Use of FACTS for System Performance Improvement
G. Beck, W. Breuer, D. Povh, D. Retzmann*
Siemens, Germany
ABSTRACT
The performance of power systems decreases with the size, the loading and the complexity of the
networks. This is related to problems with load flow, power oscillations and voltage quality. Such
problems are even deepened by the changing situations resulting from deregulation of the electrical
power markets, where contractual power flows do not follow the initial design criteria of the existing
network configuration any longer. Power Systems have not been designed for “wide-area” energy
trading with daily varying load patterns, and the systems are “close to their limits”.
Flexible AC Transmission Systems (FACTS) based on power electronics have initially been developed
to improve the performance of long distance AC transmission. Later, the technology has been
extended to the devices which can also control power flow. Excellent operating experiences are
available world-wide, and the technology became mature and reliable. FACTS are applicable in shunt
connection, in series connection, or in a combination of both.
In this paper, solutions with FACTS for system enhancement and for system interconnections are
presented, and their advantages are explained. Examples of large project applications in Asia,
America and Europe are depicted, including hybrid configurations with parallel operation of FACTS
and HVDC (High Voltage Direct Current).
KEY WORDS:
System Stability, Blackout Prevention, Increase of Transmission Capacity, Power-Flow Control,
Short-Circuit Current Limitation, Parallel Operation of FACTS and HVDC
1. INTRODUCTION
The development of power systems follows the requirements to transmit power from generation to the
consumers. With an increased demand for energy and the construction of new generation plants, first
built close and then at remote locations from the load centers, the size of power systems has grown.
Examples of large interconnected systems are the Western and Eastern European systems UCTE
(installed capacity 530 GW) and IPS/UPS (315 GW), which are planned to be interconnected in the
future [1-3].
With an increasing size of the interconnected systems, the technical and economical advantages
diminish. This is related to problems regarding load flow, power oscillations and voltage quality. If
*dietmar.retzmann.@siemens.com
1
power is to be transmitted through the interconnected system over long distances, transmission needs
to be supported.
Fig. 1 and Fig. 2 summarize the perspectives of power system developments. In the future, an
increasing part of the installed capacity, however, will be connected to the distribution levels
(dispersed generation), which poses additional challenges on planning and safe operation of the
systems, see Fig. 2. In such cases, power electronics can clearly strengthen the power systems and
improve their performance [2].
Globalisation/
Privatisation
Liberalisation
Deregulation - Privatization: Opening of the
Markets, Independent Transmission Companies
ITCs, Regional Transmission Organisations RTOs
Bottlenecks in
Privatisation
Transmission
Problem of uncontrolled Loop Flows
Overloading & Excess of SCC* Levels
System Instabilities & Outages
System Enhancement & Interconnections:
Investments in
Power Systems
Š Higher Voltage Levels **
Š New Transmission Technologies
Š Renewable Energies
* SCC = Short-Circuit Current
**
**Example
ExampleUCTE:
UCTE:400
400kV
kVisisactually
actuallytoo
toolow
low
Fig. 1: Trends in High Voltage Transmission Systems
Tomorrow:
Today:
G
G
G
G
G
G
G
G
G
G
G
Use of Dispersed Generation
G
Load Flow will be “fuzzy”
Fig. 2: Perspectives of Transmission and Distribution Network Developments
Problems with congestion and transmission bottlenecks are even deepened by the deregulation of the
electrical power markets, where contractual power flows do not follow the initial design criteria of the
existing network configuration any longer. Large blackouts in America and Europe confirmed clearly
that the favorable close electrical coupling might also include the risk of uncontrollable cascading
effects in large and heavily loaded interconnected systems [2], see Fig. 3.
2
Blackout: a large Area is out of Supply
Source: Blackout Summary, U.S./Canada
Power Outage Task Force 9-12-2003
Québec's HVDCs assist
for Power Supply and
System Restoration
However, some Islands still have local Supply
Before the Blackout
a)
Source: EPRI 2003
Giant Loop Flows
2.2 - 4.8 GW
*
System Enhancement necessary !
Source: ITC 8/2003 – “Blackout”
Source: National Transmission Grid Study; U.S. DOE
5/2002 – “Preview”
b)
* PTDF
Problems only in
the synchronous
interconnected
Systems
= Power Transfer Distribution Factor
Fig. 3: Blackouts 2003 - Example United States
a) The Blackout Area - and a Satellite View
b) Congestion and Loop Flows - Forecasting Studies and Cascading Events
3
Additional problems are expected when renewable energies such as large wind farms have to be
integrated into the system, especially when the connecting AC links are weak and when there is no
sufficient reserve capacity in the neighboring system available [3].
Based on the global experience with large blackouts [2], strategies for the development of large power
systems go clearly in the direction of hybrid transmissions consisting of DC and AC interconnections
including FACTS. Such hybrid interconnected systems offer significant advantages, both technical
and in terms of reliability [2, 4].
