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Facts controller -notes

Sub Code: 18EPE23 CIE:50
Hrs/Week: 04
Total Hrs: 50
Exam Hours: 03
1. The objective of this course is to introduce variety of controllers with new technology
based on power electronics.
2. The students have an opportunity to learn principle of operation, modeling and
applications of different FACTS controllers in AC transmission systems.
The Student will be able to
1. Acquire the basic concept of general power system considerations and knowledge of
different FACTS controllers. .
2. Describe the principle of operation, modeling, steady state and dynamic
characteristics of TCSC,SVC,STATCOM,SPST and UPFC controllers.
3. Analyze, determine and investigate the effect of individual FACTS controllers on power
flow and voltage profile in line.
Course Content:
Module-1: FACTS: Concept and General System Considerations: Basics of Power
Transmission Networks and Interconnection, Flow of power in AC system, - Control of Power
Flow in AC - Transmission Line, Limits of loading capability, Power flow and dynamic stability
consideration of a Transmission Interconnection, Relative importance of controllable
parameters, and Basic types of FACTS controllers, Brief description and Definitions of FACTS
controllers, Application of FACTS Controllers in Distribution Systems.
10 Hours
Module-2: Shunt Compensators: Analysis of Uncompensated AC Line - Passive Reactive
Power Compensation - - Shunt Compensation Connected at the Midpoint of the Line. Basic
principle of operation of StaticVAr Compensator (SVC)- Voltage control by SVC – advantages
of slope in dynamic characteristics – influence of SVC on system voltage. Modeling of SVC.
Applications – enhancement of transient stability – steady state power transfer – prevention
of voltage instability.
10 Hours
Module-3: Series Compensators: Compensation by a Series Capacitor Connected at the
Midpoint of the Line, Basic model concept of Thyristor Controlled Series Capacitor (TCSC),
Operation of the TCSC – different modes of operation – modeling of TCSC – variable reactance
model – modeling for stability studies. Applications – improvement of the system stability
limit – enhancement of system damping – voltage collapse prevention.
10 Hours
Module-4: Static Phase Shifting Transformer (SPST): General - basic principle of a PST configurations of SPST, Improvement of transient stability using SPST - damping of low
frequency power oscillations -applications of SPST- Some Representative Examples.
Comparison between Series and Shunt Capacitor.
10 Hours
Module-5: Emerging FACTS Controllers: Static Synchronous Compensator (STATCOM) –
operating principle – V-I characteristics , applications: Steady state power transferenhancement of transient stability – prevention of voltage instability. SSSC-operation of SSSC
and the control of power flow –modeling of SSSC in load flow and transient stability.Unified
Power Flow Controller (UPFC) – Principle of operation – modes of operation – applications –
modeling of UPFC for power flow studies.Special Purpose FACTS Controllers - Interline Power
Flow Controller - operation and control.
10 Hours
1. NarainG.Hingorani, Laszio. Gyugyl, “Understanding FACTS Concepts and Technology
of Flexible AC Transmission System”, IEEE Press, Standard Publishers, Delhi 2001.
2. K.R.Padiyar,” FACTS Controllers in Power Transmission and Distribution”, New
Age International(P) Limited, Publishers, New Delhi, 2008.
1. Mohan Mathur, R., Rajiv. K. Varma, “Thyristor – Based Facts Controllers for Electrical
Transmission Systems”, IEEE press and John Wiley & Sons, Inc.
Module-1: FACTS: Concept and General System Considerations:
Basics of Power Transmission Networks and Interconnection, Flow of power in AC system, Control of Power Flow in AC - Transmission Line, Limits of loading capability, Power flow and
dynamic stability consideration of a Transmission Interconnection, Relative importance of
controllable parameters, and Basic types of FACTS controllers, Brief description and
Definitions of FACTS controllers, Application of FACTS Controllers in Distribution Systems.
Power Transmission Networks and Interconnections
• Purpose of the transmission network is to pool power plants and load centers in
order to minimize the total power generation capacity and fuel cost.
• Transmission interconnections enable taking advantage of diversity of loads,
availability of sources, and fuel price to supply electricity to the loads at
minimum cost with a required reliability.
• With that perspective, transmission is often an alternative to a new generation
• Less transmission capability means that more generation resources would be
required regardless of whether the system is made up of large or small
• small distributed generation becomes more economically viable if there is a
backbone of a transmission grid.
• One cannot be really sure about what the optimum balance is between generation
and transmission unless the system planners use advanced methods of analysis
which integrate transmission planning into an integrated valuetransmission/generation planning scenario.
• Cost of transmission lines and losses, as well as difficulties encountered in
building new transmission lines, would often limit the available transmission
• It seems that there are many cases where economic energy or reserve sharing is
constrained by transmission capacity, and the situation is not getting any better.
• On the other hand, as power transfers grow, the power system becomes
increasingly more complex to operate and the system can become less secure for
riding through the major outages.
• It may lead to large power flows with inadequate control, excessive reactive
power in various parts of the system, large dynamic swings between different
parts of the system and bottlenecks, and thus the full potential of transmission
interconnections cannot be utilized.
• Power systems of today, by and large, are mechanically controlled.Problem with
mechanical devices is that control cannot be initiated frequently, because these
mechanical devices tend to wear out very quickly compared to static devices.
• There is a widespread use of microelectronics, computers and high-speed
communications for control and protection of present transmission systems;
• Power system planners, operators, and engineers have learned to live with this
limitation by using a variety of ingenious techniques to make the system work
effectively, but at a price of providing greater operating margins and
• In recent years, greater demands have been placed on the transmission network,
and these demands will continue to increase because of the increasing number of
nonutility generators and heightened competition among utilities themselves.
• Increased demands on transmission, absence of long-term planning, and the need
to provide open access to generating companies and customers, all together have
created tendencies toward less security and reduced quality of supply.
• The FACTS technology is essential to alleviate some but not all of these
difficulties by enabling utilities to get the most service from their transmission
facilities and enhance grid reliability.
• It must be stressed, that for many of the capacity expansion needs, building of
new lines or upgrading current and voltage capability of existing lines and
corridors will be necessary.
In ac power systems, given the insignificant electrical storage, the electrical
generation and load must balance at all times.
If generation is less than load, the voltage and frequency drop, and thereby the
load, goes down to equal the generation minus the transmission losses.
However, there is only a few percent margin for such a self-regulation. If voltage
is propped up with reactive power support, then the load will go up, and
consequently frequency will keep dropping, and the system will collapse.
Alternately, if there is inadequate reactive power, the system can have voltage
When adequate generation is available, active power flows from the surplus
generation areas to the deficit areas, and it flows through all parallel paths
available which frequently involves extra high- voltage and medium-voltage
Often, long distances are involved with loads and generators along the way.
There are in fact some major and a large number of minor loop flows and uneven
power flows in any power transmission system.
Power Flow in Parallel Paths
• Consider a simple case of power flow [Figure 1.1(a)], through two parallel paths
(possibly corridors of several lines) from a surplus generation area, shown as an
equivalent generator on the left, to a deficit generation area on the right.
• Without any control, power flow is based on the inverse of the various
transmission line impedances.
• Apart from ownership and contractual issues over which lines carry how much
power, it is likely that the lower impedance line may become overloaded and
thereby limit the loading on both paths even though the higher impedance path
is not fully loaded.
• There would not be an incentive to upgrade current capacity of the overloaded
path, because this would further decrease the impedance
Figure 1.1 Power flow in parallel paths: (a) ac power flow with parallel paths;
(b) power flow control with HVDC;
(c) power flow control with variable impedance; (d) power flow control with variable phase
• Figure 1.1 (b) shows the same two paths, but one of these has HVDC
transmission. With HVDC, power flows as ordered by the operator, because with
HVDC power electronics converters power is electronically controlled. Also,
because power is electronically controlled, the HVDC line can be used to its full
thermal capacity if adequate converter capacity is provided. Furthermore, an
HVDC line, because of its high-speed control, can also help the parallel ac
transmission line to maintain stability. However, HVDC is expensive for general
use, and is usually considered when long distances are involved, such as the
Pacific DC Intertie on which power flows as ordered by the operator.
• As alternative FACTS Controllers, Figures 1.1(c) and 1.1(d) show one of the
transmission lines with different types of series type FACTS Controllers. By
means of controlling impedance [Figure 1.1(c)] or phase angle [Figure 1.1(d)],
or series injection of appropriate voltage (not shown) a FACTS Controller can
control the power flow as required. Maximum power flow can in fact be limited
to its rated limit under contingency conditions when this line is expected to carry
more power due to the loss of a parallel line.
1.2.2 Power Flow in a Meshed System
• To understand the free flow of power, consider a simplified case in which
generators at two different sites are sending power to a load centre through a
network consisting of three lines in a meshed connection (Fig. 1.2).
• Suppose the lines AB, BC, and AC have continuous ratings of 1000 MW, 1250
MW, and 2000 MW, respectively, and have emergency ratings of twice those
numbers for a sufficient length of time to allow rescheduling of power in case of
loss of one of these lines.
Figure 1.2 Power flow in a mesh network: (a) system diagram; (b) system diagram with Thyristor-Controlled Series Capacitor
in line AC; (c) system diagram with Thyristor-Controlled Series Reactor in line BC; (d) system diagram with ThyristorControlled Phase Angle Regulator in line AC.
• If one of the generators is generating 2000 MW and the other 1000 MW, a total
of 3000 MW would be delivered to the load center. For the impedances shown,
the three lines would carry 600, 1600, and 1400 MW, respectively, as shown in
Figure 1.2(a). Such a situation would overload line BC (loaded at 1600 MW for
its continuous rating of 1250 MW), and therefore generation would have to be
decreased at B, and increased at A, in order to meet the load without overloading
line BC.
• Power flows in accordance with transmission line series impedances (which are
90% inductive) that bear no direct relationship to transmission ownership,
contracts, thermal limits, or transmission losses. If, a capacitor whose reactance
is —5 ohms at the synchronous frequency is inserted in one line [Figure 1.2(b)],
it reduces the line's impedance from 10 ohm to 5 ohm, so that power flow through
the lines AB, BC, and AC will be 250,1250, and 1750 MW, respectively.
• It is clear that if the series capacitor is adjustable, then other power-flow levels
may be realized in accordance with the ownership, contract, thermal
limitations, transmission losses, and a wide range of load and generation
schedules. Although this capacitor could be modular and mechanically switched,
the number of operations would be severely limited by wear on the mechanical
components because the line loads vary continuously with load conditions,
generation schedules, and line outages.
• Other complications may arise if the series capacitor is mechanically controlled.
A series capacitor in a line may lead to subsynchronous resonance (typically at
10-50Hz for a 60 Hz system). This resonance occurs when one of the mechanical
resonance frequencies of the shaft of a multiple-turbine generator unit coincides
with 60 Hz. minus the electrical resonance frequency of the capacitor with the
inductive impedance of the line. If such resonance persists, it will soon damage
the shaft.
• Also while the outage of one line forces other lines to operate at their emergency
ratings and carry higher loads, power flow oscillations at low frequency
(typically 0.3-3 Hz) may cause generators to lose synchronism, perhaps
prompting the system's collapse.
