These slides present the introduction to FACTS devices. Later we will discuss about its modelling and control aspect applications. This comes under the topic on power electronics application in smart and microgrid systems.

1. Class-11: Power electronics
application: FACTS controller
Course: Distribution Generation and Smart Grid
Prof. (Dr.) Pravat Kumar Rout
Department of EEE, ITER,
Siksha ‘O’Anusandhan (Deemed to be University),
Bhubaneswar, Odisha, India
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2. FACTS: Flexible AC Transmission System
Flexibility of Electric Power Transmission:
The ability to accommodate changes in the electric transmission
system or operating conditions while maintaining sufficient steady state
and transient margins.
Flexible AC Transmission Systems:
Alternating current transmission systems incorporating power electronics
based and other static controllers to enhance controllability and increase
power transfer capability.
FACTS Controller:
A power electronics based system and other static equipment that
provide control of one or more AC Transmission system parameters.
2

3. Basic concept of power flow control in
transmission lines
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4. Why FACTS?
The advantages from FACTS device installation includes:
 Steady-state and dynamic reactive power compensation and voltage
regulation.
 Steady state and dynamic stability enhancement.
 Increasing power transfer capability of the existing assets.
 Reduced fault current.
 Reduced transmission losses.
 Improving power quality.
4

5. Basic switching control techniques
In FACTS applications, the power ratings of the converters are in the
range of some MW to hundreds of MW, and the switching frequency is
lower than the switching frequency used in industrial application
converters to avoid excessive switching losses. However, there are
various switching control types. The most known so far are as follows:
 Multipulse converters switched at line frequency,
 Pulse-width modulation (PWM) with harmonic elimination technique
 Sinusoidal PWM
 Cascade converters , including the multilevel modular converter
(MMC)
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6. Types of FACTS Controller
 Series Controller
 Shunt Controller
 Combined Series-series controller
 Combined Series-shunt Controller
6

7. Type-1 Series connected controllers
 In series compensation, the FACTS is connected in series with the
power system.
 It works as a controllable voltage source.
 Series inductance exists in all AC transmission lines. On long lines,
when a large current flows, this causes a large voltage drop. To
compensate, series capacitors are connected, decreasing the
effect of the inductance.
7

8. Continue…
 a variable impedance such as capacitor, reactor etc. are inserted in
series or a power electronics based variable source of main
frequency, sub synchronous and harmonic frequencies (or a
combination ) to serve the desired need all series controllers inject
voltage in series with the line (working principle)
1. as long as the voltage is in phase quadrature with the line current,
the series controller only supplies or consumes variable reactive
power
2. any other phase relationship will regulate the real power as well
3. even a variable impedance multiplied by the current flow
through it, represents an injected series voltage in the line 8

9. Examples of Series Compensation
1. Static synchronous series compensator (SSSC)
2. Thyristor-controlled series capacitor (TCSC)
3. Thyristor-controlled series reactor (TCSR)
4. Thyristor-switched series capacitor (TSSC)
5. Thyristor-switched series reactor (TSSR)
9

10. 1: Static synchronous series compensator (SSSC)
10

11. Continue….
 A static synchronous generator operated without an external electric energy source as a series
compensator whose output voltage is in quadrature with and controllable independently of the line
current for the purpose of increasing or decreasing the overall reactive voltage drop across the line
and there by controlling the transmitted electric power.
 Thus, the SSSC may include transiently rated energy storage or energy absorbing devices to enhance
the dynamic behavior of the power system by additional temporarily real power compensation to
increase or decrease momentarily, the overall real (resistive) voltage drop across the line.
 It can be based on voltage source converter or current source converter
 Without an extra energy source, SSSC can only inject a variable voltage, which is 90 degree leading or
lagging the current
 Battery storage or superconducting magnetic storage can also be connected to a series controller to
inject a voltage vector of variable angle in series with the line.
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12. 2: Thyristor-controlled series capacitor (TCSC)
12

13. Continue...
 A capacitive reactance compensator which consists of a series capacitor
bank shunted by a thyristor controlled reactor in order to provide a
smoothly variable series capacitive reactance.
 A series capacitor bank is shunted by a thyristor-controlled reactor.
 The TCSC is based on thyristors without the gate turn off capability.
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14. 3: Thyristor-controlled series reactor (TCSR)
14

15. Continue….
 An inductive reactance compensator which consists of a series reactor
shunted by a thyristor controlled reactor in order to provide a smoothly
variable series inductive reactance.
 A series reactor bank is shunted by a thyristor-controlled reactor.
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16. 4: Thyristor-switched series capacitor (TSSC)
 A capacitive reactance compensator which consists of a series
capacitor banks shunted by a thyristor switched reactor to
provide a stepwise control of series capacitive reactance.
 a series capacitor bank is shunted by a thyristor-switched
reactor
 Instead of continuous control of capacitive impedance, this
approach of switching inductors without firing angle control
could reduce cost and losses of the controller.
16