Fig. 4 shows schematically such a hybrid system using FACTS as well as HVDC. Power exchange in
the neighboring areas of interconnected systems offering most advantages can be achieved by AC
links, preferably including FACTS for increased transmission capacity and for stability reasons [4].
The transmission of large power blocks over long distances should, however, be utilized by the HVDC
transmissions directly to the locations of power demand. HVDC can be implemented as direct coupler
– the “Back-to-Back” solution (B2B) – or as point-to-point long distance transmission via DC line. In
addition to FACTS, the HVDC links can strengthen the AC interconnections at the same time in order
to avoid possible dynamic problems which exist in such huge interconnections [3]. The “Firewall”
function of HVDC [2], as mentioned in Fig. 4, is explained in the next section.
System
G
System
A
System
B
System
C
System
D
System
E
System
F
Large
LargeSystem
SystemInterconnections,
Interconnections,using
usingHVDC
HVDCand FACTS
HVDC - Long Distance DC Transmission
HVDC B2B - via AC Lines
High Voltage AC Transmission & FACTS
DC – the Stability Booster and
“Firewall” against “Blackout”
“Countermeasures”
against large
Blackouts
Fig. 4: Large Power System Interconnections - Benefits of Hybrid Solutions
2. ELIMINATION OF TRANSMISSION BOTTLENECKS BY MEANS OF POWER
ELECTRONICS
Fig. 5 shows an “Application Guide” for grid enhancement with power electronics (ref. to Fig. 3 b).
Depending on the grid structure, there are four basic cases:
•
Load displacement in case of parallel lines by impedance variation (series compensation)
•
Fast load-flow control in meshed structures with HVDC/GPFC (or very slow with phase
shifting transformer)
•
Voltage collapse: reactive/active power injection by means of FACTS/HVDC
•
Excess of allowed short-circuit level: short-circuit current limitation (FACTS/HVDC)
4
Short-Circuit Current Limitation for Connection of new Power Plants
*
SVC & HVDC for Prevention of Voltage Collapse
Load Management by HVDC
The FACTS & HVDC “Application Guide”
Load Displacement by Series Compensation
* PTDF
= Power Transfer Distribution Factor
Fig. 5: Use of Power Electronics for System Enhancement
GPFC (Grid Power Flow Controller) is a special DC back-to-back link which is designed for fast
power and voltage control at both terminals [4]. The GPFC features are explained in the following.
The basic equation for power transmission (Fig. 6) explains the solutions for system enhancement in a
more detailed way. The power transmitted between two subsystems depends on voltages at both ends
of the connecting line, the line impedance and the phase angle difference between the connecting
points. Power electronics can actively influence one ore more of these parameters and control or direct
the power flow through the interconnection.
P
V1 , δ 1
V2 , δ 2
G~
G~
X
P=
V1 V2
X
sin (δ
( 1 - δ 2)
Power-Flow Control
Parallel Compensation
Series Compensation
Each of these Parameters can be used for LoadFlow Control and Power Oscillation Damping
Fig. 6: Power Transmission – The basic Equation
5
By using FACTS for reactive power compensation, the impedances and voltages of the system can be
influenced: by adding series compensation (fixed or controlled) into the line, its reactance X can be
reduced or modulated (for power oscillation damping, ref. to the equation); with FACTS parallel
compensation, e.g. SVC (Static Var Compensator), the voltage can be stabilized (at constant values, or
modulated for damping of oscillations). The transmission angle can be influenced by using HVDC for
power-flow control. These methods are explained in Figs. 7-8.
Power-Flow Control
C>
~ 1/X
L>
P
Series
Compensation
Voltage
Control
V
C>
Parallel
Compensation
Fig. 7: FACTS for
Reactive Power Compensation
Section of a Transmission Line
L>
V
Fault-Current
Blocking
V1
G~
I1
Q1
P
α and γ
V2
G~
I2
Q2
L and C
L and C
Fast Functions
Slow Functions
Slow Functions
Fig. 8: HVDC as Grid Power Flow
Controller – The “FACTS” B2B
Power & fast Voltage Control
Fault-Current Blocking
Fig. 8 shows that HVDC is also well suitable for short-circuit current limitation (fault current
blocking). Furthermore, in case of cascading events, HVDC acts like an automatic “Firewall” by fast
decoupling of the interconnected systems during a disturbance and by immediate restarting power
transmission after the fault. Systems directly coupled by AC links need time-consuming resynchronization, which can take many hours. Alongside its main function of power-flow control, the
HVDC incorporates also voltage control (by reactive power injection) for both sides of the system. It
decouples the transmission equation by forcing the power to flow in a similar way like the well known
phase-shifting transformer, however, much faster and independent from the frequencies and angles of
the two coupled systems.
Using an extended control range of HVDC, the B2B can fully “feature” FACTS functions, e.g. fast
voltage control, in the same way as an SVC. This new idea of GPFC as a “FACTS B2B” is explained
in Fig. 9, in comparison to the “standard” HVDC control range. As indicated in the figure, these
features have been successfully applied in a project at Lamar substation, USA.
