PMU for Detection of Short-circuit Point in the Transmission Line
K. V. Suslov, V. S. Stepanov and N. N. Solonina
Irkutsk State Technical University, Irkutsk, Russia
Keywords: SmartGrid, Phasor Measurement Units, Short-circuit Point, Transmission Lines.
Abstract: One of the challenges facing energy systems is failure of overhead and cable transmission lines. Short
circuits pose a particular danger. The improvement of the quality of installation, and reliability of insulators
and conducting materials decreases the probability of short circuits but does not eliminate it. Short circuits
result in disconnection of certain companies and even regions. The suggested method is based on the fact
that the short circuit current at the source end of the line depends on the distance to the point of short circuit.
The paper considers the following issues: theoretical possibility of detecting the point of fault on the basis of
time when the short circuit response arrives at the source and load ends of the line; development of an
algorithm for primary data processing, development of a structural scheme of additional devices which are
not envisaged within phasor measurement units.
1 INTRODUCTION
Electric power industry is one of the most dynamic
sectors which employs the advances of fundamental
and applied sciences. At this very stage special
attention is paid to the global system of universal
time. Even two decades ago the universal time was
the province of special organizations dealing with
space, industry, defence, astrophysics, etc. The
adoption of the Global positioning system has made
it possible to receive the standard time signals at an
individual substation and use this precise time for
solving a wide range of problems. Based on these
technologies it has become possible to develop and
adopt intelligent electric systems, Smart Grids, and
phasor measurement units (PMU).
A serious problem facing energy systems is
failure of overhead and cable transmission lines.
Short circuits represent a particular danger. The
probability of short circuits decreases but does not
disappear with an increase in the quality of
installation, reliability of insulators and conducting
materials. Short circuits lead to disconnection of
individual consumers and the entire regions. This
imposes high requirements on reliability and fast
operation of relay protection. However, if a short
circuit has occurred and relay protection has
successfully operated there remains the task to
promptly and where possible accurately detect
coordinates of the short circuit. This, in the end
allows us to quickly restore the transmission line and
place it into operation which in turn will minimize
economic losses.
There can be two types of research in the electric
power industry: active and passive experiments. An
active experiment can be exemplified by probing the
faulty transmission line with the help of short pulses
of current and assessing the response time. This time
depends on the distance between the source end of
the line and a short circuit point. A disadvantage of
active experiments is the necessity to apply special
equipment and the time to get prepared to the
experiment.
There is a great variety of methods for detection
of overhead and cable line fault locations. Let us
enumeratethem in brief.
The pulse method is based on measuring time
intervals between the moment of transmitting a
probe pulse ofalternating current and the moment of
receiving a reflected pulse from the fault location.
To make measurements by themethod of oscillation
discharge the voltage supplied to the faulted cable
conductor is gradually raised to the voltage of cable
fault. The loop method is based on measuring
resistances by the direct current bridge. The
capacitance method suggests measuring capacitance
of a broken conductor by measuring bridges. The
acoustic method supposes creation of a spark
discharge at the faultlocation and listening to sound
vibrations that occur above the fault point. There is
63
Suslov K., Stepanov V. and Solonina N..
PMU for Detection of Short-circuit Point in the Transmission Line.
DOI: 10.5220/0004796500630067
In Proceedings of the 3rd International Conference on Smart Grids and Green IT Systems (SMARTGREENS-2014), pages 63-67
ISBN: 978-989-758-025-3
Copyright
c
2014 SCITEPRESS (Science and Technology Publications, Lda.)
also the inductionmethod and others.
Passive experiment however makes it possible to
use modern high speed digital technologies, and
obtain data (instant values of current and voltage,
voltage and current phases) directly from the signals
associated with the processes that occur in
transmission lines after a short circuit, i.e. in real
time. One of the possible solutions to this problem is
related to the dependence of currents at the source
end of a line on the distance to the short circuit point
(Gilany, 2005, Hajjar, 2004, Ferreira, 2012,
Chengjiang Wang, 2011, Provoost 2005, Suslov
2012). However, there are reasons that decrease the
accuracy of the fault point detection, namely:
variations in the effective values of voltage at the
transmission line connection point, as well as the
dependence of current amplitude on voltage phase at
the time of short circuit. The authors suggest the use
of time factors related to the final velocity (v
F
) of
power (electric signal) transmission along the
transmission line. It is obvious that there will be a
response (echo) spreading in both directions along
the line, that will have the form of a front of
increasing or declining voltage or current and the
time of the response arrival at the source end or load
end of the line can be recorded with a high accuracy.
The paper focuses on the following issues:
- Consideration of the theoretical possibility of
determining the fault place on the basis of time when
responses come to the source end and load end of
the line;
- Determination of the possibility of using the
available infrastructure of PMU to determine the
above time instants and transfer the data to the
processing center;
- Development of an algorithm for primary
data processing;
Development of a block diagram of additional
devices which are not envisaged within PMU.
2 THE MAIN PRINCIPLES OF
THE APPROACH
We will consider the idea of the suggested method
on the example of a transmission line without
branches with one-way supply.
Figure 1 shows the calculation scheme for the
determination of the short circuit place, taking into
consideration the time of signal arrival at the source
and load ends of the line.
Let the short circuit occur at time t
sc
at point K
(Fig.1) and a transient process start. For the sake of
simplification we make the following assumptions:
- voltage at point К drops to zero;
- length of the considered line is much shorter
than the length of the incident current wave λ
(λ≈5000 km at the frequency of 50 Hz).
We can also assume that the instant values of
current and voltage in the steady state are constant
along the whole line.
Figure 1: Design scheme of a short circuit in a line.
