Flexible Ac Transmission Systems 2Mark Materials and Question Bank

FLEXIBLE AC TRANSMISSION SYSTEMS
QUESTION BANK
(2 MARK Q &A, 16 MARK Q)
UNIT I INTRODUCTION
1. What is meant by FACTS?
FACTS devices are made by advanced power electronic control equipments.
The Flexible ac transmission systems (FACTS) give solutions to the problems and
limitations which were introduced in the power system with the introduction of
power electronics based control for reactive power.
2. What is real power?
The real power flows from source to load. Unit: Watts (W)
3. What are the sources of real power?
All dc and ac generators
4. What is the need for reactive power?
The reactive power is essential for the operation of electromagnetic energy
devices; it provides required coupling fields for energy devices.
5. What is reactive power?
The reactive power flows from load to source. The average value for reactive
power is zero. It does not result in any active power consumption. Unit: Volt Ampere
Reactive (VAR).
6. What are the sources of reactive power?
i) Capacitor
ii) Synchronous generator (it can generate both real and reactive power)
7. Main objectives of FACTS?
i) The power transfer capability of transmission system is to be increased.
ii) The power flow is to be kept over designated routes.

8. Who implemented the FACTS concept? For what?
The concept of FACTS was first defined in 1988 by N.G.Hingorani. It
controls the interrelated parameters which are involved in power system operation
such as series and shunt impedance, current, voltage and phase angle. Also it damps
the oscillations at various frequencies below the rated frequency.
9. What are the types of FACTS devices?
SVC - Static Var Compensator
TCSC - Thyristor Controlled Series Capacitor
UPFC - Unified Power Flow Controller
IPFC - Interline Power Flow Controller
SSSC - Static Synchronous Series Compensator
SSC - Static Synchronous Compensator (STATCOM)
10. What are the types of FACTS controllers?
i) Series controller
ii) Shunt controller
iii) Combined series-series controllers
iv) Combined series-shunt controllers
11. Where the first STATCOM was implemented?
Tennesee Valley Authority (TVA) installed the first static synchronous
compensator (STATCOM) in 1955 to strengthen transmission line ties between its
Sullivan substation and the rest of its network. It reduces the need of additional
transformer bank and avoiding more labours.
12. Where the first UPFC was implemented?
In 1998 installation of the first unified power flow controller (UPFC) was
completed at the Inez Substations owned by American Electrical Power (AEP). It
represents the first controller capable of providing complete control of all the three
basic transmission system parameters that is voltage, line impedance, phase angle.

13. Write down the advantages of FACTS.
i) It controls line impedance angle and voltage which helps in controlling the power
flow in transmission lines.
ii) The power flow in the transmission lines can be made optimum.
iii) It helps in damping out the oscillations and avoids damage of various
equipments.
iv) It limits the impacts of faults and equipment failures.
14. Define reactive power control.
To make transmission networks operate within desired voltage limits,
methods of making up or taking away reactive power is called reactive power
control.
15. What is the reactive power value ‘Q’ for electromagnetic devices?
Electromagnetic devices store energy in their magnetic fields. These devices
draw lagging current, there by resulting in positive values of Q; therefore they are
frequently referred to as the absorbers of reactive power.
16. What is the reactive power value ‘Q’ for electrostatic devices?
Electrostatic devices store energy in their electric fields. These devices draw
leading current and result in a negative value of ‘Q’; thus they are seen to be
suppliers of reactive power.
17. What is reactive power compensation? Compensators?
Reactive power control for a transmission line is often called reactive power
compensation. External devices or sub systems that control reactive power on
transmission line are known as compensators.
18. What is shunt compensation?
The Shunt compensators are connected parallel to the transmission lines with
the help of Circuit breakers. Shunt reactors compensate for the line capacitance, and
they control over voltages at no loads and light load conditions. The shunt
compensators need careful system design because of high charging in-rush currents.

