FACTS Technology for
Reactive Power Compensation and System Control
H. K. Tyll, SM IEEE
Abstract
In the early days of power transmission in the late 19th
century problems like voltage deviation during load
changes and power transfer limitation were observed
due to reactive power unbalances. Today these
problems have even higher impact on reliable and
secure power supply in the world of globalisation and
privatisation of electrical systems and energy transfer.
Fast and highly reliable power electronic devices
(thyristor valves) in Static Var Compensators (SVC)
and HVDC applications proved their effectiveness in
HV transmission systems to reduce energy transfer
limitations. Further development in semiconductors
(GTO and IGCT) allowed new power electronic
configurations to be introduced to the tasks of power
transmission and load flow control. An overview is
given on existing shunt and series compensation
FACTS devices like SVC, Statcom, UPFC and
TCSC/TPSC. In more detail operating characteristics,
response times, space requirements, costs etc are
discussed for the shunt compensation devices. Future
applications based on IGBT devices are discussed.
Introduction
From the past towards the future the supply of
electrical energy developed from separated utilities to
large interconnected systems. In former times
distributed power generation supplied load centers
within a limited supply area. These smaller systems
were operated at lower voltage levels. Nowadays
there is increased power exchange over larger
distances at highest system voltages allowing reserve
sharing and competition. Electrical energy shall be
made available at most locations at minimum cost and
at highest reliability.
Following problems have been observed in threephase-systems:
ƒ Voltage control at various load conditions
ƒ Reactive power balance (voltage, transmission
losses)
ƒ Stability problems at energy transfer over long
distances
ƒ Increase of short circuit power in meshed systems
ƒ Coupling of asynchronous systems
ƒ Coupling of systems with different system
frequencies
The last two problems can be solved using HVDC
technology and the upper ones can be solved by
proper use of reactive power compensation based on
FACTS devices.
Types of Var Sources
System components
ƒ Inductances in electrical machines,
transmission lines, transformers, reactors
ƒ Capacitances in transmission lines, cables
Compensation components
ƒ Mechanically switched reactors and capacitors
ƒ Synchronous condensers
ƒ Thyristor controlled shunt and series
compensation
ƒ Converter controlled shunt and series
compensation
Types of reactive power compensation
and application
Shunt compensation
Application
Short -circuit Transmission
phase angle
level
voltage
stabilisation
at heavy load
nearly
unchanged
slightly
increased
voltage
stabilisation
at light load
nearly
unchanged
slightly
decreased
nearly
unchanged
controlled
Short-circuit
level
Transmission
phase angle
MSC
MSR
fast voltage
control
reactive
power control
damping of
power swings
SVC
Static Var Compensator
Statcom
Series compensation
Application
long
transmission
lines
increased
much smaller
decreased
much larger
controlled
controlled
bulk power
transmission
Series Capacitor
short
transmission
lines,
limitation of
short-circuit
currents
Series Reactor
TCSC
SSSC
PFC
POD
SSR mitigation
FCL
Thyristor Controlled Series
Compensation, SSSC
Power transfer equation and influence of FACTS
devices
System
1
U 1 δ1
X
U 2 δ2
System
2
Static Var Compensator
(STATCOM)
U
U
P
I
P=
U1 U2
Phase Shifting Transf.
sin ( δ 1 - δ 2 )
U T UB
UA
UB
UT
UA
α
Paper for IEEE/PES panel session
on FACTS, Nov 8-11, 04, Sao Paulo
Heinz K. Tyll is with Siemens AG - PTD H 166,
P.O. Box 3220, D-91050 Erlangen, Germany
e-mail: heinz.tyll@siemens.com
Unified Power Flow Controller
TCSC (SSSC)
UA
X
UT
UB
UB
Im
UT
UA
α
Re
The figure above shows influence of various devices.
