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

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956 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 2, APRIL 2013

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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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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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

transmission-line protection a direction

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