wide area measurement technology using gps

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A

Seminar Report on

WIDE AREA MEASUREMENT TECHNOLOGY USING GPS

Submitted In Partial Fulfillment of the Requirement

For The Award of Degree

of

Electrical Engineering

Rashtrasant Tukadoji Maharaj, Nagpur University

Submitted By

Shadab Umran Sayyad

Under the guidance of

Prof. R. B. Sharma

Department Of Electrical Engineering

Government College of Engineering, Chandrapur

2013-2014


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i

GOVERNMENT COLLEGE OF ENGINEERING,

CHANDRAPUR

Certificate

This is certifying that Shadab Umran Sayyad, final year student of

Electrical Engineering, has satisfactorily completed the seminar report on

WIDE AREA MEASUREMENT TECHNOLOGY USING GPS. A

seminar report is submitted by him in partial fulfillment of degree in Electrical

Engineering, as prescribed by Rashtrasant Tukadoji Maharaj Nagpur University,

Nagpur during academic year 2013-2014.

His work is found to be satisfactory and hereby approved for final

submission.

Date:

Place: Chandrapur

PROF. R. B. SHARMA

Electrical Engineering Department

Government College of Engineering,

Chandrapur

DR. V.N. GHATE

Head of Electrical Engineering Department

Government College of Engineering,

Chandrapur


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ii

ACKNOWLEDGEMENT

It is with great pleasure and effort that I am able to present this seminar

report. I have tried all my best to make this report complete in all aspects.

I would like to acknowledge my project guide Prof. R. B. Sharma of

Electrical Engineering Department Government College of Engineering

Chandrapur and Dr. V.N. Ghate Head of Electrical Engineering Department

Government College of Engineering Chandrapur for providing necessary

guidance and supervision in making the seminar report.

Last but certainly not least I would like to thank my colleagues and my

friends for their inspiration and motivation and also those who helped me

directly and indirectly in seminar work.

Date:

Place: Chandrapur Shadab Umran Sayyad


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iii

INDEX

Contents Page No.

1. Introduction 2

2. Global Positioning System (GPS) 4

2.1 What is GPS 4

2.2 Principle of GPS 5

2.3 GPS Time Information 5

2.4 GPS Accuracy 6

2.5 GPS Time Synchronization 6

2.5.1 GPS Receiver LeadtekGPS-9543LP 6

2.6 The Need For Time Synchronization 7

2.7 Applications of GPS 8

3. Time Synchronization 9

3.1 Introduction to Time Synchronization 9

3.2 GPS Time Synchronization 9

3.3 Configuring for GPS Time Synchronization 10

3.4 Supported GPS Receivers 10

3.5 GPS Time Synchronization Format 11

3.6 Methods of Time Synchronization 12

3.6.1 Point to Point Protocol (PTP) 12

3.6.2 Time Stamp Points 13

4. Sampling 14

4.1 Introduction to Sampling 14

4.2 Sampling Rate 15

4.3 Nyquist- Shannon Sampling Theorem 16

5. Synchronization of Sampling 17

5.1 Definition 17

5.2 Need Of Sampling Clock In Synchronization 17


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5.3 Problems of Distributed Sampling 18

5.4 Proposed Solution 18

5.5 Synchronization of the Sampling Process 18

5.6 Uses of Synchronized Phasors 19

5.6.1 State Estimation with Phasors 19

5.6.2 Protection with Phasors 19

6. Applications of the synchronized PMU 20

6.1 Monitoring Equipments 20

6.2 State Vectors 20

6.3 State Estimators 21

6.4 Fault location 21

7. Conclusion 22

References 23


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LIST OF FIGURE

FIGURE

NO.NAME OF FIGURE

PAGE

NO.

1.1 GPS satellites in orbit 3

2.1Three measurements place the observer somewhere along

the intersection of the three spheres.5

2.2 GPS Receiver Module 6

2.3 Block diagram of the Phasor Measurement Unit 8

3.1 Time synchronization in GPS 9

3.2 Data interchange in PTP 12

3.3 Different possible time stamp points 13

4.1 Signal sampling representation. 14

5.1 Distributed DSP system 17

6.1 Voltage Phase Angle 20

6.2 Fault Location 21


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LIST OF TABLES

TABLE

NO. NAME OF TABLE PAGE NO.

