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Impact of Facts Devices on Distance Relay
Naresh Patnana1, Kontham Hari Krishna2
Asst.professor, Department of EEE, Gitam University, 2Asst.professor,
Department of EIE, Centurion University, Odisha, India
1
[email protected],[email protected]
1
Abstract
In the power transmission system FACT controller is incorporated in the transmission line in
order to increase power transfer capability as well as reactive power control. At the same time with the
increase in the number of FACTS devices like TCSC (Thyristor Control Series Capacitor) and STATCOM (Static
Synchronous Compensator) in transmission lines, operation of distance relays are affected. It is very
important that the distance relays do not mal operate under system fault conditions, as this will result in
the loss of stability or the security of the system, which defeats the main objective of installing a FACT device.
In this context the impacts of FACT device on the performance of distance relay needs to be addressed and a
suitable pattern classifier is necessary for proper classification of the fault in the transmission line as well. In
the present work two types of FACT devices is considered i.e. TCSC and STATCOM which addresses the issues
of adaptive protection of a transmission line where these devices are located at the middle of transmission line.
Design and simulation of TCSC incorporated in the Transmission line is done with the occurrence of
fault before as well as after TCSC when it is located in the middle of line. The dynamic operation of novel
control scheme for STATCOM based on a model comprising a 48-pulse Gate Turn-Off thyristor voltage source
converter for combined voltage stabilization and reactive power compensation is also addressed. The impact of
STATCOM on the distance relay is described by means of impedance trajectory using S-transform with
quadrilateral trip boundary and apparent impedance calculation is carried out in presence of STATCOM for LG fault case. In the present work the modular neural network is proposed and the design of classifier is
subsequently done in presence of STATCOM and TCSC which is also another contribution in the above work. In
this work STATCOM and TCSC are connected to the 230 kV interconnected system with 400 Km transmission
line. Further the system model, control strategy, impedance trajectory and design of neural classifier are done in
MATLAB/Simulink environment.
KEYWORDS: Distance relay, Modular Neural Network, STATCOM, S-Transform, TCSC
1. Introduction
The concept of flexible ac transmission
systems (FACTS) in power systems envisages the use
of power electronic devices for increasing the power
transfer and providing the optimum utilization of
system capability In principle, series controllers
inject voltage in series with the line and the shunt
controllers inject current into the system at the point
of connection[1].
The presence of the TCSC/ STATCOM in
fault loop not only affects the steady state
components but also the transient components. Due
to presence of these devices during fault condition
the change in apparent impedance may lead to mal
operation of a distance relay. It is very important
that the distance relays do not mal operate under
system fault conditions, as this will result in the
loss of stability or the security of the system. Some
research has been done to evaluate the performance
of a distance relay for transmission systems with
FACTS controllers.
The work in [2,3] has presented some analytical
results based on steady-state model of STATCOM,
and the authors have studied the impact of FACTS on
a distance relay at different load levels and its
tripping boundaries, and both the parameters of
FACTS controllers and their location in the line have
an impact on the trip boundary. The work in [4]
shows that thyristor-controlled series capacitor
(TCSC) has a major influence on the mho
characteristic and discussed about stable operating
region.
In this paper for observing impact of
STATCOM /TCSC on distance relay, polygon
tripping boundary characteristics are shown and the
apparent impedance calculation is determined at the
fault point using S-transformation. Further modular
neural network is proposed for proper classification
of the fault in the transmission line.
2.Modeling of FACT Devices :
Fig. 1 Single-line diagram of Power system with
FACTS device.
1
To study the impact of FACT devices on
distance relay a 230 kV,50Hz interconnected power
system is considered in which STATCOM is installed
at mid point of transmission line as shown in the
Fig.1 also the same power system is considered with
TCSC located at different locations in transmission
line for fault identification , classification and
localization.
Fig. 3 Switching charcterstics of STATCO
2.2 48 pulse conveter
2.1 Modeling of STATCOM :
Fig.4 48- pulse converter
Fig.2 Transmission line with STATCOM
The static compensator (STATCOM) as shown
in Fig.2 is a device used to regulate voltage and
improve dynamic stability of power systems. GTObased STATCOM are multilevel line-commuted
voltage-sourced inverters that are shunt-connected to
a power system bus through a set of transformers.
By varying the amplitude of the output voltage E,
the reactive power exchange between the converter
and the AC system can be controlled. If E is the
STATCOM voltage and V is the System voltage,
then
E > V Capacitive Mode, Q generated
E<V Inductive Mode, Q delivered.
The switching characteristics of STATCOM are
shown in Fig.3.
k  0 ,1, 2
2.3.Control Circuit
Transient
rating
VT
Transient
rating
The STATCOM shown in Fig.4 is built
with four 3-phase 3-level inverters coupled with four
phase shifting transformers introducing phase shift
of +/- 7.5°. This transformer arrangement neutralizes
all odd harmonics up to the 45th harmonic, except
for the 23rd and 25th harmonics (for a perfectly
balanced network). Those two harmonics are
minimized choosing an appropriate conduction angle
for the three-level inverters (σ= 172.5°).
