TCSC FOR PROTECTION OF
TRANSMISSION LINE
P.S.Chaudhari#i,
#,,3,4
P. P.Kulkarni#2, R.M.Holmukhe#3, Mrs.P.A.Kulkarni #4
#iScientist, DRDO, Pune, India, #2DRDO, Pune, India
Bharati Vidyapeeth University College of Engineering, Pune, India.
#3
rajeshmholmukhe@hotmail.com,
Abstract: A grid of transmission
lines operating at high or
extra high voltages is required to transmit power from
generating stations to load. In addition to transmission
lines that carry power from source to load, modern power
systems are highly interconnected
for economic reasons.
The large interconnected transmission networks are prone
to faults due to the lightning discharges
and reduce
insulation
strength.
Changing
loads and atmospheric
conditions are unpredictable
factors. This may cause
overloading of lines due to which voltage collapse takes
place. All the above said things are undesirable for secure
and economic operation of a line. These problems can be
eased
by providing
sufficient
margin
of working
parameters and power transfer, but it is not possible due
to expansion of transmission
network. Still the required
margin is reduced by introduction of fast dynamic control
over reactive and active power by high power electronic
controllers.
Here my seminar explains the discussion on the
effect of this device on protection of transmission line. The
TCSC is considered
as a dynamical
device and its
transient process is modeled in order to have the response
to disturbances based on its own control strategy.
It is shown that not only the TCSC affects the protection
of its line, but also the protection of adjacent lines would
experience problems. The study is done first analytically
by using simple models, then the simllated results of power
system and the protective relays in Real Time Digital
Simulator (RTDS) are used. Finally, the simulation results
are varified by using a commercial relay. The results are
reviewed for the study of the impact of TSCS on
protection of transmission line
Keywords- TCSC, Transmission
line
I.INTRODUCTION
Today, transmission-level voltages are usually considered to
be 110 kV and above. Lower voltages such as 66 kV and 33
kV are usually considered sub-transmission voltages but are
occasionally used on long lines with light loads. Voltages less
than 33 kV are usually used for distribution. Voltages above
230 kV are considered extra high voltage and require different
designs
compared
to
equipment
used
at
lower
voltages. Overhead transmission lines are uninsulated wire, so
design of these lines requires minimum clearances to be
observed to maintain safety.
Engineers design transmission networks to transport the
energy as efficiently as feasible, while at the same time taking
into account
economic
factors,
network
safety and
redundancy. These networks use components such as power
lines, cables, circuit breakers, switches and transformers.
Failure protection
Under excess load conditions, the system can be designed to
fail gracefully rather than all at once. Brownouts occur when
the supply power drops below the demand. Blackouts occur
when the supply fails completely. Rolling blackouts,or load
shedding,
are intentionally-engineered
electrical power
outages, used to distribute insufficient power when the
demand for electricity outstrips the supply.
Communications
Operators
of long transmission
lines require reliable
communications for control of the power grid and, often,
associated generation and distribution facilities. Fault-sensing
protection relays at each end of the line must communicate to
monitor the flow of power into and out of the protected line
section so that faulted conductors or equipment can be quickly
de-energized
and the balance of the system restored.
Protection of the transmission line from short circuits and
other faults is usually so critical that common carrier
telecommunications are insufficiently reliable. In remote areas
a common carrier may not be available at all. Communication
systems associated with a transmission project may use:
Microwaves or Power Line Communication.
Much analysis is done by transmission companies to
determine the maximum reliable capacity of each line, which,
due to system stability considerations, may be less than the
physical or thermal limit of the line. Deregulation of
electricity companies in many countries has led to renewed
interest in reliable economic design of transmission networks.
Transmission line protection depends upon a core
group of protective elements. These elements must be
dependable and secure for all power system conditions as any
single weakness can cause problems with a relaying scheme.
Some common concerns related to a line distance relay are
operating speed, load and fault impedance induced element
under- and overreach, out-of-step (OOS) conditions, and
Capacitive Voltage Transformer (CVT) transients.
