Protection Of Transmission Line Using GPS

 INDEX

 1.ABSTRACT……………………………………………………………….5          

 

2.INTRODUCTION…………………………………………………………6

 

3.TRANSMISSION SYSTEM…...…………………………………………8

 

4.PROTECTION OF TRANSMISSION SYSTEM ………………………..9

 

5.TRAVELING WAVE FAULT LOCATION……………...……………..10 

 

6.BENEFITS OF TRAVELING WAVE FAULT LOCATION……………11

           

7.TRAVELING WAVE FAULT LOCATION THEORY .……...………..12

 

8.POSSIBLE CAUSES OF FAULT……………………………………….14          

 

9.WHAT IS GPS?………………………………………………………….15

 

10.HOW IT WORKS?……………………………………………………..17

 

11.THE GPS SATELLITE SYSTEM..........................................................19

 

12.IMPLEMENTATION AND TESTING………………………………..20

 

13.WHAT’S THE SIGNAL?.......................................................................23

 

14.HOW ACCURATE IS GPS?..................................................................24

 

15.SOURCES OF GPS SIGNAL ERRORS……………………………….25

 

16.CONCLUSION…………………………………………………………27

 

17.REFERENCES…………………………………………………………28

 

CHAPTER 1

 ABSTRACT

This 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.

At 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.

 

CHAPTER 2

INTRODUCTION

 

Accurate 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.

To 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.

The 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 location information.

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.

   

CHAPTER 3

TRANSMISSION SYSTEM  

 

GENERATION        TRANSMISSION            DISTRIBUTION

Electric 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.

 

CHAPTER 4

TRANSMISSION LINE PROTECTION

   

 

 

 

 

 

 

 

CHAPTER 5

WHAT 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 operator needs.

   

CHAPTER 6

BENEFITS 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.  

           

 

CHAPTER 7

TRAVELING 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 microsecond.

Precisely synchronized 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.

 The voltage and current at any point x obey the partial differential

Equations

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 wave’s 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. and b represent the travel time from the fault to the

discontinuity.

 With GPS technology, and b can be determined very precisely.

By knowing the length (l) of the line and the time of arrival difference

(– b), one can calculate the distance (x) to the fault from

substation A by: 

    where c= the wave propagation of 299.79 m/sec (1ft/ns) .

 

   

 

   

 

CHAPTER 8

POSSIBLE CAUSES OF FAULT

 

 

 

 

 

 

   

 

 

 

   

CHAPTER 9

WHAT IS GPS?

 

The 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

consumer costs.

 

CHAPTER 10

HOW IT WORKS?

 

GPS 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.

 

CHAPTER 11

THE GPS SATELLITE SYSTEM

The 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.

Here are some other interesting facts about the GPS satellites (also called

NAVSTAR, the official U.S. Department of Defense name for GPS):

The first GPS satellite was launched in 1978.

A full constellation of 24 satellites was achieved in 1994.

Each satellite is built to last about 10 years. Replacements are

constantly being built and launched into orbit.

A GPS satellite weighs approximately 2,000 pounds and is about 17

feet across with the solar panels extended.

Transmitter power is only 50 watts or less.

 

 

CHAPTER 12

IMPLEMENTATION AND TESTING

 

Evaluation 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 microsecond.

 

DISTORTION AND ATTENUATION

OF TRAVELING WAVES

 

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 substations.

 

Fault Locator System Test

Calculated cumulative arc length from NIC substation to the fault = 13 1,694.5 meters.

  Fault Locator        Difference

               Output          from Est. Value

   Test            (meters)       (meters)

 

 

Fault Locator Response to Traveling Waves Generated by Routine Switching of Substation Equipment

Line Estimated Tp    Measured Tp

                 

 

 

The distance to the fault from the line terminals is given by:

Where Vp is the velocity of propagation for the line and

Denotes stations with travelling wavedetector installations

Figure 2.0 Fault Locator Lnstallations and Testing

CHAPTER 13

WHAT’S THE SIGNAL?

GPS 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.

 

 

 

CHAPTER 14

HOW ACCURATE IS GPS?

Today's 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 with WAAS (Wide Area 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 their GPS.

 

   

CHAPTER 15

SOURCES OF GPS SIGNAL ERRORS

 

Factors that can degrade the GPS signal and thus affect accuracy include

the following:

Ionosphere 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.

Signal multipath — This 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.

Receiver 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.

Receiver clock errors — A receiver's built-in clock is not as accurate as

the atomic clocks onboard the GPS satellites.

Number 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.

Satellite 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.

 

Intentional 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.

 

CHAPTER 16

CONCLUSION

 

Thus the use of GPS in protection of transmission systems is beneficial with respect to  

Value regarding programmatic goals:more reliable monitoring using GPS related technologies.  

Technical merit: new fault location algorithm based on new input data.  

Emphasis on transfer of technology: CCET partnership aimed at commercialization.  

Overall performance: on time, with all goals met so far.