Numerical Relaying (2)

8/18/2019 Numerical Relaying (2)

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Numerical Relaying.


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Why Numerical Relaying

•The first and foremost driving force for advances in relaying systems is the need to improve reliability.

•In turn, this implies increase in dependability as well as security.

•This need to improve reliability propelled the development of solid state relays.

•Solid state relays have inherent self checking facility which was not available with electromechanical

relays.

•This feature is also available with numerical relays

•For example, when we boot a computer, it goes through a self checing phase where in it checs R!",

hard dis, etc.

•!lso, with the reduced cost of computer hardware, and an exponential growth in processing capability,numerical relays can provide high performance at moderate costs.

• Since, numerical relays are based on digital technology, they are more or less immune to variation or

drift in parameters of individual components lie #$%!"$S etc. due to changes in temperature, ageing

etc.

• Numerical relays also help in reducing burden &volt%amperes' of (urrent Transformer &(T' and )oltage

Transformer &)T'.

•This is desirable because ideally sensors should not consume any power.

•If a sensor consumes energy from the measure end, it will automatically distort the signal.

•This problem is further aggravated in (Ts due to non%linearity of iron core.

• Numerical relays offer very low impedance to the secondary of (T and hence reduce burden on (T.

• Numerical protection devices offer several advantages in terms of protection, reliability, troubleshooting

and fault information.


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• Numerical relays permit much more flexibility than their electromechanical and solid state

counterparts.

•

In electromechanical relays, the constructional details lie magnetic path, air gap etc., are usedto design various operating characteristics.

• Since, solid state relays mainly use analog circuit, they permit more innovation than

corresponding electromechanical relays which are no doubt robust.

• *owever, solid state relays can not have the ind of flexibility that computer aided relaying

can have.

•

For example, providing magnitude scaling and phase shift to a voltage signal to generate lineto line voltage from phase to neutral voltage is much simpler with computer aided relaying

because it can be handled by the program.

• ! computer relay can be programmed . Further, due to the programming feature, it is possible

to have generic hardware for multiple relays, which reduces the cost of inventory.

• The first protection devices based on microprocessors were employed in +-.

•

The widespread acceptance of numerical technology by the customer and the experiences ofthe user helped in developing the second generation numerical relays in +/.

• "odern power system protection devices are built with integrated functions.

• "ulti%functions lie protection, control, monitoring and measuring are available today in

numeric power system protection devices.

• !lso, the communication capability of these devices facilitates remote control, monitoring

and data transfer.


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• Traditionally, electromechanical and static protection relays offered single%function, single

characteristics, whereas modern numeric protection offers multi%function and multiple

characteristics.

• Some protections also offer adaptable characteristics, which dynamically change the

protection characteristic under different system conditions by monitoring the input parameters.

• The measuring principles and techni0ues of conventional relays &electromechanical and static'

are fewer than those of the numerical techni0ue, which can differ in many aspects lie the type

of protection algorithm used, sampling, signal processing, hardware selection, software

discipline, etc.• First generation numerical relays were mainly designed to meet the static relay protection

characteristic, whereas modern numeric protection devices are capable of providing complete

protection with added functions lie control and monitoring.

• Numerical protection devices offer several advantages in terms of protection, reliability, and

trouble shooting and fault information.

•

Numerical protection devices are available for generation, transmission and distributionsystems.

• Digital protection can be physically smaller , and almost always requiresless panel wiring than equivalent functions implemented using analogtechnology.


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• Numerical relaying along with developments in fiber optic communication have pioneered development of

automated substations.

• #nce, the analog signals from (Ts and )Ts are digiti1ed, they can be converted to optical signals and

transmitted on substation 2!N using fiber optic networ.

• 3ith high level of 4"I immunity offered by fiber optic cable, it has become the transmission medium by

choice in substation environment.

• Numerical relays can be nicely interfaced with a substation 2!N.

• This in turn should be contrasted with legacy substations where in lead wires have to run from each (T and

)T to the control panel.

• This not only reduces wiring complexity in the substation but also reduces burden on the (T as resistances of

long lead wires are eliminated.

• Further, a single fiber optic 2!N permits multiplexing of multiple analog signals which is not possible with

legacy arrangement.

• Numerical relays also permit development of new functions as well as development ofadaptive relaying

schemes.

• Traditionally, relaying systems are designed and set in a conservative manner.

• They represent compromise between5

• economy and performance

• dependability and security

• complexity and simplicity

• speed and accuracy

•credible and conceivable


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• !daptive relaying is meant to minimi1e such compromises and also allow relays to fine%tune

to existing system conditions.