17. 5: Thyristor-switched series reactor (TSSR)
 An inductive reactance compensator which consists of a
series reactors shunted by a thyristor controlled switched
reactor in order to provide a step wise control of series
inductive reactance.
 A series reactor bank is shunted by a thyristor-switched
reactor.
 This is a complement of TCSR, but with thyristor switches
fully on or off (without firing angle control) to achieve a
combination of stepped series inductance.
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18. Type-2: Shunt Controller
 The shunt controllers may be variable impedance, variable source, or a
combination of these
 All shunt controllers inject current into the system at the point of connection
(working principle )
1. as long as the injected current is in phase quadrature with the line voltage,
the shunt controller only supplies or consumes variable reactive power
2. Any other phase relationship will involve handing of real power as well as
reactive power
3. even a variable impedance connected to the line voltage causes a variable
current flow and hence represents injection of current in the line
18

19. Shunt Capacitive Compensation
 This method is used to improve the power factor.
 Whenever an inductive load is connected to the transmission line, power
factor lags because of lagging load current.
 To compensate, a shunt capacitor is connected which draws current leading
the source voltage. The net result is improvement in power factor.
 Reactive current is injected into the line to maintain voltage magnitude.
 Transmittable active power is increased but more reactive power is to be
provided.
19

20. Types of shunt compensation
1: Static synchronous compensator (STATCOM)
2: Static VAR Compensator (SVC). Most common
SVCs are:
a) Thyristor-controlled reactor (TCR):
b) Thyristor-switched reactor (TSR):
c) Thyristor-switched capacitor (TSC):
d) Mechanically-switched capacitor (MSC):
20

21. 1: Static synchronous
compensator (STATCOM)
 A static synchronous generator operated as a shunt
connected static VAR compensator whose capacitive or
inductive output current can be controlled independent of
the ac system voltage.
 STATCOM can be designed based on both voltage sourced or
current sourced converter
 Can be designed to act as active filter to absorb system
harmonics
 can be designed integrated with active power sources or
storage on the DC side so that the injected current may
include active power 21

22. 2: Static VAR Compensator (SVC)
 A shunt connected static VAR generator or absorber whose
output is adjusted to exchange capacitive or inductive
current so as to maintain or control specific parameters of
the electrical power system (typically bus voltage)
 based on thyristors without the gate turn off capability
 it includes separate equipment for leading and lagging VARS
 SVC is considered as an alternative to STATCOM
22

23. 23

24. 2(a): Thyristor-controlled reactor (TCR)
 A shunt connected thyristor controlled inductor whose effective
reactance is varied in a continuous manner by partial conduction
control of the thyristor valve.
 reactor is connected in series with a bidirectional thyristor valve.
 The thyristor valve is phase-controlled (firing angle control).
 Equivalent reactance is varied continuously.
 TCR is a subset of SVC in which conduction time and hence, current
in a shunt reactor is controlled by a thyristor based ac switch with
firing angle control.
24

25. 2(b): Thyristor-switched reactor (TSR)
 A shunt connected thyristor switched inductor whose
effective reactance is varied in a stepwise manner by full or
zero conduction operation of the thyristor valve
 Same as TCR but thyristor is either in zero- or full- conduction.
(without firing angle control)
 Equivalent reactance is varied in stepwise manner.
 TSR is another a subset of SVC
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26. 2(c): Thyristor-switched capacitor (TSC)
 A shunt connected thyristor switched capacitor whose effective
reactance is varied in a step wise manner by full or zero
conduction operation of the thyristor valve.
 capacitor is connected in series with a bidirectional thyristor valve.
 Thyristor is either in zero- or full- conduction.
 Equivalent reactance is varied in stepwise manner.
 TSC is also a subset of SVC in which thyristor based ac switches are
used to switch in and out (without firing angle control) shunt
capacitor units, in order to achieve the required step change in the
reactive power supplied to the system.
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27. 2(d): Mechanically-switched capacitor (MSC)
 In this scheme MSC’s are also used with TCR’s
 Uses conventional mechanical or SF6 switches
instead of thyristors to switch the capacitors .
 More economical when there are a large no of
capacitors to be switched than using TSCs
 The speed of switching is however longer and
this may affect transient stability
 This method is suitable for steady load
conditions , where the reactive power
requirements are predictable
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28. Type-3: Combined Series-series Controller
 Combination of separate series controllers which are controlled in a
coordinated manner in a multiline transmission system
 or as a unified way in which series controllers provide independent
series reactive compensation for each line, but also transfer real
power among the lines via the power link
 the term unified means that the dc terminals of all controller
converters are connected together for real power transfer
Unified series-series controller (Interline Power flow controller )
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29. Interline power flow controller (IPFC)
 The combination of two or more static synchronous series compensators
which are coupled Via a common DC link to facilitate bidirectional flow of
real power between the ac terminals of the SSSCs, and are controlled to
provide independent reactive compensation for the adjustment of real
power flow in each line and maintain the desired distribution of reactive
power flow among the lines.
 The IIPC structure may also include a STATCOM, coupled to the existing to
provide shunt reactive compensation and supply or absorb the overall real
power deficit of the combined SSSCs.
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30. Type-4 Combined Series-shunt Controller
 a combination of separate shunt and series controllers which are
controlled in a coordinated manner
 or as a unified power flow controller with series and shunt elements
 combined shunt and series controllers inject current into the system
with the shunt part of the controller and voltage in series in the line
with the series part of the controller
 when the shunt and series controllers are unified, there can be a real
power exchange between the series and shunt controllers via the
power link
Coordinated series and shunt
controllers
Unified Series and shunt
controllers
30

