Relay operation principles

RELAY OPERATION PRINCIPLES
11/26/2014 1

 2.1 Detection of fault :-
 In general, as faults (short-circuit) occur, current increase in magnitude, and
voltages go down. Besides these magnitude changes of the ac quantities, other
changes may occur in one or more of the following parameters:
 Phase angles of current and voltage phasors.
 Harmonic components.
 Active and reactive power.
 Frequency of the power system.
 Relay operating principles may be based upon detecting these changes, and
identifying the changes with the possibility that a fault may exist inside its
assigned zone of protection . We will divide relays into categories based upon
which of these input quantities a particular relay responds to :-
 Level detection :- this is the simplest of all relay operating principles. Fault
current magnitudes are almost always greater than the normal load current
that exist in a power system.
 As an example, consider the motor connection to a 4kv power system. The
full load current for the motor is I=245 ampers.
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Allowing for an emergency overload capability of 25%, a current of I
=1.25×245=306 amperes or lower should correspond to normal operation .
Then any current above a set level(346 amperes ) may be taken to mean
that a fault, or some other abnormal condition exists inside the zone of
protection of the motor. The relay should be designed to operate and trip
the circuit breaker for all currents above the setting value.
The level above which the relay operate is known as the pickup setting of
the relay . For all currents above the pickup , the relay operates, and for
currents smaller than pickup value, the relay takes no action.
The operating characteristic of an overcurrent relay can be presented as a
plot of the operating time of the relay versus the current in the relay.
It is best to normalize the current as a ratio of the actual current to the
pickup setting(Ip).
In=
퐼
퐼푃
,the operating time for In less than 1 is infinite, which for values>1 ,
the relay operates. The actual time for operation will depend upon the
design of the relay.
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The characteristics show the ideal and practice relay level
detector relay also known as over current relay ( OV relay )
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b) Magnitude comparison
This operation principle is based upon the comparison of one
or more operating quantities with each other. For example , a
current balance relay may compare the current in one circuit
with the current in another circuit ,which should have equal
or proportional magnitudes under normal operating
conditions . The relay will operate when the current division
in the two circuit varies by a given tolerance .
The figure below shows two identical parallel lines which are
connected to the same bus at either end . One could use a
magnitude comparison relay which compares the magnitude
of the two line current 퐼퐴 > 퐼퐵 + ϵ
Then with the line B is not open , would declare a fault on
line A and trip it ,where ϵ is a suitable tolerance .
Similar logic would be used to trip line B .
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c) Differential comparison
It is one of the most sensitive and effective methods of providing
protection against the faults . Consider the generator winding
shown in the figure , then as the winding is electrically continuous
, current entering one end I1`must equal the current leaving the
other end I2` .
One could use a magnitude comparison relay described below to
test for a fault on the protected winding .
When a fault occurs between tow ends the two current are no
longer equal alternatively , one could from an algebraic sum of the
two current entering the protected winding ,(I1-I2) and use a level
detector relay to detect the presence of a fault. In either case , the
protection is termed a differential protection . In general , the
differential protection principle is capable of detecting very small
magnitudes of a fault current .
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Thy only drawback of this technique it requires current
from both ends of zone of protection which restricts its
application to power apparatus ( Transformer ,
Generators , Motors , buses , capacitors , reactors …) .
d) Phase angle comparison
This type of relay compares the relative phase angle
between to Ac quantities . Phase angle comparison is
commonly used to determine direction of a current (
w.r.) to a reference quantity . For instance , the normal
power flow in given direction will result in the phase
angle between the voltage and the current varying
around its power factor angle say approximately ±30˚ .
When power
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flow in opposite direction ,this angle will become (180˚±30˚)
similarly , for a fault in forward or revers direction the phase
angle of the current ( w.r.t ) the voltage will be –Φ and ( 180˚-
Φ ) respectively ( Φ is impedance angle of the fault circuit )
as shown in figure .
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This difference in phase relationships created by a fault is
given by making relay which response to phase angle
differences between two inputs ( fault voltage and fault
current ) .
E ) Distance Measurement :-
As discussed above , most positive and reliable type of
protection compares the current entering the circuit with the
current leaving the it on transmission lines and feeders , the
length ,voltage, and configuration of the line may make this
principle uneconomical. Instead of comparing the local line
current with the far end line current ,the relay compares the
local current with the local voltage .
This in effect , is a measurement of the impedance of the line
as seen from the relay terminal .
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An impedance relay relies on the fact that the length of the
line (its distance ) for a given conductor diameter spacing
determines its impedance .
F ) pilot relaying :-
Certain relaying principles are based upon information
obtained by the relay from a remote location , the
information could be in the form of the contact status ( open
or closed ). The information is sent over a communication
channel using power line carrier , microwave , or telephone
circuits .