• Numerical relays also permit storage of pre and post fault data &of the order of few cycles'.

• This data can also be time stamped, now%a%days by 6eographical $ositioning System &6$S'.

• 6$S systems &a cluster of 78 satellites of pentagon, 9S!' not only provides positional

information but also a time pulse every second for synchroni1ation of sampling.

• ! phasor measurement unit &$"9', also nown as synchrophasor, is a device capable of

measuring power system voltage and current phasors at a rate of thousands of samples per

second

• The samples are time%stamped with + microsecond or better accuracy to a common absolute

time reference provided by the 6lobal $ositioning System &6$S' receivers attached to $"9s

• IRI6%: is the most commonly used time standard in 9S substations.

• IRI6%: is a standard for representing time

• It is done now with the IRI6%: $rotocol available with the 6$S (loc.

•

It does not define the physical port.• (an be also be transmitted over RS7;7, RS8- and serial fiber opt

• IRI6%: is used to synchroni1e the internal clocs of RT9s and I4


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• 4fficient power transmission and distribution would benefit from synchroni1ed near%real%time

measurements of voltage and current phasors at widely dispersed locations in an electric power

grid.

• Such measurements also have the promise to enable effective real%time system monitoring and

control, which have been considered to be the ey to preventing wide%scale cascading outages.

• Thus, in principle, every sample and every event lie closing or opening of breaers can be time

stamped.

• This helps in postmortem analysis which is used to determine whether &+' a relay operatedcorrectly &or incorrectly' and &7' any other relaying system or device &lie circuit breaer' has

failed to operate.

• Time stamping of relay operation allows us to capture the se0uence of relay operations.

• Thus, in a complex situation lie catastrophic failure of the power system &brown out or blac

out', it is now possible to precisely determine the se0uence of relay operations.

• This helps engineers to capture and simulate the disturbance using transient stability, &4"T$' programs.

• Such simulation studies help in understanding shortcomings of the existing systems and thereby

improvising them.

• In this role, a numerical relay is analogous to a fault data recorder &F


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• Numerical relays also simplify

interfacing with (Ts and )Ts.•(onsider a protective function which

re0uires 1ero se0uence voltage.

•Traditionally, it would be generated

by open delta )T connection shown in

fig.

•If 1ero se0uence current is also

re0uired, it is obtained by using an

additional (T in the ground wire.

• 3ith numerical relays, 1ero se0uence

voltages and currents can be derived

inside the processor from the phase

voltage &)a, ) b, )c' and line currents

&Ia, I b and Ic'.


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• In differential protection e.g., three phase transformer protection, traditional protection

schemes also re0uire additional care to handle polarity, scaling and phase shifting problems.

• This may even necessitate use of an auxiliary (T.

• Such complications can be resolved with ease when numerical relays are used.


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Advantages of numerical relays

• Compact Size

•In the case of electromechanical relay, there is a need for mechanical comparison devices. Thisamounts for the buly si1e of the relay. Then, there is a need for a flag system for activation

confirmation of relay.

• !s opposed to this, the numerical relay relies on one system for all approach and use

indication on 2(< for relay activation, ensuring less space.

• !n important fact to note is that digital protection can be made physically smaller. This is on

the account that it needs less panel wiring than e0uivalent functions implemented using analogtech.

• Flexibility

• Since the numerical relay system relies on software, customi1ed modifications can be made for

getting the desired protection features. This saves you the cost of replacing hardware.

• Reliability

•

#ne basic problem with electromechanical relays is that because of larger components andmass interconnection, component non reliability can be an issue. In the case of numerical

relays, fewer interconnections ensure reliability.


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• Multiple relay characteristics

• The range of operation of traditional models is narrow while numerical relays are diverse and

evolution adaptable. This property of multitasing is further strengthened on the account that

the numerical system can accommodate different types of relay characteristics.

• Since, it is possible to provide better compatible protection characteristics, the efficiency

improves. This is achieved with the memory save feature in the microprocessor

• Communication capacity

• !mong the most significant advantages of a numerical relays is its ability to cater to digital

communication. !n interface is added which brings microprocessor based relay property.Substation 2!N coupled with fiber optics complete the communication capacity.

• The property is directly lined to the data history feature of the numerical relay system.

:ecause of data storage systems, the availability of fault data and disturbance record can be

made. This helps in finding the nature=magnitude=duration of the fault.

• Auto reset

• It has also been highlighted how the system is flexible. In addition to this, it also has thefeature of auto resetting and self%diagnosis.

