Of Transmission Line Using GPS
Protection Of Transmission Line Using GPS
OF TRANSMISSION SYSTEM
WAVE FAULT LOCATION
OF TRAVELING WAVE FAULT LOCATION
WAVE FAULT LOCATION THEORY .
CAUSES OF FAULT
OF GPS SIGNAL ERRORS
is a new technique for the protection of transmission systems by
using the global positioning system (GPS) and fault generated
transients. In this
scheme the relay contains a fault transient detection system
together with a communication unit, which is connected to the
power line through the high voltage coupling capacitors of the
CVT. Relays are installed at each bus bar
in a transmission network.
These detect the fault generated
high frequency voltage
transient signals and record the time instant corresponding to
when the initial traveling wave generated by the fault arrives
at the busbar.
The decision to trip is based on the components as they
propagate through the system. extensive simulation studies of
the technique were carried out to examine the response to
different power system and fault condition.
The communication unit is used to transmit and receive
coded digital signals of the local information to and from
associated relays in the system.
each substation relay
determine the location of the fault by comparing the GPS time
stay measured locally with those received from the
adjacent substations, extensive simulation studies
presented here demonstrate feasibility of the scheme.
location of faults on power transmission systems can save time
and resources for the electric utility industry. Line searches
for faults are costly and can be inconclusive. Accurate
information needs to be acquired quickly in a form most useful
to the power system operator communicating to field personnel.
achieve this accuracy, a complete system of fault location
technology, hardware, communications, and software systems can
be designed. Technology is available which can help determine
fault location to within a transmission span of 300 meters.
Reliable self monitoring hardware can be configured for
installation sites with varying geographic and environmental
conditions. Communications systems can retrieve fault location
information from substations and quickly provide that
information to system operators. Other communication systems,
such as Supervisory Control and Data Acquisition (SCADA),
operate fault sectionalizing circuit breakers and switches
remotely and provide a means of fast restoration. Data from
SCADA, such as sequence of events, relays, and oscillographs,
can be used for fault location selection and verification.
Software in a central computer can collect fault information and
reduce operator response time by providing only the concise
information required for field personnel communications. Fault
location systems usually determine distance to fault from
a transmission line end. Field personnel can use this data to
find fault locations from transmission line maps and drawings.
Some utilities have automated this process by placing the
information in a fault location Geographical Information System
(GIS) computer. Since adding transmission line data to the
computer can be a large effort, some utilities have further
shortened the process by utilizing a transmission structures
location database. Several utilities have recently created these
databases for transmission inventory using GPS location
technology and handheld computers.
inventory database probably contains more information than
needed for a fault location system, and a reduced version would
save the large data-collection effort. Using this data, the
power system operator could provide field personnel direct
Field personnel could
use online information to help them avoid spending valuable time
looking for maps and drawings and possibly even reduce their
travel time. With precise information available, crews can
prepare for the geography, climatic conditions, and means of
transport to the faulted location. Repair time and resources
would be optimized by the collected data before departure.
Accurate fault location can also aid in fast restoration of
power, particularly on transmission lines with distributed
loads. Power system operators can identify and isolate faulted
sections on taploaded lines and remove them by opening circuit
breakers or switches remotely along the line, restoring power to
the tap loads serviced by the unfaulted transmission sections.
power transmission, a process in the delivery of electricity to
consumers, is the bulk transfer of electrical power. Typically,
power transmission is between the power plant and a substation
near a populated area.Electricity distribution is the delivery
from the substation to the consumers.Electric power transmission
allows distant energy sources (such as hydroelectric power
plants) to be connected to consumers in population centers, and
may allow exploitation of low-grade fuel resources that would
otherwise be too costly to transport to generating facilities.
Due to the large amount of power involved, transmission normally
takes place at high voltage (110 kV or above). Electricity is
usually transmitted over long distance through overhead power
transmission lines. Underground power transmission is used only
in densely populated areas due to its high cost of installation
and maintenance, and because the high reactive power produces
large charging currents and difficulties in voltage management.A
power transmission system is sometimes referred to colloquially
as a "grid"; however, for reasons of economy, the
network is not a mathematical grid.Redundant paths and lines are
provided so that power can be routed from any power plant to any
load center, through a variety of routes, based on the economics
of the transmission path and the cost of power. Much analysis is
done by transmission companies to determine the maximum reliable
capacity of each line, which, due to system stability
considerations, may be less than the physical or thermal limit
of the line.
