Copy of Global Positioning Systems

8/8/2019 Copy of Global Positioning Systems

http://slidepdf.com/reader/full/copy-of-global-positioning-systems 1/48

B.Tech,EEE-R07 SBIT,Khammam

Page 1

1.0 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 sof tware systems can be

designed. Technology is available which can help to determine fault

location to within a given transmission span. 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.




Page 2

Fault location systems usually determine ´distance to faultµ 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.

The 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.




Page 3

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 tap loaded lines and

remove them by opening circuit breakers or switches remotely along the

line, restoring power to the taploads serviced by the unfaulted

transmission sections.




Page 4

2. 0 TRANSMISSION SYSTEM

2.1 GENERATION ² TRANSMISSION ² DISTRIBUTION:

Fig.1: A typical transmission ² distribution system

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.




Page 5

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 ge nerating 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 referre d 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 o f the line.




Page 6

2.2 TRANSMISSION LINE PROTECTION

Transmission lines are a vital part of the electrical distribution

system, as they provide the path to transfer power between generation

and load. Transmission lines operate at voltage levels from 69kV to

765kV, and are ideally tightly interconnected for reliable operation.

Factors like de-regulated market environment, economics, right of-way

clearance and environmental requirements have pushed utilities to

operate transmission lines close to their operating limits. Any fault, if not

detected and isolated quickly will cascade into a system wide disturbance

causing widespread outages for a tightly interconnected system operating

close to its limits. Transmission protection systems are designed to

identify the location of faults and isolate only the faulted section. The key

challenge to the transmission line protection lies in reliably detecting and

isolating faults compromising the security of the system.






Page 8

The more detailed factors for transmission line protection directly

address dependability and security for a specific application. The

protection system selected should provide redundancy to limit the impact

of device failure, and backup protection to ensure dependability.

Reclosing may be applied to keep the line in service for temporary faults,

such as lightning strikes. The maximum load current level will impact the

sensitivity of protection functions, and may require adjustment to

protection functions settings during certain operating circumstances.

Single-pole tripping applications impact the performance requirements of

distance elements, differential elements, and communications schemes.

The physical construction of the transmission line is also a factor in

protection system application. The type of conductor, the size of

conductor, and spacing of conductors determines the impedance of the

line, and the physical response to short circuit conditions, as well as line

charging current. In addition, the number of line terminals determines

load and fault current flow, which must be accounted for by the

protection system. Parallel lines also impact relaying, as mutual coupling

influences the ground current measured by protective relays. The

presence of tapped transformers on a line, or reactive compensation

devices such as series capacitor banks or shunt reactors, also influences

the choice of protection system, and the actual protection device settings.




Page 9

2.4 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 wave s 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.

Fig.3: Fault location in transmission line




Page

10

2.5 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 acc uracies

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.




Page

11

2.6 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.

Position is Based on Time

T + 3

Distance between satellite

and receiver = ³3 times the

speed of light´

T

Signal leaves satelliteat time ³T´

Signal is picked up by the

receiver at time ³T + 3´




Page

12

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 .




Page

13

The solutions of 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, leave the disturbed

area ´xµ traveling in different directions at ´vµ, 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. Fig.4, 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 the characteristic- impedance ratios at the discontinuities.

a and b represent the travel time from the fault to the discontinuity.

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

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

b ), one can calculate the distance (x) to the fault from substation A by:




Page

14

where c= the wave propagation of 299.79 m/sec .

Fig.4: Bewley Lattice Diagram




Page

15

3.0 GLOBAL POSITIONING SYSTEM

3.1 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




Page

16

resources. Intelligent fault location systems will provide efficient routes to

our destinations, saving millions of dollars . Businesses with large

amounts of outside plant (railroads, utilities) will be able to manage their

resources more efficiently, reducing consumer costs.

Therefore, GPS systems help to determine the accurate location of

faults on power transmission systems which can save time and resources

for the electric utility industry. Line searches for faults are costly and can

be inconclusive. Accurate information needs can be acquired quickly in a

form most useful to the power system operator communicating to field

personnel.