• If series capacitor is thyristor-controlled,
o can be varied as often as required.
o can rapidly damp any SSR conditions, a
o can damp low frequency oscillations in the power flow.
o allow the transmission system to go from one steady-state condition
o Avoid risk of damage to generator shaft and system collapse
• In other words, a thyristor-controlled series capacitor can greatly enhance the
stability of the network.
• it is practical for part of the series compensation to be mechanically controlled
and part thyristor controlled.
• Similar results may be obtained by increasing the impedance of one of the lines
in the same meshed configuration by inserting a 7 ohm reactor (inductor) in
series with line AB [Figure 1.2(c)]. Again, a series inductor that is partly
mechanically and partly thyristor-controlled, it could serve to adjust the steadystate power flows as well as damp unwanted oscillations.
• Another option, a thyristor-controlled phase-angle regulator could be installed
instead of a series capacitor or a series reactor in any of the three lines to serve
the same purpose.
• In Figure 1.2(d), the regulator is installed in the third line to reduce the total
phase-angle difference along the line from 8.5 degrees to 4.26 degrees.
• The same results could also be achieved by injecting a variable voltage in one
of the lines.
Note that balancing of power flow in the above case did not require more than one
FACTS Controller, and indeed there are options of different controllers and in different
To make the best use of the transmission asset, and to maximize the loading capability
(taking into account contingency conditions), it is important to know the limitation of
transmission loading capability
There are three kinds of limitations for loading capability:
• Thermal
• Dielectric
• Stability
A large current flow increases the losses in the form of heat. This results in increased
conductor temperatures. Excessive temperature may result in expansion and resultant
sag of conductors causing decreased clearance to ground. Temperature extremes have
an "annealing effect" causing reduced mechanical strength of aluminium.
Thermal capability of an overhead line is a function of the a)ambient temperature, b)
wind conditions, c) condition of the conductor, and d) ground clearance.
It varies due to the variable environment and the loading history.
The nominal rating of a line is generally decided on a worst ambient environment case
There are also off-line computer programs that can calculate a line's loading capability
based on available ambient environment and recent loading history.
There are the on-line monitoring devices that provide a basis for on-line real-time
loading capability. These methods have evolved over a period of many years, and,
given the age of automation (typified by GPS systems and low-cost sophisticated
communication services), it surely makes sense to consider reasonable, day to day,
hour to hour, or even real-time capability information.
Normal loading of the lines is decided on a loss evaluation basis under assumptions
which may have changed for a variety of reasons;.
Increasing the rating of a transmission circuit involves consideration of the real-time
ratings of the transformers and other equipment.
Realtime loading capability of transformers is also a function of ambient temperature,
aging of the transformer and recent loading history.
Off-line and on-line loading capability monitors can also be used to obtain real time
loading capability of transformers.
There is the possibility of upgrading a line by changing the conductor to that of a higher
current rating, which may in turn require structural upgrading.
Finally, there is the possibility of converting a single-circuit to a double-circuit line.
Once the higher current capability is available, then the question arises of how it should
be used. Will the extra power actually flow and be controllable? Will the voltage
conditions be acceptable with sudden load dropping, etc.? The FACTS technology can
help in making an effective use of this newfound capacity.
The dielectric limit refers to the insulation capability of the transmission line
From an insulation point of view, many lines are designed very conservatively
conservatively based on worst ambient environment case scenario.
Exceeding dielectric limits (maximum electric field strength) results in failure of
insulation, causing faults
For a given nominal voltage rating, it is often possible to increase normal operation
by +10% voltage (i.e., 500 kV-550 kV) or may be even higher.
Care is then needed to ensure that dynamic and transient overvoltages are within
Modern gapless arresters, or line insulators with internal gapless arresters, or
powerful thyristor-controlled overvoltage suppressors at the substations can enable
significant increase in the line and substation voltage capability.
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The FACTS technology could be used to ensure acceptable over-voltage and power
flow conditions.
Stability There are a number of stability issues that limit the transmission capability.
These include:
• Transient stability - corresponds to the stability attained after a fault occurs & suddenly a
large part of load is bypassed. Then there is a large unbalance in the system. Then also
gradually the system attains the stability.
• Dynamic stability- like transient stability but here help of an external device is taken to
regain the stability whereas in transient stability the stability was attained within the power
system itself without the help of any external device.
• Steady-state stability - is the stability the power system attains after slight unbalance.
suppose a small amount of load is disconnected, but after a very short duration again the
power system will regain its steady state.
• Frequency collapse- If the frequency deviates from desired value , then generators start to
go out of synchronism which triggers events in the power system causing voltage, frequency,
power imbalance due to which a power system will collapse.
• Voltage collapse- is typically associated with reactive power demand of load not being met
due to shortage in reactive power production and transmission.
• Subsynchronous resonance- Sub-Synchronous Resonance is an electrical power system
condition where, electrical network exchanges energy with turbine generator at one or more
natural frequency of combined system, below the synchronous frequency of the system
Power flow and dynamic stability consideration of a Transmission
• Figure 1.3(a) shows a simplified case of power flow on a transmission line.
Locations 1 and 2 could be any transmission substations connected by a
transmission line.
• Substations may have loads, generation, or may be interconnecting points on
the system and for simplicity they are assumed to be stiff busses.
• Ex and E2 are the magnitudes of the bus voltages with an angle δ between the
two. The line is assumed to have inductive impedance X, and the line resistance
and capacitance are ignored.
• As shown in the phasor diagram [Figure 1.3(b)] the driving voltage drop in the
line is the phasor difference EL between the two line voltage phasors, E1 and
E2. The line current magnitude is given by I = EL/X,
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/ = E/X, and lags EL by 90°
• For a typical line, angle δ and voltage drop along the line, is small compared to
the line voltages.
• Given that a transmission line may have a voltage drop at full load 1% per10
km, and assuming that a line between two stiff busbars (substations) is 200 km
long, the voltage drop along this line would be 20% at full load, and the angle δ
would be small.
• If we were to assume, for example, that with equal magnitudes of Ex and E2,
and X of 0.2 per unit magnitude, the angle 8 would be only 0.2 radians or 11.5
• The current flow on the line can be controlled by controlling EL or X or δ.
• Figure 1.3(b) shows that the current flow phasor is perpendicular to the driving
voltage (90° phase lag). If the angle between the two bus voltages is small, the
current flow largely represents the active power. Increasing or decreasing the
inductive impedance of a line will greatly affect the active power flow. Thus
impedance control, which in reality provides current control, can be the most
cost-effective means of controlling the power flow.
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• Figure 1.3(c), corresponding to Figure 1.3(b), shows a phasor diagram of the
relationship between the active and reactive currents with reference to the
voltages at the two ends.
because it is assumed that there are no active power losses in the line.
• Thus, varying the value of X will vary P, Q1 and Q2 in accordance with Eq.(1.1),
Eq.(1.2), and Eq.(1.3), respectively. Assuming that E1 and E2 are the magnitudes
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of the internal voltages of the two equivalent machines representing the two
systems, and the impedance X includes the internal impedance of the two
equivalent machines, Figure 1.3(d) shows the half sinewave curve of active power
increasing to a peak with an increase in δ to 90 degrees.
• Power then falls with further increase in angle, and finally to zero at δ = 180°. It is
easy to appreciate that without high-speed control of any of the parameters E1, E2,
Ei — E2, X and δ, the transmission line can be utilized only to a level well below
that corresponding to 90 degrees. This is necessary, in order to maintain an
adequate margin needed for transient and dynamic stability and to ensure that the
system does not collapse following the outage of the largest generator and/or a
• Increase and decrease of the value of X will increase and decrease the height of the
curves, respectively, as shown in Figure 1.3(d). For a given power flow, varying of
X will correspondingly vary the angle between the two ends.
• Power/current flow can also be controlled by regulating the magnitude of voltage
phasor E1 or E2. However, it is seen from Figure 1.3(e) that with change in the
magnitude of E1 the magnitude of the driving voltage phasor E1 — E2 does not
change by much, but its phase angle does. This also means that regulation of the
magnitude of voltage phasor E1 and/or E2 has much more influence over the
reactive power flow than the active power flow, as seen from the two current phasors
corresponding to the two driving voltage phasors E1 — E2 shown in Figure 1.3(e).
• Current flow and hence power flow can also be changed by injecting voltage in
series with the line. It is seen from Figure 1.3(f) that when the injected voltage is in
phase quadrature with the current (which is approximately in phase with the driving
voltage, it directly influences the magnitude of the current flow, and with small angle
influences substantially the active power flow.
• Alternatively, the voltage injected in series can be a phasor with variable magnitude
and phase relationship with the line voltage [Figure 1.3(g)]. It is seen that varying
the amplitude and phase angle of the voltage injected in series, both the active and
reactive current flow can be influenced.
Relative importance of controllable parameters
It is worth noting a few basic points regarding the possibilities of power flow control:
_ Control of the line impedance X (e.g., with a thyristor-controlled series capacitor) 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 (with a Phase Angle Regulator, for example), which in turn
controls the driving voltage, provides a powerful means of controlling the current flow
and hence active power flow when the angle is not large.
_ Injecting a voltage in series with the line, and perpendicular to the current flow, can
increase or decrease the magnitude of current flow. Since the current flow lags the
driving voltage by 90 degrees, this means injection of reactive power in series, (e.g.,
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with static synchronous series compensation) can provide a powerful means of
controlling the line current, and hence the active power when the angle is not large.
_ 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. This means
that injecting a voltage phasor with variable phase angle can provide a powerful means
of precisely controlling the active and reactive power flow. This requires injection of
both active and reactive power in series.
_ Because the per unit line impedance is usually a small fraction of the line voltage, the
MVA rating of a series Controller will often be a small fraction of the throughput line
_ When the angle is not large, controlling the magnitude of one or the other line voltages
(e.g., with a thyristor-controlled voltage regulator) 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 the active and reactive power flow between the two systems
Basic types of FACTS controllers
In general, FACTS Controllers can be divided into four categories:
_ Series Controllers
_ Shunt Controllers
_ Combined series-series Controllers
_ Combined series-shunt Controllers
Figure 1.4(a) shows the general symbol for a FACTS Controller: a thyristor arrow inside a
Series Controllers: [Figure 1.4(b)]
The series Controller could be a variable impedance, such as capacitor, reactor, etc., or a power
electronics based variable source of main frequency, subsynchronous and harmonic
frequencies (or a combination) to serve the desired need. In principle, all series Controllers
inject voltage in series with the line. Even a variable impedance multiplied by the current flow
through it, represents an injected series voltage in the line. As long as the voltage is in phase
quadrature with the line current, the series Controller only supplies or consumes variable
reactive power. Any other phase relationship will involve handling of real power as well.
Shunt Controllers: [Figure 1.4(c)] As in the case of series Controllers, the shunt Controllers
may be variable impedance, variable source, or a combination of these.
In principle, all shunt Controllers inject current into the system at the point of connection.
Even a variable shunt impedance connected to the line voltage causes a variable current flow
and hence represents injection of current into the line. As long as the injected current is in phase
quadrature with the line voltage, the shunt Controller only supplies or consumes variable
reactive power. Any other phase relationship will involve handling of real power as well.
