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Content uploaded by Seghir Samira
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All content in this area was uploaded by Seghir Samira on May 10, 2019
Content may be subject to copyright.
Electrical Short Circuit Arc Effect on the Distance Protection
in the Transmission Line
Samira Seghir
1
, Tahar Bouthiba
1
, Rebiha Boukhari
1
, Abdelhakim Bouricha
1
and Samia Dadda
2
Département d’électrotechnique
1
Laboratoire Optimisation des Réseaux Electriques(LORE)
2
Laboratoire de Génie Électrique d'Oran (LGEO)
Université des sciences et de la technologie d'Oran - Mohamed-Boudiaf
B.P. 1505 El-Mnaouar, Oran 31000 Algérie
Abstract: Overhead transmission lines are an essential
component of power systems and being the most
economical means of transmitting electricity from
remote sources of generation to the customer. It is,
therefore, vital that they be protected and maintained in
service to ensure the security of the supply to the end-
user, and to protect the capital investment of the power
companies. Arcing faults are a common phenomenon on
power systems and being able to understand the effects
of fault arcs is important for power system protection.
Numerical algorithms can be developed that analyses
and locate the position of arc faults on power systems.
The aim of this works is to develop a model of arcing
faults and to develop numerical algorithms that can
operate successfully on different power system
topologies under various fault conditions.
Index Terms –Transmission lines, Arcing faults,
Distance protection.
I.
I
NTRODUCTION
Arcing faults on transmission lines can be divided
into two separate periods: the primary arc and the
secondary arc. The primary arc exists during the fault
period and is maintained by the large short circuit
current; it is extinguished when the circuit breakers on
the transmission line have operated and isolated the
fault. If the line has three-pole switching, a secondary
arc will not occur. Single-pole switching is where only
the faulted phase is isolated and the other two phases are
left in operation; this is usually used on very long lines
to improve transient stability. Three-pole switching is
where all three phases are isolated when a fault occurs.
The secondary arc occurs on lines with single-pole
switching after the circuit breakers have isolated the
faulted phase; it is maintained by mutual capacitive
coupling between the healthy phases and the faulted
phase (this is where the magnetic fields surrounding the
healthy phases and induced by the current flowing
through those phases in turn induce a relatively small
current in the isolated faulted phase) and it has a much
smaller current flowing through it than the primary arc.
The secondary arc is extinguished when the arc path
through the insulating gas becomes too long and too de-
ionized for sufficient maintaining current to flow. The
length of the arc is influenced by factors such as the
arc’s supply current, the magnetic forces acting on the
arc column caused by the supply current, and the wind
speed and atmospheric conditions surrounding the arc;
this makes modeling the arc a particularly complicated
matter [1].
Distance relay is the main protection of power
transmission lines and makes an important role in power
system stabilization if it operates selectively and
instantly.
In this paper, the fault impedance is determined. It is
shown that in the end of the first zone or in the middle
of the second zone, distance relay could not make a
correct decision for operating if the fault occurs with
arc[2]. So to overcome this problem a new technique
using the compensation method is proposed in this
paper. The proposed method makes distance relay more
selective and instantaneous.
II.
DYNAMIC
CHARACTERISTIC
OF
FAULT
ARCS
Fault arcs can be considered as purely resistive
elements between the faulted line and ground (Figure
1); the simplest form of arc model can take the form of a
short circuit resistance between the transmission line
and ground. However, as the length of the arc increases
continuously and non-linearly throughout its duration,
the arc has a non-linearly increasing value of resistance.
This means that the arc voltage also increases non-
linearly throughout the arc’s duration.
The extended theory of the switching arc can be applied
to the model unconstraint fault arc in air, as shown in
[3]. Both, the primary and secondary arcs are modeled
on the basis of differential equations of arc conductance:
(1)
with G = stationary arc conductance
g = time varying arc conductance
τ = time parameter (time constant) of the arc
Fig. 1 Equivalent circuit diagram of an arc fault.