G ) Harmonic content :-
Current and voltage in a power system usually have a
sinusoidal waveform of the fundamental power system
frequency . There are however , deviations from a pure
sinusoid , such as the third harmonic voltage and currents
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produced by generators , that are present during
normal system operation , other harmonic occur
during abnormal system conditions , such as the odd
harmonics associated with transformer saturation or
transient components . These abnormal conditions
can be detected by sensing the harmonic content
through filters in electromechanical or solid – state
relays , or by calculation in digital relays. After the
detection of these harmonics , a decision can be
made and control action is required .
H ) Frequency sensing :-
Normal power system operation is at 50 or 60 Hz . Any
deviation from these values indicates that a problem exists .
Frequency can be measured by filter circuit , by counting of
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zero crossing wave forms in a unit of time , or by
special sampling and digital computer technique
frequency – sensing relays may be used to bring
system frequency back to normal by some corrective
actions .
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 Relay design
The various input quantities described above , upon which
fault detection is based ,may be used either single or in any
combination to calculated power ,power factor ,
directionality , impedance ….. Some relays are also designed
to respond to mechanical devices such as fluid detectors ,
pressure or temperature sensors . Relay may be constructed
from electro mechanical element such as solenoids ,
induction discs , solid – state elements , digital computers
using analog – to – digital converter and microprocessors .
However , the construction of a relay does not inherently
change the protection of a relay dose not inherently change
the protection concept , although there are advantages and
disadvantages associated with each type .
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Some relay design aspects are :
Relays are devices requiring low level inputs ( voltages ,
currents , or contacts ).
 Derive their inputs from transducers such as current or
voltage transformers and switch contacts .
 They are fault detecting devices only and require an
associated interrupting device ( a circuit breaker ) to clear
the fault .
 Separating the fault detection function from the
interruption function gave the relay designer an ability to
design a protection system that match the needs of the
power system .
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 Fuses
 The fuse is the oldest and simplest of all the
protective device . The main characteristics are :-
1. It is a level detector .
2. It is both the sensor and the interrupting device .
3. It is installed in series with the equipment being
protected .
4. Operating by melting a fusible element in response to the
current flow . The melting time is inversely proportional
to the magnitude of the current flowing in the fuse .
5. It is one – shot device since the fusible link is destroyed in
the process of interrupting the currents.
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6. Fuse may only be able to interrupt current up to
their maximum short–circuit rating before it
reaches its maximum value .
7. Its application is restricted for radial feeders such as
distribution lines or auxiliary systems of power
plants .
 The two major dis advantages are :-
a) The single –shot feature requires that a blown fuse be
replaced before service can be restored ,this means a
delay ,and the need to have the correct spare fuses . To
avoid this, it is possible a multi-shot feature by installing
a number of fuses in parallel with a control unit to
transfer to another one.
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b) In a three-phase circuit ,a single phase-to- ground fault
will cause one fuse to blow, de energizing only one phase,
permitting the connected equipment-such as motors to stay
connected to the remaining phases , with excessive heating
and vibration because of the unbalanced voltage supply .
2.3 Electromechanical Relays :-
The early replay designs utilized actuating forces that were
produced be electromagnetic interaction between currents
and fluxes. Some relays were also based upon the forces
created by expansion of metals caused by temperature rise
due to a flow of current. Two main types of
electromechanical replays and in use , the plunger-type relays
are usually driven by a single actuating quantity , while the
induction –type relays may be activated by single or multiple
inputs .
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a) plunger –Type Relay
Consider a round moving a stationary electromagnet as
shown in the figure. With no current in the coil, the plunger
is held partially out side the coil by the force Fs produced by a
spring. Let X be the position of the plunger tip inside the
upper opening of the coil. When the coil is energized by a
current i , and saturation phenomena are neglected ,the
energy W(λ, i) and the co-energy w1(i,x),stored in the
magnetic field are given by :
푤 휆 , 푖 = 푤1 푖, 푥 =
1
2
푙푖2 ______(2.1)
Where λ is the flax linkage of the coil and L is the inductance
of the coil which can be given for this magnetic circuit as
퐿 =
훍̻휋 푑2푁2
4(푋+
푔푑
4푎
)
, N :number of turns in the coil
a: height of the pole –face
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The force which tries to pull the plunger inside the coil given
by
푓푚 =
훅
훅푥
w` 푖, 푥 = 푘
푖2
푔푑
4푎
푥+
2 ……………(2.2)
Where k is a constant depending upon constants of the
magnetic circuit shown on the figure .
