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Transcript of Transmission-Line Protection a Direction
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 2, APRIL 2013 955
Transmission-Line Protection: A DirectionalComparison Scheme Using the Average of
Superimposed ComponentsS. M. Hashemi, M. Tarafdar Hagh, Member, IEEE, and H. Seyedi
AbstractIn this paper, a new protection scheme for transmis-sion lines is presented. The method has some advantages in com-parison with conventional line protection schemes. Faster fault de-tection and instantaneous coverage of almost 100% of the line arethe main advantages of the new method. A full-cycle averaging
window is used for fault detection. While the power system is innormal operation conditions, this average is approximately equalto zero. As soon as the faulty signals enter the window, the averageis changed to a nonzero value. It is shown that the product of thisaverage value for voltage and current of the faulty phase, in a spe-
cific time interval after fault inception, is negative for the forwardfaults and positive for the reverse faults. The fault is detected bycommunication between the local and the remote relays. Simula-tion and experimental results show the efficiency of the proposedmethod in fast detection of line faults in less than a half cycle.
Index TermsDirectional comparison, protection, relaying, su-
perimposed component, transmission line.
I. INTRODUCTION
T RANSMISSION lines are prevalently protected by dis-tance relays as the main protection, and overcurrent re-lays as the backup protection. Both distance and overcurrent
protections use fundamental or power frequency components
to detect the faults. In microprocessor relays, the extraction of
fundamental frequency voltages and currents is, conventionally,
provided by phasor estimation methods such as the Fourier al-
gorithm [1], [2]. The common required time for fault detection
in these relays is approximately one to two cycles. Fast detec-
tion and clearing of faults improves the stability of power sys-
tems, especially in extremely high voltage (EHV) transmission
lines. Therefore, the trend is toward faster protection schemes
in modern integrated power systems.
Superimposed components are changesin voltage and current
signals with respect to the normal or steady-state conditions.
These changes cause voltage and current traveling waves (TWs)to propagate away from the fault location. Protection schemes
using TWs are capable of detecting the fault in the first millisec-
onds following the fault inception [3], [4]. TW-based protec-
tion has some outstanding features, such as immunity to power
Manuscript received May 08, 2012; revised September 07, 2012; acceptedOctober 16,2012. Date of publication February 05,2013; date of current versionMarch 21, 2013. Paper no. TPWRD-00476-2012.
The authors are with the Faculty of Electrical and Computer Engineering,University of Tabriz, Tabriz 51666-15813, Iran (e-mail: [email protected]; [email protected]; [email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPWRD.2012.2226609
swings, current-transformer (CT) saturation, and long lines ca-
pacitance which make it more robust than traditional distance
protection [5]. TW-based protection may determine the fault di-
rection by comparing the polarities of superimposed quantities
of voltages and currents. Besides, the protection can measure
the fault distance using the time difference between forward and
backward TWs. A common feature is observed among many
TW-based protection schemes in the literature [6][9]. These
schemes usually require high sampling frequency between sev-eral hundreds of kilohertz to 1 MHz, which is more than the
sampling rate of conventional digital relays [7]. Furthermore,
limitations in the bandwidth of conventional VTs and CTs, are
also introduced and there are some difficulties in measuring su-
perimposed components for TW applications [10], [11].
Superimposed-based protection is not limited to TWs. It is
shown in [12][14] that distance protection using superimposed
components, so-called delta quantities, instead of fundamental
frequency impedance, may solve some problems in the conven-
tional distance relaying. Superimposed currents are proposed
in [15] for the phase comparison protection to remove the
sensitivity of this protection to heavy load conditions. In [16],
superimposed components are used for providing high-speeddirectional comparison bus protection. The transient energy
produced by superimposed components is used in [17] for
directional comparison relaying. Positive-sequence superim-
posed components are used in [18] for directional protection
of EHV transmission lines. In [19], it has been shown that su-
perimposed-based directional comparison offers some benefits
compared with the line differential protection.
This paper introduces a directional comparison protective
scheme using the average value of superimposed components.
The method is able to detect the faulty phase and the fault
direction in less than a half-cycle. Applying the sampling rate
of 64 samples/cycle makes the proposed method compatiblewith the commercial relays. Moreover, using the average of
voltage and current signals in a full cycle for fault detection, the
buffer size applied in the proposed method is reduced compared
with some superimposed-based protection schemes where the
samples of two or four full cycles are required to be saved [11],
[20].
