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Flexible AC Transmission
Systems (FACTS) Overview and
Applications
Claudio Cañizares
Department of Electrical &
Computer Engineering
Power & Energy Systems
(www.power.uwaterloo.ca)
WISE (www.wise.uwaterloo.ca)
Outline
•
•
Compensation.
Thyristor Control:
–
–
–
–
•
Voltage-Sourced Converters (VSC):
–
–
–
–
–
–
–
•
VSC operation.
Shunt Static Synchronous Compensator (STATCOM).
Series Static Synchronous Compensator (SSSC).
Unified Power Flow Controller (UPFC).
Interline Power Flow Controller (IPFC).
Convertible Static Compensator (CSC).
HVDC light.
D-FACTS:
–
–
•
Thyristor Controlled Reactor–Fixed capacitor (TCR-FC).
Static Var Compensator (SVC).
Thyristor Controlled Series Capacitor (TCSC).
Thyristor Controlled Voltage Regulator (TCVR) and Thyristor Controlled Phase Angle Regulator
(TCPAR).
DSTATCOM.
DSMES.
Applications.
2
Compensation
• Generator-system (generator-infinite bus)
model:
3
Motivation
– In steady state:
• Maximum power that can be transmitted is Pmax = E'V2/X
• The operating point is defined by Pm = PG = PL   = o
4
Compensation
• Series compensation:
5
Compensation
– Steady state:
6
Compensation
– The system is more “stable” because:
• The maximum power that can be transmitted from
the generator to the system is increased.
• The generator operating angle o is reduced.
• These can be associated with an increase in
decelerating energy in the system (equal area
criterion), i.e. a larger stability region.
7
Compensation
• Shunt Compensation:
8
Compensation
– Steady state:
9
Compensation
– As with series compensation, the system is
more “stable”:
• The maximum power transfer for the system is
larger.
• The generator operating angle o is smaller.
• The stability region is larger.
10
Compensation
• For a phase shifter (phase-shifting
compensation):
11
Compensation
• Hence for  = 10o:
8
7
PGmax
Compensated
System
2
6
5
4
3
Base System
PGmax
2
P
1
L
1
0
0
20  o2 40

o1 60
80
100
120
140
160
180
12
Compensation
• As in the case of shunt and series
compensation, the system is more stable
because:
– The maximum power that can be transmitted
from the generator to the system is increased.
– The generator operating angle o is reduced.
13
FACTS
14
TCR-FC
• SVC and TCSC controllers are based on
the following basic circuit topology:
FC
+
v(t)
-
TCR
15
TCR-FC
• Each thyristor is “fired” every half cycle.
• The firing angle α is “synchronized” with
respect to the zero-crossing of the voltage
(or current).
• As α increases, the TCR-FC equivalent
impedance changes from inductive to
capacitive.
16
SVC
17
SVC
• The controller is connected in shunt
through a step-down transformer to
reduced the voltage level on the thyristors.
• The thyristor firing is synchronized with
respect to the bus voltage.
• The main objective is to control the bus
voltage magnitude.
• Filters may be used to reduced harmonics.
18
SVC
• The steady state control characteristics
are:
19
TCSC
20
TCSC
• Somewhat similar to the SVC controller but
connected in series with a transmission line.
• The thyristor firing is synchronized with
respect to the line current.
• Filters are usually not used in this case,
which lead to stringent limits on the firing
angle α.
• In steady state, the device controller operates
in the capacitive region; the inductive region
is only used during transient operation.
21
TCSC
• The controller has a resonant point that must to
be avoided, as the controller becomes an open
circuit:
22
TCSC
• The typical controls for the TCSC are:
23
TCSC
• The power flow or “slow” control is designed to
maintain a constant controller impedance.
• The stability or “fast” control is usually designed to
reduced system oscillations after contingencies.
• The typical use of this type of controller in practice
is for the control of inter-area oscillations (e.g.
North-South ac interconnection in Brazil).
• For simple series compensation, MSC are a much
cheaper option; however, these can lead to Subsynchronous Resonance (SSR) problems.
24
TCVR & TCPAR
• The typical topology is:
25
TCVR & TCPAR
• TCVR and TCPAR are basically ULTC and
phase-shifters, respectively, with thyristor
switching as opposed to electromechanical
switching.
• Thus, these controllers have better dynamic
response, i.e. smaller time constants, than
the corresponding electromechanical-based
devices.
• Controls are typically discrete, but with
certain designs these can be continuous.
26
VSC
• A typical six pulse VSC with GTO switches
(IGBTs are used for “low” voltage applications):
27
VSC
• To reduce harmonics, multi-pulse converters and
filters are used.
• For example, for a 12-pulse VSC:
28
VSC
29
VSC
30
VSC
• Pulse-width modulation (PWM) control
techniques may also be used (“popular” in
low voltage level applications).
• Beside the control advantages, this
technique eliminates certain lower
harmonics, although it creates high level
harmonics.
• For example, for a 6-pulse VSC:
31
VSC
Fire valves when carrier and
modulation signals cross
CARRIER:
MODULATION:
32
VSC
•
This leads to:
•
Changing the modulation ratio, i.e. the magnitude of the modulation signal,
results in changes of the ac voltage magnitudes.
Shifting the modulation signal leads to phase shifts on the ac voltages.
•
33
STATCOM
34
STATCOM
• It is basically a VSC controlling the bus
voltage.
• The phase-locked loop (PLL) is needed to
reduce problems with spurious zero
voltage crossings associated with the high
harmonic content of the signals for this
controller, especially with PWM controls.