Stationary arc conductance itself can be physically
explained as arc conductance when the arc current is
maintained at the same value for sufficiently long
enough periods under constant external situations. Thus,
the stationary arc conductance G is given by:
(2)
(3)
(4)
(5)
(6)
Where:
u
st
= stationary arc voltage (arc voltage gradient).
u = total characteristic arc voltage in V.
r = total characteristic arc resistance in mΩ.
u
0
= characteristic arc voltage per arc length in V/cm.
r
0
= characteristic arc resistance per arc length in
mΩ/cm.
l
arc
= instantaneous arc length in cm.
l
0
= initial length of the arc column (primary arc length)
in cm.
In a primary arc model, the arc length l
arc
and the time
parameter τ are relatively constant. The arc length in
this situation is represented by the gap length of the
arcing horn (l
0
). While in a secondary arc current, the
arc length and the time parameter will be changed in
time as expressed in equation:
(7)
(8)
Where
v
l
= speed of arc elongation in cm/ms.
τ
0
= initial time constant in ms.
v
τ
= speed of the time constant decrease in ms/cm.
Based on previous expressions, the arc length will
increase linearly when the secondary arc period begins.
By contrast, the time constant is inversely proportional
to the arc length.
III.
DISTANCE PROTECTION PRINCIPLE
Distance protection determines the fault impedance
from the short-circuit voltage and current at the location
where the relay is installed (Fig. 2).
Fig. 2 Distance protection principle, measurement of fault
impendence.
The measured fault impedance is compared with the
known line impedance. If the measured fault impedance
is smaller than the set line impedance, a fault is detected
and a trip signal sent to the circuit-breaker. This means
that the distance protection in its simplest form operates
by measuring the voltage and current at the relay
location [4]. No additional information is required for
this basic distance protection, and the protection does
not have to depend on any additional equipment or
transmission signal. Because of inaccuracies in distance
measurement, which are the result of measurement
errors, transformation errors and inaccuracies in line
impedance, in practice it is impossible to set the
protection to 100% of the line length. A security limit
(10% to 15%) from the end of the line must be
determined for the so-called under-reaching zone (1
st
zone) in order to ensure protection selectivity due to
internal and external faults, which can be seen in Figure
3. The rest of the line is covered by an over-reaching
zone (2
nd
zone) which, in order to ensure selectivity,
must have a time delay with respect to the protection of
the neighboring line.
Fig. 3 Distance protection principal, division of the distance grades.
In the case of an electro-mechanical protection, this
difference in time is 400 to 500 ms, and 250 to 300 ms
in the case of analog static and numerical protection.
This time delay includes the operating time of the
circuit breaker, delay of the distance measuring
elements as well as the security limit.
In contrast to differential protection which is completely
selective (its protection zone is entirely defined by the
location of the current transformers at both line
ends),the distance protection in its simplest form
(without telecommunication supplements)does not
provide absolute selectivity. Selective tripping must be
ensured by time delay relative to the neigh boring
protection. However, distance protection has the
possibility of reserve protection for the neigh boring
lines. The second stage (over-reaching stage)is used for
this purpose. It reaches the neigh boring busbars and a
part of neigh boring lines. The next, 3
rd
stage(3
rd
zone)
is usually used for protecting the entire length of the
neigh boring lines (Fig.3). The arrangement of the
stages and time settings is obtained with a time-distance
diagram.
IV.FAULT LOCATION METHOD
There are several techniques for locating faults on a
transmission line [5, 6]. In this study, we assume that
the current and voltage waves are sinusoidal after the
fault [7-9]. The signals are filtered and sampled.The
proposed method is based on the use of measurements
of the fundamentalcomponent of the current and voltage
signals at one end of the line. Figure 4 represents a fault
in transmission line at location “m”.
Fig. 4Fault in transmission line at location m.
From Figure 4 we can write the following equation:
(9)
:Positive line impedance.