II. BRIEF REVIEW ON THE FAULT SUPERIMPOSEDCOMPONENTS
Consider a simple transmission system, shown in Fig. 1(a).
This fault can be modeled by a voltage source, which is equal
in magnitude and opposite in sign to the prefault voltage at
the fault point [2]. According to the superposition theorem, the
0885-8977/$31.00 2013 IEEE
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Fig. 1. Simple transmission system with (a) steady-state (prefault) and (b) su-perimposed-state representation. A three-phase fault is assumed to occur in theforward direction, with respect to the relay R.
changes in the relay point (R) voltage and current might be com-
puted by zeroing all prefault voltage sources and representing
all network components and loads with their impedances [21].
The obtained circuit, which is depicted in Fig. 1(b), is known
as a superimposed network. For simplicity, only the inductive
portion of impedances is considered in Fig. 1(b). Therefore, the
postfault voltage and current at any point of the network can be
acquired by superposition of their prefault values and superim-
posed values, as
(1)
where is the superimposed voltage. A similar relation
may be written for the current. The superimposed components
contain dc offset, harmonics, and high-frequency transients.
Their salient feature is that the product of superimposed voltage
and current at the relay point is negative for the forward faults
and positive for the reverse faults. These properties, providing
an excellent criterion for directional comparison relaying,form the basis of TW-based protective schemes [2]. In digital
protection, the superimposed components may be extracted
by subtracting each sample from its corresponding sample in
the previous cycle. This process extracts the superimposed
components in only one cycle after fault inception, and the
relay would take its decision in this interval.
III. PROPOSEDPROTECTIVEMETHOD
In the steady-state conditions, the voltage and current signals
in the transmission lines are almost pure sinusoidal. This im-
plies that the average value of voltage and current signals, in
steady-state conditions, is almost equal to zero. Referring to (1),
it may be concluded that the average value of postfault voltage
(or current) is equal to that of superimposed voltage (or current)
(2)
where denotes the average value of the periodic signal
, with the period , which is represented in the continuous
time form by . In the discrete time
form, the average value can be represented as
(3)
where is the number of samples per cycle. Selecting a data
window with the length of one cycle, the above equation may be
Fig. 2. Output of the averagingfilter for a typical current waveform.
called the averaging filter. The filter proceeds sample by sample
along the input signal. While the protected transmission system
operates in healthy conditions, the input signal, whether voltage
or current, has a pure sinusoidal waveform, and the output of the
filter is near zero. As soon as the faulty samples enter the filter,
its output changes to a nonzero value. As mentioned before,this value is equal to the average of the superimposed compo-
nents. Referring to Fig. 1(b), the superimposed components can
be considered as the zero-state response of the electric circuit.
According to the electrical circuit theory, this response consists
of two parts, which are transient and steady-state responses, re-
spectively. For example, if the transmission line is modeled by
a series branch, the superimposed current consists of one
decaying dc component and one steady-state sinusoidal com-
ponent. The average of this current is, therefore, equal to its dc
value. However, as shown in Fig. 2, since the input signal passes
sample by sample through the averaging filter, one cycle should
be elapsed after the fault inception instant in order for that outputoffilter to become equal to the signal dc value. Moreover, the
output of the filter during this cycle has an interesting feature
which is investigated for forward and reverse faults, as follows.
A. Forward Faults
Considering Fig. 1, suppose that a three-phase fault occurs
in the forward direction, with respect to the relay R, at point F.
Shifting the time origin to the fault inception instant, the super-
imposed voltage and current can be computed as
(4)
(5)
(6)
The discrete time representations of (5) and (6), assuming
and
can be written as
(7)
(8)
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HASHEMIet al.: TRANSMISSION-LINE PROTECTION 957
Fig. 3. One-line diagram of a simple transmission system with (a) steady-state(prefault) and (b) superimposed-state representation. A three-phase fault is as-sumed to occur in the reverse direction, with respect to the relay R.