Combined series-series Controllers: [Figure 1.4(d)] This could be a combination of separate
series controllers, which are controlled in a coordinated manner, in a multiline transmission
system. Or it could be a unified Controller, Figure 1.4(d), in which series Controllers provide
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independent series reactive compensation for each line but also transfer real power among the
lines via the power link.
The real power transfer capability of the unified series-series Controller, referred to as Interline
PowerFlow Controller, makes it possible to balance both the real and reactive power flow in
the lines and thereby maximize the utilization of the transmission system. Note that the term
"unified" here means that the dc terminals of all Controller converters are all connected together
for real power transfer.
Combined series-shunt Controllers: [Figures 1.4(e) and 1.4(f)] This could be a combination of
separate shunt and series Controllers, which are controlled in a coordinated manner [Figure
1.4(e)], or a Unified Power Flow Controller with series and shunt elements [Figure 1.4(f)]. In
principle, 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.
However, 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. It is important to appreciate that
the series-connected Controller impacts the driving voltage and hence the current and power
flow directly. Therefore, if the purpose of the application is to control the current/power flow
and damp oscillations, the series Controller for a given MVA size is several times more
powerful than the shunt Controller.
As mentioned, the shunt Controller, on the other hand, is like a current source, which draws
from or injects current into the line. The shunt Controller is therefore a good way to control
voltage at and around the point of connection through injection of reactive current (leading or
lagging), alone or a combination of active and reactive current for a more effective voltage
control and damping of voltage oscillations.
This is not to say that the series Controller cannot be used to keep the line voltage within the
specified range. After all, the voltage fluctuations are largely a consequence of the voltage drop
in series impedances of lines, transformers, and generators. Therefore, adding or subtracting
the FACTS Controller voltage in series (main frequency, subsynchronous or harmonic voltage
and combination thereof) can be the most cost-effective way of improving the voltage profile.
Nevertheless, a shunt controller is much more effective in maintaining a required voltage
profile at a substation bus. One important advantage of the shunt Controller is that it serves the
bus node independently of the individual lines connected to the bus.
Series Controller solution may require, but not necessarily, a separate series Controller for
several lines connected to the substation, particularly if the application calls for contingency
outage of any one line. However, this should not be a decisive reason for choosing a shuntconnected Controller, because the required MVA size of the series Controller is small
compared to the shunt Controller, and, in any case, the shunt Controller does not provide
control over the power flow in the lines.
On the other hand, series-connected Controllers have to be designed to ride through
contingency and dynamic overloads, and ride through or bypass short circuit currents. They
can be protected by metal-oxide arresters or temporarily bypassed by solid-state devices when
the fault current is too high, but they have to be rated to handle dynamic and contingency
The above arguments suggest that a combination of the series and shunt Controllers [Fig(e)
and (f)] can provide the best of both, i.e., an effective power/ current flow and line voltage
For the combination of series and shunt Controllers, the shunt Controller can be a single unit
serving in coordination with individual line Controllers [Figure 1.4(g)]. This arrangement can
provide additional benefits (reactive power flow control) with unified Controllers.
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FACTS Controllers may be based on thyristor devices with no gate turn-off (only with gate
turn-on), or with power devices with gate turn-off capability. Also, in
general, as will be discussed in other chapters, the principal Controllers with gate turn-off
devices are based on the dc to ac converters, which can exchange active and/ or reactive power
with the ac system. When the exchange involves reactive power only, they are provided with a
minimal storage on the dc side. However, if the generated ac voltage or current is required to
deviate from 90 degrees with respect to the line current or voltage, respectively, the converter
dc storage can be augmented beyond the minimum required for the converter operation as a
source of reactive power only.
This can be done at the converter level to cater to short-term (a few tens of main frequency
cycles) storage needs. In addition, another storage source such as a battery, superconducting
magnet, or any other source of energy can be added in parallel through an electronic interface
to replenish the converter's dc storage. Any of the converter-based, series, shunt, or combined
shunt-series Controllers can generally accommodate storage, such as capacitors, batteries, and
superconducting magnets, which bring an added dimension to FACTS technology [Fig(h), (i),
and (j)]·
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Figure 1.4 Basic types of FACTS Controllers: (a) general symbol for FACTS Controller;
(b) series Controller; (c) shunt Controller; (d) unified seriesseries ontroller; (e) coordinated series and shunt Controller; (f)
unified series-shunt Controller; (g) unified Controller for multiple lines; (h) series Controller with storage; (i) shunt
Controller with storage; (j) unified series-shunt Controller with storage.
The benefit of an added storage system (such as large dc capacitors, storage batteries,
or superconducting magnets) to the Controller is significant. A Controller with storage
is much more effective for controlling the system dynamics than the corresponding
Controller without the storage. This has to do with 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. Here also, engineers have to
rethink the role of storage, particularly the one that can deliver or absorb large amounts
of real power in short bursts.
A converter-based Controller can also be designed with so-called high pulse order or
with pulse width modulation to reduce the low order harmonic generation to a very low
level. A converter can in fact be designed to generate the correct waveform in order to
act as an active filter. It can also be controlled and operated in a way that it balances
the unbalance voltages, involving transfer of energy between phases. It can do all of
these beneficial things simultaneously if the converter is so designed.
This section briefly describes and defines various shunt, series, and combined
• For the converter-based Controllers there are two principal types of converters
with gate turn-off devices.
• These are the so-called voltage-sourced converters and the current-sourced
converters. As shown in the left hand side of Figure 1.5(a), the voltage-sourced
converter is represented in symbolic form by a box with a gate turn-off device
paralleled by a reverse diode, and a dc capacitor as its voltage source. As shown
in the right-hand side of Figure 1.5(a), the current-sourced converter is
represented by a box with a gate turn-off device with a diode in series, and a dc
reactor as its current source.
• For the voltage-sourced converter, unidirectional dc voltage of a dc capacitor is
presented to the ac side as ac voltage through sequential switching of devices.
18 | P a g e
• Through appropriate converter topology, it is possible to vary the ac output
voltage in magnitude and also in any phase relationship to the ac system voltage.
The power reversal involves reversal of current, not the voltage.
• When the storage capacity of the dc capacitor is small, and there is no other
power source connected to it, the converter cannot supply or absorb real power
for much more than a cycle. The ac output voltage is maintained at 90 degrees
with reference to the ac current, leading or lagging, and the converter is used to
absorb or supply reactive power only.
• For the current-sourced converter, the dc current is presented to the ac side
through the sequential switching of devices, as ac current, variable in amplitude
and also in any phase relationship to the ac system voltage. The power reversal
involves reversal of voltage and not current. The current-sourced converter is
represented symbolically by a box with a power device, and a dc inductor as its
current source
• From overall cost point of view, the voltage-sourced converters seem to be
preferred, and will be the basis for presentations of most converter-based FACTS
19 | P a g e
Figure 1.5 Shunt-connected Controllers: (a) Static Synchronous Compensator (STATCOM) based on
voltage-sourced and current-sourced converters; (b) STATCOM with storage, i.e., Battery Energy
Storage System (BESS) Superconducting Magnet Energy Storage and large dc capacitor; (c) Static
VAR Compensator(SVC), Static VAR Generator (SVG), Static VAR System (SVS), ThyristorControlled Reactor (TCR), Thyristor- Switched Capacitor (TSC), and Thyristor-Switched Reactor
(TSR); (d) Thyristor-Controlled Braking Resistor.
on new Controllers or variations of known Controllers. The IEEE PES Task Force of
the FACTS Working Group defined Terms and Definitions for FACTS and FACTS
Controllers. Along with a brief description of FACTS Controllers, appropriate IEEE
Terms and Definitions are also presented in this section in italic for reference.
IEEE terms and definitions.
Flexibility of Electric Power Transmission. The ability to accommodate changes in
the electric transmission system or operating conditions while maintaining sufficient
steadystate and transient margins.
Flexible AC Transmission System (FACTS). Alternating current transmission
systems incorporating power electronic-based and other static controllers to enhance
controllability and increase power transfer capability.
It is worthwhile to note the words "other static Controllers" in this definition of
FACTS implying that there can be other static Controllers which are not based on
power electronics.
FACTS Controller. A power electronic-based system and other static equipment that
provide control of one or more AC transmission system parameters
Application of FACTS Controllers in Distribution Systems
20 | P a g e
FACTS was developed originally for transmission network; has been extended for
improvement of Power Quality (PQ) in distribution systems operating at low or medium
The power quality referred primarily to the continuity of power supply at acceptable voltage
and frequency.
However, the prolific increase in the use of computers, microprocessors and power electronic
systems has resulted in power quality issues involving transient disturbances in voltage
magnitude, waveform and frequency.
The nonlinear loads not only cause PQ problems but are also very sensitive to the voltage
In the modern context, PQ problem is defined as Any problem manifested in voltage, current
or frequency deviations that result in failure or misoperation of customer equipment"
The PQ problems are categorized as follows
1. Transients
(a) Impulsive
(b) Oscillatory
2. Short-duration and Long-duration variations
(a) Interruptions
(b) Sag (dip)
(c) Swell
3. Voltage unbalance
4. Waveform distortion
(a) DC offset
(b) Harmonics
(c) Interharmonics
(d) Notching
(e) Noise
5. Voltage Flicker
6. Power frequency variations
FACTS controllers for improving PQ termed them as Custom Power Devices (CPD). These
are based on VSC and are of 3 types given below.
1. Shunt connected Distribution STATCOM (DSTATCOM)
2. Series connected Dynamic Voltage Restorer (DVR)
3. Combined shunt and series, Unified Power Quality Conditioner (UPQC).
DVR is similar to SSSC while UPQC is similar to UPFC. In spite of the similarities, the
control strategies are quite different for improving PQ.
A major difference involves the injection of harmonic currents and voltages to isolate the
source from the load.
For example, a DVR can work as a harmonic isolator to prevent the harmonics in the
source voltage reaching the load in addition to balancing the voltages and providing
voltage regulation.
A UPQC can be considered as the combination of DSTATCOM and DVR.
A DSTATCOM is utilized to eliminate the harmonics from the source currents and also
balance them in addition to providing reactive power compensation (to improve power
factor or regulate the load bus voltage).
The terminology is yet to be standardized. The term `active ¯filters' or `power
21 | P a g e
is also employed to describe the custom power devices. ABB terms DSTATCOM as `SVC
light'. Irrespective of the name, the trend is to increasingly apply VSC based compensators
for power quality improvement.
Application of FACTS
1. Submarine cables. Cables have a large capacitance, and hence ac cables require
a large charging current (reactive power) an order of magnitude larger than that of
overhead lines.
As a result, for over a 30 km or so stretch of ac submarine cable, the charging current
supplied from the shore will fully load the cable and leave no room for transmitting real
The charging current flowing in the cables can only be reduced by connecting shunt
inductors to the cable at intervals of 15-20 km, thus requiring appropriate land location.
With HVDC cable on the other hand, distance is not a technical barrier. Also, the cost
of dc cable transmission is much lower than that of ac which works to HVDC's
advantage to cover new markets for long distance submarine transmission.