6
Fig. 9: HVDC Operating Ranges – and the new GPFC Solution as “FACTS B2B”
3. FACTS TECHNOLOGIES AND APPLICATIONS
The main shunt connected FACTS application is the Static Var Compensator (SVC). SVC provides
fast voltage control, reactive power control and power oscillation damping features. As an option,
SVC can control unbalanced system voltages. World-wide, there are hundreds of these devices in
operation. For decades, it has been a well developed technology, and the demand for SVCs is further
increasing.
For long AC lines, series compensation is used for reducing the transmission angle, thus providing
stability enhancement. Fixed series compensation (FSC) is widely used to improve the stability and to
increase the transmission capacity for long distance transmissions. A huge number of these
applications are in operation. In case of more complex system conditions, Thyristor Controlled Series
Compensation (TCSC) is used if fast control of the line impedance is required to adjust the load flow
on parallel lines, or for damping of power oscillations. TCSC has already been applied in different
projects for load-flow control, stability improvement and to damp oscillations in interconnected
systems.
The rating of shunt connected FACTS controllers is up to 800 MVAr, series FACTS devices are
implemented on 550 and 735 kV levels to increase the transmission capacity of the lines up to several
GW.
Fig. 10 shows the basic configurations of FACTS devices. The SVC uses line-commutated thyristor
technology, where the maximum switching frequency in each phase element is limited by the
“driving” system frequency. A further development is STATCOM (Static Synchronous Compensator)
using voltage-sourced converters (VSC, [4]). Both devices provide fast voltage control, reactive power
control and power oscillation damping features (POD). As an option, SVC can control unbalanced
system voltages. The developments of FACTS technologies are depicted in Fig. 11. Special FACTS
devices are UPFC (Unified Power Flow Controller) and the GPFC [4]. UPFC combines a shunt
connected STATCOM with a series connected STATCOM (also named S3C, Solid-State Series
7
Compensator), which can exchange energy via a coupling capacitor. GPFC is, at lower costs, less
complex than UPFC. For most applications in AC transmission systems and for network
interconnections, SVC, FSC, TCSC and GPFC/B2B are fully sufficient to match all requirements of
the grid. STATCOM and UPFC are tailored solutions for special needs.
FACTS devices consist of power electronic components and conventional equipment which can be
combined in different configurations. It is therefore relatively easy to develop new devices to meet
extended system requirements.
Recent developments are the TPSC (Thyristor Protected Series Compensation, Fig. 10) and the ShortCircuit Current Limiter (SCCL) [4, 5], both innovative solutions that use high power thyristor
technology.
FACTS - Flexible AC Transmission Systems: Support of Power Flow
z
SVC - Static Var Compensator (Standard for Parallel Compensation)
z
STATCOM - Static Synchr. Compensator (Fast SVC, Flicker Compensation)
z
FSC - Fixed Series Compensation
z
TCSC - Thyristor Controlled Series Compensation
z
TPSC - Thyristor Protected Series Compensation
z
GPFC - Grid Power Flow Controller (FACTS-B2B)
z
UPFC - Unified Power Flow Controller
SVC / STATCOM
GPFC/UPFC
/ UPFC
FSC
AC
AC
AC
AC
AC
60 Hz
60 Hz
60 Hz
60 Hz
60 Hz
AC
50 or 60 Hz
/ TPSC
TCSC/TPSC
Fig. 10: FACTS – Basic Configurations
Response Time
Breaker Delay
2 - 3 Cycles
1st Generation
2nd Generation
1- 2 Cycles
V-Control
3rd Generation
I-Control:
< 1 Cycle
Mechanically
Switched Devices
Thyristor Controlled
Components
VSC Technology
GTO, IGBT, IGCT
Fast VARs
Slow VARs
Fig. 11: FACTS – Technology Developments
8
Figs. 12-13 show today’s FACTS applications including mechanically switched devices such as
MSC/MSR, which are frequently used for voltage support and blackout prevention [2]. Actual ratings
and voltage levels of the solutions are also indicated in the figures.
MSC / MSR
SVC
Mechanical Switched
Capacitors / Reactors
STATCOM
Static Var
Compensator
Static Synchronous
Compensator
Switchgear
Thyristor Valve(s)
GTO/IGBT Valves
Capacitors
Control & Protection
Control & Protection
Reactors
Transformer
Transformer
Capacitors
DC Capacitors
Reactors
52 ≤ kV ≤ 800
50 ≤ MVAr ≤ 500
~
52 ≤ kV ≤ 800
50 ≤ MVAr ≤ 800
52 ≤ kV ≤ 800
50 ≤ MVAr ≤ 800
~
~
Fig. 12: FACTS for Parallel Compensation
FSC
TPSC
Fixed Series
Compensation
Thyristor Protected
Series Compensation
TCSC
Thyristor Controlled
Series Compensation
Capacitors
Capacitors
Capacitors
Protection
Protection
Control & Protection
Arresters
Circuit Breakers
Thyristor Valves
Thyristor Valves
Circuit Breakers
Circuit Breakers
220 ≤ kV ≤ 800
200 ≤ MVAr ≤ 800
220 ≤ kV ≤ 800
100 ≤ MVAr ≤ 500
220 ≤ kV ≤ 800
100 ≤ MVAr ≤ 200
~
~
~
ILim
Fig. 13: FACTS for Series Compensation
9
α
A large number of different FACTS and HVDC controllers have been put into operation either as
commercial projects or as prototypes. Fig. 14 gives an example of the Siemens applications worldwide. Thus it appears that some areas are still “blank”, which is expected to change in the future. For
comparison reasons, the number and the increase of large HVDC long-distance transmission projects
are also indicated in the figure. The CSC (Convertible Synchronous Compensator), as mentioned in
Fig. 14, uses a flexible combination of two STATCOMs, of which each controller (+/- 100 MVAr) can
be switched individually from shunt to series mode. By these means, CSC provides a multiple of
operation modes including UPFC operation for the two transmission lines passing Marcy substation in
the area of New York, USA.