19. What is series compensation?
The Series compensators are connected series with the transmission lines.
Series compensators are used to partially offset the effects of the series inductances
of transmission lines. It provides automatic adjustment of reactive power
compensation.
20. What are the design factors considered for the series compensators?
i)The voltage magnitudes across the capacitor bank
ii) The fault currents at the terminals of a capacitor bank
iii) The fault currents at the terminals of a capacitor bank
iv) The placement of shunt reactors in relation to the series capacitors
21. What are the methods used for compensating the uncompensated
transmission lines?
i) Load compensation- One capacitor is connected parallel across the load
ii) System compensation - In addition with the parallel capacitor the power
utility devices are also connected.
22. Compare the conventional series controller with the advanced series
controller (IPFC).
Conventional series controller Advanced series controller
(IPFC)
1. Load balancing of transmission
lines is very Poor.
1. It has better load balancing.
2. It has very low X/R ratio. 2. It has high X/R ratio.
3. Transmission line losses are
very high.
3. Transmission line losses are
low.
4. It controls both real and
reactive power with low operating
efficiency.
4. It controls both real and
reactive power with high
operating efficiency.

16 MARKS:
1. Describe briefly the load and system compensation schemes.
2. Explain the operation of UPFC with diagram.
3. Explain the reactive power compensation at the sending, mid-point and receiving
ends of the transmission lines.
4. Explain the principle, working and characteristics of static VAR compensator with
a neat circuit diagram.
5. Explain the working and characteristics of Thyristor switched series capacitor with
a neat diagram.
6. Discuss briefly about the variation of the TCSC reactance with firing angle
‘alpha’.
7. Explain the V-I capability characteristics of single module TCSC.
8. Explain with neat circuit diagram about fixed capacitor-Thyristor controlled
reactor.
UNIT II STATIC VAR COMPENSATOR (SVC) AND APPLICATIONS
1. Short notes on voltage control by SVC?
The transmission line voltage is maintained by connecting static var
compensator (SVC) in the receiving end side. The comparator will measure the
actual and reference values of transmission line voltage; depends on the comparator
output the reactive power is injected into the transmission line, and the transmission
line voltage will be controlled.
2. Write down the equation for SVC bus voltage.
VS = VSVC + ISVC XS
3. Give the advantages of the slope in the SVC dynamic characteristics.
i) The reactive power rating is reduced
ii) SVC is prevented from reaching its reactive power limit too frequently
iii) It provides effective parallel operation of two parallel connected SVC’s

4. What is SVC slope in the dynamic characteristics?
To improve the power system operating performance 2.5% voltage de-
regulation will be provided in SVC operation. So this voltage de-regulation results in
5% slope in the SVC dynamic characteristics.
5. How the SVC prevents the reactive power rating, reaching its limit too
frequently?
Due to slope in the SVC dynamic characteristics the no load to change in
load variation limit will be increased, so the SVC is prevented from reaching its
reactive power limit too frequently. Thus the total reactive power needed is reduced
to certain limit.
6. Explain the load sharing of two parallel connected SVC’s.
Without slope in the SVC dynamic characteristics there is a discontinuous
gap between capacitive and inductive region. This gap will be reduced by operating
two parallel connected SVC’s with slope (2.5% voltage de-regulation) in the SVC
dynamic characteristics.
7. What are the conditions involved for influence of the SVC on system voltage?
i) Coupling transformer ignored
ii) With coupling transformer
iii) System gain
8. What is ESCR?
Effective short circuit ratio (ESCR) = 1 / XS
= 1 / (-Δsvc / ΔIsvc)
= BS(equivalent system susceptance)
9. What is system gain?
System gain KN = Vs / ESCR
= Vs / BS
10. What is per unit system gain?
Per unit System gain KN = ΔVsvc / Bsvc

