IJCER (www.ijceronline.com) International Journal of computational Engineering research

International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 5

Reducing Powersystem Oscillations Using Facts Controller(Tcsc)
N Mohan, M.Ramesh
(Student of EEE Department i, Madanapally Institute of Technology & Science)
(Associate Professor, EEE Department, Madanapally Institute of Technology & Science)

Abstract Studies have investigated the potential of using FACTS
The recently proposed phase imbalanced series capacitive Controllers’ capability in damping inter-area oscillations.
compensation concept has been shown to be effective in The use of Thyristor Controlled Series Capacitor (TCSC),
enhancing power system dynamics as it has the potential and Static Synchronous Series Compensator (SSSC) have
of damping power swing as well as sub synchronous been the subjects of several studies evaluating their
resonance oscillations. In this paper, the effectiveness of a respective effectiveness in enhancing power system
“hybrid” series capacitive compensation scheme in dynamics The recently proposed phase imbalanced series
damping power system oscillations is evaluated. A hybrid capacitive compensation concept has been shown to be
scheme is a series capacitive compensation scheme, where effective in enhancing power system dynamics as it has
two phases are compensated by fixed series capacitor (C) the potential of damping power swing as well as
and the third phase is compensated by a TCSC in series subsynchronous resonance oscillations [7], [8]. Fig. 1
with a fixed capacitor (Cc). The effectiveness of the shows a scheme for a phase imbalanced capacitive
scheme in damping power system oscillations for various compensation. It is a “hybrid” series compensation
network conditions, namely different system faults and tie- scheme, where the series capacitive compensation in one
line power flows is evaluated using the EMTP-RV time phase is created using a single-phase
simulation program. TCSC in series with a fixed capacitor (Cc), and the other
two phases are compensated by fixed series capacitors (C).
Index Terms—FACTS Controllers, phase imbalance, The TCSC control is initially set such that its equivalent
series compensation, thyristor controlled series capacitor.
compensations at the power frequency combined with the
fixed capacitor yield a resultant compensation equal to the
Introduction:
other two phases. Thus, the phase balance is maintained at
FLEXIBLE AC Transmission Systems (FACTS)
the power frequency while at any other frequency, a phase
technology provides unprecedented way for controlling
imbalance is created. To further enhance power
transmission grids and increasing transmission
oscillations damping, the TCSC is equipped with a
capacity.FACTS Controllers provide the flexibility of
supplementary controller.
controlling both real and reactive power which could
result in an excellent capability for improving power
system dynamics. A problem of interest in the power
industry at which FACTS Controllers could play a
significant role in it is Increasing damping of low
frequency power oscillations that often arise between
areas in large interconnected power networks. These
oscillations are termed inter-area oscillations, which are
normally characterized by poor damping [2]. Inter-area Fig. 1. A schematic diagram of the hybrid series
oscillations can severely restrict system operations by compensation scheme
requiring the curtailment of electric power transfers level The phase imbalance of the proposed scheme can be
as an operational measure. These oscillations can also lead explained mathematically as follows: 1) At the power
to widespread system disturbances, e.g. cascading outages frequency, the series reactances between buses X and Y,
of transmission lines and, therefore, system wide voltage in Fig. 1, in phases a, b, and c are given by:
collapse Several

Where −jX TCSCo is the effective capacitive reactance
of the TCSC at the power frequency such that