A major program to develop new technologies for enhancing
power transmission has been initiated. This program, named
"FACTS'
for "Flexible AC Transmission
Systems," is
intended to provide new systems and methods of operation to
help electric utilities get the most from their investments in
transmission
networks. Scoping studies were conducted in
1989-1990 to identify the benefits of FACTS concepts and to
provide direction for the development of hardware systems.
One outcome of those studies was to show that a ThyristorControlled Series Compensation (TCSC) system should be
developed, as a number of benefits could be achieved in a
cost-effective manner [1,3].
What is FACTS?
The FACTS technology is a collection of controllers, which
can be applied individually or in coordination with others to
control one or more of the interrelated system parameters,
such as series impedance, shunt impedance, current, voltage,
and damping of oscillations.
Fig 1.
FACTs Controller
FACTS controller is defmed as power electronic based system
and other static equipment that provide control of one or more
AC transmission system parameters. Out of no. of FACTS
devices the one which is very efficient is the TCSC.
roles in the operation and control of power
scheduling
power
flow;
decreasing
components; reducing net loss; providing
limiting short-circuit currents; mitigating
resonance (SSR); damping the power
enhancing transient stability.
systems, such as
unsymmetrical
voltage support;
sub synchronous
oscillation; and
Advances in high-power, high-efficiency power electronics
have led to the development of thyristor-controlled series
compensators in power systems. In contrast to capacitors
switched by circuit breakers, TCSC will be more effective
because thyristors can offer flexible adjustment, and more
advanced control theories can be easily applied.
Series capacitor is used in a long distance EHV lines for
increasing power transfer. Use of series capacitor is for the
most economic enhancing power flow though series have a
problem of SSR. Series is only is used for power transfer as
compared to shunt. Shunt has the main problem of location
not of SSR. To provide variable series compensation thyristor
control is used due to which the major problem of SSR is been
reduced to much lesser extent. The first demo project of
TCSC was done at west Virginia USA. In 1991.in October
1992 l" TCSC was installed at Arizona substation. and then
in1993 in Oregon. Series compensation can be achieved in
two ways discrete and continuos .Discrete by using TCSC and
continuos by using TCSC or GTO.
TCSC- thyristor
controlled
series capacitor
Classification of FACTS controllers
a) shunt connected
b) series connected
c) combined series-series
d) combined series - shunt
Classification
of FACTS controllers
by use of power
devices
A) variable impedence type
B) voltage source converter based which includes
VCS includes
i. SVC- static var compensator
ii. TCSC- thyristor controlled series capacitor
iii. TCPSFthyristor
controlled
phase
shift
transformer
iv. STATCOM- static synchronous compensator
v. SSSC - static synchronous series compensator
vi. IPFC- interline power flow controllers
vii. UPFC- unified power flow controllers
Why TCSC only?
There have been significant activities and achievement in the
research and application of flexible AC transmission systems
(FACTS). Thyristor-controlled series compensation (TCSC) is
an important device in the FACTS family. It can have various
Fig 2.
Basic TCSC diagram
Fig 3. TCSC with sries inductor for current control
The above two figures shows that TCR is used in parallel with
a fixed capacitor to enable continuos control over series
compensation. Harmonics are present in thyristor switches can
be mitigated by TCSC along with SSR. GCSC is same as
shown above only SCR is replaced by a GTO. In between two
SCR is more advantageous as it has lower cost.
Equivalent circuit of TCSC
Xc
~rJ
Xtcr
~
<
Iter
capacitive
operation
Xc
~r~
Xtc:r
~
Iter
;;.
Inductive
operation
Fig 4. Equivalent circuit ofTCSC
Operation of TCSC
Ld
circuit breaker
B
capacitor, MOV and bypass switches are used for its
protection, while for the TCSC, co-operative operation of
MOV, thyristors and bypass switches are applied for a reliable
protection. This is the main difference in the behaviour of the'
distance relays in the two compensation methods. The
apparent impedance seen by the distance relay depends largely
on the TCSC mode of operation. The TCSC mode of
operation during a fault is not unique and it may transit from a
mode to others sequentially; meanwhile the distance relay
experiences various apparent impedances in the fault period.