: Fault arc resistance.
: Current at source S.
: Voltage at source S.
: Fault current.
m: Fault location.
The value of the impedance
measured from S can
be determined by dividing Eq. (9) by the measured
current I
S
.
(10)
We take only the imaginary part of
, Eq. (10) can be
written as:
(11)
The fault location is:
(12)
For a single-phase fault (phase "a" to ground), the
calculation of fault location m is as follows:
(13)
Where the residual current I
R
is given by:
(14)
and the ground factor is :
(15)
: Zero line impedance.
The fault location is:
(16)
V.THE COMPENSATION METHOD
This section presents the methodologies used to
compensate the effect of arc fault resistance on the
accuracy of apparent impedance measurement. The
process began with determining the fault location during
the occurrence of fault. There are many techniques
currently and previously used to locate the fault point.
The most and commonly used technique to locate fault
at transmission line is impedance based technique [10].
In IV. We have presented a proposed method to locate
the fault in transmission line. After the fault location,
the relay calculated the arc fault resistance. The next
step is to compensate the effect of fault resistance on
Mho type distance relay. This is done first by measuring
the apparent impedance at the relaying point. The
measurement of apparent impedance is done by using
Eq. (17). The apparent resistance
andreactance
are the real and imaginary values of
respectively.
After that, the apparent resistance will be subtracted
with fault resistance as shown in (21).
A. Distance Protection without Compensation
The apparent impedance see by the relay is:
(17)
(18)
(19)
: measured resistance.
: measured reactance.
B. Distance Protection with Compensation:
Using reactance method[10] we can calculate the
fault arc resistance:
(20)
(21)
(22)
(23)
(24)
: Estimated fault resistance.
: Compensated resistance.
: Compensated reactance.
: Resistance calculated by the relay.
: Reactance calculated by the relay.
: Fault location using reactance method
The compensation is only on the resistance.
VI.
SIMULATION RESULTS
A. Study Network
For analyzing the effects of arcing faults on the
operation of distance relay, a simple power system is
selected and its three phase diagram is shown in Figure
5.
Fig. 5Power system model in Matlab Simulink.
This network is composed with a transmission line of
100 km + a line adjacent to 80% assumed between two
sources, of voltage 400 kV and50Hz. Tables 1 and 2
give the transmission line and arc parameters,
respectively.
TABLE
1:
T
RANSMISSION LINE PARAMETERS
Transmission line parameters data
Direct resistance 0.01165 Ω/km
Direct inductance
0.008679
H/km
Zero resistance 0.2676 Ω/km
Zero inductance 0.003008 H/km
TABLE
2:
A
RC PARAMETERS
Arc parameters data
l
0
3.5 m
u
0
965
V/m
r
0
0.162 Ω/m
τ
0
0.001 s
v
l
10
m/s
v
t
0.285e
-
4
s/m
B. Obtained Results
Figure 6shows the simulated arc voltage and current
throughout the arc’s duration.
Fig. 6 Voltage and current of fault Arc
It can be seen in this figure that the amplitude of the
voltage of the arc increases non-linearly throughout the
duration of the arc.
Fig. 7 shows the voltage-current characteristic of the arc
and confirms its nonlinear nature.
Fig. 7 Voltage-Current characteristic of fault Arc
In this characteristic, the relation between arc voltage
and current are shown in a hysteresis loop. The
hysteresis loop of the arc occurs in the first and third
quadrant of the plane, and indicates the predominantly
resistive nature of the arc. In general for primary and
secondary arcs, when the current increases, the arc
voltage also increases proportionally with the current.
Until a certain value (about 30 % of the current peak in
primary arc in this case), the arc voltage becomes
constant although the current rises continuously. With
the decrease of the current, the arc voltage decreases
approximately linear to the current, viewing constant arc
resistance characteristic. Two different voltage
behaviors during increase and decrease current occur.