• !s opposed to numerical relay systems, electromechanical systems do not have the ability to

chec if normal condition has been attained once activated. The self%diagnose and self%reset

features provide less dependence on operating personnel.


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• :y combining several functions in one case, numerical relays also save capital cost and

maintenance cost over electromechanical relays

• Separate connection is not re0uired, 1ero se0uence voltages and currents can be derived inside

the processor.

• :asic hardware is shared between multiple functions, the cost of individual protection

functions can be reduced significantly.

• 2oss of voltage feature helps loc the relay in case of momentary=permanent loss of voltage.

•

Low burden, The microprocessor based relays have minimum burden onthe instrument transformers.

• Sensitivity, Greater sensitivity and high picup ratio.

• Speed, !ith static relays, tripping time of " cycle or even less can beobtained.

• #ast $esetting, $esetting is less.

•

%as Self checing facility• &ermit %istorical data storage

• #iber optical communication with substation L'(

• 'llow G&S )Geographical &ositioning System* Time stamping

• (umerical relays simplify interfacing with +Ts and Ts


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Drawback o numerical relay

Short lie cycle

Susceptibility to transients


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Numerical relay models

• Numerical relay models can be divided into two categories.

•

The models of the first category consider only the fundamental fre0uencycomponents of voltages and currents.

• $hasor based models were the first to be widely used to design and apply relays.

• The models of the second category tae into consideration the high fre0uency and

decaying


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!asic structure o numerical relays

• "wo main parts

• #$% &ar'ware #(% Sotware

• software embedded in a relay decides not only its characteristics but its function as well, i.e.,

whether it is an over current, differential or impedance%based measuring device.

• !n integral and important part of the software is the algorithm, which is a set of mathematical

instructions used to process input currents and=or voltages to estimate system parameters such as

the R"S values of the signal components, measured impedance, fundamental fre0uency and

differential currents etc.

• These calculated parameters are then used to decide whether the system is sound or faulty, and

conse0uently initiate the action necessary to isolate the faulted section.

•


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Relay &ar'ware

• :loc


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Analog )nput Subsystem

• The ;%> voltage and current signals are analog in nature.

•)solation an' analog signal scaling

• (urrent and voltage waveforms from instrument transformers are ac0uired and scaled down to convenient

voltage levels for use in the digital and numerical relays.

• Finally, an antialiasing filter is used after signal conditioning hardware.

• Analog anti*aliasing iltering

• 2ow%pass filters are used to avoid the phenomena of aliasing in which the high fre0uency components of

the inputs appear to be parts of the fundamental fre0uency components.• !nti aliasing filter is a low pass filter &2$F' used to cut off the high fre0uency content &including noise'

in the input signal.

• The analog inputs must be applied to low%pass filters and their outputs should be sampled and 0uanti1ed.

• The use of low%pass filter is necessary to limit the effects of noise and unwanted components of

fre0uencies.

• The cutoff fre0uency of 2$F and the sampling rate have to be properly matched.


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Signal low 'iagram o numerical relay


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Analog*to*'igital con+ersion #ADC%

• :ecause digital processors can process numerical or logical data only, the waveforms of inputs

must be sampled at discrete times. To achieve this, each analog signal is passed through a

sample% and%hold module, and conveyed, one at a time, to an !nalog%to%


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Sample an' &ol' Circuit

• The analog information is held by a Sample and *old circuit.

• !ny !=< converter re0uires a finite conversion time.

• ! S @ * circuit which conceptually is a shunt capacitor with a switch holds the information

&in terms of voltage'.

• 3hile the conversion taes place, switch is in open position.

• This is nown as the Ahold? state. 3hen the switch is closed, the )out of S and * follows the

)in.


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Sampling Scheme

• There are two commonly used schemes for configuring the analog input subsystem.

• #ne is nown as the ?simultaneous? @ other ?non%simultaneous? scheme.• Non*Simultaneous Sampling Scheme

• Fig below illustrates non%simultaneous sampling scheme.

• In this scheme, a multiplexer selects the analog channel se0uentially.

• Typically, power system applications involve more than one analog input.

• To reduce the cost of the hardware, multiple channels are multiplexed through analog

multiplexer to a single !


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Non*Simultaneous Sampling Scheme

In the scheme illustrated in figure, it can be observed

that In the Non%simultaneoussampling scheme, relative pharos information between

two signals is not preserved.

•This is because the samples from different inputs are

not obtained at same instant of time.

•#ne way to overcome, this hardware limitation is to

interpolate the value of the sample from previous values.