In normal operating mode, called capacitive-boost mode or
vernier mode, the thyristor are firing properly and the bypass
switch is open. From the system point of view, this mode
inserts capacitors into the line up to nearly three times the
fixed capacitor.
TCSC controls fire pulse to change thermistor's fire
angle and make TCSC to operate in thyristor blocked mode,
thyristor bypassed mode and vernier mode with partial
thyristor conduction. The vernier mode let a inductive loop
current to flow through capacitor produce a capacitive
reactance which is larger than that of fused capacitor.
Normally, TCSC's stability fundamental frequency impedance
is relative only to fire angle a. When a is between 145 degree
and 180 degree ,the equivalent reactance is capacitive, larger a
is corresponding to less capacitor reactance. When a is
between 90deg and l40deg , the equivalent reactance is
inductive, larger a is corresponding to larger inductive
reactance. When a fault occurs, TCSC's control system would
react swiftly to take some protection ensures. If short circuit
current is large enough, MOV would be fired, TCSC's control
system would rapidly send its commands to bypass thyristor
and make TCSC become inductive. So TCSC's capacitive
reactance would decrease and gradually change into inductive
reactance. This characteristic makes it possible for distance
relay with memory polarizing voltage and proper setting value
to be used in TCSC line. If short circuit current is not large
enough, TCSC's control system will not send its commands to
bypass the thyristors.
x
A
capacitive
C
B
Une cul'tent
Fig. 1
Simple power system compensated by TCSC
Figure
shows a TCSC module with different protective
elements [6] in the middle of a simple power system.
Basically, it comprises a series capacitor C in parallel with a
thyristor-controlled reactor (TCR), Ls. A metal-oxide varistor
MOV is connected across the series capacitor to prevent the
occurrence of high capacitor over voltages. A circuit breaker
is also installed across the TCSC module to bypass it if a
severe fault or equipment malfunction occurs. A currentlimiting inductor Ld is incorporated in the circuit to restrict
both the magnitude and the frequency of the capacitor current
during the capacitor-bypass operation. There are different
modes of operation of TCSC in the normal and fault
conditions. During a fault, an overvoltage appears across the
TCSC due to the fault current. For a conventional series
o
_--~G
Inductive
E
Fig.2
OIK'>"<UUt<;NIIIJI<Oj'TCSC
~~[h~~~~t~~r:J
c :::~u~imum
D ""
run
H)hagc limit
Ihyristur
F..;:; lirin¥ anale vnit
F ::. h!irmonic
G = thyri<oto(
heating limit
current limit
TCSC's
impedance
characteristic
become
complicated.
Considering
the situation shown in Fig,
substituting TCSC for FSC, if relay l's PT is on line side, it
can be used reliably. To relay II, Only if TCSC's capacitive
reactance is not larger than a certain value, for example, the
set impedance of relay II for phase to phase fault, it can be
used reliably. Under normal conditions, the current waveform
in TCSC's inductor is symmetrical about zero voltage point of
capacitor,
capacitor
voltage
itself is also symmetric,
harmonics of voltage at relay and current in line is very little,
so TCSC has no influence on protection relay. But when a
fault occurs, transient voltage and current's waveform is not
Symmetrical, and properly leads to TCSC confusional firing.
TCSC's harmonic become very complicated.
Harmonic
characteristic is relative to compensation degree, fault location,
fault time and many other factors. Harmonic components are
various, amplitudes are large, and amplitudes don't decrease
monotonously. It is very difficult to find accurate regulation
for TCSC's harmonics.
Fig 7.
R-X diagram
across the capacitor. The MOV is fast enough to conduct and
reset within a half cycle. The MOV would not shortcircuit out
the capacitor as the circuit breaker would. This condition is
usually very short but may be repeated several times during
the fault period. During MOV conduction, Ztcsc is the
impedance of the parallel combination of the TCR, capacitor
and the MOV in a lower resistance mode.In considering the
equivalent MOV circuit proposed
in the combination
characteristic can be considered as a resistance in series with a
boost capacitor (fixed capacitor and the TCR branch). At this
moment, the relay overreaches but less than is the case
without MOV conduction.