Shunt compensation
Statcom (GTO converter technology)
Tasks of dynamic shunt compensation
ƒ Steady state and dynamic voltage control
ƒ Reactive power control of dynamic loads
ƒ Damping of active power oscillations
ƒ Improvement of system stability
VTr
∆V
I
Examples of shunt compensation devices
VCon
SVC (Thyristor technology)
Conventional SVCs consist of thyristor controlled
(TCR) and thyristor switched branches (TSC / TSR)
together with filter branches for harmonic current
absorption.
C o n tro l
TCR
TSC
The operating principle is based on an inverter
configuration as shown above. The DC voltage of a
storage capacitor is applied to the network by
electronic switches forming a stair case waveform.
FC
The figure above shows a simplified single line of an
SVC
The figure below shows an example of an SVC for
(500 kV, ± 250 MVar) the East-West interconnection
in North-East Brazil.
Site View of SVC Bom Jesus da Lapa, Brazil
The figure above shows the first Statcom project
138 kV / ± 100 MVar by TVA / EPRI / Westinghouse
(Sullivan, AEP, USA)
The major component in a Statcom is the GTO valve
configuration (Statcom
Statcom versus SVC
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
The figure above shows a light triggered thyristor
(LTT) valve module using LTT with integrated
overvoltage protection on the silicon wafer.
V / I - characteristic
Control range
Modularity
Response time
Transient behaviour
Operating losses
Space requirements
Availability
Investment costs
Above table lists the major areas of differences
between Statcom and SVC.
Series compensation
Basic differences
Vprim
(pu)
Tasks of dynamic series compensation
ƒ Reduction of load dependent voltage drops
ƒ Reduction of system transfer impedance
ƒ Reduction of transmission angle
ƒ Increase of system stability
ƒ Load flow control for specified power pathes
ƒ Damping of active power oscillations
1.8
2 cycles
SVC
1.5
transient
operation
SVC
transient
operation
SVC
12 cycles
SVC
Uref= 1.035 pu
slope =3%
Design
point
cap
cont operation
SVC
Design
point
ind
1.0
Basic fixed capacitor scheme
0.5
Operating area STATCOM
continous and
transient
Operating characteristic SVC
continous and
transient
VBase = 115 kV = 1.0 pu
IBase = 150 MVar =1.0 pu
1.5
0
1.0 0.5
0.5
Iprim(pu)
cap
1.0 1.5
ind
The figure above shows the different V / I –characteristics of Statcom and SVC. A Statcom is advantageous at severe undervoltage conditions.
Loss Comparison
1750
1500
Above figure shows the basic components of a fixed
series capacitor built up by two segments. MOV
arresters provide the fast overvoltage protection for
the capacitors. Spark gaps are used to limit the
energy absorption of the MOVs in case of severe
systems faults (so-called internal faults on series
compensated line sections).
STATCOM
245 Mvar cap to
0 (20) Mvar ind
Pvtot (MVar)
1250
1000
SVC (TSC/FC/TCR)
245 Mvar cap
to 0 Mvar ind
750
500
250
SVC (FC/TCR)
245 Mvar cap to
0 Mvar ind
0
-250
-200
-150
-100
-50
0
50
Qtot (MVar)
The figure above shows the loss – characteristics of a
Statcom and conventional SVC solutions. Losses of a
GTO Statcom are typically higher. Even higher losses
occur in a Statcom in case of IGBT applications.
Thyristor protected series capacitor (TPSC)
The use of a TCR branch in a series capacitor results
in:
ƒ Improved capacitor protection
ƒ Substitution of spark gaps
ƒ Removal of high energy absorption by MOV
8
7
7
1 series capacitor
Issue
Statcom
SVC
V/I characteristic
good undervoltage performance
Current source
Symmetrical
otherwise Hybrid solutions
Same converter usable for various
applications (STATCOM, UPFC,
CSC, B2B etc)
Redundancy
no degraded mode
1 to 2 cycle
Self protecting at critical system
faults
40 to 50 %
96 to 98 %
120 to 150 %
good overvoltage performance
Impedance
freely adjustable to any range
by TCR/TSR /TSC branches
TCR/TSR/TSC branches
used in SVC and TCSC/TPSC
Control range
Modularity
Response time
Transient behaviour
Space requirements
Availabilty
Investment costs
2 thyristor valve
as fast bypass - device
1
3 current limiting reactor
4
4 MOV
5 bypass circuit breaker
3
Redundancy
Degraded mode operation
2 to 3 cycle
Available before, during and
after critical system conditions
100 %
> 99 %
100 %
The table above summarises the major differences
between Statcom and SVC.
