Flexible AC Transmission (FACTS)

TWO Broad Classes:
(i) STEADY STATE STABILITY
(ii) TRANSIENT STABILITY
Steady State Stability
A power system is said to be steady state
stable for a particular operating condition
if, following a small disturbance, it
reaches a steady state operating condition
which is identical or close to the pre-
disturbance operating condition.
POWER SYSTEM
STABILITY

Transient Stability
Transient stability is the ability of the power
system to maintain synchronism when
subjected to a severe transient disturbance.
Large disturbance in the normal operating
condition can result from a change in system
parameters; i.e. impedance variation due to
short circuits, outage of transmission lines,
generator or transformers, large variation in
load or receiving end voltage, loss of
excitation of any generator etc.

Factors Influencing Transient Stability
How heavily the generator is loaded .
The generator output during the fault. This depends
on the fault location and type.
The fault clearing time.
The post fault transmission system reactance.
 The generator inertia. The higher the inertia, the
slower the rate of change in rotor angle. This
reduces the kinetic energy gained during fault.
 The generator internal voltage magnitude.
 The infinite bus voltage magnitude.

• Dynamic braking
• High speed excitation systems
• Series capacitor insertion
• Regulated shunt compensation
• High speed fault clearing
• Independent pole operation of circuit breakers
Methods to Improve Transient
Stability

FACTS-DEFINITION
Flexible AC Transmission Systems (FACTS) are the
name given to the application of power electronic
devices to control the power flows and other
quantities in power systems.
IEEE Definitions
FACTS: AC transmission systems incorporating the
power electronic-based and other static controllers
to enhance controllability and increase power
transfer capability.
FACTS Controllers: A power electronic based system
& other static equipment that provide control of
one or more AC transmission parameters.

Limitations of Large AC Systems
 Long distance
transmission
systems
• Voltage stability
• Reactive power
problems
• Steady state stability
• Transient stability
Interconnected
systems
• Load flow problems
(needs management
of congestion)
• Voltage stability
• Frequency control
• Oscillation stability
• Inter-area oscillations

WHY FACTS CONTROLLERS?
 Enhances controllability and power
transfer capability.
 High speed response.
 Control line impedance, voltage and
optimum power flow.
 Increases system security.
 Provides greater flexibility.
 Balance reactive power (voltage,
transmission losses)
 Increase the stability of power
transmission over long distances

Dynamics of Electric Power Systems
∑Pi
∑Qi
= PGenerator + PLoad + PCompensation
= QGenerator + QLoad + QCompensation
S = P + jQ

Load Considerations - Transmission

CONTROLLABLE PARAMETERS FOR
FACTS DEVICES
•Control of the line impedance X can provide a
powerful means of current control.
•When the angle is not large, which is often the
case, control of X or the angle substantially
provides the control of active power
•Control of angle, which in turn controls the
driving voltage.
•Injecting a voltage in series with the line, and
perpendicular to the current flow, can increase
or decrease the magnitude of current flow.

•Injecting voltage in series with the line and with any
phase angle with respect to the driving voltage can
control the magnitude and the phase of the line
current.
•When the angle is not large, controlling magnitude of
one or the other line voltages can be a very cost
effective means for the control of reactive power flow
through the interconnection.
•Combination of the line impedance control with a
series controller and voltage regulation with a shunt
controller can also provide a cost effective means to
control both active and reactive power flow between
the two systems
CONTROLLABLE PARAMETERS FOR
FACTS DEVICES

Generations
• 1st
Generation of FACTS
(SVC & TCSC)
• 2nd
Generation of FACTS
(STATCOM & SSSC)
• 3rd
Generation of FACTS
(UPFC & IPFC)

INTRODUCTION TO
FACTS CONTROLLERS

Thyristor based FACTS Controllers
 Employs reactive impedances or a tap-
changing transformer with thyristor
switches as controlled elements.
SVC and TCSC
 Necessary reactive power required for
compensation is generated or absorbed
by capacitor or reactor banks
 Thyristor switches are used only for the
control of the combined reactive
impedances, these banks present to the
system.

Phase shifter
 Does not inherently need capacitor or
reactor banks
 Not able to supply or absorb the reactive
power it exchanges with the ac system
SVC and TCSC act indirectly on the
transmission network
SVC is applied as a shunt impedance to
produce the required compensating
current.

Thus the shunt compensation provided by
the SVC is a function of the line voltage.
TCSC is inserted in series with the line
Develops a compensating voltage to
increase the voltage across the series
impedance of the line
Ultimately determines the line current and
the power transmitted
Thus the actual series compensation
provided is a function of the line current.