IS TRAVELING WAVE FAULT LOCATION?
Faults on the power transmission system
cause transients that propagate
along the transmission line as waves. Each wave is a composite of frequencies, ranging from a few
kilohertz to several megahertz, having a fast rising front and a
slower decaying tail. Composite waves have a propagation
velocity and characteristic impedance and travel near the speed
of light away from the fault location toward line ends. They
continue to travel throughout the power system until they
diminish due to impedance and reflection waves and a new power
system equilibrium is reached. The location of faults is
accomplished by precisely time-tagging wave fronts as they cross
a known point typically in substations at line ends. With waves
time tagged to sub microsecond resolution of 30 m, fault
location accuracy of 300 m can be obtained. Fault location can
then be obtained by multiplying the wave velocity by the time
difference in line ends. This collection and calculation of time
data is usually done at a master station. Master station
information polling time should be fast enough for system
OF TRAVELING WAVE FAULT LOCATION
Early fault locators used pulsed radar.
This technique uses reflected radar energy to determine the
fault location. Radar equipment is typically mobile or located
at substations and requires manual operation. This technique is
popular for location of permanent faults on cable sections when
the cable is de-energized. Impedance-based fault locators are a
popular means of transmission line fault locating. They provide
algorithm advances that correct for fault resistance and load
current inaccuracies. Line length accuracies of ±5% are typical
for single-ended locators and 1-2% for two-ended locator
systems. Traveling wave fault locators are becoming popular
where higher accuracy is important. Long lines, difficult
accessibility lines, high voltage direct current (HVDC), and
series-compensated lines are popular applications. Accuracies of
<300 meters have been achieved on 500 kV transmission lines
with this technique. Hewlett-Packard has developed a GPS-based
sub microsecond timing system that has proven reliable in
several utility traveling wave projects. This low-cost system
can also be used as the substation master clock.
WAVE FAULT LOCATION THEORY
Traveling wave fault
locators make use of the transient signals generated by the
fault. When a line fault occurs, such as an insulator flashover
or fallen conductor, the abrupt change in voltage at the point
of the fault generates a high
frequency electromagnetic impulse called the traveling wave
which propagates along the line in both directions away from the
fault point at speeds close to that of light. The
fault location is determined by accurately time-tagging
the arrival of the traveling wave at each end of the line and
comparing the time difference to the total propagation time of
the line. Refer to Figure 1.0
Unlike impedance-based fault
location systems, the traveling wave fault locator is unaffected
by load conditions, high ground resistance and most notably,
series capacitor banks. This fault locating technique relies on
precisely synchronized clocks at the line terminals which can
accurately time-tag the arrival of the traveling wave. The
propagation velocity of the traveling wave is roughly 300 meters
per microsecond which in turn requires the clocks to be
synchronized with respect to each other by less than one
clocks are the key element in the implementation of this fault
location technique. The required level of clock accuracy has
only recently been available at reasonable cost with the
introduction of the Global Positioning System.
voltage and current at any point x obey the partial differential
where L and C are the inductance and
capacitance of the line per unit
length. The resistance is assumed to be
negligible. The solutions of these equations are
these equations are
where Z = (L/C ) is the characteristic
impedance of the transmission line and v=1/(LC) is the velocity
of propagation. Forward (ef and if) and reverse (er and ir)
waves, as shown in Figure 1, leave the disturbed area x
traveling in different directions at v, which is a little less than the speed of light, toward transmission line ends.
Transmission line ends represent a discontinuity or
impedance change where
some of the waves energy will reflect back to the disturbance.
The remaining energy will travel to other power system elements
or transmission lines. Figure 2, a Bewley lattice diagram,
illustrates the multiple waves
(represented by subscripts 2 and 3) generated at line ends. Wave
amplitudes are represented by reflection coefficients
ka and kb which are determined by characteristic impedance
ratios at the discontinuities. ta and tb represent the travel time
from the fault to the
With GPS technology,
ta and tb can be determined very precisely.
By knowing the length (l)
of the line and the time of arrival difference
one can calculate the distance (x) to the fault from
where c= the wave propagation of 299.79 mm/msec
CAUSES OF FAULT
Global Positioning System (GPS) is a satellite-based navigation
system made up of a network of 24 satellites placed into orbit.