In this area, FACTS technology (e.g., the UPFC) can provide an improvement by
controlling the magnitude of one of the end (e.g., the receiving-end) voltages so as to
keep it identical to that of the other one. In this way, the effective length of the cable
from the standpoint of the charging current can be halved. This approach may provide
an economical solution for moderate submarine distances, up to about 100 km, but for
long distance transmission HVDC will remain unchallenged.
2. Long distance overhead transmission.
If the overhead transmission is long enough, say 1000 km, the saving in capital costs
and losses with a dc transmission line may be enough to pay for two converters (note
that HVDC represents total power electronics rating of 200% of the rated transmission
This distance is known as the break-even distance. This break-even distance
is very subject to many factors including the cost of the line, right-of-way, any need to
tap the line along the way, and often most important, the politics of obtaining permission
to build the line.
Nevertheless, it is important to recognize that while FACTS can play an important role
in an effective use of ac transmission, it probably does not have too much influence on
the breakeven distance.
Thus, the principal role of FACTS is in the vast ac transmission market where HVDC
is generally not economically viable.
3. Underground transmission.
Because of the high cost of underground cables, the break even distance for HVDC is
more like 100 km as against 1000 km for overhead lines.
22 | P a g e
Again, FACTS technology probably does not have much influence in this break-even
In any case, to date there have been no long distance underground projects, either ac or
dc, because, in an open landscape, overhead transmission costs so much less than
underground transmission (about 25% of the costs of underground transmission).
Cable transmission, on the other hand, has a significant potential of cost reduction, both
in the cost of cables and construction cost.
4. Connecting ac systems of different or incompatible frequencies.
For historical reasons, the oceans in effect separate the globe's electric systems into 50
Hz and 60 Hz groups.
The 60 Hz normal frequency pervades all the countries of the Americas, excepting
Argentina and Paraguay.
Those two countries and all the rest of the world have a 50 Hz frequency except Japan,
which is partly 50 Hz and partly 60 Hz. In general, the oceans are too huge and deep to
justify interconnections of 50 and 60 Hz systems.
Thus there is a limited market for HVDC for connecting 50 and 60 Hz systems.
Questions Module 1
1. Explain power flow in ac power system for cases a). Parallel network b) Mesh
20 M
2. Elaborate the applications of FACTS
3. Describe in detail FACTS and FACTS controller
4. Elaborate the basic types of FACTS controller
16 M
5. Elaborate Power flow and dynamic stability consideration of Transmission 14M
6. Elaborate the series and shunt types of FACTS controller
7. Elaboarte types of limitations for loading capability of transmission
8. Explain importance of controllable parameters
9. Explain Power Transmission Networks and Interconnections
10. Elaborate the applications of FACTS in distribution Systems
One mark question
1. What are the factors that affects Thermal capability of an overhead line.
2. What limits the loading capability of transmission lines?
3. What is open access?
4. State three kinds of limitations for loading capability
5. What are stability issues that limit the transmission capability.
6. Define Flexible AC Transmission System (FACTS) as per IEEE definition.
7. State and draw types of voltage source converter-based Controllers
8. What is frequency collapse in power system?
23 | P a g e
9. What is voltage collapse in power system?
10. What is subsynchronous resonance?
11. What is steady state stability?
12. What is transient stability?
13. State the classification of the power quality problem
Module-2: Shunt Compensators: Analysis of Uncompensated AC Line - Passive
Reactive Power Compensation - - Shunt Compensation Connected at the
Midpoint of the Line. Basic principle of operation of StaticVAr Compensator
(SVC)- Voltage control by SVC – advantages of slope in dynamic characteristics
– influence of SVC on system voltage. Modeling of SVC. Applications –
enhancement of transient stability – steady state power transfer – prevention
of voltage instability.
Analysis of Uncompensated AC Line
Lossless Distributed Parameter Lines
• Most power-transmission lines are characterized by distributed parameters:
24 | P a g e
series resistance, r; series inductance l; shunt conductance, g; and shunt
capacitance, c—all per-unit (pu) length.
• These parameters all depend on the conductors’ size, spacing, clearance
above the ground, and frequency and temperature of operation.
• Characteristic behavior of a transmission line is dominated by its l and c
parameters. Parameters r and g account for the transmission losses.
• Consider a small element of the line of length (dx) at a distance x from the
receiving end,
I(x + dx) = I(x) + (ydx)V (x + dx)
V (x + dx) = V (x) + (zdx)I(x)
where y = g + jb; z = r + jx;
• we get the following differential equations for V and I.
The fundamental equations governing the propagation of energy along a line are
the following wave equations:
25 | P a g e
These equations are used to calculate voltage and current anywhere on line,
at a distance x from the sending end, in terms of the sending-end voltage and
current and the line parameters
26 | P a g e
Likewise, power at the receiving end is given as
It is concluded that for a lossless line, Ps = − Pr ,
However, Qs ≠ Qr because of the reactive-power absorption/ generation in the
Midpoint Conditions of a Symmetrical Line
The magnitude of the midpoint voltage depends on the power transfer. This
voltage influences the line insulation and therefore needs to be well understood.
For a symmetrical line where the end voltages are held at nominal values, the
midpoint voltage shows the highest magnitude variation.
In terms of the midpoint voltage Vm, the receiving-end voltage of a
symmetrical line, from Eq., is given as
27 | P a g e
Figure shows the load angle δ and the midpoint-normalized per-unit voltage
𝑣̃𝑚 as functions of P/ P0 per-unit power transfer on the line.
For this line, it is observed that for light loading below the surge-impedance load,
the midpoint voltage exceeds Vnom and reaches its highest value at no load.
Furthermore, for stability considerations should an operating load angle of, say,
δ =30 be chosen to define the full-load rating of the line (0.588P0), the midpoint
voltage of the line will be 1.1058 pu. These expected overvoltages in the range
0.1–0.2 pu from full load to no load are not within acceptable limits.
Therefore, special techniques must be used to control these overvoltages
It is possible to control the overall voltage profile of such a line
connecting a controllable reactive-power source, called a var compensator, to it
so that below the surge-impedance loading P0, the var compensator absorbs
reactive power, and above P0, it supplies reactive power
28 | P a g e
the var requirement from the midpoint var controller is Qv = 2Qm
The conclusions are as follows:
1. As the length of the line increases on account of the line-charging
capacitances, the line experiences significant overvoltages at light-load
2. Overvoltages can be limited by using fixed- or switched-shunt reactors at the
line ends as well as at intermediate buses where needed.
3. The application of midpoint or intermediate bus-voltage controllers (var
compensators) enhances the power-transmission capacity of a long line.
4. In practical cases, var controllers are sized by carefully selecting their
continuous-operating range to hold the connecting-bus voltage within an
acceptable range of values in the normal line-loading range.
• Reactive-power control for a line is often called reactive-power
• External devices or subsystems that control reactive power on
transmission lines are known as compensators.
29 | P a g e
A compensator mitigates the undesirable effects of the circuit parameters
of a given line.
The objectives of line compensation are invariably
1. to increase the power-transmission capacity of the line,
2. to keep the voltage profile of the line along its length within acceptable
bounds to ensure the quality of supply to the connected customers as well
as to minimize the line-insulation costs.
Because reactive-power compensation influences the power-transmission
capacity of the connected line, controlled compensation can be used to
improve the system stability
Reactive-power compensators are dimensioned, and their types are
selected on the basis of both their technical and cost effectiveness.
Passive reactive-power compensators include series capacitors and shuntconnected inductors and capacitors.
Shunt Compensation
• Shunt devices may be connected permanently or through a switch. Shunt
reactors compensate for the line capacitance, and because they control
overvoltages at no loads and light loads, they are often connected
permanently to the line, not to the bus.
• Figure shows the arrangements of shunt reactors on a long-distance, highvoltage ac line.
30 | P a g e
• Many power utilities connect shunt reactors via breakers, thereby
acquiring the flexibility to turn them off under heavier load conditions.
• Shunt reactors are generally gapped-core reactors and, sometimes, aircored.
• Shunt capacitors are used to increase the power-transfer capacity and to
compensate for the reactive-voltage drop in the line.
• The application of shunt capacitors requires careful system design.
• The circuit breakers connecting shunt capacitors should withstand highcharging in-rush currents and also, upon disconnection, should withstand
more than 2-pu voltages, because the capacitors are then left charged for
a significant period until they are discharged through a large timeconstant discharge circuit.
• Also, the addition of shunt capacitors creates higher-frequency–resonant
circuits and can therefore lead to harmonic overvoltages on some system
Series Compensation
• Series capacitors are used to partially offset the effects of the series
inductances of lines.
• Series compensation results in the improvement of the maximum powertransmission capacity of the line.
• The net effect is a lower load angle for a given power-transmission level
and, therefore, a higher-stability margin.
• The reactive-power absorption of a line depends on the transmission
current, so when series capacitors are employed, automatically the
resulting reactive-power compensation is adjusted proportionately.
• Also, because the series compensation effectively reduces the overall line
reactance, it is expected that the net line-voltage drop would become less
susceptible to the loading conditions.
• In an interconnected network of power lines that provides several parallel
paths, for power flow between two locations, it is the series compensation
of a selected line that makes it the principal power carrier.
• Series compensation is defined by the degree of compensation; for
example, a 1-pu compensation means that the effective series reactance of
a line will be zero. A practical upper limit of series compensation, on the
other hand, may be as high as 0.75 pu.
• One impact of the passive compensation of lines is that whereas the shuntinductive compensation makes the line electrically resonant at a
supersynchronous frequency, the series compensation makes the line
resonant at a subsynchronous frequency.
31 | P a g e
• The subsynchronous resonance (SSR) can lead to problematic situations
for steam turbine–driven generators connected to a series-compensated
transmission line.
• These generators employ multiple turbines connected on a common shaft
with the generator. This arrangement constitutes an elastically coupled
multimass mechanical system that exhibits several modes of lowfrequency torsional resonances, none of which should be excited as a result
of the subsynchronous-resonant electrical transmission system.
• The application of series compensation requires several other careful
• The application of series capacitors in a long line constitutes placing a
lumped impedance at a point. Therefore, the following factors need careful
1. The voltage magnitude across the capacitor banks (insulation);
2. The fault currents at the terminals of a capacitor bank;
3. The placement of shunt reactors in relation to the series capacitors
(resonant overvoltages); and
4. The number of capacitor banks and their location on a long line
(voltage profile).
Effect on Power-Transfer Capacity
• The consideration of series compensation invariably raises the issue of its
comparison with shunt compensation.
• A simple system analysis to develop a basic understanding of the effect
✓ shunt compensation on power-transmission capacity.
✓ series compensation on power-transmission capacity.
Series compensation on power-transmission capacity Consider a short, symmetrical electrical line as shown in Fig.below. For an
uncompensated line, and assuming Vs = Vr = V, the power equation becomes
32 | P a g e
Figure The series compensation of a short, symmetrical transmission line
CASE1 :Series Compensation If the effective reactance of a line is controlled
by inserting a series capacitor, and if the line terminal voltages are held
unchanged, then a ΔXl change in the line reactance will result in a ΔIl change
in the current, where
33 | P a g e
Therefore, from above Eq. , the corresponding change in the power transfer
will be
Using above Eqs.. we can write
Shunt Compensation Reconsider the short, symmetrical line described in Fig.
