Maximum
(w/out D-STAT)
(w/ D-STAT)
Ratio
1
1.30
0.58
55.1%
2
1.12
0.47
58.3%
3
1.09
0.42
61.7%
4. D-STATCOM SUMMARY
The Compact D-STATCOM has been developed and its
innovative design incorporates a DC voltage source
behind a thynstor-based voltage source inverter set
connected to the power system via a multi-winding
inverter transformer. The device is 1/3 the area and 1/5
the volume of a conventional D-STATCOM.
Operation of the Compact D-STATCOM provides an
advanced, high-speed control technology for reactive
power compensation
in order to provide flicker
suppression, voltage stabilization, power factor control,
and other distribution system mitigation measures for
power quality improvement.
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5. REFERENCES
6. BIOGRAPHIES
[1] Reed, G., Takeda, M., Yamamoto, H., Aritsuka, T.,
Kamiyama, I., “Development of a Novel Hybrid
Switch Device and Application to a Solid-State
Transfer
Switch;
accepted
for
Conference
Proceedings, LEEWPES 1999 Winter Meetings, New
York, NY.
[2] Schwartzenberg, J.W., DeDoncker, R.W., “15 kV
Medium Voltage Static Transfer Switch; IEEE,
May/June, 1995.
[3] Reason, John, “Solid-State
Transfer
Electrical World, August, 1996.
Switch:
[4] T.akeda, M., et. al., “Development of SVG Series for
Voltage Control over Three-Phase
Unbalance
caused by Railway Load,” IPEC, Yokohama, 1995.
[5] Mori, S., et. al., “Development of a Large Static Var
Generator Using Self-Commutated
Inverters for
IEEE
Improving
Power
System
Stability;’
Transactions on Power Systems, Vol. 8, No. 1,
February, 1993.
[6] Iyodaj
L,
Hosokawa,
Y.,
Kinoshita,
H.,
“Development of Compact STATCOM (Active
Type Static
Reactive
Power
Compensator):
Mitsubishi Electric Corporation.
Gregory F. Reed, Ph.D. - Dr. Reed received his Ph.D. in
Electric Power Engineering from the University of Pittsburgh
in 1997. He is currently Manager of the Power Systems
Division at Mitsubishi Electric Power Products, Inc. Dr. Reed
is a Member of the IEEE since 1985 and is a member of the
Custom Power Task Force (IEEE 15.06.06.01) and the Voltage
Source Inverters Working Group (JVG 15).
He is also
Chairman of the IEEWPES Pittsburgh Chapter.
Masatoshi Takeda, Ph.D. - Dr. Takeda received his Ph.D. in
Electrical Engineering from Osaka University, Japan in 1996.
He is currently Manager of HVDC Projects, Power Electronics
Department at Mitsubishi Electric Corporation in Kobe, Japan.
Dr. Takeda is a member of the IEE Japan since 1973, and is a
full participating member of the CIGRE Working Group on
STATCOM.
Lsao Iyoda, Ph.D. - Dr. Iyoda received his Ph.D. in Electric
Power Engineering from Kyoto University, Kyoto, Japan in
1992. He is currently Manager of the Power Electronics
Technologies
Section, Power System & Transmission
Engineering Center at Mitsubishi Electric Corporation in
Kobe, Japan. Dr. Iyoda is a Member of the IEEE, JIEE, and
the Japan Society of Power Electronics. He is also a member
of the CIGRE Task Force on Modeling of Power Electronics
Equipment (FACTS) in Load Flow and Stability Programs.
Tomohiko “Tomo” Aritsuka - Mr. Aritsuka received his
B.S. in Mechanical Engineering from Nagoya University in
1986. He is currently an Assistant Manager in the Power
Conwoller Systems Engineering Section, Power Electronics
Systems Department in Kobe, Japan.
Mr. Arhsuka is a
member of the IEE Japan since 1986, and is a full participating
member of the CIGRE Working Group on Active Filter.
KatsuhH Tokuhara - Mr. Tokuhara received his B.S. in
Electrical Engineering from Hiroshima University in 1991. He
is currently an Engineer in the System Analysis Technologies
Section, Power System & Transmission Engineering Center at
Mitsubishi Electric Corporation in Kobe, Japan. Mr. Tokuhara
is a Member of the IEEE since 1998.
0-7803-4893-1/99/$10.00 (c) 1999 IEEE