Lamar 2005
Virginia Smith 1988
Inez 1998
Spring Valley 1986
2 Dominion 2003
2 Marcy 2001-2003
Lugo 1985
3, 2 El Dorado
2006
Devers 2006
••
Nine Mile 2005
Dayton 2006
• •
• • •
•
•••••• • • •
•
••• •• •
• ••
••
•
Porter 2006
Paul Sweet 1998
2 Adelanto 1995
2 Midway 2004
3 Vincent 2000
Clapham 1995,
Refurbishment
Military Highway 2000
2 Tecali 2002
Eddy County 1992
La Pila 1999
Sullivan 1995
Jacinto 2000
Cano Limón 1997
Imperatriz 1999
Serra de Mesa 1999
Samambaia 2002
Acaray 1981
•
•
•
•
• • ••
• •
• ••
•
•
Status: 12-2005
In total:
over 150
SVCs
.
2 Zem Zem 1983
Ahafo 2006
••
•
•
Baish 2005, •
Milagres 1988
Maputo 2003
Funil 2001
Sao Luiz 2006
Sinop 2006
Bom Jesus da Lapa 2002
•
••
Illovo
Athene
Impala
2 Gooty 2003
2 Chuddapah 2003
•
2 Sabah 2006
• •
•
9 Powerlink 2007,
Refurbishment
Nebo 2007,
Refurbishment
Parallel
2 Hechi 2003
2, 2 Tian Guang 2003
3 Puti 2005
Kapal 1994
1994-1995
Muldersvlei 1997
2 Yangcheng 2000
Jember 1994
Barberton 2003
••
Limpio 2003
Series
.
K.I. North 2004
Campina Grande 2000
2 Fengjie 2006
•
•
•
••
Samitah 2006
Iringa,
Shinyanga 2006
•
Fortaleza 1986
Ibiuna 2002
… and over 110 Industry
SVCs all over the World
•
2 Lucknow 2006
•
•
2006
Kanjin (Korea) 2002
P. Dutra 1997
Chinú 1998
Atacama 1999
2, 2 Gorakhpur
2, 2 Purnea
••
••
•
Hoya Morena,
Jijona 2004
Nopala 2006
Cerro Gordo 1999
3 Juile 2002
.
1997
Dürnrohr 1983
Welsh 1995
Laredo 2000 Benejama,
Seguin 1998 Saladas 2006
Kayenta 1992
•
Wien Südost 1993
.
.•
Moyle MSC 2003
Ghusais,
Hamria,
Mankhool,
Satwa
Etzenricht 1993
Siems 2004
2 Pelham, 2 Harker,
2 Central, 1991-1994
Brushy Hill 1988 Willington 1997
3 Montagnais 1993
Châteauguay 1984
••
Radsted 2006
Rejsby Hede 1997
2 Kemps Creek 1989
Load Flow
FSC
SVC
B2B/GPFC
NGH
MSC/R
UPFC
TPSC
STATCOM
CSC
TCSC
Flicker STATCOM
Plus 16 Projects for
HVDC Long Distance
Transmission …
8 alone between 2000 &
2005 in 4 Continents
Fig. 14: FACTS & HVDC worldwide – Example Siemens (ref. to Text)
4. USE OF FACTS FOR TRANSMISSION ENHANCEMENT
In Great Britain, in the course of deregulation, new power stations where installed in the north of the
country, remote from the southern load centers; and some of the existing power stations in the south
were shut down due to environmental constraints and for economic reasons, see Fig. 15-1). To
strengthen the transmission system, a total number of 27 SVCs have been installed because there was
no right of way for new lines or higher transmission voltage levels [3]. Fig. 15-1c) shows the very
effective power oscillation damping (main control function) with two of these SVCs, installed in
Harker Substation in a parallel configuration. Additional SVCs were implemented in the southern part
of the grid, of which Fig. 15-2) shows a view of one of the two Pelham SVCs (left side of the figure).
The single line diagram for both Harker and Pelham SVCs is attached in the right part of Fig. 15-2).