Mechanical Switched Devices
MSC MSR

Comparison Chart- MSC, MSR, SVC

ABB’s SVC installation at Viklandet, Norway, on behalf of Statnett,
Norway’s state-owned transmission system operator. One of eight FACTS
solutions that ABB has delivered to Statnett since 1981, the SVC solution
has improved the capacity and reliability of power supplies in central
Norway.

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 the bus
voltage.
Regulate the line voltage by electronically switching an
inductor or a capacitor in shunt with the transmission
line
SVC is a combination of TCRs and TSCs connected in
shunt with the transmission line.

Static VAR Compensator (SVC)
Main Components
 Thyristor Controlled Reactor (TCR)
 Thyristor Switched Capacitor (TSC)
 TCR + TSC
 TCR + TSC +Filter Circuit (FC)
 Controller
- Hardware
- Software

Thyristor Controlled Reactor (TCR)
 Continuous control
 No transients
Elimination of harmonics by
tuning the FCs as filters
 Compact design

TCR – Bus Voltage and Current
α : firing angle
σ : conduction angle
σ = 2(π-α)

Thyristor Controlled Reactor (TCR)

Thyristor Switched Capacitor (TSC)
 Stepped control
 No transients
 No harmonics
 Low losses
 Redundancy and flexibility

TCR + TSC
Continuous control
 No transients
 Elimination of harmonics via
filters or TSR control
(thyristor switched reactor)
 Low losses
 Redundancy
 Flexible control and operation

TCR + TSC + Filter Circuit (FC)

VI Characteristics of SVC
• The regulation with a given slope
around the nominal voltage can be
achieved in the normal operating
range defined by the maximum
capacitive and inductive currents
of SVC
• The maximum obtainable
capacitive current (Ic) decreases
linearly with system voltage
• The voltage support capability of
the SVC deteriorates with the
decreasing system voltage

Applications
(i)Damping of Power
Oscillations
(ii)Voltage Stability
Enhancement
(iii)Maximum Power Transfer
Improvement
(iv)Transient Stability Margin
Enhancement
STATIC VAR COMPENSATOR (SVC)

Other types  Direct connected SVC
 Re-locatable SVC
61

THYRISTOR CONTROLLED SERIES
COMPENSATOR (TCSC)
 TCSC is 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 reactance.
 TCSC can control the line impedance through the
introduction of a thyristor controlled capacitor in
series with the transmission line.
T 1
T 2
L S
C
IC
IT
Ilin e
(a )
Figure 2. (a) A basic module
C+ -
( b )
L S
T 1
T 2
C B
M O V
G
U H S C
L d
Figure 2.(b) A practical module

Operation of TCSC Controller
A TCSC is a series controlled capacitive reactance and
can be considered as a variable inductor connected
in parallel with a fixed capacitor as in figure.
C
L
+ -
L
C
jLj
C
jZeq
ω
ω
ω
ω 1
1
)(||
1
−
−=





−=
Equivalent impedance is given by:
1.If or , the reactance of the fixed
capacitor is less than that of the parallel connected variable
reactor and this combination provides a variable capacitive
reactance.
2.If , a resonance develops that result in infinite
capacitive impedance, and this is an unacceptable condition.
3.If , then the combination provides a variable
inductive reactance.
0)/1( >− LC ωω )/1( CL ωω >
0/1 =− LC ωω
0)/1( <− LC ωω

Modes of TCSC Operation
1. Bypassed Thyristor Mode: Thyristors are made to fully
conduct with a conduction angle of 180 degrees.
A continuous sinusoidal flow of current
Net current through the module is inductive, for the
susceptance of the reactor is greater than that of the
capacitor
2. Blocked Thyristor Mode: (waiting mode)
Firing pulses to the thyristor valves are blocked
Net TCSC reactance is capacitive
3. Partially Conducting Thyristor or Vernier Mode:
By varying the thyristor pair firing angle in an
appropriate range
Allows TCSC to behave either as a continuously
controllable capacitive reactance or as an inductive
reactance