31. Unified Power Flow Controller (UPFC)
● A combination of STATCOM and SSSC coupled via a common dc link
● Bi-directional flow of real power between the SSSC and the STATCOM
 Unified Power Flow Controller = Static Synchronous Series
Compensator + STATCOM
31

32. Relative Importance of Different Types of
Controller (series controller)
 series connected controller impacts the driving voltage and hence
the current and power flow directly
 application is to control the current/power flow and damp
oscillations
 the series controller for a given MVA size is several times more
power full than the shunt controller
 series controller can be used for contingency and dynamic overload
conditions
32

33. Relative Importance of Different Types of
Controller (Shunt controller)
 shunt controller is like a current source which draws from or
injects or injects current into the line
 it control voltage at and around the point of connection through
injection of reactive current for a more effective voltage control
and damping of voltage oscillation
 main advantage of the shunt controller is that it serves the bus
node independently of the individual lines connected to the
bus
 shunt controller does not provide control over the power flow in
the lines
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34.  Can provide best of the both i.e. an effective power/current
flow and line voltage control
 can provide additional benefits (reactive power flow control )
with unified controllers
Relative Importance of Different Types of
Controller (Combination of series and shunt
controller)
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35. Converters for FACTS
 Controllers with gate turned off devices are based on dc to ac
converters and exchange active/reactive power with the ac
lines
 This requires energy storage device
 A converter based controller can be designed with high pulse
order or pulse width modulation to reduce the low order
harmonic generation to a very low level
 A converter can be designed to generate the correct waveform
in order to act as an active filter
35

36. Storage system with FACTS
 storage may be a large dc capacitor, storage batteries,
superconducting magnets
 Energy storage devices are needed when active power is involved in
the power flow
 storage is effective for controlling the system dynamics than the
corresponding controller without the storage
 dynamic pumping of real power in or out of the system as against only
influencing the transfer of real power within the system as in the case
with controllers lacking storage
 storage can deliver or absorb large amounts of real power in short
bursts
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37. Relative Importance of FACTS Controller
 For a given MVA size, the series controller is several times more powerful than the
shunt controller in application of controlling the power/current flow.
 Drawing from or injecting current into the line, the shunt controller is a good way
to control voltage at and around the point of connection.
 The shunt controller serves the bus node independently of the individual lines
connected to the bus.
 Series connected controllers have to be designed to ride through contingency and
dynamic overloads, and ride through or bypass short circuit currents.
 A combination of series and shunt controllers can provide the best of effective
power/current flow and line voltage.
 FACTS controllers may be based on thyristor devices with no gate turn-off or with
power devices with gate turn-off capability.
 The principle controllers are based on the dc to ac converters with bidirectional
power flow capability.
37

38. Continue…
 Energy storage systems are needed when active power is involved in
the power flow.
 Battery, capacitor, superconducting magnet, or any other source of
energy can be added in parallel through an electronic interface to
replenish the converter’s dc storage.
 A controller with storage is more effective for controlling the system
dynamics.
 A converter-based controller can be designed with high pulse order
or pulse width modulation to reduce the low order harmonic
generation to a very low level.
 A converter can be designed to generate the correct waveform in
order to act as an active filter.
 A converter can also be controlled and operated in a way that it
balances the unbalanced voltages, involving transfer of energy
between phases.
 A converter can do all of these beneficial things simultaneously I the
converter is so designed.
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39. References
 Hingorani, N. G. (1993). Flexible AC transmission. IEEE
spectrum, 30(4), 40-45.
 Song, Y. H., & Johns, A. (Eds.). (1999). Flexible ac transmission
systems (FACTS) (No. 30). IET.
 Hingorani, N. G. (1988, January). High power electronics and flexible
AC transmission system. In Proceedings of the American Power
Conference;(USA) (Vol. 50, No. CONF-880403--).
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40. Questions
 Differentiate between the shunt and series FACTS controllers
operational(application) point of view.
 What are the different types of FACTS devices?
 Compare the different types of FACTS devices according to their
application point of view.
 Why extra storage device is used in FACTS devices?
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