An increasing number of SVCs are also going to be installed on other continents. In Fig. 16, an
example of a large SVC in South America is depicted. The SVC was implemented to improve system
stability of the extended transmission grid. The installed containerized solution offers additional
benefits such as reduction in installation and commissioning time, as well as space and cost savings
compared with conventional building technologies.
10
The Transmission System:
Results of Dynamic
System Tests:
a) No SVC connected
b) Both SVCs in
Voltage Control Mode
c) Both SVCs in Power
Harker: 2 SVCs
Oscillation Damping
Mode
Fully confirmed by
Site Experience
Increase of Transmission Capacity
Prevention of Outages
1)
Benefits
Verified by Computer and Real-Time Simulation
Harker: 275 kV
Pelham: 400 kV
„
„
„
„
Voltage Control
Reactive Power Control
Power Oscillation Damping
Unbalance Control (Option)
Benefits:
o Improvement of Voltage Quality
o Increased Stability
2)
Deregulation caused Transmission Problems
Fig. 15: Europe - UK goes ahead with FACTS - 27 SVCs
1) Harker Substation, 1993 – 2 SVCs for Power Oscillation Damping
2) Pelham Substation, 1991 - 2 SVCs for Voltage Control (ref. to Text)
11
2002
Valves &
Control
Benefits:
o Improvement of Voltage Quality
o Increased Stability
o Avoidance of Outages
Fig. 16: SVC Bom Jesus da Lapa, Enelpower, Brazil - 500 kV, +/-250 MVAr
Containerized Solution
In Figs. 17-19, the features and cost savings of series compensation due to grid enhancement are
summarized. The mentioned SSR (subsynchronous resonances) topic is a crucial issue for large
thermal generators with long shafts [5].
Fixed Series Compensation:

~
~
Increase of Transmission
Capacity
Controlled Series Compensation:
Damping of Power Oscillations
 Load-Flow Control
 Mitigation of SSR

TCSC/TPSC
α
FSC
Fig. 17: FACTS - Application of Series Compensation
The flexibility of modern FACTS technologies under extremely harsh environmental conditions is
indicated in Figs. 18-19: the operating range for FSC begins at -500 C, for TCSC it can reach up to
+850_C. This is necessary due to the outdoor installation on high voltage potential, with the isolated
platform mounted directly in series with the transmission line.
In Fig. 20, two projects with series compensation in China are presented. Picture a) gives a view of
one phase element of the two Pingguo TCSCs. The 3D view b) and the photo c) (from Barberton FSC,
RSA) demonstrate how easily series compensation can be mounted to the existing line: when the
equipment installation is finished but not yet connected to the line, a line interruption and a jumper
connection from the line to the platform is made with a short interruption of power transmission of 1-3
days only.
For Thyristor Protected Series Compensation TPSC, innovative developments in ThyristorTechnology have been applied: LTT (Light-triggered Thyristors, now state-of-the-art for FACTS and
HVDC) by applying a special heat-sink to enable very fast self-cooling of the valves, within half a
12
second only. By these means, TPSC is fully suitable for multiple fault conditions as it is often the case
under hot climate conditions due to brush-fires leading to repetitive line faults. In the TPSC, the
thyristor replaces the conventional MOV (metal oxide arrester) for fast capacitor protection against
over-voltages due to short-circuit currents. During faults, the MOV heats up heavily. Due to an upper
temperature limit, the MOV must cool down before the next current stress can be absorbed. Cooldown requires a substantial amount of time, time constants of several hours are typical. During this
time, the series compensation must be taken out of service (bypass breaker closed) and consequently
the power transfer on the related line needs to be reduced dependent on the degree of compensation,
leading to a significant loss in transmission capacity. Therefore, it appears that by using the TPSC with
fast cooling-down time instead of conventional series compensation with MOV, a significant amount
of money for each application can be saved.
Fig. 21 shows a site-view of one of the 5 TPSCs installed at 500 kV in California, USA.
1993
- 50 o C
Poste Montagnais, Canada - FSC
Fig. 18: FSC at EHV 735 kV plus harsh Environment
„ Current Control
„ Impedance Control
„ Power Oscillation
Damping (POD)
„ Mitigation of SSR
(Option)
Up to 500 POD
Operations per Day
for saving the
System Stability
A System Outage of
24 hrs would cost
840,000 US$ *
* 25 US$/MWh x 1400 MW x 24 hrs
1999
> + 60 o C
Benefits:
o Increase of Transmission Capacity
o Improvement of System Stability
up to 85 o
Fig. 19: 500 kV TCSC Serra da Mesa, Furnas/Brazil – Essential for Transmission
Fast-growing generation in high load density networks on one hand, and interconnections among the
systems on the other hand, increase the short-circuit power. If the short-circuit capacity of the
13
equipment in the system is exceeded, the switchgears must be uprated or replaced, which is a very cost
and time-consuming procedure. In such cases, short-circuit current limitation offers clear benefits.
Limitation by passive elements, e.g. reactors, is a well known practice. It reduces, however, the system
stability, and there is an impact on the load flow.