The line-to-neutral 48-pulse ac output
voltage from the STATCOM model is expressed as:
8 
Vab48 (t ) 
Vabn sin( nt  18.5n  18.75i) ------- (1)
3 n 48k 1
1.0 pu
0.75 pu
0.5 pu
Fig. 5 GTO firing pulse generator
0.25 pu
IC
IC max
IL max
Capacitive
Inductive
IL
Control circuit is used to operate the voltage
source inverter to inject or absorb reactive power to
regulate the connecting point voltage to the setting
value Vref.
(The dead angle of STATCOM is kept fixed at
γ=π/48). The process of generating firing pulses by
voltage source inverter shown in Fig. 5
2
Where  is the firing angle, XL is the reactance of
the inductor.
The parameters considered are shown below
Voltage Regulator
gain
Current
Regulator Gain
2.5 Impact of FACTS device on Distance
Relay
Kp
15
6
Ki
3000
40
Vshunt = Three phase voltages at the connecting point
I shunt = Three phase currents of STATCOM
2.4. Modeling of TCSC
The TCSC is one of the main FACTS
devices, which improves the stability and power
transfer capability of the existing transmission system
which not only affects the steady state components
but also the transient components.
Simulink model of open loop TCSC device
connected in series with the interconnceted three
phase transmission system is shown in Fig.6.
The circuit thus has an equivalent impedance (
)
under steady state conditions which is represented by
The operation of distance relay depends on
apparent impedance measurement which is mainly
influenced by the type of FACTS device, location,
control parameters of the device and fault resistance.
From equation (2) X net varies due to
changes in firing angle and during fault condition it
effects the apparent seen by the relay leading to its
maloperation.
Apparent impedance during Lg fault in presence of
STATCOM.
For the analysis associated with the
operation of a distance relay, the power system
shown in Fig. 3 is used, the relay is installed on the
before the STATCOM. The apparent impedance is
calculated
from
symmetrical
component
transformation at relay point [7].
When Lg fault occurs at the right side of
STATCOM and the distance is n*L from the relay
point, the apparent impedance is given by [6]
Z=
-------------------- (2)
Where
X c capacitive reactance of the parallel
capacitor and X1 is the reactance of the
---------------- (4)
are quantities at relay point
the relaying current
On simplification, the impedance seen by this relay
can be expressed as:
Z  nz1 
Where
I sh
I relay
(n  0.5) z1 
If
I relay
R f ------ (5)
R f = fault resistance, z1 =positive sequence
impedance, n=distance in PU
The second part of equation (5) shows impact of
STATCOM on the apparent impedance and results
from the shunt current
I sh injected by the
STATCOM.
2.6 S-Transform
Fig. 6 Simulink diagram of TCSC
Variable reactor which is given by
------------------- (3)
S-transform is an invertible time-frequency
spectral localization technique that combines the
elements of wavelet transform and short Fourier
transform.
In this application S-transform used for
calculating amplitude and phase of the faulted
current and voltage signals.
The impedance to the fault point is
calculated by using the phasor information from stransform. The impedance trajectories(R-X plot) for
different operating conditions are found out and the
circuit breaker tripped when the trajectory enters in
to the relay operating zone and thus protects the line.
3
2.7 Classification scheme by Modular neural
network
Table 2 Test result for fault location for AG fault
Cases
Fig. 7 Modular structure
The modular concept to neural network is
borrowed from the principle of divide and conquers.
Such a strategy in neural network is applied to
directional relay for a line in [5].In the approach any
task is divided into number of possible subtasks
where each one is accomplished by an individual
neural network. Finally all network outputs are
integrated to achieve the overall task.
In this work such a strategy is applied to
identification of fault section using probabilistic
neural network. When the fault is beyond the TCSC
from relay side the network should provide 1 else 0.
After the net work is trained it is tested at different
section within or beyond TCSC to identify the
section of fault. Test result for identification for fault
section for different types of fault given Table 1
Table 1 Identification of fault
Cases
Fault at 15% AG.fault
( before TCSC)
Fault at25% (before TCSC) BG fault
Fault at 35%( before TCSC)
AG fault with fault inception angle
900
Fault at 65% (after TCSC)
AB fault
Fault at 75% (after TCSC)
BCfault
Fault 55% (after TCSC)
CG.fault with change in frequency
52 Hz
Network
output
0
Network
output
% Error
Fault at 17% before
TCSC
0.1734
2
Fault at 27% before
TCSC
0.2684
0.59
Fault at 32% before
TCSC
0.3206
0.19
Fault at 41% before
TCSC
0.4123
0.56
Fault at 63% before
TCSC
0.6277
0.0037
Fault at 77% before
TCSC
0.7704
0.05
Fault at 88% before
TCSC
0.8841
0.47
Accuracy of distance estimation for different values
of fault resistance as given in Table 3
Table 3 Accuracy Estimation
Distance
(km)
51
81
96
123
231
264
Error (%)
(Rf=0.1Ω)
3.65
1.41
1.61
2.54
1.43
2.11
Error(%)
(Rf=10.Ω)
1.81
2.52
1.31
1.98
1.26
2.24
3. Results and Discussion:
0
0
1
1
1
The table 2 indicates a test result for a-g fault at
different location of the transmission line using
standard back propagation neural network.