The plunger moves when 푓푚 exceeds 푓푠 .if the current is
sinusoidal with an rms value of I ,the average force is
proportional to I² . The valve of the current (퐼푝) at which the
plunger just begins to move is known as the pickup setting of
the replay is given by :
퐼²
푓푚 = 푓= 푘
푝푠 푔푑
4푎
(푥 +
)²
then
퐼푝=[
푓푠
푘
] x +
푔푑
4푎
… … … … ……(2.3)
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Where x is the displacement of the plunger when no
current is flowing in the coil (stating position ). The
plunger travels some distance from x to x1 before it
closes its contacts and hits a stop . The energizing
current must drop below a value 퐼푑 , known as the
dropout current before the plunger can return to tis
original position x . The dropout current is given by :
퐼푑=[
푓푠
푘
] 푥1 +
푔푑
4푎
… … … … ……(2.4)
퐼푑<퐼푝 푎푠 x < 푥1
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The operating time of the relay depends upon the mass of the
plunger .
For a normalized current of magnitude 퐼푛 (actual current divided
by the pickup current ), the accelerating force on the plunger is :
퐼퐼2
F=퐹푛 푃
푚 − 퐹푆 = 푘
푔푑
4푎
푋+
2 − 퐹푠 , substituting for 퐼푝 from equation
(2.3) then ,
F= 퐹푠[
(푥0+푔푑/4푎)²
푔푑
4푎
(푥+
)²
퐼푛² − 1] … … … … (2.5)
The equation of motion for the plunger is ,
M푋 = -F …………….(2.6)
Where m is the mass of the plunger.
Equation (2.6)can be integrated twice to provide the operating
time of the relay (the time it takes the plunger to travel from
푥 푡표 푥1). The integrals in eqn (2.6) are elliptic integrals and must be
evaluated numerically for given displacements .
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The general shape of the replay characteristic list operating
time plotted as a function of the current through the coil is as
shown in this figure. Operating time calculation is given in
the next example.
Ex: consider a plunger-type relay with a pickup current of 5
amperes (rms).the pole face has a height of 1.5 cm ,while the
spring holds the plunger 1cm out of the coil when the
current is below the pickup value. The air gap g=0.2cm , and
gd/4a=0.05 , let the spring force be a constant ,with a value of
0.001 Newton , and let the mass of the plunger be 0.005kg .
Let the travel of the plunger be 3mm before it hits a stop and
closes its contacts . Calculate :
a)The dropout current to its pickup current
b)The accelerating force on the plunger for 퐼푛=2, and x=0.8
cm
c)The operating time of the relay for closes its contacts.
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Sol: use the sam construction of plunger-type relay was given
in section (2.3)then :
a)From eqns (2.3)and(2.4),we have for dimension in cm:-
푔푑
퐼(푥+
)
푑
1 4푎
(0.7 + 0.05)
=
=
= 0.714
퐼푝
(푥0+푔푑/4푎)
(1 + 0.05)
b)From eqn (2.5),the accelerating force F is :
F=퐹푠
푥0+
푔푑
4푎
2
푥+
푔푑
4푎
2 퐼푛
2
− 1 = 0.001
1.05 2
푥+0.005 2 2 2 − 1
F=0.001
4.41
0.7225
− 1 = 5.1 × 10−3N.
c)Using eqns (2.5)and (2.6) them
0.005 푥 =-0.001[
(1.05)²
(푥+0.005)²
퐼푛² -1 ]= -F
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The operating time can be calculated using a constant force
equal to the average taken over its travel from 푥0푡표 푥1 then
from the above eqn use x= 푥0 =1cm and x= 푥1=0.7 cm and
calculate F in both cases.
For x= 1cm
퐹1 = 0.001( 퐼푛² - 1 ) newton
For x= 0.7cm
퐹2 = 0.001(1.96 퐼푛² - 1 ) newton
Fav = 0.001(1.48 퐼푛² - 1 )newton
Using this expression for the force ,then the approximate
equation of motion for the plunger is :
0.005푥 = −0.001[1.48 퐼푛² - 1 ]
푥 = 0.2[1.48 퐼푛² − 1 ]
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푥1 푑푥 = 0.2[1.48 퐼푛² - 1 ] 0
푥0
푡
푑푡
t²=
10(푥0−푥1)
1.48 퐼푛² − 1
Or t =
10(푥0−푥1)
1.48 퐼푛² − 1
=
0.3
1.48 퐼푛² − 1
=
0.3
0.48
= 0.79 sec. approximately
This relation shows the inverse –time behavior of the relay for
larger values of 퐼푛 and can be drown for 퐼푛 ≥ 1 .
Most plunger relays also have several taps available on the
winding of the coil to adjust the pickup current over a wide
range (tap setting 1,2,….10 amperes ). Also the pickup can be
controlled by adjusting the plunger within the coil. Plunger –
type relays will operate on dc as well as on ac current .