T 1
T 2
L S
C
Bypassed thyristor mode
T 1
T 2
L S
C
Blocked thyristor mode

Modes of TCSC Operation
Capacitive – Vernier Control Mode: Thyristors are fired
when the capacitor voltage and capacitor current have
opposite polarity.
TCR current has a direction opposite to that of the
capacitor current resulting in a loop current
Loop current increases the voltage across the C ,
effectively enhancing the equivalent capacitive
reactance .
To preclude resonance, the firing angle of the forward
facing thyristor is constrained in the range αmin ≤ α ≤ 180
Inductive – Vernier Control Mode: TCSC can be operated
by having a high level of thyristor conduction.
The direction of the circulating current is reversed
The controller presents a net inductive impedance

L S
C
T 1
T 2
TCSC partially conducting capacitive vernier mode
T 1
T 2
L S
C
TCSC partially conducting inductive vernier mode

THYRISTOR CONTROLLED SERIES
CAPACITOR (TCSC)
 Without
TCSC
 With TCSC
)sin(
X
VV
P 21
L
21
δ−δ=
)sin(
XX
VV
P 21
CL
21
TCSC δ−δ
−
=

π
σσσ
π
σσ ))2/tan()2/tan((
)1(
)2/(cos
)(
4sin
)( 2
222
−
−−
+
+
−
−==
kk
kXX
X
XX
X
X
I
V
X
PC
C
PC
C
C
m
CF
TCSC
Thyristor Controlled Series Capacitor (TCSC)
K is the degree of
compensation
L
TCSC
X
X
=K
XC Nominal reactance of the fixed
capacitor only.
XP Inductive reactance of inductor
connected in parallel with
fixed capacitor.
σ Conduction angle
P
Cr
X
X
k ==
ω
ω

Converter Based FACTS Controllers
 Employs self commutated voltage sourced
switching converters to realize rapidly
controllable, static synchronous ac voltage and
current sources.
 Provides superior performance characteristics
and uniform applicability for transmission voltage
 Effective line impedance
 Angle control
 Offers unique potential to exchange real power
directly with the ac system
 Provide independently controllable reactive
power compensation

VSC BASED FACTS CONTROLLERS
(i) Static Synchronous Compensator (STATCOM)
(ii) Static Synchronous Series Compensator (SSSC)
(iii) Unified Power Flow Controller (UPFC)
(iv) Interline Power Flow Controller (IPFC)
Voltage Source Converter

Switching Converter Based
Synchronous Voltage Source
 Generates a balanced set of three sinusoidal
voltages at the fundamental frequency with
controllable amplitude and phase angle.
 No inertia
 Its response is practically instantaneous
 It does not alter the existing system impedance
 It can internally generate reactive power (both
capacitive and inductive)
 It can exchange real power with the ac system if
it is coupled to an appropriate energy source

Switching Converter Based
Synchronous Voltage Source
 The Synchronous Voltage Source (SVS)
facilitates a forcing function approach to
transmission line compensation and power flow
control.
 It can apply a defined voltage to force the desired
line current
 It can apply a defined current to force the desired
terminal voltage
 The compensation provided by an SVS remains
largely independent of the network variables (line
current, voltage or angle)

(i) Static Synchronous Compensator (STATCOM)
(ii) Static Synchronous Series Compensator (SSSC)
(iii) Unified Power Flow Controller (UPFC)
(iv) Interline Power Flow Controller (IPFC)
STATCOM controls transmission voltage by reactive
shunt compensation.
SSSC provides series compensation by directly
controlling the voltage across the series impedance
of the transmission line,
thereby controlling the effective transmission
impedance.

• UPFC can control individually or in combination all
three effective transmission parameters (voltage,
impedance and angle)
• Directly can control the real and reactive power
flow in the line.
• IPFC is able to transfer real power between lines
• To provide reactive series compensation
• Can facilitate a comprehensive overall real and
reactive power management for a multiline
transmission system

θ= sin
X
VV
P
t
cs
t
scs
X
)VcosV(V
Q
−θ
=
Control Variables for Power Flow Direction
(i)Active Power Flow ⇒ Phase Difference θ
(ii)Reactive Power Flow ⇒ Voltage Magnitude Vc

 Reactive Power exchange between the
converter and the ac system can be controlled
by varying the amplitude of the output voltage
produced (Vc )
 If Vc > Vs then current flows through the
reactance Xt, from the converter to the ac
system
 Converter generates reactive (capacitive) power
for the ac system
 If Vc < Vs then reactive current flows from the ac
system to the converter
 Converter absorbs reactive (inductive) power
 If Vc = Vs then reactive power exchange is zero