3.1 List of GPS Receiver 10

3.2 Protocol Registers 11

3.3Explanation of GPS: ARBITER & GPS: True time/Datum

ASCII Time Strings11


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WIDE AREA MEASUREMENT TECHNOLOGY USING GPS

ABSTRACT

Electric Power Systems are an essential infrastructure of modern society and have

been characterized as the largest man-made systems. Catastrophic failures of power systems

popularly known as blackouts occur infrequently, but when they occur they cause great

trouble to the industrial companies. This paper presents an adaptive transmission line

protection scheme based on synchronized phasor measurement units. This scheme uses the

positive-sequence voltage and current phasors at both ends of a transmission line to

determine the parameter of the transmission line and the location of a faulton the transmission

line. This scheme can be used for the protection of both single and double-circuit

transmission lines. This scheme is also robust against power swing conditions. A novel

adaptive single pole auto recloser is introduced based on the proposed scheme due to its

capability of differentiating transient and permanent faults. System simulation studies show

that the proposed scheme is able tooperate fast and accurately for transmission line

protection.

When disturbances occur in power grid, monitoring, control and protection systems

are required to stop the grid degradation, restore it to a normal state, and hence minimize their

effects. However, in wide area power grid resulting from large extension and interconnection

with neighbor grids, classical systems based on local independent measurements and

decisions are not able to consider the overall power grid disturbances and then they are not

able to avoid the blackout. The introduction of the advanced measurement and

communication technologies in these systems may provide better ways to detect rapidly these

disturbances and protect the overall grid from the propagation of the fast-cascading outages.

Indeed, the observability of the wide area power system dynamics becomes feasible throughthe use of these recent developed technologies. Using wide area real-time synchro-phasor

measurement system based on Phasor Measurement Units (PMUs), different types of wide

area protection, emergency control and optimization systems can be designed and

implemented.


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CHAPTER 1

INTRODUCTION

Have you ever been lost and wished there was an easy way to find out which way you

needed to go? How about finding you out hiking and then not knowing how to get back to

your camp or car? Ever been flying and wanted to know the nearest airport?

Our ancestors had to go to extreme measures to keep from getting lost. They erected

monumental landmarks, laboriously drafted detailed maps and learned to read the stars in the

night sky.

GPS is a satellite based radio navigation system which provides continuous, all

weather, worldwide navigation capability for sea, land and air applications. Therefore, things

are much, much easier today. For less than $100, you can get a pocket-sized gadget that will

tell you exactly where you are on Earth at any moment. As long as you have a GPS receiver

and a clear view of the sky, you will never be lost again.

Navigation in three dimensions is the primary function of GPS. Navigation receivers

are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. Precise

positioning is possible using GPS receivers at reference locations providing corrections and

relative positioning data for remote receivers. Time and frequency dissemination, based on

the precise clocks on board the SVS and controlled by the monitor stations, is another use for

GPS. Astronomical observatories, telecommunications facilities, and laboratory standards can

be set-to precise time signals or controlled to accurate frequencies by special purpose GPS

receivers.

Modern power and control systems are complex and extensively interconnected.

Events at a single location in the system can have diverse trickle down effects in other areas

of the larger system. This complexity can make it difficult to analyze and diagnose system

events and problems.

Fortunately, in today system, plenty of Intelligent Electronic Devices (IEDs) are

recording and storing even the minute details of system of operations. This may include

protective relays, communication processors, Digital Fault Recorders (DFRs), Remote

Terminal Units (RTUs), Voltage regulator controls and Programmable Automation

Controllers (PACs). The IEDs log and stamp data from the power system, including analog

waveforms, contact status, internal device binary state, trip and reclose signals and many

more.


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Introduction

3

Each IEDs places time stamps on data items related to some internal time source.

Thus, the accuracy of the internal time source is critically important if the recorded data are

to be useful in a larger system analysis.

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 blackouts have led many companies to place GPS-based time

synchronization devices in power plants and substations. Typical ratings of commercial

available GPS clocks range from 50ns to 1ms.

By synchronizing the sampling processes for different signals, which may be,

hundreds of kilometers apart it is possible to put their phasors in the same phasor diagram.

Synchronized phasor measurements (SPM) have become a practical proposition. As such,

their potential use in power system applications has not yet been fully realized by many

of power system engineers.