B. Reverse Faults
For the reverse faults condition, Fig. 3 can be investigated
(9)
(10)
(11)
Here, assuming
and , the discrete time
representations of (9) and (10) can be written as
(12)
(13)
Comparing (7), (8), (12), and (13) and considering that
, and are positive values, it can be realized that
the sign of in the forward faults is opposite that sign in
the reverse faults, while the sign of is similar in both
forward and reverse faults. This property may be used as a cri-
terion for discriminating between the forward and the reverse
faults. For this purpose, let us compute the product of
and . For the forward faults, we have
(14)
For the reverse faults
(15)
The sign of in the first cycle after fault
inception is given in Table I, where it can be shown that this
sign is dependent on the sample number . Moreover, is re-
stricted by the number of samples per cycle and the value of
fault inception angle . For particular values of , the interval
TABLE IDETERMINING THESIGN OF IN THEFIRST CYCLEAFTER
FAULT INCEPTION
where the sign of is invariant, becomes veryshort and may include only one or two samples. It means that
the dependability of the relay would decrease. Since the value
of is variable, can be controlled only by . In other words,
for increasing the interval where the sign of
is negative for forward faults and positive for reverse faults, the
sampling frequency should be increased. As before, this incre-
ment is a shortcoming for practical implementation in the con-
ventional relays. This problem can be solved by the proposed
method, as follows.
The average value of the voltage and current signal in each
phase is calculated using (3). Assume is the number of super-
imposed samples entering the averaging window. The output ofthe window for forward faults is given by (16) and (17) at the
bottom of the next page. For the reverse faults, similar equations
result by replacing in (16) with , and in (17) with
. It can beshown that the first interval after the fault incep-
tion instant, where the sign of
is invariant, is equal to for and
for . Comparing these values with
Table I demonstrates that the interval becomes almost twice. In
Fig. 4, theproduct of (16) and(17) is compared with (14) fortwo
different fault inception angles. It is shown that the mentioned
interval is extended. The proposed method consists of (16) and
(17)of the following stages. The rate of sampling is assumed tobe 64 samples/cycle. The relay detects the fault using eight con-
secutive superimposed samples.
1) Phase Selection: Phase selectivity provides the capability
of single-phase automatic reclosing. On overhead lines, most
faults are of a transient nature and disappear when the infeed is
switched off. Therefore, following the fault clearance, the line
can be returned to service [12]. This means that the single-phase
tripping is preferred for single-phase-to-ground faults. Single-
phase automatic reclosing basically improves the transient sta-
bility of power systems. The process of phase selection in the
proposed method is performed by comparing the absolute value
of the average of superimposed currents in the eight consecutive
windows with a threshold value. This value can be selected sim-
ilar to the setting of overcurrent relays. It means that assuming
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958 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 2, APRIL 2013
Fig. 4. Normalized values of (14) and (16) (17) for various fault inceptionangles. (a) . (b) .
theminimumfault current level of 1.25 times the maximumload
current, the threshold value may be set to , where
is the rated current in the secondary side of CT which is typi-
cally 5 A. (The division to is because of the averaging nature
of the proposed method.) Single-phase tripping is executed by
the proposed algorithm if a single-phase fault is detected for the
first time. Otherwise, all three phases should be tripped.
2) Detecting the Fault Direction: A flowchart of the pro-
posed directional relaying is shown in Fig. 5. When a short-cir-
cuit fault occurs, it changes the output of the averagingfilter to a
nonzerovalue. However, because of VT and CT errors and some
environmental noises, during normal operation of the system
and in the absence of any fault, this output may not be exactly
equal to zero. A threshold level is, therefore, needed to detect
the fault conditions. This threshold for the current signals is se-
lected as the aforementioned value (i.e., ), and for the
voltage signals, is assumed to be , where is the rated
voltage in the secondary side of VT which is typically 110 V.Provided that the average of superimposed voltages and currents
Fig. 5. Directional relaying by the proposed method in phase A. A similarprocess is performed in phases B and C.
exceeds their corresponding thresholds, the relay computes the
sum of and the sum of in the eight con-
secutive windows. The product of these values forms the
basis of directional relaying: The negative indicates forward
faults, while the positive indicates reverse faults (Fig. 5). In
very rare situations where the value of becomes almost zero
, the fault direction is detected in one or two subse-
quent windows.
3) Communication With the Remote Relay: So far, the relay
discriminates between forward and reverse faults. Additional
discrimination should be performed between the internal faults
(i.e., the faults between the close-in and the remote bus and the
external faults, that is, the faults beyond the remote bus). This
can be provided by communication between local and remote
relays. Assume the forward direction for the local and the re-
mote relays as depicted in Fig. 6. If one relay detects a fault that
is in the forward direction, it will wait for the permissive signal
from the remote bus relay. The internal fault is, therefore, de-
tected if both relays detect the fault in the forward direction.This provides extremely fast protection for the entire line.
(16)
(17)
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Fig. 6. Directional comparison relaying by the proposed method.
Fig. 7. First simulated system.