= Qsvc / Sc p.u
11. What is short circuit power?
Short circuit power Sc = (base voltage) × (short circuit current)
= (Vb ) × ( BS. VS)
12. What is SMIB system?
A single synchronous generator connected with more number of transmission
lines in electric power system is called single machine infinite bus (SMIB) system.
13. What are the different cases involved in power angle curves of a SMIB
system?
i) Uncompensated case
ii) Ideal midpoint SVC unlimited rating (Qsvc > 4 Pmax)
iii) Fixed capacitor connected at its midpoint
iv) Midpoint SVC with limited rating (Qsvc = 2Pmax)
14. What is the objective of SVC enhancement in transient stability?
An SVC significantly enhances the ability to maintain synchronism of a
power system that is controlled and co-ordinated system, even when the system is
subjected to large sudden disturbances. So the SVC enhancement in transient
stability provides steady power transfer in transmission lines.
15. What is synchronizing power co-efficient?
Synchronizing power co-efficient Ks =ΔPE / Δ
Where ΔPE - change in generator power output
Δ - change in generator rotor angle
16. What is SVC susceptance?
Susceptance BS = ( αc/ xc ) - (αi / xi)
Where xi - total inductive reactance of the SVC
xc - total capacitive reactance of the SVC
αi - the conducting fraction of TCR

αc - the conducting fraction of TSC
17. Write down the equation for synchronizing torque co-efficient.
Synchronizing torque co-efficient Ks
Ks = ΔPE / Δ
= ((V1 V2 COS ) / XT ) + ((V1 V2 SIN ) / (Vm XT)) 2 × (X2 /
4XT)
18. Write short notes on prevention of voltage instability.
By connecting SVC at receiving end bus the transmission line voltage will be
maintained even when the power factor value varies ( p.f.-lag, lead, unity) with load.
Thus the SVC prevents the voltage instability in a power system.
16 MARKS:
1. Explain the design of SVC voltage regulator. Also discuss the influence of SVC
on system voltage.
2. Discuss in detail the effect of SVC for the enhancement of transient stability.
3. Using a general schematic diagram, explain the three basic modes of SVC control
in detail.
4. Explain the application of SVC for prevention of voltage instability.
5. How do you enhance the damping in power system using SVC?
6. Explain the design of SVC voltage regulator and discuss the voltage control
capability of SVC. What are the advantages of slope in dynamic characteristics of
SVC?
7. Discuss in detail about the role of SVC in improving the stability limit and
enhancing the power system damping
8. Describe the construction and operating characteristics of synchronous
condensers.
9. Derive and explain the series and shunt compensation of symmetrical transmission
lines.
UNIT III THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC) AND
APPLICATIONS

1. What is meant by TCSC?
TCSC is a thyristor controlled series capacitor. It has one parallel connected
thyristor controlled inductor and a series capacitor connected with the transmission line. It
provides continuous variable capacitive reactance and variable inductive reactance to
control the transmission line parameters.
2. Write down the expression for equivalent impedance of a TCSC.
Equivalent impedance of a TCSC, Zeq = -j ((1) / (ωc-1/ ωL))
3. What is the condition for variable capacitive reactance in a TCSC?
If (ωc - (1/ ωL)) 0 - the TCSC provides variable capacitive reactance mode.
4. What is the condition for variable inductive reactance in a TCSC?
If (ωc - (1/ ωL)) 0 - the TCSC provides variable inductive reactance mode.
5. What are the different modes of operation of TCSC?
i) Bypassed- thyristor mode
ii) Blocked - thyristor mode
iii) Partially conducting thyristor or Vernier mode
6. Give short notes on Bypassed- thyristor mode.
In this mode the TCSC module behaves like a parallel capacitor - inductor
combination. The susceptance (increase in power flow) value of inductor is higher
than the capacitor. This mode is employed for control purposes and initiating certain
protective functions.
7. Give short notes on Blocked - thyristor mode.
In this mode, also known as the waiting mode, here the firing pulses of
thyristor switches are blocked. The TCSC behaves like a fixed series capacitor and
the net TCSC reactance is capacitive. In this mode the dc-offset voltages of the
capacitors are monitored and quickly discharged using a dc-offset control without
causing any harm to the transmission system transformers.
8. Give short notes on capacitive Vernier mode.
In this mode the TCSC provides continuously controllable capacitive
reactance. It is achieved by varying the thyristor pair firing angle in an appropriate