GPS was originally intended for military applications, but in
the 1980s, the government made the system available for civilian
use. GPS works in any weather conditions, anywhere in the world,
24 hours a day. GPS Technology allows precise determination of
location, velocity, direction, and time. GPS are space-based
radio positioning systems that provide time and
three-dimensional position and velocity information to suitably
equipped users anywhere on or near the surface of the earth (and
sometimes off the earth).
Concept of satellite navigation
was first conceived after the launch of Sputnik 1 in 1957 when
scientists realized that by measuring the frequency shifts in
the small bleeps emanating from this first space vehicle it was
possible to locate a point on the earth's surface. The
NAVSTAR system, operated by the US Department of Defense, is the
first such system widely available to civilian users. The
Russian system, GLONASS, is similar in operation and may prove
complimentary to the NAVSTAR system. Current GPS systems enable
users to determine their three dimensional differential
position, velocity and time. By
combining GPS with current and future computer mapping
techniques, we will be better able to identify and manage our
natural resources. Intelligent vehicle location and navigation
systems will let us avoid congested freeways and more efficient
routes to our destinations, saving millions of dollars in
gasoline and tons of air pollution. Travel aboard ships and
aircraft will be safer in all weather conditions. Businesses
with large amounts of outside plant (railroads, utilities) will
be able to manage their resources more efficiently, reducing
satellites circle the earth twice a day in a very precise orbit
and transmit signal information to earth. GPS receivers take
this information and use triangulation to calculate the user's
exact location. Essentially, the GPS receiver compares the time
a signal was transmitted by a satellite with the time it was
received. The time difference tells the GPS receiver how far
away the satellite is. Now, with distance measurements from a
few more satellites, the receiver can determine the user's
position and display it on the unit's electronic map. By
knowing the distance from another satellite, the possible
positions of the location are narrowed down to two points (Two
intersecting circles have two points in common). A GPS receiver
must be locked on to the signal of at least three satellites to
calculate a 2D position (latitude and longitude) and track
movement. With four or more satellites in view, the receiver can
determine the user's 3D position (latitude, longitude and
altitude). Once the user's position has been determined, the GPS
unit can calculate other information, such as speed, bearing,
track, trip distance, distance to destination, sunrise and
sunset time and more. Accurate 3-D measurements require four satellites. To achieve 3-D real time
measurements, the receivers need at least four channels.
GPS SATELLITE SYSTEM
24 satellites that make up the GPS space segment are orbiting
the earth about 12,000 miles above us. They are constantly
moving, making two complete orbits in less than 24 hours. These
satellites are traveling at speeds of roughly 7,000 miles an
hour. GPS satellites are powered by solar energy. They have
backup batteries onboard to keep them running in the event of a
solar eclipse, when there's no solar power. Small rocket
boosters on each satellite keep them flying in the correct path.
are some other interesting facts about the GPS satellites (also
called NAVSTAR, the official U.S. Department of Defense name for
first GPS satellite was launched in 1978.
full constellation of 24 satellites was achieved in 1994.
satellite is built to last about 10 years. Replacements are
constantly being built and launched into orbit.
GPS satellite weighs approximately 2,000 pounds and is about
17 feet across with the solar panels extended.
power is only 50 watts or less.
of the fault locator involved the installation of GPS timing
receivers at four 500kV substations, see Figure 2.0. A
especially developed Fault Transient Interface Unit (FTIU)
connects to the transmission lines and discriminates for a valid
traveling wave. The FTIU produces a TTL-level trigger pulse that
is coincident with the leading edge of the traveling wave. A
time-tagging input function was provided under special request
to the GPS receiver manufacturer. This input accepts the TTL level logic pulse from the FTIU and time tags
the arrival of the fault-generated traveling wave. The time tag
function is accurate to within 300 nanoseconds of UTC - well
within the overall performance requirement of timing to within 1
The accuracy of fault
location depends on the ability to accurately time tagging the
arrival of the traveling wave at each line terminal. The
traveling wave once generated, is subject to attenuation and
distortion as it propagates along the transmission line.
Attenuation occurs due to resistive and radiated losses.
Distortion of the waveform occurs due to a
variety of factors including bandwidth limitations of the
transmission line, dispersion from different propagation
constants of phase-to-phase and phase-to-ground components, etc.