TABLE IISIMULATION RESULTS FOR THE STUDY ON THE EFFECT
OFFAULTLOCATION OF THE PROPOSED METHOD
IV. SIMULATION RESULTS
The proposed method is tested on two different systems under
several operating conditions. The results are given in the fol-
lowing subsections A and B.
A. System I
This system is a part of an Iranian 230-kV transmission net-
work, depicted in Fig. 7. The system data are given in Table X.
The system is simulated in PSCAD/EMTDC where the voltage
and current signals in the relay point are used for running
the proposed scheme in MATLAB. The frequency-dependentmodel is used for transmission lines in order to increase the
accuracy of simulation. Performance of the proposed method
is evaluated in many various conditions which are summarized
as follows.
1) Effect of Fault Location: Various types of fault in dif-
ferent points of the transmission line are tested. Table II shows
that the proposed algorithm is able to detect the fault in less
than a half cycle. In this table and the following tables, Ag,
AB, ABg, and ABC stand for single-phase-to-ground, double-
phase, double-phase-to-ground, and three-phase faults, respec-
tively. Moreover, F and R denote forward and reverse, and the
negative locations represent the faults occurring in the reverse
direction. For the fault Ag at 10 km far from bus B, the details of
waveforms are shown in Fig. 8, where the system frequency is
Fig. 8. Simulation result of an Ag fault in the forward direction. (a) Voltageand current of phase A. (b) Averages of the superimposed voltage and current.
50 Hz. For the internal faults as , both relays R and R should
detect the fault in the forward direction while for the external
faults as , relay R, on the contrary of relay R , should detectthe fault in the reverse direction. Neglecting the delay of the
communication channel, the total time required for fault detec-
tion is determined by the relay with longer operating time.
2) Effect of Fault Resistance: The presence of resistance
in the fault path causes the dc component of fault current in
(7) and (10) (i.e., the term to decay exponentially).
Since the fault resistance is remarkable in the case of earth
faults, performance of the proposed method is tested on some
single-line-to-ground faults with fault resistances between 5 to
100 . As shown in Table III, the proposed method seems to not
be sensitive to the fault resistance.
3) Effect of Fault Inception Angle: The presence of the dccomponent in the fault current depends on the fault inception
angle. In other words, it would be some angles, or equally some
instants, that the corresponding fault current does not have any
dc value. Nevertheless, as the averaging window moves sample
by sample, it needs at least one full cycle to compute the new
dc value after fault inception. In this interval, the estimated dc
component changes from zero to a nonzero value and returns to
zero at the end of one cycle. Fig. 9 represents an example of this
condition for a reverse three-phase fault at 0.204 s.
Theeffect of the fault inceptionangle on the proposed method
can be considered in Table IV. In order to cover the entire in-
terval of 0 to , the fault inception instant is gradually in-
creased in one full cycle. The results of Table IV demonstrate
that the required time for fault detection has increased in some
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960 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 2, APRIL 2013
TABLE IIISIMULATIONRESULTS FOR THE STUDY ONEFFECT OFFAULT RESISTANCE
ON THEPROPOSED METHOD
Fig. 9. Example of zero dc current for a particular fault inception angle. Onlyphase A is represented.
inception angles. However, the delay, in the worst condition, is
less than half a cycle.
4) Effect of Noisy Fault Signals: Since the nature of the pro-
posed method is averaging signals during one full cycle, it is
expected that the noise effect on the proposed method is not
considerable. This is due to the fact that the average of white
noise is zero. Various faults with different signal-to-noise ratios
(SNRs) are tested, and the results are shown in Table V. For this
purpose, the Gaussian white noise with the signal-to-noise ratio
(SNR) of 60, 40, and 20 dB are added to the main signals. For
increasing the security of the proposed method in the presence
of noise-polluted signals, the value of thresholds should be set
more accurately. The inception instant of all faults in Table V is
0.2 s.
B. System II
The purpose of simulation studies in this case is to evaluate
the performance of the proposed method in situations where the
line distance protection, as the prevailing protection in trans-
mission lines, is encountered with challenges and difficulties.
The IEEE Power System Relaying Committee (PSRC) proposes
the system depicted in Fig. 10 for testing most transmission-
line protection applications [22]. The system is simulated in
TABLE IVSIMULATIONRESULTS FOR THE STUDY ONEFFECT OFFAULTINCEPTION
ANGLE ON THE PROPOSED METHOD
TABLE VSIMULATIONRESULTS IN THECASE OFNOISYSIGNALS
the Electromagnetic Transients Porgram (EMTP). It should be
noted that for each of the following cases, according to [22],
some changes on the topology of Fig. 10 are applied. Thesystem
frequency is 60 Hz, and the sampling rate is 64 samples/cycle.