Fig 1.1 GPS satellites in orbit


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CHAPTER 2

GLOBAL POSITIONING SYSTEM (GPS)

2.1 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 by the U.S. Department of Defense that

continuously transmit coded information, which makes it possible to precisely identify

locations on earth by measuring the distance from the satellites. The satellites transmit very

low power specially coded radio signals that can be processed in a GPS receiver, enabling the

receiver to compute position, velocity and time thus allowing anyone one with a GPS

receiver to determine their location on earth. Four GPS satellite signals are used to compute

positions in three dimensions and the time offset in the receiver clock. The system was

designed so that receivers did not require atomic clocks, and so could be made small and

inexpensively.

The GPS system consists of three pieces. There are the satellites that transmit the

position information, there are the ground stations that are used to control the satellites and

update the information, and finally there is the receiver that you purchased. The receiver

collects data from the satellites and computes its location anywhere in the world based on

information it gets from the satellites. There is a popular misconception that a GPS receiver

somehow sends information to the satellites but this is not true, it only receives data.

2.2 PRINCIPLE OF GPS

The principle behind GPS is the measurement of distance (or "range") between the

receiver and the satellites. The satellites also tell us exactly where they are in their orbits

above the Earth. It works something like this-If we know our exact distance from a satellite in

space, we know we are somewhere on the surface of an imaginary sphere with radius equal to

the distance to the satellite radius. By measuring its distance from a second satellite, the

receiver knows it is also somewhere on the surface of a second sphere with radius equal to its

distance from the second satellite. Therefore, the receiver must be somewhere along a circle

which is formed from the intersection of the two spheres. Measurement from a third satellite

introduces a third sphere. Now there are only two points, which are consistent with being at

the intersection of all three spheres. One of these is usually impossible, and the GPS receivers


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Global Positioning System (GPS)

5

have mathematical methods of eliminating the impossible location. Measurement from a

fourth satellite now resolves the ambiguity as to which of the two points is the location of the

receiver. The fourth satellite point also helps eliminate certain errors in the measured distance

due to uncertainties in the GPS receivers timing as well.

Fig 2.1 Three measurements place the observer somewhere along the intersection of the three spheres.

2.3 GPS TIME INFORMATION

Satellites are equipped with very precise clocks that keep accurate time to within three

nanoseconds - that is 0.000000003, or three billionths, of a second. This precision timing is

important because the receiver must determine exactly how long it takes signals to travel

from each GPS satellite.

Each Block II/IIA satellite contains two Cesium (Cs) and two Rubidium (Rb) atomic clocks.

Each Block IIR satellite contains three Rb atomic clocks.

Its Composite Clock (CC) gives GPS time. The CC or "paper" clock conforms to all Monitor

Station and satellite operational frequency standards.

The system time is referenced to the Master Clock at the USNO from which system time will

not deviate by more than one microsecond.

2.4 GPS ACCURACY

The accuracy of a position determined with GPS depends on the type of receiver.

Most hand-held GPS units have about 10-20 meter accuracy. Other types of receivers use a

method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an

additional receiver fixed at a known location nearby. Observations made by the stationary

receiver are used to correct positions recorded by the roving units, producing an accuracygreater than 1 meter.


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It furnishes a common-access timing pulse which is accurate to within 1 microsecond at any

location on earth. A 1-microsecond error translates into 0.021for a 60 Hz system and 0.018

for a 50 Hz system and is certainly more accurate than any other application.

2.5 GPS TIME SYNCHRONIZATION

GPS technology can be used as an inexpensive, readily available method for high

precision timing and measurement of event simultaneously. The Global Positioning System

(GPS) provides a method of synchronized tracking that is extremely accessible due to the low

cost and easy set up involved. Most GPS receivers make this adjustment automatically, so the

time reported to the user is UTC. The satellites broadcast regularly recalculated and updated

ephemeredes, so their position in space can be accurately calculated as well. This effectively

provides researchers with an accurate timing system that is a viable alternative to acquiring

and constantly recalibrating a set of atomic clocks.

2.5.1 GPS RECEIVER LEADTEK GPS-9543LP

We used a commercially available GPS receiver, Leadtek Research, Inc. model GPS-

9543. This is a 12-channel GPS receiver chip whose small size and low power consumption

lends itself to easy integration in a hand-held module or circuit board, such as the set-up we

used.

Fig. 2.2. GPS Receiver Module

The basic specifications of this chip are:

1-pulse-per-second (1PPS) signal.

3.3 V power requirement.

Reacquisition time of 0.1 seconds.

The apparatus we used consists of two antennae connected to circuit boards we

designed for the specific purpose of housing the GPS timing chips. Each board includes a pin

for antenna power, PPS signal output, ground connection and connection to an oscilloscope.