range. A variant of this mode is the capacitive vernier mode, in which the thyristor
are fired when the capacitor voltage and current have opposite polarity. This
condition causes a thyristor controlled reactor (TCR) current that has a direction
opposite that of the capacitor current thereby resulting in a loop-current flow in the
TCSC controller. This loop current increases the voltage across the fixed capacitor
(FC), effectively enhancing the equivalent capacitive reactance and the series
compensation level.
9. Give short notes on inductive Vernier mode.
In this mode the TCSC provides continuously controllable inductive
reactance. Here the TCSC can be operated by having a high level of thyristor
conduction. In this mode the direction of the circulating current is reversed and the
controller presents a net inductive impedance.
10. What are the conclusions made from the TCSC modes of operation?
i) Thyristor switched series capacitor (TSSC), which permits a discrete
control of the capacitive reactance.
ii) Thyristor controlled series capacitor (TCSC), which offers a continuous
control of capacitive or inductive reactance. Practically TSSC is more commonly
used.
11. What are the modeling techniques involved in TCSC?
i)Variable reactance model (1. Transient stability model 2. Long term
stability model)
ii)An advanced transient stability studies model
12. What is the need for modeling of a TCSC?
A TCSC involves continous - time dynamics, relating to voltages and
currents in the capacitor and reactor, and nonlinear discrete switching behavior of
thyristors. So it is very important to derive a model for a TCSC controller to
maintain the stability of a power system.
13. Give short notes on variable reactance model.
It is a quasi-static approximation model. This model provides multimode
TCSC configuration that is the discontinuity gap between inductive and capacitive

regions are reduced by providing continous reactance range. This model is generally
used for inter-area mode analysis, and it provides high accuracy when the reactance
boost factor is less than 1.5.
14. What is reactance boost factor of a TCSC model?
Reactance boost factor = XTCSC / XC
15. Write short notes on Transient stability model.
In the variable reactance model for stability studies, a reference value of
TCSC reactance Xref, is generated from the power scheduling controller based on the
power flow specification in the transmission lines. Then Xref is converted into Xtotal
with the help of modulation controller, delay circuit, lag circuit, Xaux signal and Xfixed
signals. Finally the required Xtotal control value will be converted into per unit value
and it pass through the control unit.
16. Write down the TCSC base reactance value.
TCSC base reactance Zbase = (KVTCSC) 2 / MVAsys
Where KVTCSC - rms line - line voltage of the TCSC in KV
MVAsys - 3phase MVA base of the power system
17. What is long term stability model?
The capability curves of the TCSC depend on the duration for which the
voltage and current operating conditions persist (keep on continued) of the TCSC.
For long term dynamic simulation, an over load management function needs to be
incorporated in the control system. It determines the appropriate TCSC overload
range, for which it modifies the Xmax limit and Xmin limit. It then applies the same
modifications to the controller. This model is used widely in commercial stability
programs because of its simplicity, and it is also used for system planning studies as
well as for initial investigations of the effects of the TCSC in damping power
oscillations.
18. What is the use of advanced transient stability studies model?
This model is used to solve the TCSC time varying dynamics with the help of
differential equations. It calculates the capacitor voltage, capacitor current, inductor
voltage and inductor current for every half cycle of zero - crossing.

19. Write down the expression for proportional controller gain and integral
controller gain.
Proportional controller gain Kp =-10 exp (- ((65 - σreq) / 2)2)
Integral controller gain Ki = -23-24 exp ((σreq -65) / 2)
20. What is the need for improvement in the TCSC system stability limit?
In a power system due to critical faults, the large volume of power tends to
flow in parallel paths of a transmission lines; it would create severe overload
condition. The conventional fixed-series compensation on the parallel paths of a
transmission line provides the solution for the above problem but it increases the
total system losses. Therefore, it is advantageous to install a TCSC in key
transmission paths, which provides series compensation according to their system
requirements with lower system losses. So the improvement in the TCSC system
stability limit was very important in power system.
21. What are functions of damping controller?
i) It stabilizes both the post disturbance oscillations and growing oscillations
during normal operation.
ii) It must reduces the interaction of high frequency phenomena in power
system, such as network resonances.
iii) It eliminates the local instabilities within the controller band width.
iv) It should be robust; it provides desired damping over a wide range of
system operating conditions.
v) It should be reliable.
22. Write short notes on bang-bang control.
Bang-bang control is a discrete control form in which the thyristors of
inductor bank are either fully switched on or fully switched off. Thus the TCSC
alternates between a fixed inductor and a fixed capacitor respectively, and it is
advantageous that such control is used not only for minimizing first order swing
oscillations but for damping any subsequent swing oscillations as well.