These effects combine to degrade the quality of
the "leading edge"
of he traveling wave at large distances from the fault
inception point. The accuracy of time tagging the traveling wave
diminishes for the substations far away from the fault.
Experience with the evaluation system has shown that the
traveling wave is relatively "undistorted" for
distances less than 350 km. To effectively reduce the effects of
attenuation and distortion requires traveling wave detector
installations spaced at regular intervals. For B.C. Hydro, this
translates to installing fault location equipment at fourteen
out of nineteen 500 kV
Locator System Test
cumulative arc length from NIC substation to the fault = 13
from Est. Value
Locator Response to Traveling Waves Generated by Routine
Switching of Substation Equipment
Estimated Tp Measured
distance to the fault from the line terminals is given
Vp is the velocity of propagation for the line and
stations with travelling wavedetector installations
2.0 Fault Locator Lnstallations and Testing
satellites transmit two low power radio signals, designated L1
and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the
UHF band. The signals travel by line of sight, meaning they will
pass through clouds, glass and plastic but will not go through
most solid objects such as buildings and mountains. A GPS signal
contains three different bits of information a pseudorandom
code, ephemeris data and almanac data. The pseudorandom code is
simply an I.D. code that identifies which satellite is
transmitting information. You can view this number on your GPS
unit's satellite page, as it identifies which satellites it's
receiving. Ephemeris data tells the GPS receiver where each GPS
satellite should be at any time throughout the day. Each
satellite transmits ephemeris data showing the orbital
information for that satellite and for every other satellite in
the system. Almanac data, which is constantly transmitted by
each satellite, contains important information about the status
of the satellite (healthy or unhealthy), current date and time.
This part of the signal is essential for determining a position.
ACCURATE IS GPS?
GPS receivers are extremely accurate, thanks to their parallel
multi-channel design. 12 parallel channel receivers are quick to
lock onto satellites when first turned on and they maintain
strong locks, even in dense foliage or urban settings with tall
buildings. Certain atmospheric factors and other sources of
error can affect the accuracy of GPS receivers. GPS receivers
are accurate to within 15 meters on average. Newer GPS receivers
Augmentation System) capability can improve accuracy to less
than three meters on average. No additional equipment or fees
are required to take advantage of WAAS. Users can also get
better accuracy with Differential GPS (DGPS), which corrects GPS
signals to within an average of three to five meters. The U.S.
Coast Guard operates the most common DGPS correction service.
This system consists of a network of towers that receive GPS
signals and transmit a corrected signal by beacon transmitters.
In order to get the corrected signal, users must have a
differential beacon receiver and beacon antenna in addition to
OF GPS SIGNAL ERRORS
that can degrade the GPS signal and thus affect accuracy include
and troposphere delays
The satellite signal slows as it passes through the
atmosphere. The GPS system uses a built-in model that
calculates an average amount of delay to partially correct
for this type of error.
occurs when the GPS signal is reflected off objects such as
tall buildings or large rock surfaces before it reaches the
receiver. This increases the travel time of the signal,
thereby causing errors.
clock errors A
receiver's built-in clock is not as accurate as the atomic
clocks onboard the GPS satellites. Therefore, it may have
very slight timing errors.
clock errors A
receiver's built-in clock is not as accurate as the atomic
clocks onboard the GPS satellites.
of satellites visible The more satellites a GPS receiver can "see," the
better the accuracy. Buildings, terrain, electronic
interference, or sometimes even dense foliage can block
signal reception, causing position errors or possibly no
position reading at all. GPS units typically will not work
indoors, underwater or underground.
geometry/shading This refers to the relative position of the satellites at any
given time. Ideal satellite geometry exists when the
satellites are located at wide angles relative to each
other. Poor geometry results when the satellites are located
in a line or in a tight grouping.
degradation of the satellite signal
Selective Availability (SA) is an intentional
degradation of the signal once imposed by the U.S.
Department of Defense. SA was intended to prevent military
adversaries from using the highly accurate GPS signals. The
government turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.
the use of GPS in protection of transmission systems is
beneficial with respect to
regarding programmatic goals:more
reliable monitoring using GPS related technologies.
new fault location algorithm based on new input data.
on transfer of technology:
CCET partnership aimed at commercialization.
performance: on time,
with all goals met so far.