The signal output is sent to a serial port, of which each board has two.


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2.6 THE NEED FOR TIME SYNCHRONIZATION

Modern power and control systems are complex and extensively interconnected. Events at

a single location in the system can have diverse trickle-down effects in other areas of the

larger system. This complexity can make it difficult to analyze and diagnose system events

and problems.

Fortunately, in todays systems, a plethora of intelligent electronic devices (IEDs) are

recording and storing even the minute details of system operations. These may include

protective relays, communications processors, Digital Fault Recorders (DFRs), Remote

Terminal Units (RTUs), voltage regulator controls and Programmable Automation

Controllers (PACs).

2.7 APPLICATIONS OF GPS

Many civilian applications use one or more of GPS's three basic components: absolute

location, relative movement, and time transfer.

Astronomy: Both positional and clock synchronization data is used in Astrometry and

Celestial mechanics calculations. It is also used in amateur astronomy using small

telescopes to professional observatories, for example, while finding extra solar planets. Cellular telephony: Clock synchronization enables time transfer, which is critical for

synchronizing its spreading codes with other base stations to facilitate inter-cell

handoff and support hybrid GPS/cellular position detection for mobile emergency calls

and other applications.

Clock synchronization: The accuracy of GPS time signals (10 ns)[71] is second only

to the atomic clocks upon which they are based.

Fleet Tracking: The use of GPS technology to identify, locate and maintain contact

reports with one or more fleet vehicles in real-time.

Navigation: Navigators value digitally precise velocity and orientation measurements.

Robotics: Self-navigating, autonomous robots using a GPS sensors, which calculate

latitude, longitude, time, speed, and heading.


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GPS enables highly accurate time stamping of power system measurements, making it

possible to compute phasors.Implementation of PMUs for monitoring applications requires a

training program that includes clear explanations, real case studies, and carefully planned

scenarios that will help the engineers and operators not only understand the technology but to

trust the information it provides.

Fig. 2.3 Block diagram of the Phasors Measurement Unit


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CHAPTER 3

TIME SYNCHRONIZATION

3.1 INTRODUCTION TO TIME SYNCHRONIZATION

Time synchronization lets you synchronize the internal clocks of all networked Power

Logic ION meters and devices. Once synchronized, all data logs have timestamps that are

relative to a uniform time base. This allows you to achieve precise sequenceofevents and

power quality analyses. To synchronize clocks, use ION Setup or ION Enterprise software, a

Network Time Protocol (NTP) server, a Global Positioning System (GPS) receiver or

supported third party protocols to broadcast time signals across the network.

3.2 GPS TIME SYNCHRONIZATION

A dedicated serial network is required to implement a GPS scheme. If you are already

using a serial link for communications with ION Enterprise, you need second serial network

to transport GPS signals. Either RS232 or RS485 networks can be used for GPS time

synchronization, though RS485 is recommended if more than two meters are being

synchronized.

If your GPS receiver output is RS

232, use the COM32 or equivalent RS

232/RS

485converter that does not buffer communications. The COM128 is not recommended if used

in Repeater Mode.

Fig. 3.1 Time synchronization in GPS


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Time Synchronization

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3.3 CONFIGURING FOR GPS TIME SYNCHRONIZATION

To implement GPS time synchronization, use the Designer component of ION

Enterprise to configure the Clock module and the Communications module:

1. Start Designer (ensure Options > Show Toolboxes selected). Doubleclick the Clock

module.

2. Set the Clock modules Clock Source setup register to COMM.

3. Specify which COM port will receive time synchronization signals by setting the Time

Sync Source setup register in the meters Clock module. Note that Ethernet cannot be used

with GPS time synchronization. Only signals received on the port specified are used for

synchronization.

4. Specify the receiver you want to use by selecting it from the Protocol setup register in

the receiving ports Communications module (see table below). You may need to modify

the Time Sync Type setup register to LOCAL, if a DNP Master is sending time broadcasts

in local time.

3.4 SUPPORTED GPS RECEIVERS

The following receivers are supported. Standard models of these receivers are

sufficient, as long as they have RS232 ports additional options are available, butnot

required:

Tabel 1. List of GPS receiver

GPS receiver Comm Module Protocol Register Setting

Symmetricom XL-DC series(was True

Time XL-DC series)GPS:TRUETIME/DATUM

Arbiter 1092GPS:ARBITER

GPS:ARBITER-VORNE

Clark Associates GPS-200-ASCII GPS:TRUETIME/DATUM

3.5 GPS TIME SYNCHRONIZATION FORMAT

Any GPS receiver may be used as a time synchronization source, as long as the

receiver outputs the ASCII time string (shown below) every second and has On Time Mark

(OTM).