Fig. 8 Impedance characteristics of TCSC
From Fig. 8 it is clear that resonance occurs at a
firing angle of    / 2 and on left side of this
4
point the TCSC operates in inductive mode whereas
on the right it is capacitive mode of operation.
Fig.9 48-pulse converter output voltage
1349.38
1349.34
1349.30
1349.26
1349.22
137.02
137.01
137.01
137.01
137.01
1361.05
1361.01
1360.97
1360.92
1360.88
170.63
170.58
170.52
170.45
170.37
The impact of STATCOM on distance relay
during fault (LL fault, phase ‘a’) condition is shown
in Fig. 11 by the quadrilateral trip boundary
characteristics. It clearly shows the chance of mal
operation of distance relay with and without
STATCOM
during
fault
condition.
The
characteristic with statcom during fault condition is
observed to be not entering into the tripping zone
which may lead to mal operation of distance relay.
The reactive power injected by STATCOM
when its settings are 0.88, 1 and 1.12 respectively is
shown in Fig.10.
Mho relay and polygon shape charecteristics
100
50
X Plane
0
-50
-100
-150
-200
0
100
200
300
400
500
600
700
800
900
R Plane
Fig. 11 Quadrilateral trip boundary
characteristics of distance relay with
STATCOM (dotted line) & without
STATCOM (solid line)
Fig.10 Reactive power control in STATCOM
In the system shown in the Fig.2, an LL
fault occurs on the right side of STATCOM and the
fault distance to relay point is 200km; the setting
value in terms of the desired voltage for STATCOM
is 1.0pu.
The change of apparent impedance during
fault conditions with STATCOM and without
STATCOM is shown in Table 4.
Fault is created after 20 km from
STATCOM having
Simlary for quadrilatral trip boundry
charctarstics for phase C is shown in Fig.12.
250
200
R f =10ohms and inception angle
150
0
of 18 .
Table 4 R,X vaules at Fault Condition
XPlane
100
50
0
Without STATCOM
With STATCOM
-50
R(  )
1349.52
1349.49
1349.45
1349.42
X(  )
137.17
Rs(  )
1361.22
Xs(  )
170.78
137.05 1361.18
137.04 1361.14
137.03 1361.09
170.68
170.67
170.65
-100
-150
0
100
200
300
400
R Plane
500
600
700
800
Fig. 12 phase C Characteristics
5
4. CONCLUSION
This paper firstly presents a detailed model
of a Transmission system employing STATCOM and
TCSC. In the present work the modular neural
network is used for fault section identification and
fault localization in presence of TCSC.which is
observed to be less time consuming and efficient
technique. The simulation result shows clearly the
impact of STATCOM on distance relay performance
is done by the help of quadrilateral trip boundary
using S-transformation.
5. References:
[1] Hingorani N. G. and Gyugyi L., Understanding
FACTS Concepts and Technology of Flexible AC
Transmission Systems. New York: IEEE Press,
2000.
[2] El-Arroudi K., Joos G., and McGillis D. T.,
“Operation of impedance protection relays with
the STATCOM,” IEEE Trans. Power Del.,
vol.17, no. 2, pp. 381–387, Apr. 2002.
[3] Dash P. K., Pradhan A. K., Panda G., and Liew
A. C., “Adaptive relay setting for flexible AC
transmission systems (FACTS),” IEEE Trans.
Power Del., vol. 15, no. 1, pp. 38–43, Jan. 2000.
[4] Wang W. G., Yin X. G., Yu J., Duan X. Z., and
Chen D. S., “The impactof TCSC on distance
protection relay,” in Proc. Int. Conf. Power
SystemTechnology (POWERCON ’98), vol. 1,
Aug. 1998, pp. 18–21.
[5] Lahiri U., Pradhan A. K,Mukhopadhyaya S.,
“Modular neuralnetwork-based directional relay
for transmission line protection,” IEEE Trans. on
Power System, vol. 20, no. 4, pp. 2154-2155,
2005.
[6] Xiaoyao ZhouU, Haifeng Wang R. K. Aggarwal
and Phil Beaumont ”Performance Evaluation of
a Distance Relay as Applied to a Transmission
System With UPFC”IEEE Trans. on Power
Delivery vol 21,no.3 1137-1147,2006
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