(a). Apply a shunt capacitor at the midpoint of the line so that a shunt
susceptance is incrementally added (DBc), as shown in Fig. b. For the system in
this figure, the power transfer in terms of the midpoint voltage
on the line is
Fig The midpoint-capacitor compensation of a short, symmetrical line
34 | P a g e
Assuming an operating load angle δ=30, we get the ratio of the ratings of
series (ΔQse) to shunt (ΔQsh) compensators to be 0.072, or 7.2%.
35 | P a g e
From the discussion, it is clear that the var net rating of the series compensator is
only 7.2% of that required of a shunt compensator for the same change in power
Therefore, concludes that the series-capacitive compensation is not only achieved
with a smaller MVAR rating, but also that it is automatically adjusted for the
entire range of the line loading.
However, the cost of the compensator is not directly related only to the MVARrating.
series capacitor costs increase because they carry full line current and also both
their ends must be insulated for the line voltage.
Practical application of series capacitors requires isolation and bypass
arrangements as well as protection and monitoring arrangements.
Basic principle of operation of StaticVAr Compensator (SVC)- Voltage control
by SVC – advantages of slope in dynamic characteristics – influence of SVC on
system voltage. Modeling of SVC. Applications – enhancement of transient
stability – steady state power transfer – prevention of voltage instability.
An SVG (static var generators) is a static electrical device, system that is capable
of drawing a controlled capacitive or inductive current from an electrical power
system, thereby generating or absorbing reactive power.
An SVC is a shunt-connected static generator and or absorber of
reactive power in which the output is varied to maintain or
control specific parameters of an electrical power system.
SVS is a combination of different static and mechanically switched var
compensators (capacitors and or reactors) in which the outputs are coordinated.
A var compensating system (VCS) is a combination of SVSs and rotating var
compensators in which the outputs are coordinated.
The general characteristics of SVCs are given in the list that follows.
1. The lowering of maintenance requirements from the absence of rotating parts.
2. The very fast control-response time.
3. The feasibility of individual phase control.
4. The diminished losses.
5. The high reliability.
6. The lack of contribution to system short-circuit capacity.
36 | P a g e
7. The generation of harmonics by SVCs except thyristor-switched capacitors
8. The variation of SVC reactive-power generation as the square of terminal
voltage when it is operating outside the linear controllable range, leading to a
substantial reduction in reactive-power support at lower voltages.
A TCR is one of the most important building blocks of thyristor-based SVCs.
Although it can be used alone, it is more often employed in conjunction with fixed or
thyristor-switched capacitors to provide rapid, continuous control of reactive power
over the entire selected lagging-to-leading range.
• Single-Phase TCR
A single-phase TCR comprises an anti-parallel–connected pair of thyristor valves, T1 and
T2, in series with a linear air-core reactor, as illustrated in Fig.
The anti-parallel–connected thyristor pair acts like a bidirectional switch, with
thyristor valve T1 conducting in positive half-cycles and thyristor valve T2
conducting in negative half-cycles of the supply voltage.
The firing angle of the thyristors is measured from the zero crossing of the voltage
appearing across its terminals.
The controllable range of the TCR firing angle, a, extends from 90 0 to 180 0 .
A firing angle of 90 0 results in full thyristor conduction with a continuous sinusoidal current flow
in the TCR.
As the firing angle is varied from 90 0 to close to 180 0, the current flows in the form of
discontinuous pulses symmetrically located in the positive and negative half-cycles,
37 | P a g e
Once the thyristor valves are fired, the cessation of current occurs at its natural zero crossing, a
process known as the line commutation.
The current reduces to zero for a firing angle of 180 0 . Thyristor firing at angles below 90 0
introduces dc components in the current, disturbing the symmetrical operation of the two
antiparallel valve branches.
38 | P a g e
39 | P a g e
The TCR thus acts like a variable
However, as the firing angle is increased beyond 90 0 , the current becomes
nonsinusoidal, and harmonics are generated.
susceptance. (admittance,)
Variation of the firing angle changes the susceptance and,
consequently, the fundamental-current component, which leads to a
variation of reactive power absorbed by the reactor because the
applied ac voltage is constant.
• If the two thyristors are fired symmetrically in the positive and negative
half-cycles, then only odd-order harmonics are produced.
Three phase TCR
The Thyristor-Switched Reactor - (TSR)
• TSR is a special case of a TCR in which the variable firing-angle control
option is not exercised.
• Instead, the device is operated in two states only: either fully on or fully
• If the thyristor valves are fired exactly at the voltage peaks corresponding
to a c 90 0 for the forward-thyristor valve T1 and 2700 (90 0 + 180 0 )
for the reverse-thyristor valve T2, full conduction
40 | P a g e
The maximum inductive current flows in the TCR as if the thyristor
switches were replaced by short circuits.
However, if no firing pulses are issued to the thyristors, the TSR will
remain in a blocked-off state, and no current can flow.
• TSR ensures a very rapid availability of rated inductive reactive power to
the system.
• When a large magnitude of controlled reactive power, Q, is required, a
part of Q is usually assigned to a small TSR of rating, say, Q 2; the rest
is realized by means of a TCR also of a reduced rating Q 2.
• This arrangement results in substantially decreased losses and harmonic
content as compared to a single TCR of rating Q.
• TCR provides continuously controllable reactive power only in the
lagging power-factor range.
• To extend the dynamic controllable range to the leading power-factor
domain,(SUPPLYING REACTIVE POWER) a fixed-capacitor bank is
connected in shunt with the TCR.
41 | P a g e
• TCR MVA is rated larger than the fixed capacitor to compensate (cancel)
the capacitive MVA and provide net inductive-reactive power should a
lagging power-factor operation be desired.
• The fixed-capacitor banks, usually connected in a star configuration, are
split into more than one 3-phase group.
• Each capacitor contains a small tuning inductor that is connected in series
and tunes the branch to act as a filter for a specific harmonic order.
• For instance, one capacitor group is tuned to the 5th harmonic and
another to the 7th,
• At fundamental frequency, the tuning reactors slightly reduce the net
MVA rating of the fixed capacitors.
42 | P a g e
Operating Characteristic
1 Without the Step-Down Transformer (Directly connected to
supply line)
• The operating V-I characteristic of an FC–TCR compensator is illustrated
in Fig. below.
• The fixed capacitor extends the operating-control range of the SVC to the
leading side.
• The SVC current, ISVC, can be expressed as a function of system voltage,
V, and compensator susceptance, BSVC, as follows,
43 | P a g e
• Figures (b) and (c) show the operating characteristic and the susceptance
of this type of compensator, respectively, and both also show that
VAR(Reactive power ) production as well as absorption is possible.
• By dimensioning the ratings of the TCR and the capacitor, respectively,
the production and absorption ranges can be selected according to the
system requirements.
2 With the Step-Down Transformer – ( High power transmission line)
An FC–TCR SVC is usually connected to the high-voltage power system(
110kv , 220kv ,400kv ) by means of a step-down coupling transformer, as
shown in Fig.
44 | P a g e
From Eq. (a),
• BL is a negative quantity.
• An analysis of Eq. (a) shows that the total susceptance BSVC of the static
var compensator does not change linearly with BTCR.
• However, if (BC / Bo ) << 1 and (BL/ Bo ) << 1, which is usually the case,
the nonlinearity is relatively small.
• This assumption implies that the reactance of the coupling transformer
is greatly smaller than the reactance of either the fixed capacitor or
• Equation (a) can then be approximated by a linear relation, as follows
The Transformer-Secondary Voltage The voltage at the secondary of the
transformer is
45 | P a g e
• In certain applications, especially those involving few capacitor
switchings, an MSC–TCR has been shown to offer acceptable
performance at much lower compensating system costs than a TSC–TCR.
• The different MSC–TCR circuit configurations are shown in Fig.
• Advantages of the MSC–TCR scheme1. lower capital cost from the elimination of the thyristor switches in the
capacitor branches;
2. reduced operating costs in terms of losses.
• Disadvantage - slower speed of response.
• Mechanical switches can close in two cycles and open in about eight,
compared to one-half to one cycle with thyristor switches.
• To compensate for the slower speed and to achieve a level of transient
stability similar to a thyristor switched capacitor–thyristor-controlled
reactor, 25% higher–rated MSC–TCR SVC may be needed.
46 | P a g e
• Mechanical switches also possess a finite life, typically 2000–5000
operations, compared to the infinite switching life of thyristors.
• Problem with the MSC–TCR relates to the trapped charge that is invariably
left on the capacitor after de-energization.
• The residual charge on the capacitors is usually dissipated in about five
minutes through discharge resistors (built into the capacitor units).
• If the capacitor is switched on within five minutes after de-energization,
the trapped charge may lead to increased switching transients.
• MSCs can be switched in only when the capacitors are discharged.
• TCR in an MSC–TCR is designed to have a lower inductance .( compared
to a TCR in a TSC–TCR SVC of similar rating).
• MSCs are usually switched two to four times a day; they are connected
during heavy-load conditions and removed under light-load conditions.
• An MSC–TCR may not be very suitable for voltage-control applications in
a system experiencing frequent disturbances.
47 | P a g e
The circuit shown in Fig. consists of a capacitor in series with a bidirectional
thyristor switch, is supplied from an ideal ac voltage source (without resistance
nor reactance present in the circuit).
Figure Switching of a capacitor at a voltage source: (a) a circuit diagram and
(b) the current and voltage waveforms
Analysis of the current transients after closing the switch brings out two cases:
1. The capacitor voltage is not equal to the supply voltage when the thyristors
are fired. Immediately after closing the switch, a current of infinite magnitude
flows and charges the capacitor to the supply voltage in an infinitely short time.
The switch realized by thyristors cannot withstand this stress and would fail.
2. The capacitor voltage is equal to the supply voltage when the thyristors are
fired, as illustrated in Fig. (b). The analysis shows that the current will jump
immediately to the value of the steady-state current. The steady state condition
is reached in an infinitely short time. Although the magnitude of the current
48 | P a g e
does not exceed the steady-state values, thyristors have an upper limit of di/ dt
values that they can withstand during the firing process. Here, di/ dt is infinite,
and the thyristor switch will again fail.
It can therefore be concluded that this simple circuit of a TSC branch is not
Switching a Series Connection of a Capacitor and Reactor
To overcome the problems discussed above , a small damping reactor is added
in series with the capacitor, as depicted in Fig..
Let the source voltage be
v(t) = V sin ω0t
where ω0 is the system nominal frequency.
The switching strategies to limit the transients to acceptable limits ,are based on
very processes to decide when the thyristors should be fired.
The following two simple firing schemes are the basis for the switching
If VC0 < V at the time of demand for the capacitor, it is switched on, as soon as
the voltage across the valve reaches zero and the capacitor voltage is equal to
the supply voltage
2.If the capacitor is overcharged (VC0 > V) at the time of demand, it is
switched on, when the supply voltage reaches the crest and the voltage across
the valve is minimal. This scheme is also called a forced switch-on.