Use the table below to select the appropriate protocol register for each OTM type.


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Table 2. Protocol Registers

On Time Mark (OTM) Protocol Register

Start bit of GPS :Arbiter

Start bit of GPS:TRUETIME/DATUM

Start bit of GPS:ARBITER-VORNE

During normal operation of a GPS time synchronizing system, time signals are sent

out once per second as an ASCII string containing the time.

The ASCII time string for GPS:ARBITER and GPS:TRUETIME/DATUM is the

following:

DDD: HH:MM: SSQ

Table 3. Explanation of GPS: ARBITER & GPS: TRUETIME/DATUM ASCII Time String

ASCII Time String:DDD:HH:MM:SSQ

Start of header (ASCII 01hex)

DDD Day of the year

HH Hours

MM Minutes

SS Seconds

Q Quality flags

Carriage return (ASCII 0Dhex)

Line feed (ASCII 0Ahex)

3.6 METHOD OF TIME SYNCHRONISATION

3.6.1 PTP

PTPs operating principle is to exchange message regularly to determine the offset

between master and slave but also the message transit delay through the network. PTP

message exchange. The slave clock requires four-measured values t1, t2, t3, t4 to calculate

delay and offset. These are the send and the receive times of the Sync and the Delay_Req

messages. The Follow_up and Delay_Resp messages transport the values measured

by the master down to the slave. A simple calculation delivers delay and offset:


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Delay + Offset = t2-t1

Delay - Offset = t4-t3

Fig 3.2. Data interchange in PTP

Delay = ((t2-t1) + (t4-t3)) / 2

Offset = ((t2-t1) - ( t4-t3)) / 2

The precision of the result depends on the precision of the time stamps. They should

reflect the send and receive time as precise as possible. The slaves offset and delay

calculation is based on the difference of time stamps taken at two different places. Therefore,

the two clocks should use the same scale, i.e. the same tic interval. This is achieved by drift

compensation: the slave clocks rate is accelerated or slowed down by a control loop. A

slightly different tic interval will degrade the result. It is assumed that the message transit

delay is the same for both directions. Ethernet transceiver shave asymmetric transmit and

receive paths. If their timing characteristics are clearly specified within a small range, the

asymmetry can be taken into account by calculation as in bound and outbound latency

correction constants. In the long run, conditions may change due to reconfiguration (leading

to a totally different delay) or environmental conditions (temperature).How fast the clocks

can react depends on the frequency of sync and delay measurement and the dynamic behavior

of the servos controlling the slave clock. To sum it up, performance depends on:

The communication channels symmetry (i.e.; same delay in both directions and

constant over a longer period of time)


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Drift compensated clocks (i.e. adjusted time base in master and slave clocks), time

stamp accuracy, time stamp resolution, sync interval, clock stability, clock control

loop characteristics.

3.6.2 TIME STAMP POINTS

The PTP environment offers different possible time stamp points. In the hardware

assisted approach, time stamps are taken at the Medium Independent Interface(MII) between

MAC and PHY chips, if accessible. To acquire the frames directly on a 100Base-TXwire

pair, functions such as clock recovery, line decoding, descrambling, etc. are required, which

is essentially the purpose of a PHY. The PTP software on the application layer requires an

interface to the time stamping unit to collect the time stamps of transmitted and received PTP

messages plus additional information to correlate time stamps with the corresponding

messages.

Fig 3.3. Different possible time stamp points


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CHAPTER 4

SAMPLING

4.1 INTRODUCTION TO SAMPLING

In signal processing, sampling is the reduction of a continuous signal to a discrete

signal. A common example is the conversion of a sound wave (a continuous signal) to a

sequence of samples (a discrete-time signal). A sample refers to a value or set of values at a

point in time and/or space. A sampler is a subsystem or operation that extracts samples from

a continuous signal. A theoretical ideal sampler produces samples equivalent to the

instantaneous value of the continuous signal at the desired points.

Fig 4.1 Signal Sampling Representation.The continuous signal is represented with a green colored line while the blue vertical

lines indicate the discrete samples. Sampling can be done for functions varying in space,

time, or any other dimension, and similar results are obtained in two or more dimensions. For

functions that vary with time, let s(t) be a continuous function (or "signal") to be sampled,

and let sampling be performed by measuring the value of the continuous function every T

seconds, which is called the sampling interval. Thus, the sampled function is given by the

sequence:

S (nT), for integer values of n.