23. What are the two auxiliary signals for TCSC modulation?
i) Local signals
ii) Remote signals
24. What are the local auxiliary signals for TCSC modulation?
i) The line current
ii) The real power flow
iii) The bus voltage
iv) The local bus frequency
25. What are the remote auxiliary signals for TCSC modulation?
i) The rotor-angle/speed deviation of a remote generator
ii) The rotor-angle/speed (frequency) difference across the system
iii) The real power flow on adjacent transmission lines
16 MARKS:
1. Explain the principle of operation of TCSC. Also discuss the different modes of
TCSC.
2. Explain the effect of TCSC for the enhancement of system damping.
3. Explain the need for variable and fixed series compensation schemes.
4. Describe the variable reactance model of TCSC.
5. Explain the different modes of operation of TCSC.
6. Describe the modeling of TCSC for load flow study.
7. Explain the working, characteristics and operating modes of variable reactance
model of thyristor controlled series capacitor.
8. Explain in detail the applications of thyristor controlled series capacitor.
9. How TCSC is used for the improvement of the stability of a system.

UNIT IV EMERGING FACTS CONTROLLERS
1. What is meant by emerging facts controllers?
The emerging facts controllers exchange the reactive power to the
transmission lines with the help of phase shifting techniques. If needed the real
power is also supplied in addition to the reactive power in to the transmission line
with the help of emerging FACTS devices such as STATCOM and UPFC. Here the
need of large size capacitor bank and inductor bank are reduced, so the operating
performance will be improved.
2. What is meant by STATCOM?
The static synchronous compensator (STATCOM or SSC) is a shunt
connected reactive power compensation device that is capable of generating and/or
absorbing reactive power and in which the output can be varied to control the
specific parameters of an electric power system. It is capable of generating or
absorbing independently controllable real and reactive power at its output terminals
when it is fed from a dc energy source or energy storage device at its input terminals.
3. What are the functions of STATCOM in the improvement of power system
performance area?
i) It provides dynamic voltage control in transmission and distribution system
ii) It provides damping against the oscillation in power system.
iii) It provides better transient stability
iv) It has voltage flicker control (it withstands sudden changes)
v) It controls both real and reactive power
4. What are the common advantages of STATCOM?
i) It required small space because it replaces the passive inductor and
capacitor bank by compact electronic converters.
ii) It has modular factory build electronic equipments, so site work and
commissioning time will be reduced.
iii) It uses encapsulated electronic converters, thereby minimizing its
environmental impact.

5. Give details about first installed STATCOM device at Sullivan Sub-station.
Tennessee Valley Authority (TVA) installed the first ± 100 MVA STATCOM
in 1995 at its Sullivan substation.
6. What are the applications of first installed STATCOM device at Sullivan
Sub-station?
The application of this STATCOM is expected to reduce the TVA’s need for
load tap changing transformers, there by achieving savings by minimizing the
potential for transformer failure. This STATCOM solves the problems against off-
peak dilemma of over voltages in the Sullivan substation area while avoiding the
more labor and apace intensive installation of an additional transformer bank.
7. What are the advantages of first installed STATCOM device at Sullivan Sub-
station?
i) It increases the capacity of transmission line voltage by providing
instantaneous control.
ii) It provides greater flexibility in bulk power transactions.
iii) It also increases the system reliability by damping grids of major
oscillations in this grid.
8. Write short notes on principle of operation of STATCOM.
A STATCOM is a controlled reactive power source. It provides the desired
reactive power generation and absorption entirely by means of electronic processing
of the voltage and current waveforms in a voltage source converter.
9. Give the explanation about reactive power exchange between converter and
the ac system.
If the ac system voltage is lesser than the sending end voltage then the
converter inject the reactive power to the transmission line. If the ac system voltage
is higher than the sending end voltage then the converter absorb the reactive power
from the transmission line.
10. What is the importance of V-I characteristics of STATCOM?
The V-I characteristics of STATCOM shows that it can supply both the
capacitive and inductive compensation and is able to independently control its output
current over the rated maximum capacitive or inductive range irrespective of the