 Real Power exchange between the converter and
the ac system can be controlled by phase shifting
the converter output voltage (Vc) w.r.t the ac
system voltage (Vs)
 If Vc is made to lead Vs the converter supplies
real power from its dc energy storage to the ac
system
 The real component of current through Xt is in
phase opposition to Vs
 If Vc is made to lag Vs the converter absorbs real
power from the ac system for dc energy storage.
 The real component of current through Xt is now
in phase with Vs

STATIC SYNCHRONOUS COMPENSATOR
(STATCOM)
• For reactive shunt compensation, dc energy source is
replaced by a small dc capacitor.
• Same control mechanism

CONTROL MODES OF STATCOM
STATIC SYNCHRONOUS COMPENSATOR
(STATCOM)
• The converter keeps the capacitor charged to the required
voltage level
• This is done by keeping Vc lag Vs by a small angle.
• Converter absorbs a small amount of real power

VI Characteristics of STATCOM
• STATCOM can provide both
capacitive and inductive
compensation
• Able to control its output
current over the rated maximum
capacitive or inductive range
independently of the ac system
voltage.
• The sharp decrease of
transmitted power in the
region is avoided.
• The stability margin is
significantly improved
2
δmaxc
2
sin
2
VI
_δsin
X
V
=P
π<δ<
2
π

Comparison of Shunt Compensators
 SVC and STATCOM are very similar in their
functional compensation capability, but the
basic operating principles are fundamentally
different.
 A STATCOM functions as a shunt-connected
synchronous voltage source whereas
 A SVC operates as a shunt-connected,
controlled reactive admittance.
 This difference accounts for the STATCOM’s
superior functional characteristics, better
performance, and greater application flexibility
than those attainable with a SVC.

 In the linear operating range, the V-I characteristic and
functional compensation capability of the STATCOM and
the SVC are similar.
 Concerning the non-linear operating range, the STATCOM
can provide the full capacitive output current at any
system voltage, whereas the SVC can supply only
diminishing output current with decreasing system voltage.

 Thus, the STATCOM is more effective than the SVC in
providing voltage support under large system disturbances
during which the voltage excursions would be well outside of
the linear operating range of the compensator. The ability of
the STATCOM to maintain full capacitive output current at
low system voltage also makes it more effective than the
SVC in improving the transient stability.
 The attainable response time and the bandwidth of the
closed voltage regulation loop of the STATCOM are also
significantly better than those of the SVC.
 In situations where it is necessary to provide active power
compensation, the STATCOM is able to interface a suitable
energy storage (large capacitor, battery) from where it can
draw active power at its DC terminal and deliver it as AC
power to the system. On the other side, the SVC does not
have this capability.

STATIC SERIES SYNCHRONOUS
COMPENSATOR (SSSC)

COMPENSATOR (SSSC)
 The SSSC is able to maintain a constant compensating
voltage in face of variable line current.
 Control the amplitude of injected compensating voltage
independent of the amplitude of the line current.
 For normal capacitive compensation, the output voltage
lags the line current by 90o
 The output voltage when leads the line current by 90o
,
the injected voltage decreases the voltage across the
inductive line impedance and thus an inductive
compensation is in effect.
 SSSC increases the transmitted power by a fixed fraction
of the maximum power transmittable by the
uncompensated line, independent of δ, in the operating
range of 0 ≤ δ ≤ π/2.

COMPENSATOR (SSSC)
SSSC increases the transmitted power
by a fixed fraction of the max. power
transmittable by the uncompensated
line, independent of δ, in the operating
range of
2
δq
2
cos
X
VV
_δsin
X
V
=P
2
πδ0 ≤≤

Comparison of Series Compensators
 The SSSC is capable of internally generating a
controllable compensating voltage over an identical
capacitive and inductive range independently of the
magnitude of the line current.
 The compensating voltage of TSSC over a given control
range is proportional to the line current.
 The TCSC can maintain maximum compensating voltage
with decreasing line current over a control range
determined by the current boosting capability of the
thyristor-controlled reactor.
 The variable impedance type series compensators
(TCSC or SVC) cannot exchange active power with the
transmission line and can only provide reactive
compensation.

Comparison of Series Compensators
 The SSSC has the ability to interface with an external DC
power supply to provide compensation for the line resistance
by the injection of active power as well as for the line
reactance by the injection of reactive power.
 The SSSC with an energy storage increases the
effectiveness of power oscillation damping (i) by modulating
the series reactive compensation to increase and decrease
the transmitted power and (ii) by concurrently injecting an
alternating virtual positive and negative real impedance to
absorb and supply active power from the line in sympathy
with the prevalent machine swings.
 The variable impedance type compensators can damp power
oscillation only by modulated reactive compensation affecting
the transmitted power.