Fig.5: Constellation of 24 Satellites




Page

17

3.2 WHY SHOULD WE GO FOR GPS ?

7777777777777ffbnkjh.077

y To know the exact fault location.

y To avoid repeated power blackouts.

y As time and frequency enable efficient power transmission and

distribution.

y Very accurate fault detectors which even gives 3-D view.

y Is accurate to within 1micro-second at any point on the earth.

y To enable continuous power supply.

y Cheaper way of fault detection.

3.3 HOW DOES GPS 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




Page

18

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 , which are as

shown follows:




Page

19

SIGNAL FROM ONE SATELLITE:

Signal From One Satellite

The receiver is

somewhere on

this sphere.

Fig.6: SIGNAL FROM ONE SATELLITE




Page

20

SIGNALS FROM TWO SATELLITES:

Signals From Two Satellites

Fig.7: SIGNALS FROM TWO SATELLITES




Page

21

SIGNALS FROM THREE SATELLITES:

Three Satellites (2D Positioning)

Fig.8: SIGNALS FROM THREE SATELLITES




Page

22

SIGNALS FROM FOUR SATELLITES:

Three Dimensional (3D) Positioning

Fig.9: SIGNALS FROM FOUR SATELLITES




Page

23

3.4 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):

1. The first GPS satellite was launched in 1978.

2. A full constellation of 24 satellites was achieved in 1994.

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

constantly being built and launched into orbit.

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

feet across with the solar panels extended.

5. Transmitter power is only 50 watts or less.




Page

24

3.5 WHAT·S THE SIGNAL?

GPS satellites transmit two low power radio signals, designated L1

and L2. 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 ³ a pseudorandom code, ephemeris data and almanac data.

1) PSEUDORANDOM CODE:

The pseudorandom code is simply an I.D. code that identifies which

satellite is transmitting information. We can view this number on your

GPS unit's satellite page, as it identifies which satellites it's receiving.

2) EPHEMERIS DATA:

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.




Page

25

3) ALMANAC DATA:

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.

Fig.10: Signals of transmission of GPS satellites

14




Page

26

All GPS satellites have several atomic clocks. The signal that is

sent out is a random sequence, each part of which is different from every

other, called pseudo-random code. This random sequence is repeated

continuously. All GPS receivers know this sequence and repeat it

internally. Therefore, satellites and the receivers must be in synch. The

receiver picks up the satellite's transmission and compares the incoming

signal to its own internal signal. By co mparing how much the satellite

signal is lagging, the travel time becomes known.

3.6 GPS SIGNAL CHARACTERISTICS

The satellites transmit on two L-band frequencies: L1 =

1575.42 MHz and L2 = 1227.6 MHz. Three pseudo-random noise (PRN)

ranging codes are in use.

y The coarse/acquisition (C/A) code has a 1.023 MHz chip rate,

a period of 1 millisecond (ms) and is used primarily to acquire the P -code.

y The precision (P) code has a 10.23 MHz rate, a period of 7

days and is the principal ranging code.

y The Y-code is used in place of the P-code whenever the anti-

spoofing (A-S) mode of operation is activated.

The C/A code is available on the L1 frequency and the P -code is

available on both L1 and L2. The various satellites all transmit on the

same frequencies, L1 and L2, but with individual code assignments.




Page

27

Due to the spread spectrum characteristic of the signals, the

system provides a large margin of resistance to interference. Each satellite

transmits a message containing its orbital elements, clock behavior,

system time and status messages. In addition, an almanac is also

provided which gives the approximate data for each active satellite. This

allows the user set to find all satellites once the first has been acquired.

3.7 GPS CAPABILITIES

The GPS is a DoD developed, worldwide, satellite-based radio

transmission system that will be the DoD's primary radio transmission

system well into the next century. The constellation consists of 24

operational satellites. The U.S. Air Force Space Command (AFSC) formally

declared the GPS satellite constellation as having met the requirement

for Full Operational Capability (FOC) as of April 27, 1995. Requirements

include 24 operational satellites (Block II/IIA) functioning in their

assigned orbits and successful testing completed for operational

functionality.