By combining the proven TPSC application with an external reactor (see Fig. 22), whose design is
determined by the allowed short-circuit current level, this device can also be used very effectively as
short-circuit current limiter (SCCL, ref. to [4, 5]).
a)
* RSA = Republic of South Africa
How to
“loop” the
FSC into the
Line …
b)
… Example
Barberton –
RSA*, 2003
quite easy
Power
Outage
between
1 & 3 Days
only
c)
Enhancement of Chinas “Central Transmission Corridor”
Commercial Operation in June 2006
Fig. 20: China goes ahead – Transmission Enhancement with FACTS
a) Photo of Pingguo TCSC, commissioned in June 2003
b) 3D View on Fengjie 500 kV Fixed Series Compensation, China
2 x 600 MVAr, Line Compensation Level 35%
c) Demonstration of the FSC-Jumper Connection to the Line - from
Barberton FSC, RSA
Fig. 23 shows the basic function and the operating principle of the SCCL, including a 3D view of the
SCCL. In comparison with the TPSC site photo, it can be seen that the TPSC is complemented by just
an additional reactor for current limitation. Further details on the SCCL solution are described in [5].
This new device operates with zero reactance in steady-state conditions, and in case of short-circuit it
is switched over to the current limiting reactance within a few ms.
Fig. 24 depicts an example of an on-site fault recording of one of the Vincent TPSCs. The measured
currents and the calculated junction temperature rise of the valve in Phase B for a line fault in phase
BC are recorded. The figure shows that there is still a huge margin for higher current stresses.
14
In Fig. 25, a brief overview on today’s solutions for fault-current limitation is given, including the new
SCCL. Basically, there are two methods for fault-current reduction: limitation and interruption. The
constraints and the benefits of the different solutions are indicated in the figure.
It can be seen that the SCCL offers numerous advantages.
A comparison of the new SCCL with the conventional solution using a current limiting reactor is
depicted in Fig. 26. The main concerns are related to a risk of voltage collapse in case of dynamic
system conditions, which can lead to cascading disturbances (blackout).
TPSC Technology:
Outdoor Valves on a Platform
 LTT Thyristors, self-cooled

Fig. 21: TPSCs Vincent & Midway/USA: five Systems at 500 kV - fully proven
in Practice, plus two new Projects (El Dorado)
SCCL
Use of proven Technology
TPSC
+
The new Idea !
Reactor
AC
AC
Bus 2
Bus 1
Thyristor Protected
Series Compensation
Fig. 22: SCCL - an Innovative FACTS Solution using TPSC
15
Thyristor Valve Housing
Capacitor Bank
To Bus 1
BYPASS
Breaker
Communication
Reactor
Just one additional X !
To Bus 2
Reactance
X
Fast Increase of Coupling Reactance
Zero Ohm for best Load Flow
t
Fig. 23: SCCL - Short-Circuit Current Limitation with FACTS
A huge Margin for higher Valve
Currents - up to 110 kA peak
20000
10000
50
40
0,53000
0,58000
-10000
30
-20000
20
-30000
10
-40000
0
t [sec]
Fig. 24: TPSC and SCCL – up to 110 kA
16
Valve-Temperature
Rise only 11 o
dTj [K]
I [A]
0
0,48000
60
Risk of Voltage Collapse
 Fault
Current Limitation
Difficult or impossible at
High Voltage Levels
¾ Conventional Solution: Reactor
¾ The new FACTS Solution: SCCL
¾ Future Option: High-Temperature Superconducting FCL
 Fault
Current Interruption
Not available for HV
Levels plus Concerns
about Reliability and
Protection Co-ordination
¾ Is-Limiter
¾ Electronic Devices (“Small FACTS”)
SCCL: no Concerns
Fig. 25: FCL (Fault Current Limiters) - Principles and Applications
SCCL - The better Alternative:
 No Risk of Voltage Collapse
AC
AC
 Reactive Power remains balanced
Bus 1
 No Impact on Grid Load Flow
Bus 2
 No Impact on First Swing Stability
Only Current Limiting Reactor ?
AC
AC
Bus 1
Bus 2
 Voltage Drop - needs Compensation
 Mechanically or Thyristor
Switched Capacitor will be necessary
Fig. 26: SCCL versus Conventional Reactor
In the next section, examples for parallel operation of FACTS and HVDC in large interconnected
transmission systems are depicted.
5. FACTS AND HVDC IN PARALLEL OPERATION
With the Mead-Adelanto and the Mead-Phoenix Transmission Project (MAP/MPP), a major 500 kV
transmission system extension was carried out to increase the power transfer opportunities between
Arizona and California, USA [3]. The extension includes two main series compensated 500 kV line
segments and two equally rated Static Var Compensators (supplied by Siemens) at the Adelanto and
Marketplace substations – ref. to Fig. 27.
The SVCs enabled the integrated operation of the already existing highly compensated EHV AC
system and the large HVDC system. The SVC installation was an essential prerequisite for the overall
system stability at an increased power transfer rate.