In Figure 1: L-length of the line; l
1
, l
2
– distances
from the short circuit point to the source and load
ends of the line, respectively; i
1,
i
2
, u
1
, u
2
– current
and voltage of the first and second sections of the
line, respectively.
Figure 2 presents a model of the line with
lumped parameters for the calculation of transient
process in the line with distributed parameters.
Figure 2: Model of the line for calculation of transient
process in the line
In Figure 2:
- wave impedance of the line;
- incident current wave of the first section, i.e. from
the source end to the short circuit point;
–
reflected current wave of the first section;
–
incident voltage wave of the first section;
–
reflected voltage wave of the first section;
-
incident current wave of the second section, i.e. from
the short circuit point to the load end of the line;
– reflected current wave of the second section;
– incident voltage wave of the second section;
– reflected voltage wave of the second section.
It is easy to see from the model that at time t
sc
:
and
˗
SMARTGREENS2014-3rdInternationalConferenceonSmartGridsandGreenITSystems
64
Thus, a positive current wave front and a negative
voltage wave front travel at velocity v
F
in the
direction from point К to the source end of the line.
The fronts can be detected with the aid of current
and voltage sensors. Time t
1
of their arrival at the
source end of the line can be recorded with high
accuracy thanks to the clock showing the time
synchronized with the universal time.
At the second section of the line there is also a
transient process. At time t
sc
current i
2
and voltage
u
2
at the beginning of the second section vanish
which means that
0 ,
˗
0 ,
˗
Hence, from point К negative fronts of current
and voltage travel to the load end of the line. Time
t
2
, when these fronts arrive at the load end of the line
can be recorded with the aid of current and voltage
sensors as well as with high precision clock.
Let us show that knowing t
1
and t
2
, we can detect
the place of short circuit. To this end we will
consider a geometric model of the short-circuited
transmission line. The technique of identifying the
short circuit place is explained in Figure 3.
Figure 3: A scheme of devising an algorithm for
determining the place of point K.
In Figure 3: (1), (2) are the sites at which the
chronometers (t) and primary current (i) and voltage
(u) sensors are installed; А – geometric center of the
line; В – modules for processing the data from
current and voltage sensors; ∆– distance from the
center of Line A to the short circuit point К.
Using this scheme we determine ∆
- the time
of wave propagation from the short circuit point to
the source end of the line:
∆
ф
In the same way we find ∆
- the time of wave
propagation from the short circuit point to the load
end of the line:
∆
ф
Express time t
1
of the signal (response) arrival at
the source end of the line through the short circuit
time:
Similarly find time t
2
of the response arrival at
the load end of the line:
Determine the difference between the time of
response arrival at the source end of the line and the
time of response arrival at its load end:
–
–
ф
–
ф
–
2∆
ф
and finally determine
–
–
ф
2
(1)
Knowing
, we find
and
by the equations
2
–
,
(2)
2
–
(3)
If point К is closer to the source end of the line
(to the left of point A), then
>0. If point К is
closer to the load end of the line, then
<0.
When the time aspect is taken into consideration
the issue of accurate determination of the time
instants t
1
and t
2
is particularly important.
We suggest using the available infrastructure of
phasor measurement units (Fig. 4).
Phasor Measurement Units (PMU) make it
possible to take phasor measurements of currents
and voltages at the given points of the power system.
Phasor measurement implies simultaneous
measurement of both the effective value and the
phase of current and voltage. These parameters
allow us to calculate current values of transmitted
power, voltage drops in the sections of the
transmission line, power loss in the transmission
line, etc. In fact, the measurement of effective values
and phases of current and voltage is not a new
PMUforDetectionofShort-circuitPointintheTransmissionLine
65
problem. However, such measurements, although
possible, have not become widespread, since the
control of the system under dynamic operating
conditions requires that the data be definitely
connected with the universal time. For instance, in
order to determine losses in the line we should
simultaneously measure active power at the source
and load ends of the line precisely at the same time.
Figure 4: PMU infrastructure. (where SS1- feeding
substation; SS2- receiving substation; L– length of the
line; АС- atomic clock; GPS1, GPS2 – satellites sending
time signals; SC – control center; SS1, SS2 – network
substations;
– time pulse of the atomic clock; h –
height of the satellite above the Earth in the area, where
the substations are located; Q
1
, Q
2
– angles at which the
satellite is seen from SS1 and SS2, respectively.
The time measurement resolution of PMU is not
sufficient to accurately determine the time of the
event. Therefore, at the measurement points we
should form our own time (count) pulses with a
short time interval of, for example, 10
-9
seconds,
using additional devices.
Consider the use of the PMU and additional
devices for accurate determination of time t
1
and t
2
.
These pulses are formed by the pulse generators
installed in modules В
1
and В
2
, and received at the
input of the pulse counters located in modules .
The pulses are generated with the same frequency.
At the outputs of modules we obtain t
1
and
t
2
,
respectively, in the following form:
,
(4)
,
(5)
where t
gps
– a synchronizing pulse from GPS
satellite, that contains complete information about the
universal time, namely: year, month, day, hour,
minute, second, milliseconds;
n, m – number of count pulses from the arrival of
t
gps
to the moment, when the response to the short
circuit is received at the source end and load end of
the line, respectively.
We determine the difference between the time
the responses arrive at the source end of the
transmission line and the time they arrive at its load
end, using expressions (4) and (5):
Knowing t
1
and t
2
, we find
,
,
according to
expressions (1), (2), (3).
3 CONCLUSIONS
The proposed method is very promising, since it
mainly uses PMU devices, and the costs related to
the development and use of additional devices are
insufficient.
This method allows to determine the coordinates
of the damage in real time. The error is not more
than 50 meters. This figure will be adjusted to the
experiments conducted.
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