Prior to FOC an Initial Operational Capability (IOC) was

declared on December 8, 1993 when 24 GPS satellites (Block-Iand Block-

II/IIA) were operating in their assigned orbits, available for use and

providing the Standard Positioning Service (SPS) levels specified below.




Page

28

GPS provides two levels of service: Standard Positioning

Service and the Precise Positioning Service and are as follows:

1) STANDARD POSITIONING SERVICE :

The Standard Positioning Service (SPS) is a positioning and

timing service which will be available to all GPS users on a continuous,

worldwide basis with no direct charge. SPS will be provided on the GPS L1

frequency which contains a coarse acquisition (C/A) code and a data

message. SPS provides a predictable positioning accuracy of 100 meters

(95 percent) horizontally and 156 meters (95 percent) vertically and time

transfer accuracy to UTC within 340 nanoseconds (95 percent).

2) PRECISE POSITIONING SERVICE:

The Precise Positioning Service (PPS) is a highly accurate

positioning, velocity and timing service which will be available on a

continuous, worldwide basis to users authorized by the U.S. P(Y)

code capable user equipment provides a predictable positioning accuracy

of at least 22 meters (95 percent) horizontally and 27.7 meters vertically

and time transfer accuracy to UTC within 200 nanoseconds (95 percent).

PPS will be the data transmitted on the GPS L1 and L2 frequencies. PPS

was designed primarily for U.S. military use. It will be denied to

unauthorized users by the use of cryptography. PPS will be made

available to U.S. and U.S. Federal Government users. Limited, non-




Page

29

Federal Government, civil use of PPS, both domestic and foreign, will be

considered upon request and authorized on a case -by-case basis,

provided:

It is in the U.S. national interest to do so.

Specific GPS security requirements can be met by the

applicant.

A reasonable alternative to the use of PPS is not available.

SELECTIVE AVAILABILITY, ANTI-SPOOFING:

Selective Availability (SA), the denial of full accuracy, is

accomplished by manipulating navigation message orbit data (epsilon)

and/or satellite clock frequency (dither).

Anti-spoofing (A-S) guards against fake transmissions of

satellite data by encrypting the P-code to form the Y-code.

SA will be implemented on Block II at the SPS levels, as soon as

each Block II satellite is operational.

HAPTER-14CCCCC

3.8 HOW ACCURATE IS GPS?

Today's GPS receivers are extremely accurate, due 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




Page

30

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.




Page

31

4.0 FAULT LOCATION METHODS

4.1 TIME DOMAIN REFLECTOMETERS

Early fault locators were based on pulse radar techniques. These

devices known as time domain reflectometers (TDRs), are commonly used

for locating faults in buried cables where visual inspection is not possible

without excavation. This technique makes use of a "probe" pulse that is

launched into one end of the cable and relies on the pulse being reflected

back to the source due to an electrical discontinuity a t the faulted

location. The fault location is determined by measuring the time delay

from launching the pulse and receiving its reflection. There is, however,

little interest in applying this method to overhead transmission lines.

Several factors make this technique impracticable.

Unlike cables which are physically and electrically uniform

throughout its length, overhead lines have inherent discontinuities such

as tower structures, conductor variations, etc. that cause unwanted

secondary reflections which interfere with detecting the reflected "probe"

pulse.

The faulted line must be taken out of service to conduct testing and

special precautions must be taken to ensure mutual inductance effects

from healthy adjacent lines and nearby energized equipment does not

pose a safety hazard.




Page

32

The fault must be permanent (e.g.. solid short or open

conductors) to ensure a strong reflected signal. Difficult to find faults

such as ice build-up, galloping conductors, etc., conveniently disappear

by the time the fault location equipment can be mobilized and put into

service.