17
Each SVC: 388 MVAr for
Voltage and POD Control
Upgrade of a large AC and
DC Transmission System
with 2 SVCs &
FSCs
ƒ Increase of Transmission Capacity
ƒ Improvement of System Stability
Fig. 27: HVDC plus SVC - Mead-Adelanto, USA
E dc Adelanto (volts)
1100
1.4
1000
1.0
900
0.8
800
Design by Computer Studies
0.6
700
Adel Bsvc (Mvar)
a)
Mkplc 500kV Bus Vlt (pu)
1.2
400
400
200
200
0
0
Mkplc Bsvc (Mvar)
1100
Mkplc 500kV Bus Vlt (pu)
E dc Adelanto (volts)
1.4
1.2
1000
1.0
900
0.8
800
0.6
700
400
a) Both SVCs in Voltage Control Mode
b) Both SVCs in Coordinated Voltage
& Power Oscillation Damping
Control Mode
Mkplc Bsvc (Mvar)
Adel Bsvc (Mvar)
400
200
200
0
0
0
10
Time (sec)
20
0
10
20
Time (sec)
b)
Fig. 28: Mead-Adelanto Studies – Comparison of SVC Voltage- and POD-Control Mode
18
An example of the intensive project testing with computer and real-time simulator facilities for a fault
application at Marketplace 500 kV bus is given in Fig. 28. The figure shows the computer test results
with both SVCs active. The influence of the HVDC can be seen from the DC voltage E dc. Figure a)
is with both SVCs only in voltage control mode (POD blocked); Figure b) shows an improved
damping with the coordinated POD function enabled.
In Fig. 29, a view on the SVC installation in Marketplace is given.
Benefits:
ƒ Increase of Transmission Capacity
ƒ Improvement of System Stability
ƒ Support of existing HVDC
1995
Features:
o Coordinated Voltage Control &
o Damping of Power Oscillations
Fig. 29: Static Var Compensators Mead-Adelanto – View on Marketplace Substation
Similar studies have been carried out for a number of large transmission projects world-wide. In Figs.
30-32, an innovative FACTS application with SVC in combination with HVDC for transmission
enhancement in Germany is shown [3, 6].
It’s a matter of fact that this project is the first high voltage FACTS controller in the German network.
The reason for the SVC installation at Siems substation nearby the landing point of the Baltic Cable
HVDC were unforeseen right of way restrictions in the neighboring area, where an initially planned
new tie-line to the strong 400 kV network for connection of the HVDC was denied. Therefore, with
the existing reduced network voltage of 110 kV (see the dotted black lines in Fig. 31), only a limited
amount of power transfer of the DC link was possible since its commissioning in 1994, in order to
avoid repetitive HVDC commutation failures and voltage problems in the grid. In an initial step
towards grid access improvement, an additional transformer for connecting the 400 kV HVDC AC bus
to the 110 kV bus was installed (see the figure). Finally, in 2004, with the new SVC equipped with a
fast coordinated control, the HVDC could fully increase its transmission capacity up to the design
rating of 600 MW. In addition to this measure, a new cable to the 220 kV grid was installed to increase
the system strength with regard to performance improvement of the HVDC controls.
In Fig. 32, a view of the Siems SVC in Germany is depicted.
19
HVDC and FACTS in
parallel Operation
Source:
Fig. 30: SVC Siems, Germany - Support of HVDC Baltic Cable
Initially planned Connection
400 kV Grid
2
Access denied
HVDC
Signals
1994
only
110 kV
220 kV Land Cable
350 MVA, 11 km
1
Now, the HVDC
can operate at
full Power
Rating
2
2004
2
Final Solution: new
SVC with TCR & TSC
100 MVAr ind.
200 MVAr cap.
1 Initial Step for Grid Access
Enhancement
2 and a new 220 kV Cable
Fig. 31: The Problem – no Right of Way for 400 kV AC Grid Access of
Baltic Cable HVDC - and Solutions
Prior to commissioning, intensive studies have been carried out; first with the computer program
NETOMAC and then with the RTDS real-time simulator by using the physical SVC controls and
simplified models for the HVDC [3].
20
Essential for
enhanced Grid
Access of the HVDC
… fully
confirmed by Site Experience
2004
Verified by Computer and Real-Time Simulation …
Fig. 32: The Solution – the first HV SVC in the German Grid at Siems Substation
6. POWER ELECTRONICS FOR HVDC AND FACTS – MARKET EXPECTATIONS
AND RELIABILITY ISSUES
Table 1 summarizes the market expectations for FACTS and HVDC solutions today and in the future.
Today, the market for series compensation, for SVC and for B2B/GPFC for load-flow control is in fact
large and, as a result of liberalization and deregulation in the power industry, is developing fast in the
future. Further, the market in the HVDC long distance transmission field is progressing fast. A large
number of high power long distance transmission schemes using either overhead lines or submarine
cables have been put into operation or are in the stage of installation.