(iii) Blocking mode: When the thyristors are not triggered and
are kept in nonconducting state, the TCSC is operating in
blocking mode. In this mode, the TCSC performs like a fixed
series capacitor. When the TCSC detects an overvoltage by
the MOV current, the TCR branch stops its firing sequence by
a protection function. The process is effective for avoiding the
over current of the thyristors or capacitor caused by
fluctuation of the firing angle under the condition that the
voltage phase of the capacitors changes suddenly. Herein, the
equivalent MOV circuit will be the combination of a
resistance in series with the fixed series. At this moment the
relay overreaches less than is the case with the thyristor-fIring
mode. By the process, if the overvoltage and overcurrent are
cleared, the system returns to the normal mode, otherwise the
energy absorbed in the MOV exceeds its limitation and the
TCSC transits from the blocking mode to the bypass mode to
protect theMOV and $capacitor.
(iv) Bypass mode: In this mode, the thyristors are triggered
continuously and the TCR branch conducts in the whole
General TCSC
~, {~7
.TC~-j-i
8~~
t
Fig. J TCR compensated transmission line
Fig 8. General TCSC
Fig 8.
TCSC modes of operation
According to above fig different possible TCSC modes of
operation during a fault and the behaviour of the relays
can be summarised as follows:
(i) Capacitive-boost
mode without MOV conduction: For a
low fault current, the protection function of the TCSC device
does not work; therefore the TCSC remains constantly in its
vernier mode of normal operation; i.e. the TCR branch is
triggered by its prespecified firing angle. In this case a
significant compensation exists, so the conventional distance
relay overreaches considerably;
or may even lose its
directional integrity.
(ii) Capacitive-boost
mode with MOV conduction: For a
high fault current, MOV operates for decreasing the voltage
Sample test system arrangement
fa
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~
11<,.''''
X,
lUQ
~
lIQ
P:IiW 2!ItIlVA.!pI.!
Fig 9. Test system arrangement
Simulated results
Few simulated results are as follows
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20
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-20
9
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{=;\1~
I'
-40 I
-60 L
o
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0.3
0.2
Figur4f B.
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H.o'U
Results
of t!/~ctricQl
daITlping
COnvt'lO" oJut6far <."O"apuri.-rOIJ..
tt!3t.~ plott4!!d 011
Tim e (s)
<'
Fig 12. Simulated results
2 10.0327(5)
~
~
8
~
:.:l
0.3
0.2
0.1
Time (s)
Fig. 2 Capacitor voltage and line current waveforms
in the thyristors-blocked operating mode
Fig 10. Simulated results
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unil
I
--.
-...--- 111111111111111111111111111
o.oa
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•••.•
9..,"
0.'.
o...,~ •..••••
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0.1'" •. ,.
cin:k:
-I+-----~~~--~~--~
...1
-0.5
o
0.'
Real
Fig. 5 Variations in eigenvalues with conduction angle.
(OO~ conduction angle a ,; 18(0)
'(<.1)
impedance
F""tg..,Z ~~.
••••~e
.,....,....u.,reftl.cut.
..nib bypass
la:upeclanc:e
conttol
'Versus
Fig 11. Simulated results
Fig 13. Simulated results
Thyristor Conduction angle table for 5 cycles
I\~
cycles
I
o
Fig. 6
Wavef'onns
O.OS
T~me
using
0.1
($)
the proposed
equations.
(ex.-600)
2
3
4
5
Conduction anzles (u, =60u u2-24OO)
60° -108.0°
240° - 289.2°
60° -116.5°
240° - 298.5°
60° -119.1°
240° - 299.6°
60° -120.0°
2400 - 300.0°
60° -120.0°
240u - 300.0u
d.ate
40
-~
..
... ~ ~.
- ... -
.~ ~---.
,_., _.
I
'r-
,
._,_.
