6. Comparator

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Different types of relay and

characteristics

Overcurrent relay

Directional overcurrent relay Differential relay

Distance relay

Overvoltage and under voltage relay

Digital relay

Comparators


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Comparator

Phasors are fundamental quantities in the analysis of

ac systems.

A comparator is a design element used in relays to

compare two phasors either in magnitude or phase.A distance relay will always have a phase comparator

or magnitude comparator regardless of the technology

used, i.e., electromechanical, solid-state, and

microprocessor-based relays.


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Cont..

A comparator will give the relay system an output when

the conditions for operation are satisfied.

Two types comparator- phase and magnitude

Phase comparators are used widely in distance relaydesigns.


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Phase Comparator Phase comparators are used to detect when one a.c.

input is more or less than 90 out of phase with a

second a.c. input.

Phase-angle comparator logic circuitry produces anoutput when the phase angle between two quantities

is within certain critical limits.

Either of these two quantities, the polarizing (or

reference quantity) and operating quantity, may becurrent or voltage.


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Phase Comparator

In Figure, given two arbitrary phasors, S1 and S2, the

output of a phase comparator is a logic 1 (the

comparator has operated) if

0901

2

jS Me

S

0 01 290 90S S


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Cont..

The quantity

for the condition to be met and it does not

affect the operation of the phase comparator.

is any arbitrary magnitude


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Characteristic of phase

comparator


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Phase comparator


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Generalized Use of Phase Comparators

Phase comparators are used widely in distance relay

designs The input phasors are generally a combination of

voltages and currents. From these inputs, the ratio

V/I, or impedance, is proportional to the distance to

the fault

In general the input a phase comparator is given by

Constants k1, k2, k3, and k4 are design constants

that may be complex and introduce a phase shift.


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Cont..A general procedure to derive the impedance

characteristic of the comparator on the Z=V/I planewill be developed.

In most applications, k1

and k3 are real numbers


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Cont..

The quantities a and b are vectors and do have the same

units as Z, i.e., they are impedances as well.

In general, a and b will be sufficient to define the operating

characteristics of the unit


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Basic Application Example of a Phase

Comparator

For this purpose, let V and I be the voltage and

current input to the relay and the inputs to the phase

comparator be

where Zc is the relay setting

k1 =1, k2 =Zc, k3 = 1, k4 = 0.


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Characteristic of directional relay on the R-X plane


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Directional property of mho relay


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Cont


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Inputs to cosine-type phase comparator

resulting in trip output


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Inputs to cosine-type phase comparator

causing it to restrain


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Inputs to cosine-type comparator causing it to

be on threshold


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The Sine-type Phase Comparator

00 180m pS S then trip; else restrain


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Magnitude Comparator

Given two arbitrary phasors, SAand SB, the output of

a magnitude comparator is a logic 1 (the

comparator has operated) if

jrA

B

SCe

S

A BS S


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Characteristics of amplitudecomparator

1A Bif S S theoutput is


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Inputs to amplitude comparator resulting intrip output


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Inputs to amplitude comparator causing it torestrain


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Inputs to amplitude comparator causing it to

be on the threshold


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Generalized Use of Magnitude

Comparators

If the inputs SA and SB are expressed in terms of

impedances, either the power-system or relay setting

impedances

with C=1, the operating characteristic of thecomparator in the R-X diagram can be defined.

jrA

B

SCe

S

AS

BS

A BS S


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compare the above with the general equation for the circle


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Basic Application Example of a Magnitude

Comparator

Let V and I be the voltage and current input to the

relay, and the inputs to the magnitude comparator be

where Zc is the relay setting

For a magnitude comparator, the characteristic is

determined by


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Mho unit derived from a magnitude

comparator

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6. Comparator