MSC/R
zzz
SVC
zzz
STATCOM
z
FSC
zzz
TCSC / TPSC
zz
Combined Device
UPFC
z
Power Transmission
HVDC
zzz
Shunt Compensation
Series Compensation
Excellent Market
Upcoming Market
Small Market
zzz
zz
z
Table 1: Markets for FACTS and HVDC
21
Concerning reliability of high voltage power electronics, Table 2 gives an example of two SVC
projects installed in South Africa. The same high reliability is also achieved for HVDC as the
technology applied uses the same components. Excellent on-site operating experience is being
reported, and the FACTS and HVDC technology became mature and reliable.
Illovo SVC
1995
1996
1997
1998
Availability (%)
99.9
99.45
100
100
Forced and
deferred Outages
2
5
2
1
Off-line
Maintenance
0h
80h00
102h26
162h00
On-line
Maintenance
10h15
2h00
3h00
0
MDT in hrs
2h13
9h40
36h40
1h00
Athene SVC
1995
1996
1997
1998
Availability (%)
99.78
99.71
99.92
99.77
Forced and
deferred Outages
4
9
1
2
Off-line
Maintenance
4h00
81h00
62h00
60h15
On-line
Maintenance
1h00
0
0
0
MDT in hrs
4h40
3h20
4h40
10h30
Recordings from NATAL SVCs / RSA (2 TCR & 3 Filter)
Guarantied Availability: 98 - 99 %
Table 2: Availability of Power Electronics - Example FACTS: close to 100 % - same
for HVDC
7. CONCLUSIONS
Deregulation and privatization is posing new challenges on high voltage transmission systems. System
elements are going to be loaded up to their thermal limits, and wide-area power trading with fast
varying load patterns will contribute to an increasing congestion. To keep the power supply reliable
and safe, system enhancement will be essential.
In conclusion to the previous sections, Table 3 summarizes the impact of FACTS on load flow,
stability and voltage quality when using different devices. The evaluation is based on a large number
of studies and experiences from projects. For comparison, HVDC as well as mechanically switched
devices (MSC/R) are included in the table.
As a consequence of “lessons learned” from the large blackouts in 2003, FACTS and HVDC will play
an important role for the system developments, leading to “Smart Grids” [4] with better controllability
of the power flows.
High voltage power electronics provide the necessary features to avoid technical problems in the
power systems, they increase the transmission capacity and system stability very efficiently and they
assist in prevention of cascading disturbances.
22
Principle
Devices
Scheme
Impact on System Performance
Load Flow
Variation of the
Line
Impedance:
Series
Compensation
FSC
Stability
Voltage
Quality
z
zzz
z
z
zzz
z
zz
zzz
z
(Mechanically
Switched Capacitor /
Reactor)
{
z
zz
SVC
{
zz
zzz
{
zz
zzz
zzz
zzz
zz
zz
zzz
zzz
(Fixed Series
Compensation)
TPSC
(Thyristor
Protected Series
Compensation)
TCSC
(Thyristor
Controlled Series
Compensation)
MSC/R
Voltage
Control:
Shunt
Compensation
(Static Var
Compensator)
STATCOM
(Static Synchronous
Compensator)
Load-Flow
Control
HVDC (B2B, LDT)
UPFC
(Unified Power
Flow Controller)
Based on Studies & practical Experience
Table 3: FACTS & HVDC – Overview of Functions & “Ranking”
Influence: *
{
z
zz
zzz
no or low
small
medium
strong
8. REFERENCES
[1] W. Breuer, X. Lei, D. Povh, D. Retzmann, E. Teltsch: “Role of HVDC and FACTS in
future Power Systems”; 15th CEPSI, October 18-22, 2004, Shanghai, China
[2] G. Beck, D. Povh, D. Retzmann, E. Teltsch: “Global Blackouts – Lessons Learned”;
Power-Gen Europe, June 28-30, 2005, Milan, Italy
[3] G. Beck, D. Povh, D. Retzmann, E. Teltsch: “Use of HVDC and FACTS for Power System
Interconnection and Grid Enhancement”; Power-Gen Middle East, January 30 – February 1,
2006, Abu Dhabi, United Arab Emirates
[4] U. Armonies, M. Häusler, D. Retzmann: “Technology Issues for Bulk Power EHV and UHV
Transmission”; HVDC Congress 2006 – Meeting the Power Challenges of the Future using
HVDC Technology Solutions, July 12-14, 2006, Durban, Republic of South Africa
[5] V. Gor, D. Povh, Y. Lu, E. Lerch, D. Retzmann, K. Sadek, G. Thumm: “SCCL – A new Type of
FACTS based Short-Circuit Limiter for Application in High Voltage Systems”; CIGRÉ Report
B4-209, Session 2004, Paris
[6] H. Waldhauer: “Grid Reinforcement and SVC for full Power Operation of Baltic Cable HVDC
Link”; The 38th Meeting and Colloquium of Cigré Study Committee B4 “HVDC and Power
Electronics”, Technical Colloquium, September 25, 2003, Nuremberg, Germany
23