Referring to Table X, the protection in two challenging con-
ditions (i.e., the presence of short line and the maximum line
loading) has been considered in the previous subsection. The
other cases are investigated as follows.
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Fig. 10. IEEE PSRC recommended model for testing transmission-line protec-tion in EMTP.
TABLE VISIMULATIONRESULTS FOREVALUATING THE PROPOSED METHOD IN
DOUBLE-CIRCUITLINES
1) Double-Circuit Lines: Mutual inductive coupling be-
tween transmission lines on the same tower or parallel alongthe same right of way introduces some errors in the impedance
measured by distance relays. These errors are negligible in
positive- and negative-sequence impedances. Nevertheless, as
the zero-sequence mutual impedance of parallel lines can be
50% to 70% of zero-sequence self-impedance, the impact of
mutual coupling is more significant in the case of ground faults
[12], [23]. Therefore, the performance of the proposed method
in parallel lines is evaluated only for the earth faults. For this
purpose, the switch SW in Fig. 10 remains open and the relay
is tested for faults on 66% of the lower line AB, and
on 11% of the upper line AB. The results are given in Table VI.
2) Three-Terminal Lines: Sometimes, usually due to eco-nomic restrictions, transmission lines are tapped to provide in-
termediate connections to loads, or to reinforce the underlying
lower voltage network through a transformer. These connec-
tions introduce some problems in distance protection, especially
when sources of generation exist behind the tap points [24]. The
fundamental problem with this line configuration is the inter-
mediate infeed to the fault location from the third terminal [12].
Besides, the application of travelling-wave-based protection in
three-terminal lines requires careful study, since the travelling
waves are strongly affected by the connected taps.
To create this condition in Fig. 10, the upper line AB is
opened and the switch SW is closed. The tap point is located
at 33% of the length of line AB. Afterwards, the performance
of relays R1 and R2 is evaluated for the faults located at F1
TABLE VIISIMULATIONRESULTS FOREVALUATING THE PROPOSED METHOD IN
THREE-TERMINAL LINES
TABLE VIIISIMULATIONRESULTS FOREVALUATING THEPROPOSED METHOD INPOWER
SWING CONDITIONS
(at 66% of the length of line AB) and F3 (at 33% of the length
of line BD). As considered in Table VII, the results show the
efficiency of the proposed algorithm in this case.
3) Power Swings:From the reliability point of view, the dis-
tance relays have two fundamental problems in the presence of
power swings. The first problem is the possibility of detecting
power swing as a fault, which causes the loss of security. For
preventing these conditions, distance relays are equipped withthe power swing blocking (PSB) units, which blind the relay
to see the faults while the power swing persists. This function,
however, leads to the second problem, which is loss of de-
pendability for the faults occurring during a power swing. The
problem is considerable only for symmetrical or three-phase
faults [25], since asymmetrical faults can be detected by other
protective approaches, like applying the negative-sequence
components. The proposed method is, however, immune to
the power swing conditions. According to [22], power swing
can be created in Fig. 10 by applying a three-phase fault at
bus A and removing the fault before the generator loses
synchronism. The second cited problem is investigated by
applying three-phase faults at points and (defined in
Subsection B.2), where the results are presented in Table VIII.
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TABLE IXSIMULATIONRESULTS FOREVALUATING THEPROPOSED METHOD INSERIES-COMPENSATEDLINES
Fig. 11. Experimental prototype setup.
4) Series-Compensated Lines: Compensation of transmis-sion lines by series capacitors imposes some significant effects
on both directionality and reach of distance relays. The details
are out of the scope of this paper and can be found in [26] and
[27]. These effects become more severe in double-circuit se-
ries-compensated lines. In Fig. 10, the source -connected line
is removed and thedouble-circuitline AB is compensated by the
degrees of 70%.
The performance of relays and is tested for the faults
at points and , where the results are given in Table IX.
With protection of series-compensated lines, the location of the
fault hasa key role, since the well-known phenomena of voltage
and current inversions are affected by the fault location. Forthe sake of briefness, these phenomena are not investigated fur-
ther. However, they are considered in Table IX. The presence of
metaloxide varistors (MOVs) is also considered in this table.