49 | P a g e
The TSC Configuration
A basic single-phase TSC consists of an anti-parallel–connected thyristor-valve
pair that acts as a bidirectional switch in series with a capacitor and a current
limiting small reactor, as shown in Fig
• The thyristor valves are turned on at an instant when minimum voltage is
sensed across the valves to minimize the switching transients.
• Besides the initial transients, the TSC current is sinusoidal and free from
harmonics, thus no need for any filters.
• The small-series inductor is installed to limit current transients during
overvoltage conditions and planned switching operations, as well as
when switching at incorrect instants or at the inappropriate voltage
• The inductor magnitude is chosen to give a natural resonant frequency
of four to five times the system nominal frequency, which ensures that
the inductance neither creates a harmonic-resonant circuit with the
• A 3-phase TSC unit comprises three single-phase TSCs connected in a
delta, which are usually supplied by the delta secondary winding of a stepdown transformer, as depicted in Fig. (b). An alternative 3-phase, 4-wire
star-connected TSC configuration is shown in Fig. (c).
50 | P a g e
Figure Different TSC configurations: (a) a single-phase TSC branch; (b) a 3phase delta-connected TSC; and (c) a 3-phase Y-connected transformer
secondary with neutrals connected.
Operating Characteristic
The TSC has a discrete voltage–current operating characteristic as shown in
Fig.. The shape of this characteristic is a function of the number of TSC units, their
individual ratings, and a hysteresis voltage ΔV, which is built in to avoid
undesirable frequent switchings of capacitors.
In a closed-loop voltage control operation, the TSC regulates the bus voltage
within the range Vref ± ΔV/ 2.
51 | P a g e
52 | P a g e
1 Configuration
• TSC–TCR compensator usually comprises n TSC banks and TCR that are
connected in parallel.
• The number of branches n, is determined by practical considerations that
include the operating voltage level, maximum var output, current rating of
the thyristor valves, bus work and installation cost, etc
• TSC bank // TCR
• The rating of the TCR is chosen to be 1/ n of the total SVC rating.
• The TSC is switched in using two thyristor switches (connected back to
back) at the instant in a cycle when the voltage across valve is minimum
and positive. This results in minimum switching transients.
• In steady state, TSC does not generate any harmonics.
• To switch off a TSC, the gate pulses are blocked and the thyristors turns
off when the current through them fall below the holding currents
• More flexible than FC-TCR - The feature of disconnecting the capacitor
in is not available with FC–TCRs.
53 | P a g e
V-I characteristic of the operation of a TSC-TCR
• Fig. shows the V-I characteristic of the operation of a TSC-TCR.
Fig – V-I characteristics of TSC-TCR
54 | P a g e
• The capacitors of the TSC can be switched in discrete steps, whereas
continuous control within the reactive power span of each step is
controlled by the TCR.
• The total capacitive output range is divided into n intervals. In the
first interval, the output of the var generator is controllable in the zero
to QCmax/n range, where QCmax is the total rating provided by all
TSC branches
• In this interval, one capacitor bank is switched in (by firing, , thyristor
of TSC1, ) and, simultaneously, the current in the TCR is set by the
appropriate firing delay angle so that the sum of the var output of the
TSC (negative) and that of the TCR (positive) equals the capacitive
output required.
• In the second, third, ..., and nth intervals, the output is controllable
in the QCmax/n to 2QCmax/n, 2Qcmax/n to 3QCmax/n, ..., and (n 1)Qcmax/n to QCmax range by switching in the second, third, ..., and
nth capacitor bank and using the TCR to absorb the surplus capacitive
• TSC-TCR can quickly operate to disconnect all the capacitors from
the compensator, preventing the resonant oscillations.
• The main function of the TCR is for generating inductive VAR to
increase the flexibility of VAR compensation.
• VAR rating of the TCR has to be larger than that of one TSC to
provide enough overlap for TSC switching in and out action.
• The VAR rating of the TCR in this type of SVC is lower than that of
the TCR in FC-TCR
• The maximum inductive range of the SVC corresponds to the rating
of the relatively small TCR.
Four Types of Power Losses –
1. Reactor resistive loss ,
2. Capacitor losses
3. Thyristor losses
4. Controller and filter losses - (In electronic control part of TCR and TSC
• The advantages of TSC-TCR type SVC over FC-TCR type are
(i) the reduction in the reactor size and consequently the harmonics
(ii) greater flexibility in control
55 | P a g e
(iii) better performance under system fault conditions.
Calculation of the Operating-Range Limits
• BC3 as the susceptance of all three TSC branches in parallel and
• BC3 is considered zero for the TSC scheme at the absorption limit, for all
capacitors are switched off.
• Susceptance at production limit is
• SVC susceptance in the TSC–TCR scheme as follows:
Current Characteristic
TSCs and the TCR share the current and contribute to the total SVC current..
Generally, the TSC and TCR currents
IC = V2nBCn,
n - number of TSC branches turned on ,
56 | P a g e
where nc = 1, 2, . . . is the number of TSC circuits in operation and BCn is the
total susceptance of n TSC branches.
Substituting BTCR = 0 and BTCR = BL in the above equations, respectively,
results in the currents at the absorption limit and at the production limit for
different numbers of TSCs.
57 | P a g e
Figure-a gives the total susceptance BSVC as a function of the susceptance of the
controlled reactor BTCR for the example data. These characteristics are of
importance for control design, for the controls vary BTCR and the effect on the
system is caused by BSVC.
58 | P a g e
59 | P a g e
The voltage-control action can be explained through a simplified block
representation of the SVC and power system shown below. The power system is
modeled as an equivalent voltage source Vs behind equivalent system impedance
Xs as viewed from the SVC terminals.
Figure 5.2 (a) A simplified block diagram of the power system and SVC control
system; (b) a phasor diagram of the ac system for the inductive SVC current
system impedance Xs corresponds to the short circuit MVA at the SVC bus ,is
obtained as
Xs = (Vb / Sc). MVAb in p.u.
Where, Sc = the 3 phase short circuit MVA at the SVC bus
Vb = the base line-line voltage
60 | P a g e
MVAb = base MVA
SVC bus voltage is given by , Vs = VSVC + ISVC Xs
The SVC current results in a voltage drop of ISVCXs in phase with the system
voltage Vs.
The SVC bus voltage decreases with the inductive SVC current and increases
with the capacitive current.
Equation (1) represents the power-system characteristic or the system load
The intersection of the SVC dynamic characteristic and the system load line
provides the quiescent operating of the SVC as illustrated in the below figure.
Characteristics of the simplified power system and the SVC
The voltage control action in the linear range is described as
61 | P a g e
Where ISVC is positive if inductive and ISVC is negative if capacitive.
It is emphasized that the V-I characteristics described here relate SVC current
or reactive power to the voltage on the high-voltage side of the coupling
62 | P a g e
SVC Applications
• Static var compensators (SVCs) constitute a mature technology that is
finding widespread usage in modern power systems for load compensation
as well as transmission-line applications.
• In high-power networks, SVCs are used for voltage control and for
attaining several other objectives such as damping and stability control.
1. Enhancement of Transient Stability - Power-angle curves
➢ An enhancement in transient stability is achieved primarily through voltage
control exercised by the SVC at the interconnected bus.
➢ enhancement in transient stability can be obtained from the power-angle
curves,of uncompensated and midpoint SVC–compensated SMIB system
as shown in Fig.
➢ Assume that both systems are transmitting the same level of power and are
subject to an identical fault at the generator terminals for an equal length
of time.
➢ The power-angle curves for both systems are depicted in Fig.
63 | P a g e
➢ Iinitial operating point in the uncompensated and compensated systems are
indicated by rotor angles d1 and dc1. These points correspond to the
intersection between the respective power-angle curves with the
mechanical input line PM, which is same for both the cases.
➢ In the event of a 3-phase-to-ground fault at the generator terminals, even
though the short-circuit current increases enormously, active-power output
from the generator reduces to zero.
➢ Because the mechanical input remains unchanged, generator accelerates
until fault clearing, by which time the rotor angle has reached values d2 and
dC2 and the accelerating energy, A1 andAC1, has been accumulated in the
uncompensated and compensated system, respectively.
➢ When the fault is isolated, the electrical power exceeds the mechanical
input power, and the generator starts decelerating.
➢ The rotor angle, however, continues to increase until δ3 and δc3 from the
stored kinetic energy in the rotor.
64 | P a g e
➢ The decline in the rotor angle commences only when the decelerating
energies represented by A2 and AC2 in the two cases, respectively, become
equal to the accelerating energies A1 and AC1.
➢ The power system in each case returns to stable operation if the post-fault
angular swing, denoted by d3 and dC3, does not exceed the maximum limit
of dmax and dc max, respectively.
➢ The far the angular overswing from its maximum limit, the more transient
stability in the system.
➢ An index of the transient stability is the available decelerating energy,
termed the transient-stability margin, and is denoted by
areas Amargin and Ac margin in the two cases, respectively.
➢ Clearly, as Ac margin significantly exceeds Amargin, the system-transient
stability is greatly enhanced by the installation of an SVC. The increase in
transient stability is thus obtained by the enhancement of the steady-state
power-transfer limit provided by the voltage-control operation of the
midline SVC.
2. Steady State Power Transfer Capacity
➢ An SVC can be used to enhance the power-transfer capacity of a
transmission line, which is also characterized as the steady-state power limit.
➢ Consider a single-machine infinite-bus (SMIB) system with an
interconnecting lossless tie line having reactance X shown in Fig.
The SMIB system: (a) an uncompensated system (b) an SVC-compensated
65 | P a g e
➢ Let the voltages of the synchronous generator and infinite bus be V1∠ δ
and V2 ∠ 0, respectively. The power transferred from the synchronous
machine to the infinite bus is expressed as
➢ Power varies as a sinusoidal function of the angular difference of the
voltages at the synchronous machine and infinite bus,
➢ The maximum steady-state power that can be transferred across the
uncompensated line without SVC corresponds to δ = 90 0; it is given by
Fig - variation of line real-power flow and SVC reactive-power flow in a SMIB
system. (double Pmax , reactive-power rating of the SVC is four times the
maximum power transfer in an uncompensated case
66 | P a g e
➢ The variation of linear real-power flow and SVC reactive-power flow in a
SMIB system
➢ Let the transmission line be compensated at its midpoint by an ideal SVC.
➢ The term ideal corresponds to an SVC with an unlimited reactive-power
rating that can maintain the magnitude of the midpoint voltage constant for
all real power flows across the transmission line.
➢ SVC bus voltage is then given by Vm ∠–δ/2.The electrical power flow
across the half-line section connecting the generator and the SVC is
expressed as ,(Assuming Vm= V1 =V2 =V)
➢ The power transfer in the other half-line section interconnecting the SVC, and
the infinite bus is also described by a similar equation
Hence , maximum transmittable power across the line (occurs at δ/ 2 =900 ) is
given by
which is twice the maximum power transmitted in the uncompensated case
➢ Midpoint-located ideal SVC doubles the steady-state power limit and increases
the stable angular difference between the synchronous machine and the infinite
bus from 900 to 1800 .