6 bypass damping reactor
2
6
PLATFORM
5
7 platform disconnects
with grounding switch
8 bypass disconnect
Above figure shows the simplified single line of a
TPSC.
Thyristor controlled series capacitor (TCSC)
The figure below shows a simplified single line of the
CSC Marcy.
Above figure shows the TCSC Kayenta, Wapa, USA,
installed in 1992. TCSCs can provide continuous
control of the series capacitor impedance up to four
times the nominal impedance. TCSCs are also useful
for power oscillation damping, dynamic power flow
control and mitigation of subsynchronous resonance
conditions. Newer installations exist also in Brazil,
China and India.
Static synchronous series compensation
Series compensation can also be built up by the use
of Statcom converter technology. Similar valve
configurations are used.
SSSC
Two inverters can be connected via transformers TRSE 1 and 2 to the different lines. Without interconnection on the DC bus they can be operated as
single SSSC.
With the chosen arrangement shown above various
operating modes like CSC, Statcom and UPFC are
also possible and allow flexible use of the whole
device. The installation was finally commissioned in
early 2004.
Dynamic shunt and series compensation
combined in Unified power flow controller (UPFC)
Using the converter technology series and shunt
devices can be combined by interconnection of the
DC bus. Such a UPFC can practically take over all
requirements on reactive power compensation.
Above figure shows the connection principle of an
SSSC. A series voltage formed by the DC storage
capacitor and the converter configuration will be
introduced to the system in quadrature to the line
current. Capacitive as well inductive compensation is
possible.
Such SSSC configurations are also used in the
Unified Power Flow Controller (UPFC, described later)
as series part of the whole device.
Two or more of the SSSC can be installed in a system
in parallel lines or at major substations with several
lines leaving to different areas. Such arrangement
allows power flow control under severe system
conditions.
SSSC configurations are used in the CSC (convertible
Static Compensator) project at Marcy, NYPA, USA.
Electronic generator to
provide reactive power
and insert real power
Line
Load
Electronic generator to
provide reactive power
and extract real power
The figure above shows the simplified single line
diagram of an UPFC. UPFCs are installed at Inez
(AEP/USA) and Kanjin (KEPRI/Korea). Also the CSC
Marcy kann be used as UPFC.
Semiconductor technology
The development of FACTs devices mainly depends
on the development of the available semiconductor.
semiconductor devices like GTO / IGCT or IGBT (or
ever newer?) must still improve with regard to energy
handling of single devices.
Future aspects
Siemens
Electrically
triggered
thyristor
wafer
5.2 kV
3500 Aeff
Siemens
Light
triggered
thyristor
wafer
7.5 kV
3500 Aeff
Toshiba
Gate-turn-off
thyristor
(GTO)
4.5 kV
appr 1500 Aeff
Fuji
Press Pack
IGBT
4,5kV
appr. 800Aeff
The development of FACTS devices is best described
by the four semiconductor types shown in the figures
above.
High power semiconductors are always essential to
built installations with lowest costs. The newer
ƒ
Exchange of electrical energy in extended
systems requires flexible transmission
systems to provide solutions
with regard to reactive power balance
ƒ
Conventional system components only
provide limited adjustments
ƒ
Up to now power electronics like HVDC, SVC
and TCSC have proven reliable functioning
ƒ
Further development of semiconductor
devices and configurations will increase the
use of power electronics in case of economic
manufacturing for high power applications