The more the current increases, the arc temperature
becomes higher due to the accumulated arc energy, and
also as a result of increasing arc conductivity [5].
During the primary arc stage, the characteristic
approximately follows the same path. Unlike previous
stage, the situation is different for the secondary arc
stage. The characteristic has different value in each
cycle of secondary arc because at this stage the arc
elongates and the arc resistance becomes inconsistent.
A closer view of the conductance of the primary arc is
given in Figure 8.
Fig. 8 Instantaneous conductance of fault Arc
As would be expected, the conductance of the low
voltage heavy current primary arc is relatively high, and
the conductance of the high voltage low current
secondary arc is much lower.
The resistance of the arc is presented in Figure 9, it can
be seen that the primary arc resistance is very low.
Fig. 9 Instantaneous resistance of fault Arc
C. Distance Protection
To analyze the effects of arc faults on the operation
of the distance relay, a simple power system is selected
in Figure 5. Relay zones are defined as 80% of line 1 as
zone 1, 100% of line 1 and 20% of line 2 as zone 2 and
100% of line 1 and 100% of line 2 and 60% of line 3 as
zone 3.
Figure 10 represents distance protection characteristic
for an electric arc with ro = 0.162Ω/cm for a fault at
different distances in the transmission line: 80km in
zone 1, 110km in zone 2 and 150km in zone 3.
From Figure 10, the results obtained show that despite
the existence of the arc, there is not influence on the
distance protection because the resistance of the arc is
very low, so we can act on the parameters to increase
the resistance of the arc.
Figure 11 represents distance protection characteristic
for an electric arc with ro = 2.162Ω/cm for a fault at different distances in the transmission line: 80 km in
zone 1, 110km in zone 2 and 150km in zone 3.
Fault at 80 km (zone 1) Fault at 110 km (zone 2) Fault at 150 km (zone 3)
Fig. 10 Distance protection characteristic for an electric arc with ro = 0.162 Ω/cm
Fault at 80km (zone 1) Fault at 110 km (zone 2) Fault at 150 km (zone 3)
Fig. 11 Distance protection characteristic for an electric arc with ro = 2.162 Ω/cm
From Figure 11 we see that the resistance of the arc
affect the distance protection. In this case we can apply
the compensation method to correct the protection.
D. Compensation Method to Correct the Protection
In Figure 12 we represent the results of the
compensation method used in paragraph V for an
electric arc with ro = 2.162Ω/cm for a fault at different
distances in the transmission line: 80 km in zone 1, 110
km in zone 2 and 150 km in zone 3.
This method makes the distance relay more selective
and instantaneous. As shown in Figure 11, we can see
that if an arc fault occurs at the end of the first zone or
near this one, the relay cannot make a correct decision,
and the fault is seen in the second zone. This means, for
example, that an arc fault occurring at 80km, which is in
zone 1, is seen by the relay in the second zone and also
with a time delay, because zone 2 is not instantaneous.
Whereas after the use of the compensation technique, it
can be seen that the relay can select the fault zone (zone
1) as shown in Figure 12. The same procedure is used
for faults in zone 2 and faults in zone 3
Fault at 80 km (zone 1) Fault at 110 km (zone 2) Fault at 150 km (zone 3)
Fig. 12 Compensation method results on distance protection for an electric arc with ro = 2.162 Ω/cm.
VII.CONCLUSION
In this paper, we have simulated a fault arc that can
appear in the transmission line. We have verified that
the variation of arc resistance is nonlinear and the
distance protection is influenced by the high resistance
of the arc fault. The operation of distance relay in arcing
fault situation is analyzed. It shows that if an arcing
fault occurs at the end of each zone, for example zone 1,
the distance relay will mistake and the distance relay see
the impedance fault in the second zone, so the relay
cannot operate in a selective and instantaneous manner.
We have used a compensation method to correct the
distance protection. This method makes the distance
relay more selective and instantaneous.
REFERENCES
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