➢ If the transmission line is divided into n equal sections, with an ideal SVC at
each junction of these sections, maintaining a constant-voltage magnitude (V),
then the power transfer (P′c) of this line can be expressed theoretically by
67 | P a g e
➢ Maximum power, P′c max = nV2/ X.
Hence , with n sections the power transfer can be increased n times that of
the uncompensated line.
➢ It can be shown that the reactive-power requirement, QSVC, of the midpoint
SVC for the voltage stabilization is given by
3. Enhancement of Power System Damping
➢ The power-transfer capacity along a transmission line is limited by several
factors; such as , the thermal limit, the steady-state stability limit, the transientstability limit, and system damping.
➢ In certain situations, a power system may have —even negative— damping;
therefore, a strong need arises to enhance the electrical damping of power
systems to ensure stable, oscillation-free power transfer.
➢ A typical scenario of the magnitude of various limits, especially where
damping plays a determining role , is depicted graphically in Fig.
➢ The behavior of generator oscillations is determined by the two torque
components: the synchronizing torque and damping torque.
➢ The synchronizing torque ensures that the rotor angles of different generators
do not drift away following a large disturbance.
68 | P a g e
➢ Magnitude of the synchronizing torque determines the frequency of oscillation.
Meanwhile, damping torque influences the decay time of oscillations.
➢ Even if a power system is stable, the oscillations may be sustained for a long
period without adequate damping torque.
➢ SVCs are employed primarily for voltage control; they do not contribute to
system damping. However by incorporating auxiliary control, an SVC can
significantly improve the electrical damping of power systems
69 | P a g e
4. Prevention of Voltage Stability
➢ Voltage instability is caused by the inadequacy of the power system to supply
the reactive-power demand of certain loads, such as induction motors.
➢ Drop in the load voltage leads to an increased demand for reactive power, if
not met by the power system, leads to a further decline in the bus voltage.
This decline eventually leads to a rapid decline of voltage which may have a
cascading effect on neighboring regions that causes a system voltage collapse.
➢ Consider an SVC connected to a load bus, (Fig. a). The load has varying power
factor and is fed by a lossless radial transmission line.
➢ The voltage profile at the load bus, which is situated at the receiver end of
the transmission line, is shown in Fig. (b).
➢ For a given load-power factor, as the transmitted power is gradually
increased, a maximum power limit is reached beyond which the voltage
collapse takes place.
➢In this system, if the combined power factor of the load and SVC is
appropriately controlled through the reactive-power support from the SVC, a
70 | P a g e
constant voltage of the receiving-end bus can be maintained with increasing
magnitude of transmitted power, and voltage instability can be avoided.
Modelling of SVC
For a detailed study of SVC control interactions, it is necessary to perform transient simulation for which SVC is modelled in detail including the
switching of the thyristor valves in TCR and TSC.
The transient network is modelled by differential equations rather than algebraic
equations .
However for stability study it is not necessary to consider the switching of
valves and assume that SVC generates only fundamental current.
In addition, the network transients are neglected and represented by algebraic
equations of the type:
[Y ]V = I
( 1)
Module-3: Series Compensators: Compensation by a Series Capacitor
Connected at the Midpoint of the Line, Basic model concept of Thyristor
Controlled Series Capacitor (TCSC), Operation of the TCSC – different modes of
71 | P a g e
operation – modeling of TCSC – variable reactance model – modeling for stability
studies. Applications – improvement of the system stability limit – enhancement
of system damping – voltage collapse prevention
Compensation by a Series Capacitor Connected at the Midpoint of the Line
1 Fixed-Series Compensation
• Series capacitors offer certain major advantages over their shunt
• With series capacitors, the reactive power increases as the square of line
• whereas with shunt capacitors, the reactive power is generated
proportional to the square of bus voltage.
• For achieving the same system benefits as those of series capacitors,
shunt capacitors that are three to six times more reactivepower–rated than
series capacitors need to be employed .
• Furthermore, shunt capacitors typically must be connected at the line
midpoint, whereas no such requirement exists for series capacitors.
• Let Qse and Qsh be the ratings of a series and shunt capacitor,
• To achieve the same level of power transfer through a line that has a
maximum angular difference of δmax across its two ends.
• Then
• Specifically, for δmax of 350, Qse will be approximately 10% of Qsh.
• Even though series capacitors are almost twice as costly as shunt
capacitors (per-unit var) because of their higher operating voltages, the
overall cost of series compensation is lower than shunt compensation
2. The Need for Variable-Series Compensation
Compensation of transmission lines by series capacitors is likely to result in the
following :
72 | P a g e
1. enhanced power flow and loadability of the series-compensated line
2. additional losses in the compensated line from the enhanced power flow
3. increased sensitivity of power flow in the series-compensated line
from the outage of other lines in the system.
(Also, the increased sensitivity of the compensated line to other network outages
may cause a line loading that exceeds the enhanced loadability level of the line
itself. )
These undesirable effects can be avoided by employing variable levels of series
compensation instead of fixed compensation.
Series compensation can be varied, depending on the enhancement of power
transfer desired at that time, without affecting other system-performance criteria
• Basic conceptual TCSC module comprises a series capacitor, C, in parallel
with a thyristor-controlled reactor, LS, [Fig.(a)].
• However, a practical TCSC module also includes protective equipment
normally installed with series capacitors,[Fig. (b)].
• Metal-oxide varistor (MOV), - nonlinear resistor, is connected across the
series capacitor to prevent the occurrence of high-capacitor over- voltages.
MOV allows the capacitor to remain in circuit even during fault conditions
and helps improve the transient stability
73 | P a g e
• Also installed across the capacitor is a circuit breaker, CB, for controlling
its insertion in the line. Also CB bypasses the capacitor if severe fault or
equipment-malfunction events occur.
• A current-limiting inductor, Ld, is incorporated in the circuit to restrict both
the magnitude and the frequency of the capacitor current during the
capacitor-bypass operation.
• If the TCSC valves are required to operate in the fully “on” mode for
prolonged durations, the conduction losses are minimized by installing an
ultra–high-speed contact (UHSC) across the valve.
• UHSC metallic contact offers a lossless feature similar to circuit breakers.
The metallic contact is closed shortly after the thyristor valve is turned on,
and it is opened shortly before the valve is turned off.
• During a sudden overload of the valve, and also during fault conditions,
the metallic contact is closed to relieve the stress on the valve.
74 | P a g e
• An actual TCSC system usually comprises a cascaded combination of
many such TCSC modules, together with a fixed-series capacitor, CF. This
fixed series capacitor is provided primarily to minimize costs.
Fig C – Typical TCSC system
• A conceptual TCSC system with basic TCSC modules [see Fig. C]. The
capacitors—C1, C2, . . . , Cn—in the different TCSC modules may have
different values to provide a wider range of reactance control.
• The inductor in series with the antiparallel thyristors is split into two halves
to protect the thyristor valves in case of inductor short circuits.
Basic Principle
• A TCSC is a series-controlled capacitive reactance that can provide
continuous control of power on the ac line over a wide range.
• From the system viewpoint, the principle of series compensation is to increase
the fundamental-frequency voltage across an fixed capacitor (FC) in a series
compensated line through appropriate variation of the firing angle, α .
• This enhanced voltage changes the effective value of the series-capacitive
75 | P a g e
• A simple understanding of TCSC functioning can be obtained by analyzing
the behavior of a variable inductor connected in parallel with an FC, [see
Fig. d.]
Figure d- A variable inductor connected in shunt with an FC.
• The equivalent impedance, Zeq, of this LC combination is expressed as
• The impedance of the FC is given by -j(1/ ωC).
• If ωC − (1/ ωL) > 0 or, in other words, ωL > (1/ ωC), the reactance of the
(XL>Xc ) FC is less than that of the parallel-connected variable reactor .
• Inductor increases the equivalent reactance of the LC combination above
that of the FC.
• If ωC− (1/ ωL)=0, a resonance develops that results in an infinitecapacitive impedance—an unacceptable condition.
• If, ωC − (1/ ωL) < 0, the XL< Xc
• In the variable-capacitance mode of the TCSC, as the inductive reactance
of the variable inductor is increased, the equivalent-capacitive reactance
is gradually decreased.
• The minimum equivalent-capacitive reactance is obtained for extremely
large inductive reactance or when the variable inductor is open-circuited,
in which the value is equal to the reactance of the FC itself.
• The behavior of the TCSC is similar to that of the parallel LC combination.
• The difference is that the LC-combination analysis is based on the presence
of pure sinusoidal voltage and current in the circuit, whereas in the TCSC,
because of the voltage and current in the FC and thyristor-controlled
reactor (TCR) are not sinusoidal because of thyristor switchings.
76 | P a g e
3 Advantages of the TCSC (Thyristor Controlled Series Capacitor)
Use of thyristor control in series capacitors potentially offers the following
1. Dynamic control of power flow in selected transmission lines within network
to enable optimal power-flow conditions and prevent the loop flow of power.
2. Damping of the power swings from local and inter-area oscillations.
3. Suppression of subsynchronous oscillations.
At subsynchronous frequencies, TCSC presents an inherently resistive–inductive
reactance. subsynchronous oscillations cannot be sustained in this situation and
consequently get damped.
4. Decreasing dc-offset voltages. The dc-offset voltages, invariably resulting
from the insertion of series capacitors, can be made to decay very quickly (within
a few cycles).
5. Enhanced level of protection for series capacitors.
A fast bypass of series capacitors can be achieved through thyristor control when
large over-voltages develop across capacitors following faults. Likewise,
capacitors can be quickly reinserted by thyristor action after fault clearing to aid
in system stabilization.
6. Voltage support.
TCSC can generate reactive power that increases with line loading, thereby
aiding regulation of local network voltages and improve of any voltage
7. Reduction of the short-circuit current.
During events of high short-circuit current, TCSC can switch from the
controllable-capacitance to controllable-inductance mode, thereby restricting the
short-circuit currents.
77 | P a g e
Modes of TCSC Operation
• There are essentially three modes of TCSC operation
(a) the bypassed-thyristor mode;
(b) the blocked-thyristor mode;
(c) the partially conducting thyristor (capacitive and inductive) mode;
• Modes of TCSC operation are illustrated in Fig. and described in the
following section .
78 | P a g e
Figure 1 Different operating modes of a TCSC: (a) the bypassed-thyristor
mode; (b) the blocked-thyristor mode; (c) the partially conducting thyristor
(capacitive) mode; and (d) the partially conducting thyristor (inductive) mode
1 Bypassed-Thyristor Mode (α=900)
• In this bypassed mode, both thyristors conduct 180 degree consecutive
when they have right condition of conducting.
• Gate pulses are applied as soon as the voltage across the thyristors
reaches zero and becomes positive, resulting in a continuous sinusoidal of
flow current through the thyristor valves.
• The TCSC module behaves like a parallel capacitor–inductor combination.
• The TCR branch is in circuit completely in this mode. The TCSC
impedance is:
In above Equation , negative X TCSC means the overall impedance is
capacitive and positive X TCSC means the overall inductive impedance.