amount of ac system voltage. That is, the STATCOM can provide full capacitive
reactive power at any ac system voltage even as low as 0.15 p.u.
11. How will you determine the maximum attainable transient over current
region?
The maximum attainable transient over current in the capacitive region is
determined by the maximum current turn-off capacity of the converter switches.
12. Why the converters (STATCOM) absorb the small amount of real power
from the ac system?
The converter absorbs the small amount of real power from the ac system to meet its
internal losses and keep the capacitor (energy storage device) voltage at the desired
level.
13. Write short on UPFC.
The unified power flow controller (UPFC) is the most versatile FACTS
controller developed so far, with all encompassing of voltage regulation, series
compensation and phase shifting. It can independently and very rapidly control both
real and reactive power flows in a transmission line.
14. What are the functions of series converter in the UPFC?
In a UPFC series converter exchanges both real and reactive power with the
transmission line. Although the reactive power is internally generated or absorbed by
the series converter, the real power generation or absorption is made feasible by the
dc energy storage device.
15. What are the functions of shunt converter in the UPFC?
In a UPFC shunt converter mainly used to supply the real power demand of
series converter, which it derives from the transmission line itself. The shunt
converter maintains constant voltage of the dc bus. In addition, the shunt converter
functions like a STATCOM and independently regulate the terminal voltage of the
interconnected bus by generating or absorbing a requisite amount of reactive power.
16. What are the operating variable constraints of UPFC?
i) The series injected voltage magnitude
ii) The line current through series converter

iii) The shunt converter current
iv) The maximum line side voltage of the UPFC
V) The minimum line side voltage of the UPFC
vi) The real power transfer between the series converter and the shunt
converter
17. What was the result of case study of an UPFC at 100 miles length
transmission line power system?
In that case study of a 100 miles long two area model transmission line the
3Ф to ground fault has been applied four times then disconnected. The result
indicates that with UPFC the power transfer is 357 MW, without UPFC the power
transfer is 176 MW. Thus the power transfer will be increased with the help of an
UPFC.
18. What was the effect of damping by using UPFC in case study power system
transmission lines?
In that case study of a 100 miles long two area model transmission line the
3Ф to ground fault has been applied four times then disconnected. The result
indicates that with UPFC the power transfer is 357 MW, without UPFC the power
transfer is 176 MW. Thus the power transfer will be increased with the help of an
UPFC. Also the oscillations in the transmission line will be damped by UPFC. That
is with UPFC the oscillation range will be -14 MW to -15 MW per second but
without UPFC the oscillation range will be -9 MW to -21 MW per second.
16 MARKS:
1. With a neat sketch, explain the implementation of UPFC.
2. Explain the working of STATCOM with a neat sketch. In what way it differs from
SVC?
3. Explain the operation of STATCOM with its V-I characteristics.
4. Explain the performance of VSC based STATCOM.
5. Describe the modeling of UPFC for power flow and transient stability studies.
6. Explain the basic principle and control capability of unified power flow controller.
7. Explain the power transfer capability of UPFC and compare its capabilities with
other FACTS controllers.