UNIFIED POWER FLOW CONTROLLER (UPFC)

 Series converter provides the main function of the
UPFC by injecting a voltage with controllable
magnitude Vpq and phase angle ρ in series with the
line via an insertion transformer
 The transmission line current flows through this
voltage resulting in reactive and real power exchange
between it and the ac system
 The reactive power exchanged at the terminal of the
series insertion transformer is generated internally
by this converter
 The real power exchanged at this terminal is
converted into dc power which appears at the dc link
as a positive or negative real power demand
UNIFIED POWER FLOW CONTROLLER

 The basic function of Shunt converter is to supply or
absorb the real power demanded by series converter at the
common dc link
 This dc link power is converted back to ac and coupled to
the transmission line via a shunt-connected transformer
 Shunt Converter can also generate or absorb controllable
reactive power, and thereby provide independent shunt
reactive compensation for the line.
 There is a closed direct path for the real power negotiated
by the action of series voltage injection, through both the
converters back to the line, the corresponding reactive
power exchanged is supplied or absorbed locally by series
converter and therefore does not have to be transmitted by
the line
UNIFIED POWER FLOW CONTROLLER

 independently control active and reactive power flows
on the line as well as the bus voltage.
 Active power can freely flow in either direction
between the ac terminals of the two converters
through the dc link.
 Although, each converter can generate or absorb
reactive power at its own ac output terminal, they
cannot internally exchange reactive power through dc
link.
 simultaneously control all three parameters (voltage,
line impedance and phase angle) of power flow.
 These are achieved through the control of the
magnitude and phase angle of the series injected
voltage
ADVANTAGES OF UPFC

 UPFC can provide multiple power flow control
functions by adding the injected voltage phasor Vpq,
with appropriate magnitude Vpq, and phasor angle, ρ,
to the sending end voltage phasor Vs.
 Terminal Voltage Regulation or Control with
continuously variable in-phase/anti-phase voltage
injection for voltage increments Vpq = ±ΔV (ρ=0)
 Series Reactive Compensation where Vpq= Vc is
injected in quadrature with the line current I. The
UPFC injected series compensating voltage can be
kept constant, independent of the line current
variation, whereas the voltage across the series
compensating impedance varies with the line
Power Control Functions of UPFC

(a) (b) (c) (d)
(a) voltage regulation
(b) line impedance compensation (c) phase shifting
(d) simultaneous control of voltage, impedance, and angle

 Phase Shifting (Transmission Angle Regulation)
where Vpq = Vσ is injected with an angular relationship
w.r.t to Vs that achieves the desired σ phase shift
(advance or retard) without any change in
magnitude. Thus UPFC can function as a perfect
phase shifter.
 Multi-function Power Flow Control, executed by
simultaneous terminal voltage regulation, series
capacitive line compensation, and phase shifting
where Vpq = ΔV + Vc + Vσ
 This functional capability is unique to UPFC.

 Controlled series compensators such as TCSC,
TSSC and SSSC provide a series compensating
voltage that is in quadrature with the line current.
 Effective overall line impedance is defined at which
the transmitted power is strictly determined by the
transmission angle.
 SSSC has a considerably wider control range at low
transmission angles than both TCSC and TSSC.
 Since UPFC is a self-sufficient voltage source it can
force upto 0.5 p.u real power flow in either direction
and also control reactive power exchange between
sending end and receiving end buses.
Comparison of UPFC to Controlled
Series Compensators

 The maximum increase attainable in actual transmitted
power is much less at small transmission angles than
at large ones
 The achievable maximum increase in transmittable
power attainable with the TCSC and TSSC is a
constant percentage defined by the maximum degree
of series compensation of the power transmitted with
the uncompensated line, at the given transmission
angle.
 The TCSC and TSSC are a series impedance and thus
the compensating voltage they produce is proportional
to the line current, which is function of angle δ
Series Compensators

 SSSC being a reactive voltage can provide
compensation over a wider range.
 SSSC cannot control the reactive line power and thus
the reactive power remains proportional to the real
power P
 The compensating voltage, the UPFC produces, is
independent of the line current and of angle δ
 The maximum change (increase or decrease) in
transmittable power, as well as in receiving-end reactive
power, achievable by the UPFC is not a function of angle
δ and is determined solely by the maximum voltage the
UPFC is rated to inject in series with the line.
Series Compensators

Flexible AC Transmission (FACTS)

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