•Fig illustrates the concept.•2et )a&t' be sampled first and then ) b&t' be sampled.

•The first two samples of ?a? @ ?b? phases are given by

points ?!? and ?(?. !fter one sampling interval, samples

?:? and ?


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Simultaneous Sampling Scheme

•Fig shows a simultaneous samplingscheme.• In this scheme, all S@* amplifiersare set to hold state simultaneously.•This preserves the relative phaseinformation between multiple analogsignals. Then, the multiplexer selectsthe channel se0uentially.• Typically, digital relays usesuccessive !


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Source Impedance

• "ost signal sources have impedances of less than +. C, so such a maximum source impedanceis usually not a problem.

•

*owever, faster multiplexer rates re0uire lower source impedances.• For example, a + "*1 multiplexer in a +7%bit system re0uires a source impedance less than +./

C.

• 3hen the source impedance exceeds this value, buffering is necessary to improve accuracy. ! buffer is an amplifier with a high input impedance and extremely low output impedance. &SeeFigure below'

• ! buffer on each channel located between the transducer and the multiplexer ensures higher

accuracies by preventing the multiplexerDs stray capacitance from discharging through theimpedance of the transducer.

Buffering signals ahead of the multiplexer increases

accuracy, especially with high-impedance sources of fast

multiplexers


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Multiplexer• Thus, multiplexer is a collection of analog switches.

• 4ach channel can be selected by supplying appropriate binary code to the multiplexer e.g. for -%

channel multiplexer, ; bit address space is re0uired.

• ! chip disable line permits parallel expansion if external logic is used to select desired multiplexer.

• ! multiplexer has two inputs &terminals' for a single channel.

• It provides better noise immunity.

• !ccuracy of the analog multiplexer depends on load impedance at the output terminal.

• Typical recommended value is +/E to +/-.

•

!s Sample &S' and *old &*' circuit has impedance in the range +/-

% +/+7

, no problem is encountered.• (hannel%to%channel cross tal is another non%ideal characteristic of analog switching networs,

especially integrated circuit multiplexers.

• (ross tal develops when the voltage applied to any one channel affects the accuracy of the reading in

another channel.

• (onditions are optimum for cross tal when signals of relatively high fre0uency and high magnitude

such as 8 to ) signals are connected to one channel while +// m) signals are connected to anadacent channel.

• *igh fre0uency multiplexing also aggravate cross tal because the signals couple through a small

capacitance between switch channels.

• 2ow source impedance minimi1es the cross tal and eliminates the charge inection.


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Digital ilter

•


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Relaying har'ware or Metering

• In principle, the hardware setup shown in previous fig. can be used for both measurement and

protection function.

•

*owever, considering the order of difference between current magnitudes in case of fault andload, there can be loss of accuracy during metering applications.

• (onsider a hypothetical case where in maximum load current is +//! and maximum fault current

is 7/ times this load current &7///!'.

• 2et a +7 bit unipolar !


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,pen System Relaying

• #pen system relaying motivated by experiences from energy management field where in a plethora

of manufacturers specific e0uipment has led to difficulty in expanding the system without changing

the entire existing S(!


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Why Digital Signal -rocessing.

•


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Why Digital Signal -rocessing.

• !t this point, a worthwhile observation is that direct analog signal processing is conceptually

much simpler.

• Some of the advantages of digital processing are as follows5

• #peration of digital circuits do not depend on precise values of digital signals.

• !s a result, a digital circuit is less sensitive to tolerances of component values.

• ! digital circuit has little sensitivity to temperature, aging and other external parameters.

• In terms of economics of volume, a digital circuit can be reproduced easily in volume

0uantities.

•

3ith )2SI circuits, it is possible to integrate highly sophisticated and complex digital signal processing systems on a single chip.

• In


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ADC

Ny/uist "heorem


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Ny/uist "heorem

• Transforming a signal from the time domain to the fre0uency domain re0uires the application of the

Ny0uist theorem.

• Ny0uist Theorem5 Sampling rate (f s' L 7 G highest fre0uency component &of interest' in the measured

signal The Ny0uist theorem states that a signal must be sampled at a rate greater than twice the highest

fre0uency component of interest in the signal to capture the highest fre0uency component of interestJ

otherwise, the high%fre0uency content will alias at a fre0uency inside the spectrum of interest &pass%

band'.

• This means that capturing a signal with a maximum fre0uency component of fmax re0uires that it must

be sampled at 7fmax or higher.