As shown, the presence of two capacitors in the path of reverse
faults at point , in some situations, makes the direction de-
tected by relays and inversed. However, since this inver-
sion occurs for both relays, it does not impact the fault detection
criterion used by the proposed method. It should be noted that
this condition is only present in double-circuit series-compen-
sated lines. Our extensive studies show that this inversion is not
present in single-circuit series-compensated lines.
It is remarkable that, in comparison with Fig. 2, the output of
the averaging filter in series-compensated lines is equal to that of
the zero-state response of the second-order circuit composed by
Fig. 12. Experimental results of an SLG fault on phase A: (a) three-phase volt-ages, (b) three-phase currents at the relaying point, (c) averages of voltage andcurrent of the faulty phase, (d) output of the proposed method in simulation, (e)experimental averages of voltage and current computed by the relay, and (f) theexperimental result of the fault direction by the relay.
the series capacitor and the line series impedance which consists
of subsynchronous oscillations.
V. EXPERIMENTALRESULTS
The proposed method is tested, also, on an experimental
prototype setup, which is depicted in Fig. 11. The relay is de-
signed using an AVR microcontroller (ATMEGA32A). In order
to make the laboratory tests compatible with the real world,
the real voltage and current signals saved by a fault recorder
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Fig. 13. Experimental results of an SLG fault on phase C: (a) three phase volt-ages, (b) three-phase currents at the relaying point, (c) averages of voltage andcurrent of the faulty phase, (d) output of the proposed method in simulation,(e) experimental averages of voltage and current computed by the relay, (f) andexperimental result of the fault direction by the relay.
TABLE XPARAMETERS OF THESYSTEMI
have been used. These data include three-phase voltages and
currents measured at the relaying point in the substation of one
side of the line. These voltages and currents are saved into a
personal computer and played back as the inputs of the relay.
Considering restrictions in the space of this paper, only the
results of two single-phase-to-ground (SLG) faults are pre-
sented here. All faults are internal and, therefore, the protective
relays of the line had to trip the line. Fig. 12 shows the result
of an SLG fault on phase A. The voltages and currents of three
phases, which are acquired by the fault recorder, are depicted
in Fig. 12(a) and (b), respectively.
The averages of voltage and current of phase A, which
are used for the detection of fault direction, are depicted in
Fig. 12(c). The output of the proposed method, which is pro-
vided by simulation, is represented in Fig. 12(d). In thisfigure,
the fault direction signal is 0, where there is no fault or there
is a reverse fault. As soon as a forward fault is detected by the
relay, this signal becomes 1. The results of the experimental test
are shown in Fig. 12(e) and (f), where the relay has detected
the fault direction within less than a quarter of a cycle after the
fault inception.
Another experimental test is carried out for an SLG fault on
phase C, where the obtained results are shown in Fig. 13.
VI. CONCLUSION
This paper presents a high-speed directional comparison pro-
tective scheme for transmission lines, using the average value
of superimposed components. Extensive simulation studies are
performed to evaluate the proposed method in different oper-
ating conditions, including double-circuit lines, three terminal
lines, power swing conditions, and series-compensated lines.
The impact of important parameters, such as fault resistance,fault location, fault inception angle, and noise-polluted fault sig-
nals on the protection systems are also considered in evaluating
the proposed method. The obtained simulation results, in addi-
tion to the experimental results, show that the proposed method
is competent for being applied to line protection.
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S. M. Hashemi received the B.Sc. degree in elec-trical engineering from the Bu-Ali Sina University,Hamedan, Iran, and is currently pursuing the M.Sc.degree in electrical engineering at the University ofTabriz, Tabriz, Iran.
His research interests include power systemprotection,flexible ac transmission systems, HVDC,power system operation, and electricity markets.
M. Tarafdar Hagh(S98M06) received the M.Sc.(Hons.) andPh.D.degrees in power engineering fromthe University of Tabriz, Tabriz, Iran, in 1992 and2000, respectively.
He has been with the Faculty of Electrical andComputer Engineering, University of Tabriz, since2000, where he is currently a Professor. He haspublished more than 150 papers in power systemand power electronics-related topics. His interesttopics include power system operation, flexible actransmission systems,and power quality.
H. Seyediwas born in Iran in 1979. He received theB.Sc., M.Sc., and Ph.D. degrees in electrical engi-neering from the University of Tehran, Tehran, Iran,in 2001, 2003, and 2008, respectively,.
Currently, he is with the faculty of Electrical andComputer Engineering, University of Tabriz, Tabriz,Iran. His areas of interest include digital protectionof power systems and power system transients