Time domain reflectors used on transmission lines for many years

before largely abandoning this technique due to its complexity and small

gains.

4.2 IMPEDENCE-BASED FAULT LOCATON

METHODS

The largest area of development on fault location methods are those

based on the measurement of system frequency (6OHz) signals. These

methods make use of information from the transmission lines that are

already available for protective relaying purposes (i.e . line voltage and

current). In the most general sense, these techniques analyze the

impedance characteristics of the line to determine the fault location.

Early fault locators were based on the reactance method where fault

location is determined by measuring the ratio of the reactance of the

faulted line at the source end to the unfaulted line reactance, the

assumption being that the fault impedance is purely resistive.




Page

33

More complex algorithms make use of pre- and post-fault

conditions to reduce the effects of sources (except for the virtual source at

the fault) and loads. The development of digital fault recorders and digital

protective relays has made it possible to implement better and

computationally more complex algorithms. Many of these methods involve

simultaneous solutions of non-linear equations which can easily be

implemented using iterative techniques. On the 500 kV system where

accurate fault location is of much greater importance, the use of series

compensation precludes the use of these relays for fault location. In long

transmission lines, reactive compensation in the form of a series capacitor

is typically inserted at the midpoint of the line to compensate for the line

inductance and increase the load carrying capacity of the line.

A fault on a series compensated line produces transient frequencies

below the power system frequency that are the result of the natural

resonance of the series capacitance and system inductance. The frequency

of these oscillations depends largely on the sou rce impedance and can lie

close to the power system frequency. These transient frequencies can

produce spurious outputs, causing inaccurate fault location.




Page

34

4.3 TRAVELLING WAVE FAULT LOCATION

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.

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.




Page

35

Precisely synchronized clocks is 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. CBasic fault locator principle

figure is as follows:

Fig.11: Travelling wave fault location




Page

36

5.0 IMPLEMENTATION AND TESTING

Evaluation of the fault locator involved the installation of GPS timing

receivers at four 500 kV substations, as in Fig.12. 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 1microsecond.

The fault locator tested by placing B-phase to ground faults on line

using a high speed ground switch. Twelve faults were applied to the line

and the fault locator successfully located all twelve faults to well within

the design goal of plus or minus one tower span (300 meters) accuracy.

The results are summarized in Table 1.




Page

37

5.1 ADDITIONAL SOURCES OF VERIFICATION :

The fault locator responds to any traveling wave impulse with

sufficient energy in the 35 kHz to 350 kHz range. These impulses are not

limited to those generated by actual transmission line faults. Routine

switching of substation equipment also produces similar fast rise time

impulses which are also detected and time tagged by the fault locator.

These station-generated traveling waves provide an opportunity to test

(and calibrate) the fault locator by comparing the measured results

against the total propagation time of the line a known quantity. Traveling

waves generated by routine operation of 500 kV equipment at NIC, KLY

and ING substations over a period of one year indicated repeatability of

fault location measurements to within the one microsecond or 300 meter

performance accuracy goal. The values are summarized in Table 2.

5.2 EFFECTS OF SERIES CAPACITOR BANKS :

A prime consideration leading to the development a traveling wave

fault locator was the use of series capacitor banks has made it very

difficult if not impossible for the system frequency (60Hz) or impedance-

based fault location techniques to operate properly i.e. fault location using

digital relays and digital fault recorders. The high frequency fault-




Page

38

generated traveling waves on the other hand, are not affected by series

capacitor banks. There are series capacitor banks installed on lines .

Testing using traveling waves generated by reactor switching at NIC and

KLY showed the capacitor banks did not have any noticeable effects on

the propagation of the traveling waves.

5.3 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 the 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.




Page

39

CASE STUDY:

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 =

131,694.5 meters.