I
·······
.. r-.....
20
M •
K
.
·20
,
;
..•
•..
~OO:-~I~OO--~IIO~~,20~~I~~~I~~L-~I~~~I~~~1~1O~,~
(~)
Fig14.
Variation ofTCSC reactance with firing angle 'a'
Advantages of TCSC
Protective distance relays, which make use of impedance
measurements in order to determine the presence and location
of faults, are "fooled" by installed series capacitance on the
line when the presence or absence of the capacitor in the fault
circuit is not known a priori. This is because the capacitance
cancels or compensates some of the inductance of the line and
therefore the relay may perceive a fault to be in its first zone
when the fault is actually in the second or third zone of
protection. Similarly, first zone faults can be perceived to be
reverse faults! Clearly this can cause some costly operating
errors. The conventional protections like distance, differential,
and by using relays power controllers are very much in use
nowadays.
In order to meet the high demand for power transmission
capacity, some power companies
have installed series
capacitors on power transmission lines. This allows the
impedance of the line to be lowered, thus yielding increased
transmission capability. The series capacitor makes sense
because it's simple and could be installed for 15 to 30% of the
cost of installing a new line, and it can provide the benefits of
increased system stability, reduced system losses, and better
voltage regulation. 1
Disadvantage of TCSC
Employment of series compensation creates certain problems
for its protective relays and fault locators using conventional
techniques because of the rapid changes introduced by the
associated
TCSC control
actions
in primary
system
parameters such as line impedances and load currents. The
most important singularity lays in the fact that the positive
sequence impedance measured by traditional distance relays is
no longer an indicator of the distance to a fault. The apparent
reactance and resistance seen by the relay are affected due to
the uncertain variation of series compensation voltage during
the fault period.
The major benefits include being able to directly
schedule power flows along desired paths, to rapidly modulate
the effective impedance in response to power swings so that
the level of secure power transfer can be increased, and to
permit higher levels of conventional series
compensation
without being restricted by sub synchronous resonance(SSR)
aspects
Applications of TCSC control
There have been studies on TCSC with models of real powt
systems.
In the US, there
are Bonneville
Powt
Administration (BPA) system American Electric Power (AE
system, 345kV Edic-Fraser transmission line in New YOl
state system , and Montanatransmission
system . Otlu
systems include the Southern Brazil interconnected network
North-Northeast
Brazilian power system, Central Ind
transmission corridor,
and Queensland-New South Walt
power system in Australia.
TCSC devices have been installed in the Slatt
Kayenta and Kanawha River [SO, 811 substations in the U~
In these TCSC devices, power swing damping control systerr
adopt linear schemes and the transient stability contn
systems adopt open-loop insertion schemes. In China, TCS'
is proposed to be installed in the Yimin-Fengtun 500k'
transmission lines at the Fengtun site . Several contn
strategies are proposed to increase both the transient stabilit
and dynamic performance, including nonlinear control ADRI
control and fuzzy control. One of the TCSC plant is locate
in Chattisgarh, Raipur in India.
Summary
There still exist several problems with the engineerin
application and implementation of TCSC stability controller
Control schemes that have been adopted in real power systerr
are usually conventional, such as in the Slatt, Kayenta an
Kanawha River substations. In contrast, many scheme
proposed in the literature are rather academic and will b
impractical. The implementation of TCSC in real powe
systems requires not only appropriate control theories, bi
also feasibility, the amount of computation and the acquisitio
of the input signals. Furthermore, some limitations in re,
operation would prevent control schemes reaching expecte
performances. As described in Section 2, the operating rang
of TCSC will be limited by its own capacity and voltag
protection of the MOV. From a purely theoretical point (
view, most studies in the literature do not consider thes
limitations. In addition, some schemes require remote inpi
signals, even transient variables in distant generators, whic
are difficult to be measured and transferred. Therefore,
would be very difficult for many control schemes to b
implemented in a practical engineering project. Howeve
efforts are being made to understand and analyse the problem
Conclusion
This paper focus on TCSC's influences on protection regior
Transmission lines can be compensated by Fixed Serie
Capacitors (FSC) or more effectively by Thyristor Controlle
Series Capacitors
(TCSC) to increase power transfe
capability, improve transient stability, reduce transmissio
losses, and dampen power system oscillations. However It i
certain that TCSC will exert influences on distance relay'
ability about reflecting ground resistance and avoiding loa
impedance and will have some new requests on filterin
performance. All these problems are worth study and ar
being studied. TCSC dynamics have a significant impact 0
the power system protection and its transition from a mode t
another can create serious problems for the conventional
relays like forward overreach,
reverse overreach,
mis
coordination in primary and back-up protection, directional
malfunction and adverse effect on distance schemes. The
work is in progress toward remedial actions such as adaptive
relaying and optimal pilot protection schemes. The results will
be reported in future.