The sampling frequency or sampling rate fs is defined as the number of samples

obtained in one second (samples per second), thus fs = 1/T. Reconstructing a continuous

function from samples is done by interpolation algorithms. The WhittakerShannon

interpolation formula is mathematically equivalent to an ideal low pass filter whose input is a

sequence of Dirac delta functions that are modulated (multiplied) by the sample values. When

the time interval between adjacent samples is a constant (T), the sequence of delta functions


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Sampling

15

is called a Dirac comb. Mathematically, the modulated Dirac comb is equivalent to the

product of the comb function with S(t). That purely mathematical function is often loosely

referred to as the sampled signal. Most sampled signals are not simply stored and

reconstructed. But the fidelity of a theoretical reconstruction is a customary measure of the

effectiveness of sampling. That fidelity is reduced when S(t) contains frequency components

higher than fs/2 Hz, which is known as the Nyquist frequency of the sampler. Therefore S(t)

is usually the output of a low pass filter, functionally known as an "anti-aliasing" filter.

Without an anti-aliasing filter, frequencies higher than the Nyquist frequency will influence

the samples in a way that is misinterpreted by the interpolation process.

4.2 SAMPLING RATE

The analog signal is continuous in time and it is necessary to convert this to a flow of

digital values. It is therefore required to define the rate at which new digital values are

sampled from the analog signal. The rate of new values is called the sampling rate or

sampling frequency of the converter. A continuously varying band limited signal can be

sampled (that is the signal values at intervals of time T, the sampling time, are measured and

stored) and then the original signal can be exactly reproduced from the discrete-time values

by an interpolation formula. The accuracy is limited by quantization error. Since a practical

ADC cannot make an instantaneous conversion, the input value must necessarily be held

constant during the time that the converter performs a conversion (called the conversion

time). An input circuit called a sample and hold performs this task-in most cases by using a

capacitor to store the analog voltage at the input, and using an electronic switch or gate to

disconnect the capacitor from the input. Many ADC integrated circuits include the sample

and hold subsystem internally.

4.3 NYQUIST- SHANNON SAMPLING THEOREM

The NyquistShannon sampling theorem, after Harry Nyquist and Claude Shannon, in

the literature more commonly referred to as the Nyquist sampling theorem or simply as the

sampling theorem, is a fundamental result in the field of information theory, in particular

telecommunications and signal processing. Sampling is the process of converting a signal (for

example, a function of continuous time or space) into a numeric sequence (a function of

discrete time or space). Shannon's version of the theorem states: If a function x(t) contains no

frequencies higher than B hertz, it is completely determined by giving its ordinates at a series


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Sampling

16

of points spaced seconds apart. In essence, the theorem shows that a band limited analog

signal can be perfectly reconstructed from an infinite sequence of samples if the sampling

rate exceeds 2B samples per second, where B is the highest frequency of the original signal.

If a signal contains a component at exactly B hertz, then samples spaced at exactly seconds

do not completely determine the signal, Shannon's statement notwithstanding. This sufficient

condition can be weakened, as discussed at Sampling of non-baseband signals below. More

recent statements of the theorem are sometimes careful to exclude the equality condition; that

is, the condition is if x(t) contains no frequencies higher than or equal to B; this condition is

equivalent to Shannon's except when the function includes a steady sinusoidal component at

exactly frequency B.

The theorem assumes an idealization of any real-world situation, as it only applies to

signals that are sampled for infinite time; any time-limited X(t) cannot be perfectly band

limited. Perfect reconstruction is mathematically possible for the idealized model but only an

approximation for real-world signals and sampling techniques, albeit in practice often a very

good one.

The theorem also leads to a formula for reconstruction of the original signal. The

constructive proof of the theorem leads to an understanding of the aliasing that can occur

when a sampling system does not satisfy the conditions of the theorem.

The sampling theorem provides a sufficient condition, but not a necessary one, for

perfect reconstruction. The field of compressed sensing provides a stricter sampling condition

when the underlying signal is known to be sparse. Compressed sensing specifically yields a

sub-Nyquist sampling criterion.