8. Describe the construction of UPFC with a block diagram and its characteristics
with phasor diagrams.
9. Explain the power flow control and oscillation damping in the two area system
using UPFC.
UNIT V CO-ORDINATION OF FACTS CONTROLLERS
1. What is meant by controller interactions?
If two or more FACTS devices are connected in same transmission line then
the operating variables between them must have better co-ordinated, that is called
controller interaction. If FACTS devices are not co-ordinated, it creates unwanted
oscillation in the transmission lines.
2. What are the types of controller interactions?
i) Multiple FACTS controller of a similar kind
ii) Multiple FACTS controller of a dissimilar kind
iii) Multiple FACTS controllers and HVDV converter controllers
3. What are the frequencies ranges of controller interactions?
i) 0 Hz for steady state interactions
ii) 0 - 3/5 Hz for electromechanical oscillations
iii) 2 - 15 Hz for small signal or control oscillation
iv) 10 - 50/60 Hz for sub synchronous resonance (SSR) interaction
v) 15 Hz for electromagnetic transients, high - frequency resonance or
harmonic resonance interactions, and network resonance interactions
4. What is meant by steady state interaction?
Steady-state interactions between different controllers (FACTS-FACTS or
FACTS-HVDC) occur between their system related controls. They are steady state in
nature and do not involve any controller dynamics. These interactions are related to
issues such as the stability limits of steady state- state voltage and steady-state
power, included are evaluations of the adequacy of reactive-power support at buses,
system strength and so on. Eg) Steady state voltage control between FACTS-HVDC
for ac voltage regulation.

5. What is the analysis method used to determine the steady state interaction?
To determine this interaction Load-Flow and Stability programs are used.
6. What is meant by electromechanical oscillation interaction?
Electromechanical oscillation interaction between FACTS controllers involve
synchronous generators, compensator machines and associated power system
stabilizer control. The oscillations include local mode oscillations typically in the
range of 0.8 - 2 Hz, and inter-area mode oscillations, typically in the range of 0.2 -
0.8 Hz. The local mode is contributed by synchronous generators in a plant or
several generators located in close vicinity, the inter-area mode results from the
power exchange between tightly coupled generators in two areas linked by weak
transmission lines.
7. What is the analysis methods used to determine the electromechanical
oscillation interaction?
To determine this interaction eigen value analysis programs are used.
8. What is meant by control or small signal oscillation interactions?
Controller interactions between individual FACTS controllers and the
network or between FACTS controllers and HVDC links may lead to the onset of
oscillations in the range of 2 - 15 Hz. These oscillations are largely dependent on the
network strength and the choice of FACTS controller parameters, and they are
known to result from the interaction between voltage controllers of multiple SVC’s,
the series resonance between series capacitors and shunt reactors in the frequency
range of 4 - 15 Hz and so forth. The emergence of these oscillations significantly
influences the tuning of controller gain.
9. What are the analysis methods used to determine the control or small signal
oscillation interaction?
These high frequency oscillation interactions are determined by frequency
scanning programs, electromagnetic transient programs (EMTP’s), Physical
simulators and eigen value analysis programs.
10. What is meant by sub synchronous resonance interactions?
Sub synchronous oscillations may be caused by the interaction between the
generator torsional system and the series compensated transmission lines, the HVDC
converters, the generator excitation control or even the SVC’s. Theses oscillations

usually in the frequency range of 10 - 50/60 Hz, can potentially damage generator
shafts.
11. What are the analysis methods used to determine the sub synchronous
resonance interactions?
These SSR oscillation interactions are determined by frequency scanning
programs, electromagnetic transient programs (EMTP’s), Physical simulators and
eigen value analysis programs.
12. What is meant by high frequency interaction?
High-frequency oscillations in excess of 15 Hz are caused by large nonlinear
disturbances, such as the switching of capacitors, reactors, or transformers for which
reason they are classified as electromagnetic transients. FACTS controllers need to
be co-ordinated to minimize such interactions.
13. What causes the geomagnetically induced currents (GIC’s) during
controller interactions?
Transformer saturation causes the geomagnetically induced currents (GIC’s)
during controller interactions.
16 MARKS:
1. Using linear control techniques, explain the co-ordination of multiple controllers.
2. Explain the controller interactions between multiple SVCs (SVC-SVC) in a large
power system.
3. Explain the FACTS controller interactions with similar, dissimilar HVDC
controller links.
4. Describe the genetic algorithm based control co-ordination of FACTS controllers.
5. Explain the co-ordination of multiple controllers using genetic algorithm.

Flexible Ac Transmission Systems 2Mark Materials and Question Bank

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