REFERENCES
[I] M. Adamiak and R. Patterson, "Protection requirements
for flexible AC transmission systems," in Proc. CIGRE,
Paris, France, 1992.
[2] T. S. Sidhu, M. Hfuda, and M. S. Sachdev, "Generating
relay models for protection studies," IEEE Comput. Appl.
Power, vol. II, no. 4, pp.
33-38, Oct. 1998. Mojtaba
Khederzadeh received the B.Sc. degree in electrical
engineering from
[3] Weiguo, W. et al.: 'The impact ofTCSC on distance
protection relay' .Proc. Int.Conf. Power System
Technology, Powercon'98, 18-21 Aug. 1998, Vol. 1, pp.
382-388,5 Real time digital simulator, RSCAD', Ver.
1.177,2003 6 Larsen, E.V., Clark, K., Miske, S.A. Jr.,
and Urbanek, J.:
[4] N. G. Hingorani, "Future Opportunities for Electric Power
Systems," LuncheonSpeech, IEEE PES Summer Power
Meeting, San Francisco, CA, July 14, 1987
[5] FACTS controllers in power transmission and distribution
by N.K. Padiyar
Operation of TCSC
A typical configuration of TCSC is shown. It consists of a
thyristor-controlled reactor (TCR), parallel capacitor and
metal oxide varistor (MOV). TCSC can operate in the
capacitive or inductive range. Its operating range is limited by
the firing angle of thyristors, the protection of the MOV and
the current limitation of capacitors. In real operation, these
limitations may affect the performance of control schemes
TCSC comprises a capacitor C in parallel with a thyristor
Controlled Reactor TCR branch. A metal Oxide Varistor
(MOV), essentially a nonlinear resistor, is connected
across the capacitor to prevent the occurrence of high
capacitor over voltages. The basic scheme of such
compensators is shown in Figure. The amount of series
compensation in the capacitive operating region is
increased (or decreased) by varying the thyristor firing
angleQ. The overall impedance of the TCSC is given in
equation. figure (2) shows variation of TCSC reactance
(XTCSC ) against of the firing angleQ. The TCSC
considers three different operation modes; the basic
principles of these modes and
the switching policy between them are described as follows
a- Vernier Mode:
In high impedance fault cases, TCSC constantly remains in its
vernier mode of normal operation therefore; the protection
function of the TCSC does not work. The TCR branch is
triggered by its pre-specified firing angle. The TCSC works
as variable impedance.
b- Block Mode:
When the TCSC detects the over-voltage, the TCR Inductive
Capacitive branch stops its firing sequence by a protection
function. The TCSC works as a series capacitor in parallel
with a MOV. The combination characteristic can be
considered a resistance series with a capacitor,since the high
resistance characteristic is introduced by the MOV.
c- Bypass Mode:
When the MOV operation is not enough to decrease the
capacitor voltage; the TCR branch enters its bypass mode to
protect the TCSC. In this mode, the TCR conducts in the
whole cycle, and the TCSC device serves as inductance.
d- Circuit Breaker bypass:
When the energy absorbed in the MOY exceeds its setting, the
fault is not cleared within a certain time, the TCSC protection
function transits the circuit breaker to bypass mode. Since the
series reactor in the circuit breaker is very small.