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CHAPTER 5

SYNCHRONIZATION OF SAMPLING

5.1 DEFINITION

A distributed signal processing system comprises numerous processor nodes (mostly

based on DSPs) which are interacting with each other to perform real-time data acquisition

and signal processing. An overview of this system is presented. Such systems are also known

as intelligent sensor networks, where processing units are placed near to the sensors.

Distributed DSP systems are used, e.g., in seismic wave measurements. In DSP-based

systems, the nodes are performing online signal processing, i.e., the DSP algorithm is

executed sample by sample. The input data for this algorithm are digital samples of a discrete

signal, usually sampled at the clock rate of the DSP algorithm. Distributed processors and

data acquisition units have separate clocks that may hurt data consistency constraints, due to

their jitter and drift. In distributed embedded systems, data consistency asks for synchronous

data acquisition and representation. This problem does not exist in centralized one-processor

systems, as generally these have only one master sampling clock that schedules all the

sampling processes.

Fig. 5.1.Distributed DSP system

5.2 NEED OF SAMPLING CLOCK IN SYNCHRONIZATION

Interfacing an ADC or a DAC to a fast DSP parallel requires an understanding of how

the DSP processor reads data from a memory-mapped peripheral (the ADC) and how the

DSP processor writes data to a memory-mapped peripheral (the DAC). It should be noted that

the same concepts presented here regarding ADCs and DACs apply equally when reading

and writing from/to external memory. It is assumed that the ADC is sampling at a continuous

rate which is controlled by the external sampling clock, not the internal DSP clock. Using a


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Synchronization of Sampling

18

separate clock for the ADC is the preferred method, since the DSP clock may be noisy and

introduce jitter in the ADC sampling process, thereby increasing the noise level.

5.3 PROBLEMS OF DISTRIBUTED SAMPLING

In distributed DSP systems, a problem arises when two or more DSP nodes having

asynchronous clocks are communicating by sending samples of real-time discrete signals.

Consider the case, when the input signal of a certain node (Drain) is the output signal of

another node (Source). This kind of real-time communication presumes that the sample rate

of the data is the same at both nodes. Crystal oscillators usually control the sampling, where

the frequency deviation is typically 50 ppm. The problem arises as its own crystal oscillator

schedules the DSP algorithm of the Drain, but the input samples are sampled by the AD of

the Source. Additionally, if we suppose that the communication is packet-based, and one

sample is sent in one packet, then due to this difference, sometimes two or zero packets arrive

during the period when the Drain expects exactly one packet. In this case, data packets are

lost.

5.4 PROPOSED SOLUTION

Any solution of this problem has to ensure that the samples of the different input data

streams of a certain DSP node are available with the same sampling frequency. In the above

shown example, the data stream (samples of discrete signal) from the Source is expected with

the sampling frequency of the AD of the Drain an obvious hardware solution is to

synchronize the sampling clocks of the ADs by e.g. a PLL. We propose a software solution

for this problem that does not influence the hardware layers of the DSP node, so the sampling

processes of the different nodes remain asynchronous.

5.5 SYNCHRONIZATION OF THE SAMPLING PROCESS

The phasor given uses the sampling instant of the first sample as the reference. The

necessary accuracy of synchronization may be specified in terms of the prevailing phase

angle differences between buses of a power network. Typically, this angular difference may

vary between a few degrees, to perhaps 60" under extreme loading conditions.

Under these circumstances, a precision corresponding to 0.1" seems to be desirable to

measure angular differences corresponding to lightly loaded systems. Allowing for other

sources of error in the measurement system, it seems certain that a synchronizing accuracy of

about1 pulse-per-second would meet the needs of this measurement technique. A superior


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Synchronization of Sampling

19

and satisfying solution to the synchronization problem is to use the1 Pulse-Per-Second (PPS)

transmission provided by the Global Positioning System (GPS) satellites.

5.6 USES OF SYNCHRONIZED PHASORS

5.6.1 STATE ESTIMATION WITH PHASORS

The collection of all positive sequence bus voltages of a power system is known as its

state vector. Knowledge of the state vector is essential in many of the central control

functions associated with Comm. Port power system operations. The present practice is to

obtain Measurements of various system quantities such as real and reactive power flow over

transmission lines real and reactive power injections, voltage magnitude at system buses, line

the state of power system with a non-linear system estimator.

A state estimation procedure such as that described above can never represent the

dynamic phenomena occurring on the power system during transient power Swings. The data

scan rates in use at present are rather slow, and the non-linear iterative Algorithm contributes

to the slow response time of the estimation process.