As XL is smaller than XC, the TCSC impedance is inductive in bypass
Hence , the net current through the module is inductive.
Also known as the thyristor-switched-reactor (TSR) mode, the bypassed
thyristor mode is distinct from the bypassed-breaker mode, in which the
circuit breaker provided across the series capacitor is closed to remove the
capacitor or the TCSC module in the event of TCSC faults or transient
overvoltages across the TCSC.
This mode is employed for control purposes and also for initiating certain
protective functions.
Whenever a TCSC module is bypassed from the violation of the current
limit, a finite-time delay, Tdelay, must elapse before the module can be
reinserted after the line current falls below the specified limit.
79 | P a g e
2 Blocked-Thyristor Mode (α=1800 )
• This mode, also known as the waiting mode, the firing pulses to the
thyristor valves are blocked.
• If the thyristors are conducting and a blocking command is given, the
thyristors turn off as soon as the current through them reaches a zero
• The TCSC module is thus reduced to 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.
3 Partially Conducting -Thyristor, or Vernier, Mode
• This mode allows the TCSC to behave either as a continuously controllable
capacitive reactance or as a continuously controllable inductive reactance.
• It is achieved by varying the thyristor-pair firing angle in an appropriate
• However, a smooth transition from the capacitive to inductive mode is not
permitted because of the resonant region between the two modes.
• A variant of this mode is the capacitive-vernier-control mode, in which the
forward voltage thyristor valve is triggered slightly before capacitor
voltage crosses zero (i.e thyristors are fired when the capacitor voltage and
capacitor current have opposite polarity) to allow current to flow through
inductive branch, adding to capacitive current.
• If firing angle is between zero and 90 degree, the impedance of TCR
branch will be
Where α is delay angle of thyristors turning on from zero crossing of line
current and σ in conducting angle σ= 2π-α ;
• The firing angle of TCR is assigned as XL(α)>XC for TCSC operation in
capacitive mode , This situation will happen in angles greater than α
min_Cap and smaller than 90 degree.
• There is a resonance angle that XL(αresonance) will be equal to XC
• This condition causes a TCR current that has a direction opposite that of
the capacitor current, (This effectively increases the observed capacitance
of the TCSC without requiring a larger capacitor within the TCS ), thereby
resulting in a loop-current flow in the TCSC controller.
80 | P a g e
Fig b
• According to Fig.b , in delay angle α , after zero crossing of the line
current, the proper thyristor (Th+) turns on in t1 when the line current and
capacitor voltage are in opposite sign. Afterwards a resonance circuit
forms from L and C that resonates half cycle. The ON thyristor turns off
in t2. capacitor voltage inversing from -VC0 to +VC0 is result of half
cycle resonance , The capacitor voltage without switching is shown also
in dash line in Fig.b.. The capacitor voltage increasing due to
resonance circuit creates larger capacitive impedance for TCSC.
• In this operating condition, the current through the TCR is opposite to the
current through the capacitor. This results in current loop inside the TCSC
module [see below fig]
• The loop current increases the voltage across the capacitor, effectively
enhancing the equivalent-capacitive reactance and the series-compensation
level for the same value of line current
81 | P a g e
Parallel resonant circuit will resonate with 1/ (2𝜋√ LC ) frequency. Parallel
resonance corresponds to high impedance ,voltage and internal currents. LC
resonance will work for half cycle because of conduction thyristors in one
• The loop current increases as α is decreased from 1800 to αmin. The
maximum TCSC reactance permissible with α = αmin is typically two-anda-half to three times the capacitor reactance at fundamental frequency.
Inductive-vernier mode - Another variant is the inductive-vernier mode,
• For TCSC operation in inductive mode, the firing angle of TCR branch
must be so that XL(α)<XC. This case happens when α be greater than
zero and smaller than α max_ Ind
• In this mode, the direction of the circulating current is reversed and the
controller presents a net inductive impedance.
82 | P a g e
Fig c- Switching in inductive mode and half cycle resonance of LC circuit
• after delay angle α from zero crossing of line current, when the polarity
of the capacitor voltage and line current are same, the switching is done
in t1. Again a resonance circuit consists of L and C. This circuit resonates
half cycle and the ON thyristor will turn off
• Angle switching thyristors can change inductive reactance controlled
choke from a minimum value (α = 0, XTCR = XL) theoretically to infinity
(α = π/2, XTCR = ꝏ_).
• For sufficiently small inductive reactance of reactor towards capcitive
reactance of capacitor (XL < XC), the operating diagram of TCSC
contains inductive and capacitive mode operation of TCSC and the
transition between areas is the resonance region
• TCSC will be in capacitive mode for the firing angle greater than
αresonance and will be in inductive mode for the firing angle smaller than
this angle.
• Because of resonance in αresonance, impedance changing of TCSC is
high around this angle and it is very sensitive to firing angle deviations.
Therefore an inhibited area always defines between capacitive and
inductive region as shown in Fig.a that it is defined by α min_Cap and α max_
Ind boundary
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A TCSC involves continuous-time dynamics, relating to voltages and currents in
the capacitor and reactor, and nonlinear, discrete switching behavior of thyristors.
Deriving an appropriate model for such a controller is an intricate task.
1. Variable-Reactance Model
A TCSC model for transient- and oscillatory-stability studies, used widely for
its simplicity, is the variable-reactance model depicted in Fig.
In this quasi-static approximation model, the TCSC dynamics during power-
swing frequencies are modeled by a variable reactance at fundamental frequency.
The other dynamics of the TCSC model—the variation of the TCSC response
with different firing angles.
It is assumed that the transmission system operates in a sinusoidal steady state,
with the only dynamics associated with generators and PSS.
This assumption is valid, because the line dynamics are much faster than the
generator dynamics in the frequency range of 0.1–2 Hz that are associated with
angular stability studies.
The reactance-capability curve of a single-module TCSC,. exhibits a
discontinuity between the inductive and capacitive regions.
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However, this gap is lessened by using a multimode TCSC. The variablereactance TCSC model assumes the availability of a continuous-reactance
This model is generally used for inter-area mode analysis, and it provides high
accuracy when the reactance-boost factor (=XTCSC/ XC) is less than 1.5.
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2. Transient – Stability Model
In the variable-reactance model for stability studies, a reference value of
TCSC reactance, Xref, is generated from a power-scheduling controller
based on the power-flow specification in the transmission line.
The reference Xref value may also be set directly by manual control in
response to an order from an energy-control center, and it essentially
represents the initial operating point of the TCSC; it does not include the
reactance of FCs (if any).
The reference value is modified by an additional input, Xmod, from a
modulation controller for such purposes as damping enhancement.
Another input signal, this applied at the summing junction, is the open-loop
auxiliary signal, Xaux, which can be obtained from an external power-flow
A desired magnitude of TCSC reactance, Xdes, is obtained that is
implemented after a finite delay caused by the firing controls and the
natural response of the TCSC. This delay is modeled by a lag circuit having
a time constant,TTCSC, of typically 15–20 ms .
The output of the lag block is subject to variable limits based on the TCSC
reactance-capability curve shown in Fig.
The resulting XTCSC is added to the Xfixed, which is the reactance of the
TCSC installation’s FC component.
To obtain per-unit values, the TCSC reactance is divided by the TCSC base
reactance, Zbase, given as
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where ,
kVTCSC = the rms line–line voltage of the TCSC in kilovolts (kV)
MVAsys = the 3-phase MVA base of the power system
The TCSC model assigns a positive value to the capacitive reactance, so
Xtotal is multiplied by a negative sign to ensure consistency with the
convention used in load-flow and stability studies.
The TCSC initial operating point, Xref, for the stability studies is chosen as
The reactance capability curve of the multimodal TCSC shown in Fig. can
be approximated by the capability curve shown in Fig.b
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Figure b A simplified reactance-capability curve of a multimodule TCSC
This figure can be conveniently used for the variable-reactance model of
TCSC, and the capability curve that the figure depicts includes the effect
of TCSC transient overload levels.
It should be noted that the reactance limit for high currents is depicted in
Figb. as a group of discrete points for the different modules.
During periods of over current, only some TCSC modules move into the
bypassed mode, for the bypassing of a module causes the line current to
decrease and thus reduces the need for the remaining TCSC modules to go
into the bypass mode.
However, for the case of modeling, only one continuous-reactance limit—
denoted by a vertical line in Figb is considered for all TCSC modules.
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All reactance are expressed in per units on XC; all voltages, in per units on
ILrated. XC and all currents, in amps. In the capacitive region, the different
TCSC reactance constraints are caused by the following:
1. The limit on the TCSC firing angle, represented by constant
reactance limit Xmax 0.
2. The limit on the TCSC voltage VCtran. The corresponding reactance
constraint is give by
3.The limit on the line current (ILtran) beyond which the TCSC transpires
into the protective-bypass mode:
The effective capacitive-reactance limit is finally obtained as a minimum of
the following limits:
In the inductive region, the TCSC operation is restricted by the following
o The limit on the firing angle, represented by a constant-reactance
limit Xmin 0.
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o The harmonics-imposed limit, represented by a constant-TCSCvoltage limit VLtran. The equivalent-reactance constraint is given by
3. Long - Term – Stability Model
The capability curves of the TCSC depend on the duration for which the
voltage- and current-operating conditions persist on the TCSC.
In general, two time-limited regions of TCSC operation exist: the transientoverload region, lasting 3–10 s, and the temporary-overload region, lasting
30 min; both are followed by the continuous region. For long-term dynamic
simulations, an overload-management function needs to be incorporated in the
control system.
The overall reactance-versus-line-current (X-I) capability curve of the TCSC
is depicted in Fig. c with the relevant data presented in Table 1
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This function keeps track of the TCSC variables and their duration of
application, and it also 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.
The variable-reactance model does not account for the inherent dependence
of TCSC response time on the operating conduction angle.
Therefore, entirely incorrect results may be obtained for the highconduction-angle operation of the TCSC or for whenever the power-swing
frequency is high (>2 Hz) .
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However, the 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 dampingpower oscillations.
A reason for the model’s widespread use lies in the assumption that controls
designed to compensate the TCSC response delay are always embedded in
the control system by the manufacturer and are therefore ideal.
Hence the response predicted by the model is a true replica of actual
In situations where this assumption is not satisfied, a more detailed stability
model is required that accurately represents the inherent slow response of
the TCSC.
Applications of TCSC(Thyristor Controlled
Series Capacitor)
1. What is meant by TCSC?
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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, capacitive and
inductive reactance of a TCSC.
Equivalent impedance of a TCSC, Zeq = -j ((1) / (ωc-1/ ωL))
If (ωc - (1/ ωL)) < 0 - the TCSC provides variable capacitive reactance mode. If
(ωc - (1/ ωL)) > 0 - the TCSC provides variable inductive reactance mode.
3. What are the different modes of operation of TCSC?
Bypassed- thyristor mode
Blocked - thyristor mode
Partially conducting thyristor or Vernier mode.
4. What are the modeling techniques involved in TCSC?
Variable reactance model (1. Transient stability model 2. Long term stability
5. What is the need for modeling of a TCSC?
A TCSC involves continuous - 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.
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