• *owever, common practice dictates that while woring in the fre0uency domain, the sampling rate

must be set more than twice and preferably between five and ten times the signalDs highest fre0uencycomponent.

• 3aveforms viewed in the time domain are usually sampled +/ times the fre0uency being measured to

faithfully reproduce the original signal and retain accuracy of the signalDs highest fre0uency

components.

Ny/uist "heorem


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Ny/uist "heorem


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Ny/uist "heorem


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Ny/uist "heorem


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Ny/uist "heorem


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Aliasing

• !ny analog signal consists of components at various fre0uencies. The simplest case is the

sinewave, in which all the signal energy is concentrated at one fre0uency.

•

In practice, analog signals usually have complex waveforms, with components at manyfre0uencies.

• The highest fre0uency component in an analog signal determines the bandwidth of that

signal.

• The higher the fre0uency, the greater the bandwidth, if all other factors are held constant.

• Suppose the highest fre0uency component, in hert1, for a given analog signal is f max.

• !ccording to the Ny0uist Theorem, the sampling rate must be at least 7 f max, or twice thehighest analog fre0uency component.

• The sampling in an analog%to%digital converter is actuated by a pulse generator &cloc'.

• If the sampling rate is less than 7 f max, some of the highest fre0uency components in the analog

input signal will not be correctly represented in the digiti1ed output.

• 3hen such a digital signal is converted bac to analog form by a digital%to%analog converter,

false fre0uency components appear that were not in the original analog signal.

• This undesirable condition is a form of distortion called aliasing.

• Aliasing occurs when a signal is under sampled . If the signal sampling rate is too low, we get

frequency-domain aliasing .


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Aliasing

• If a signal is sampled at a sampling rate smaller than twice the Nyquist frequency, false lower

fre0uency component&s' appears in the sampled data. This phenomenon is called !liasing

• 3hen sampling an ac signal at less than two times the Ny0uist fre0uency, the original

waveform cannot be reproduced faithfully.

• 3hen ac inputs are sampled more than twice the Ny0uist fre0uency of the sine wave, the

fre0uency content of the signal is preserved, and all the Fourier components of the periodic

waveform are recovered.

• 3hen input signals are sampled at less than the Ny0uist rate, ambiguous signals that are much

lower in fre0uency than the signal being sampled can appear in the time domain.

• This phenomenon is called aliasing.• (onversely, input fre0uencies of half or more of the sampling rate will also generate aliases.

• To prevent these aliases, a lowpass, anti%aliasing filter is used to remove all components of

these input signals.

• The filter is usually an analog circuit placed between the signal input terminals and the !


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Aliasing

• These types of systems primarily utili1e low%pass filters, digital filters or a combination of

both.• 3ith the analog low%pass filter, high fre0uency noise and interference can be removed from

the signal path prior to the analog%to%digital &!=


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• 4xamplef h li f i d h h i i l i h f ll i f i


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!ssume f s, the sampling fre0uency, is +// *1 and that the input signal contains the following fre0uencies5

7 *1, E/ *1, +/ *1, and +/ *1. These fre0uencies are shown in the following figure.

's shown in the following -gure, frequencies below the (yquist frequency )f s/ 0 12 %3* aresampled correctly. #requencies above the (yquist frequency appear as aliases. #or e4ample, #5)/1 %3* appears at the correct frequency, but #/ )62 %3*, #7 )582 %3*, and #9 )152 %3* havealiases at 72 %3, 92 %3, and 52 %3, respectively.

Original frequencies present in the input signal

Original and aliased frequencies that appear in the measured signal afterpassing through a ADC'lias #/ 0 :522 ; 62: 0 72 %3

'lias #7 0 : )/*522 ; 582: 0 92 %3'lias #9 0 : )1*522 ; 152: 0 52 %3


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'n interesting analog can be drawn by considering a room having manymirrors each reculty can be resolved if the observer hasan idea of location or coordinates of the real person. =n the samemanner, we can identify the original lobe from replicated lobes if wehave an idea of the frequency content of original signal. =n -g /?.1,notice that lobes are distinctly separated because @ s A /B %3 . Cn the

other hand, if @ s 0 /B %3 , then as seen in -g /?.8, lobes will ust toucheach other. =f however, @ s E /B %3, then lobes will overlap )-g /?.6* and

this will lead to distortion of replicated frequency spectrum. Thus, it isnecessary that @ s the sampling frequency should atleast equal to /B %3.


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Thus, we can classify sampling frequency into three categories.

5. Sampling at a rate

/. Sampling at a rate

7. Sampling at a rate


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Numerical Relaying (2)