5.6.2 PROTECTION WITH PHASOR

Protection is a form of control. Phasors play an important role in protection system

design. In fact, modern phasor measurement techniques originated in the field of computer

relaying. It has now become clear that synchronized phasor measurements can be of great use

in many of the protection applications. Although phasor may be used in many relaying tasks,

their full impact is felt in the new field of adaptive relaying. Adaptive Protection is a

protection philosophy, which permits and seeks to make adjustments in various protection

findings automatic allying order to make them more altered to prevailing power system

conditions. The idea of adaptive relaying is an old one.


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CHAPTER 6

APPLICATIONS OF TIME SYNCHRONIZED PMU

6.1 MONITORING EQUIPMENT

By using modern high-speed modems, several utilities have built a centrally locates

master station. The clock synchronization has to be done at the remote site by the utility.

The master station does not synchronize the remote clocks. By synchronization to UTC, any

recorded event on the utility system can be related to any other time-tagged event on another

synchronized power system.

6.2 STATE VECTORS

The complex voltages of substation busses are the state vectors of the power system

and hence are the key in the application of control theory. One purpose of large area

synchronous sampling is to obtain these voltage phasors in real time. Measuring voltage

magnitude is routine. One electrical degree of 60 Hz waveform equals about 56

microseconds. Across short transmission lines, measurements may need to be made equal to

0.1 electrical degrees. This translates to a clock synchronization of about 5 microseconds.

Actual measurement systems use positive sequence voltages.

Fig.6.1 Voltage Phase Angle


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Applications of Time Synchronized PMU

21

6.3 STATE ESTIMATORS

The purpose of state estimation is primarily to detect, identify, and correct gross

measurement errors and to compute a good estimate of the bus voltage magnitudes and angle.

Static state estimators have had to infer the voltage magnitude and transmission line active

and reactive power flows. This in turn aids system security enhancement. Direct knowledge

of system phase angles by measurement improves the performance of state estimation. A

keyword in static state estimators is static. In the next generation of system controls, dynamic

controls are envisioned where the control system operates on real-time data and takes action

needed to improve power system security.

6.4 FAULT LOCATION

Knowledge of the severity and relative location of power system short circuit faults

can be used to improve system control. As an example, reclosing into a permanent close-in

multi-phase fault may cause system instability. If this fault is farther away from the

substation, the risk of instability due to reclosing is reduced. To locate powers system faults

in the time domain" time synchronization of less than 1 microsecond is needed. Fault-

induced waves travel at the speed of fight, 300 meters per microsecond. By time-tagging the

arrival of fault-induced pulses at each end of the transmission line to within one microsecond,

the fault can be located to within 300 meters, the typical tower spacing on a high voltage

transmission line.

Fig.6.2 Fault Location


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CHAPTER 7

CONCLUSION

Most electric power system operators include a modern form of time synchronization

in new system installations and designs, usually a GPS clock. This provides good device

synchronization moving into the future. However, a large number of existing installations

have not been updated. In some cases, the technology is too old (i.e., electromechanical

relays) to support time synchronization without complete equipment replacement. In these

cases, cost can be an inhibiting factor.

Not having a time-synchronization system in place greatly increases system operating

costs; it increases the labour required to perform post disturbance analysis and system

troubleshooting and maintenance. NERC states, All digital fault recorders, digital event

recorders, and power system disturbance recorders should be time stamped at the point of

observation with a precise GPS Synchronizing Signal. While stated as a recommendation at

the time, future mandates could make the addition of GPS clocks a requirement, perhaps

imposing financial penalties for failure to comply.

For systems with equipment, that supports time synchronization but not originally

designed to include it, there are economical ways to add this capability. Not only does this

reduce the overall system operating costs, it also improves operating efficiency. GPS-

synchronized clocks capable of supplying highly accurate time synchronization to a dozen

devices or more are readily available, starting as low as $550.In most cases, this costs less

than replacing even a single existing device and is more than recovered in operating cost

savings.

While using existing SCADA communications links for time synchronization may

seem attractive due to their low cost, they have proven to be inaccurate and inconsistent.

Time-synchronization accuracy varies erratically from interval to interval, resulting in timing

accuracy with a low level of confidence. Additionally, these methods rely on communications

paths that may have additional unreliability (i.e., poor availability) of their own. These

methods are not sufficient to reduce or eliminate the need for intensive manual data

manipulation in system disturbance and troubleshooting analysis.


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wide area measurement technology using gps

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