35
STATCOM
• Two types of controls can be implemented:
– Phase control in a multi-pulse VSC: By controlling the
phase angle of the voltage, the capacitor can be
charged ( <  ) controller absorbs P) or discharged
( >  ) controller delivers P), thus controlling the
voltage output Vi.
36
STATCOM
• PWM control in a 6-pulse VSC: the voltage
output Vi can be controlled through the
modulation ratio m independently of its phase
angle , which in turn controls Vdc.
37
STATCOM
•
The typical steady state control strategy is:
•
The current limits are due to the valve current limitations.
38
STATCOM
• This device is typically model using a voltage
source, neglecting dc voltage dynamics and
losses; this is a rough approximation.
• There are several applications of this type of
converter, but most of them at distribution
voltage levels (using IGBT technology).
• Additional controls may be added to
effectively damp system oscillations (the
same applies to SVC).
39
STATCOM
• Compared to an SVC:
– The STATCOM occupies significantly less space.
– There is more control flexibility (e.g. PWM, more
reactive support at the limits).
– Costs are higher due to the cost of switching
devices, i.e. installation costs:
• MSC  10 USD/kvar
• SVC  50-60 USD/kvar (100 Mvar)
35-40 USD/kvar (200 Mvar)
• STATCOM  1.2-1.3 SVC
40
SSSC
41
SSSC
• Similar to the STATCOM but connected in
series and synchronized with respect to
the line current.
• A phase angle  control charges and
discharges of the capacitor, thus
controlling the output voltage Vi.
• PWM controls can be decoupled or
coupled:
42
SSSC
– Decoupled PWM controls:
43
SSSC
– Coupled PWM controls (better overall performance):
44
UPFC
45
UPFC
• This controller is basically the STATCOM and
SSSC combined, with independent controls,
especially for PWM:
– The STATCOM controls the sending-end voltage
Vk and dc voltage Vdc.
– The SSSC controls the power on the line Pl and
Q l.
• There is a “demo” UPFC controller in Ohio
(AEP-EPRI venture).
46
IPFC
•
A combination of 2 SSSCs connected independently to 2 lines is referred to
as an Interline Power Flow Controller (IPFC):
•
In this case the power on both lines can be controlled independently.
47
CSC
• A combination of 2 SSSC and 2 STATCOMS
connected to 2 independent lines is referred to as a
Convertible Static Compensator (CSC).
• In this case the control possibilities are many, as it can
work as a STATCOM, SSSC, UPFC and IPFC.
• The CSC has been implemented in NY to relief
congestion (NYPA-EPRI venture) [E. Uzunovic et al,
“NYPA convertible static compensator (CSC)
application phase I: STATCOM,” Proc. Trans. & Dist.
Conf. and Expo., vol. 2, 2001, pp. 1139-1143]:
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CSC
49
HVDC Light
•
Based on VSCs as opposed to the current sourced converters (CSCs) used
in classical HVDC:
•
IGBTs (Insulated-gate bipolar transistors have a FET gate and a BJT switch)
instead of GTOs are used as switches; have lower losses, higher frequency
switching capacity, are cheaper, but have less reverse voltage blocking
capacity.
These switches allow using PWM controls, which yield greater control
flexibility.
•
50
HVDC Light
• The reduced reverse voltage blocking capacity of IGBTs
versus GTOs reduces the overall power capacity of the link;
this is the reason for the “light” label.
• The overall costs of the link are lower, allowing for wider
applications of this technology.
• Classical HVDC is most cost effective at power ranges above
~250 MW, whereas HVDC Light ratings are typically in the
order of a few tens of MW (the technology current upper limits
are 1,200 MW and ±320 kV).
• Visit http://www.abb.com/industries/us/9AAC30300394.aspx
for more details and actual projects.
51
D-FACTS
• FACTS applied to distribution systems are
referred to as D-FACTS.
• Since the voltage levels are lower, IGBTS are
used as the switching valves.
• VSC-based controllers found in practice for
voltage control:
– D-STATCOM: Basically the same as the
STATCOM but VSC is based.
– D-SMES: IGBT-based Super-Magnetic Energy
Storage (SMES).
52
D-FACTS
• Transmission system SMES:
– Used for voltage and power (damping) regulation.
– Limited applications given the costs.
53
D-FACTS
• D-SMES:
– Better performance than D-STATCOM, since it has more active power
capacity.
– More expensive than D-STATCOM, so its application is more limited.
54
Applications
• Oscillation damping with SVC and
STATCOM:
N. Mithulananthan, C. A. Cañizares, J.
Reeve, and G. J. Rogers, “Comparison of
PSSS, SVC and STATCOM Controllers for
Damping Power System Oscillations,”
IEEE Transactions on Power Systems,
Vol. 18, No. 2, May 2003, pp. 786-792.
55
Examples
• IEEE 145-bus, 50-machine test system:
56
Examples
– A line 90-92 outage yields:
57
Applications
– This has been typically solved by adding
Power System Stabilizers (PSS) to the
voltage controllers in “certain” generators, but
FACTS (e.g. TCSC, SVC, STATCOM, UPFC)
may also be used to address this problem.
– A power oscillation damping (POD) controller
is added to the voltage regulation control, like
in generator AVRs.
58
Examples
– The line 90-92 outage with STATCOM POD located at
the optimal voltage-stability placement Bus 77 (fairly
similar results are obtained for the SVC POD):
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Applications
•
TCSC:
– Brazil TCSC.
•
SVC:
– Toshiba/Mitsubishi projects.
– South Australia POD.
•
STATCOM:
– Toshiba/Mitsubishi projects.
•
UPFC
– AEP.
•
CSC:
– NYPA.
•
HVDC light:
– ABB.
•
SMES:
– BPA.
– Wisconsin Public Service.
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