Issn 2250-3005(online) September| 2012 Page 1606


synchronize TCSC operation. The thyristor gating control
is based on the Synchronous Voltage Reversal (SVR)
2) During any other frequency, fe technique [9] - [11]. The TCSC impedance is measured in
terms of a boost factor kB, which is the ratio of the
apparent reactance of the TCSC seen from the line to the
physical reactance of the TCSC capacitor bank. A positive
The first terms in (2) and (3) are different because of the value of kB is considered for capacitive operation. A low-
difference in frequency. The third term in (3) represents pass filter based estimation algorithm is used to estimate
the change in the effective capacitive reactance of the the voltage and the current phasors. A boost measurement
TCSC due to the action of the TCSC supplementary block performs complex impedance calculations for the
controller. boost factor of the TCSC as kBˆˆ= Imag VC / I C / X
This scheme would, definitely, be economically attractive CTCSC , where, VC and I C are the estimated phase
when compared with a full three-phase TCSC which has voltage and current and XCTCSC is the capacitive
been used/proposed for power oscillations damping. reactance of the TCSC capacitor branch at the
Furthermore, reducing the number of thyristor valves to fundamental frequency. A proportional-integral (PI)
one third will also have a positive impact on system control based boost level controller is implemented to
reliability. The effectiveness of the scheme in damping control the TCSC boost level to the desired value by
power swings and subsynchronous resonance oscillations adjusting the instant of the expected capacitor voltage zero
is reported in [7], [8]. This paper evaluates the crossing. The integral part of the controller helps in
effectiveness of the scheme in damping power system removing the steady state errors. The controller parameters
oscillations. Time domain simulations were conducted on were determined by performing repeated time domain
a benchmark network using the EMTP-RV. simulations for the different operating conditions.
This algorithm uses the difference between the actual
Ii. Study Benchmark: boost level and the reference boost level (err) shown in
To demonstrate the effectiveness of the proposed scheme Fig. 3 as an objective function. The algorithm starts with
in power system oscillations damping, the system shown arbitrary initial values for the control parameters and
in Fig. 2 is adopted as a test benchmark. It consists of calculates the values of the objective function each time.
three large generating stations (G1, G2 and G3) supplying The control parameters are incremented for the next
two load centers (S1 and S2) through five 500 kV iteration and the procedure is repeated until the objective
transmission lines. The two double-circuit transmission function approaches a minimum value (below a threshold
lines L1 and L2 are series compensated with fixed value). The procedure described above is widely used by
capacitor banks located at the middle of the lines. The industry for tuning of controller parameters. The multiple
compensation degree of L1 and L2 is 50%. The simulations run based tuning procedure similar to the
compensation degree is defined as the ratio (XC/XL)* above was reported in [12].
100% for fixed capacitor compensated phases and
(XCc+XTCSC)/XL* 100% for the hybrid compensated
phase. The total installed capacity and peak load of the
system are 4500 MVA and 3833 MVA respectively. Shunt
capacitors are installed at buses 4 and 5 to maintain their
voltages within 1±0.05 p.u. Two loading profiles
designated as Load Profiles A and B are considered in the
investigations of this paper. In Load Profile A, S1 = 1400
+ j200 MVA and S2 = 2400 + j300 MVA while in Load
Profile B, S1 = 2000 + j200 MVA and S2 = 1800 + j300
MVA. The power flow results for the bus voltages and the
line real power flows of the system for these two loading
profiles are shown in the Appendix. The EMTP- RV is
used as the simulation study tool.

III. Modeling of The Single-Phase Tcsc:
The single-phase TCSC is modeled in the EMTP-RV as a
single module using an ideal thyristor pair and an RC
snubber circuit as shown in Fig. 3. A Phase Locked Loop
Fig. 2. Test benchmark
(PLL) is used to extract phase information of the
fundamental frequency line current, which will be used to


assumed to be placed in the test benchmark replacing the
fixed series capacitive compensation in L1 and L2.
Moreover, it is assumed that each TCSC provides 50% of
the total capacitive compensation and the disturbance is a
three-cycle, three-phase fault at bus 4. Furthermore, the
performance of the scheme is compared to the case with
only fixed capacitor compensation which is labeled in the
figures of the time responses as Fixed C.

Fig. 3. Block diagram of a TCSC controller CASE STUDY I: Load Profile A
In Fig. 3, D(t) is a supplemental signal generated from an In this case, four different combinations of stabilizing
m-stage lead-lag compensation based controller. As the signals (tabulated in Table I) are examined in order to
real power flow in the transmission line is proportional to determine the combination that would result in the best
the inverse of the total line reactance, the power swing system transient time responses. The final results of the
damping can be achieved by properly modulating the time-domain simulation studies (controllers tuning) are
apparent TCSC reactance through this controller. The shown in Fig. 5 which illustrates the generator load angles,
supplemental controller input (stabilizing) signals could be measured with respect to generator 1 load angle, during
local (e.g. real power flows) or remote (e.g. load angles or and after fault clearing. The transfer functions of the
speed deviations of remote generators). If a wide-area TCSC supplemental controllers for the four combinations
network of Synchronized Phasor Measurement (SPM) are given in Table II. Comparing the responses of the
units is available, then the remote signals can be fixed series capacitor compensation to the hybrid TCSC
downloaded at the controller in real time without delay. compensation scheme in
Local signals are generally preferred over remote signals Fig. 5, the positive contribution of the proposed
as they are more reliable since they do not depend on hybrid scheme to the damping of the system oscillations is
communications. In Fig. 3, kBref is the TCSC boost level very clear. As it can be seen from Fig. 5, the power swing
set point. TheSynchronous Voltage Reversal block solves damping controller effectively damps the system
for angle from non-linear relation,
the oscillations. It can also be seen from Fig. 5 that the best
damping of the relative load angle responses are achieved
with the 21- 21 combination. The second best damped
wher responses are obtained with the 31- 21 combination. These
e uCZ is the estimated capacitor voltage at the desired results should be expected due to the direct relationship
instant when the capacitor voltage zero crossing occurs, between the relative load angles and the generators that
iLM is the measured value of the line current iL, X0 is the yield the problem. It can also be seen from Fig. 5 that the
TCSC capacitor reactance at the TCSC resonance worst damped responses are obtained with PL1- 21
frequency, is theratio between the TCSC resonance combination which results also in the increase of the first
frequency and the system fundamental frequency and swings.
is the angle difference betweenthe firing time and the TABLE I: THE FOUR EXAMINED
voltage zero-crossing. The value of isused to calculate COMBINATIONS OF STABILIZING SIGNALS FOR
the exact firing instants of the individual thyristors. The CASE STUDY I
non-linear relationship between the boost factor and the
thyristor firing angle is shown in Fig. 4.