Table 1: Fault locator output and its difference from estimated value

TestFault Locator Output

(meters)

Difference from Estimated

Value (meters)

1 131725 30

2 131819 124

3 131721 26

4 131803 108

5 131800 105

6 131834 139

7 131730 35

8 131697 2

9 131829 134

10 131806 111

11 131810 115

12 131814 119




Page

40

Table 2: Fault Locator Response to Traveling Waves Generated by the

Routine Switching of Substation Equipment:

Line estimated T p (sec) Measured T p (sec)

501 499

66 67

850 851

900 896

901 901

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

Where Vp is the velocity of propagation for the line and




Page

41

Denotes stations with travelling wave-detector installations

Fig.12: Fault Locator Installations and Testing




Page

42

6.0 GPS SYNCHRONISATION SYSTEM

GPS time Synchronization: Power system reliability starts with

precision timing.

Precision timing is required for monitoring and control of

electrical power networks where power system reliability is important.

Circuit breaker control schemes require precise timing to ensure

proper operation, and Sequence of Events Recording systems report

events with one-millisecond precision to aid diagnostics and

troubleshooting. Typical ´critical powerµ applications include electric

utility networks, data centres, hospitals, water treatment plants,

refineries, and other process industries.




Page

43

Global Positioning Systems can be used to provide a precise

time reference for power system devices in an electrical network,

ensuring time synchronization even across great distances. A generic

GPS system architecture is shown below:

Generic GPS Synchronisation Architecture for Power Systems

A GPS antenna receives a time signal from the system of

satellites orbiting the Earth. A GPS receiver accepts raw time data

from the antenna and converts it to one or more time protocols.

Each power system device accepts a sync pulse, usually via a




Page

44

dedicated, high-speed digital input. The device then sets its internal

clock by decoding the supported time protocol.

Power companies and utilities have fundamental requirements for

time and frequency to enable efficient power transmission and

distribution. Repeated power blackouts have demonstrated to power

companies the need for improved time synchronization throughout the

power grid. Analyses of these blackouts have led many companies to place

GPS-based time synchronization devices in power plants and substations.

By analyzing the precise timing of an electrical anomaly as it propagates

through a grid, engineers can trace back the exact location of a power line

break.



B

¡

ec ¢

£

¤ ¤ ¤ - ¥ 07 SB ¦

¡

£

§ ¢ ¨ © © ¨ ©

P

45

S/S A S/S B

GRL100 GRL100

GPS communication GPS receiver System

Sampling Sampling receiver OSC OSC

Fi

13 A Con i

ion o GPS syn! h ono s Curren

Di eren i l rel y

E !

h rel y measures the time di eren! e between the output o

its own samplin oscillator and the one pulse per second (1pps) GPS

si nal

Each relay then corrects the oscillation frequency of its

samplin

oscillator so that the time difference becomes zero

The relay continuousl y updates the correction factor and stores

it as the absolute accuracy of the oscillator. In the event of the GPS

si nal bein lost, the relay is able to control the oscillator·s free-

runnin frequency error to within 0.2ppm, b y usin the correction

factor stored immediatel y prior to the loss of the GPS si nal. By this

method, the samplin synchronization error between two relays can

be limited to within 1.5" sec.




Page

46

Fig.14 shows the GPS standard time signal that is provided to

the relay from the GPS receiver. The standard time data is encoded so

that any failure or corruption of the standard time signal can be

detected by the relays, thus ensuring that sampling synchronization

control is not adversely affected.

St# $ d# % d &

' ' ( (

i) $ # l

Ti0 1 d#

t#

&

( 1 2 3 $

d

Fig.14: GPS standard time signal




Page

47

CHAPTER-1CHAPTER-204

7.0 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.

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




8.0 REFERENCES :

1. Transmission lines and networks ² Umesh Sinha

2. Communication & Control in electric power systems-applications-

M. Shahidehpeur and Yaouyu Wang, google books

3. Global Positioning Systems- Pratap Mishra and Per Enge

4. Wikipedia of transmission line protection methods and Global

Positioning Systems

5. Website of National Institute of standards &Technology (NIST) time

and frequency division.

Documents

Copy of Global Positioning Systems