Fig. 4. TCSC boost factor as a function of the thyristor
firing angle .
This section demonstrates the capability of the proposed
hybrid series compensation scheme in power system
oscillations damping. For this purpose, the scheme is



Fig. 5. Generator load angles, measured with respect to
generator 1 load angle, during and after clearing a three-
phase fault at bus 4 (Load Profile A).
CASE STUDY II: Load Profile B
In this case, 21 is used as the supplementary controllers
stabilizing signal. The transfer functions of the TCSC
supplemental controllers are given in Table III. Fig. 6 Fig. 6. Generator load angles, measured with respect to
illustrates the generator load angles, measured with respect generator 1 load angle, during and after clearing a three-
to generator 1 load angle, during and after fault clearing. It phase fault at bus 4 (Load Profile B).
can be seen from Fig. 6 that, at this loading profile, the CASE STUDY III:
hybrid single-phase-TCSC scheme provides again a better A Dual-Channel Controller:
damping performance to system oscillations compared to Any of the four signals, 21, 31, PL1 and PL2 contains the
fixed capacitor compensation. It is observed, however, that system’s two natural modes of oscillations and can be
there is a slight increase in the first swing of 21.0 TABLE used to add damping to these modes as it has been
II demonstrated in Case study I. The sum of two properly
Transfer Functions of the Tcsc Supplemental selected signals, however, should result in a more effective
Controllers for Case Study I damping. The reason is that the two natural modes of
oscillations are, in general, not in phase. A dual-channel
controller would adjust separately the gain and phase of
each mode of oscillations and, thus, provides a better
damping.

TRANSFER FUNCTIONS OF THE TCSC
SUPPLEMENTAL CONTROLLERS FOR CASE
STUDY II

The performance of the dual-channel TCSC supplemental
controller shown in Fig. 7 in damping power system
oscillations is examined using the six pairs of signals
given in Table IV. Investigations are conducted on the test
benchmark system at Load profile B.
TABLE IV
THE SIX EXAMINED COMBINATIONS OF
STABILIZING SIGNALS FOR CASE STUDY III



The final results of the time-domain simulation studies
(controllers tuning) show that the best and second best
damped responses are obtained with pairs 2 and 5. The
transfer functions of the TCSC supplemental controllers
for the six pairs of signals are given in Table V. Fig. 8 Fig. 8. Generator load angles, measured with respect to
illustrates the generator load angles, measured with respect generator 1 load angle, during and after clearing a three-
to generator 1 load angle, during and after fault clearing. phase fault at bus 4 (Load Profile B, dual-channel
These results (in red color) are compared to the hybrid controller).
case of Fig. 6 (referred to as single channel).

TABLE V: TRANSFER FUNCTIONS OF THE TCSC
SUPPLEMENTAL CONTROLLERS

Fig. 9 illustrates the three-phase voltages, VX-Y, across Fig. 9. Phase voltages, VX-Y across the hybrid single-
the hybrid single-phase-TCSC compensation scheme phase-TCSC scheme on L1 during and after clearing a
(installed in L1 and the controllers are Pair 2) during and three-phase fault at bus 4 (Load Profile B, dual- channel
after clearing the fault. The system phase imbalance supplemental controllers, Pair 2).
during the disturbance is clearly noticeable especially in
V. Conclusions:
The paper presents the application of a new hybrid series
capacitive compensation scheme in damping power
system oscillations. The effectiveness of the presented
scheme in damping these oscillations is demonstrated
through several digital computer simulations of case
studies on a test benchmark. The presented hybrid series
capacitive compensation scheme is feasible, technically
sound, and has an industrial application potential.

phase C.


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