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b) Induction – type relays
These relays are based upon the principle of operation of the single
phase ac motor. As such ,they cannot be used for dc currents. There
are two types of these fairly standard : one with an induction disc,
and the other with an induction cup. In both cases, the moving
element (disc or cup) is equivalent to the rotor of the induction
motor. The figure below shows the principle of construction of an
induction disc relay. The moving element acts as a carrier of rotor
currents, while the magnetic circuit is completed through
stationary magnetic elements. Induction type relays require two
sources of alternating magnetic flux in which the moving element
may turn. The two fluxes must have a phase difference between
them, otherwise no operating torque is produced. Let us assume
that the two currents in the coils of the relay, 푖1 푎푛푑 푖2 are
sinusoidal :
푖1 (t)=퐼푚1cos wt , and 푖2 (t)=퐼푚2cos (wt+θ)
λ1 푡 = 퐿푚 퐼푚1cos wt , and λ2 푡 ² = 퐿푚 퐼푚2cos (wt+θ) where Lm is
mutual inductance
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Each of these flux linkages in turn induces a voltage in the
rotor, and since the rotor is a metallic structure with low self-inductance,
a rotor current in phase with the inducted
voltages flows in the rotor. Assuming the equivalent rotor
resistance to be 푅1, the induced rotor current are given by :
푖푟1 푡 =
1
푅푟
푑λ1
푑푡
= -
ω퐿푚퐿푚1
푅푟
sin ωt ………….(2.7)
푖푟2 푡 =
1
푅푟
푑λ2
푑푡
= -
ω퐿푚퐿푚2
푅푟
sin (ωt+θ) ………….(2.8)
Each of the rotor current interacts with the flux produced by
the other coil, producing a force. The two forces are in
opposite direction w.r.t each other, and the net farce, or the
corresponding net torque T is given by :
T [λ1 푖푟2-λ2푖푟1 ], substituting for λ,i and simplify to get
T= k 퐼푚1 퐼푚2 [cos ωt+θ)-cos(ωt+θ)sin ωt] ……….(2.9)
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Or using trigonometric identity, the net torque is
T = k 퐼푚1 퐼푚2 sinθ ………………(2.10)
Not that the net torque is constant in this case and dose not change
with time. If the phase angle between the two coil currents is zero,
there is no torque produced. By an appropriate choice of the source
of the two coil currents, this relay could be made to take on the
characteristic of :
 A level detector (relay)
 A directional relay
 A ratio relay
For example, by using the same current to flow through the two
coil, one could make a level detector. The phase shift between
currents can be produce by placing in parallel with one of the coil a
shunt with an impedance angel that is different from that of the
coil, then the relay will produce a torque .
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For 퐼푚1 = 퐼푚2= 퐼 푡ℎ푒푛 푒푞푛 2.10 reduced to
T = 푘1I² …………(2.11)
Where 푘1=k sin θ
When the torque produced by the current (the pickup
current of the relay) just exceeds the spring torque Ts, the
disc begins to turn. After turning and angel φ ,the relay closes
its contacts .
E 풙ퟎ:- consider an induction disc relay, designed to perform
as an overcurrent relay. The spring torque Ts is 0.001 Nm, and
the pickup current of the relay is 10 amperes. The constant 푘1
is 10−5 , the moment of inertia of the disc is 10−4kg-m²
a) Give an expression for the accelerating torque.
b) Develop the inverse relation of the operating time w.r.t
the normalized current for angel of rotation (0__2˚)
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c) Calculated the operating time of the relay to close its
contacts over rotation period of (0≤φ≤2˚).
Sal:-
a) From eqn (2.11), the magnetic torque is
푇푚 = 푘1 퐼푛퐼푝
2
= 10−5 10퐼푛
2 = 10−3퐼푛²
The accelerating torque on the disc is the difference between
the magnetic torque and the spring torque :
T=푇푚 − 푇푠 = 10−3(퐼푛² -1)
b) The equation of motion of the disc is
Jθ =T where J :moment of inertia
10−4θ = 10−3 (퐼푛² -1)
(θ is angel of rotation of the disc o≤θ≤φ)
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For φ=2˚=0.035 radian, in tegreting the equation of motion
twice to get θ=5(퐼푛² − 1)푡² and the operating time of the
relay is
t=
0.035
5(퐼푛²−1)
sec
c) The operation time after relay closes its contact (퐼푛 ≥ 1)
take 퐼푛= 1.01 then
t =
0.035
= 0.59 sec
5[ 0.01 2−1]
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2.4 solid –state relays:-
The expansion and growing complexity of modern power systems
have brought a need for protective relays with higher level of
performance and more sophisticated character is tics .this has been
made possible by the development of solid-state or static relays
where all of the functions and characteristics available with
electromechanical relays has can be performed by solid-state
disadvantages as follow:
Advantages:-
1-Use low power compare and devices.
2-Springs and driving torques from the input quantities are not
presented.
3-High reliability than other electromechanical relays.
4-Performance and economic advantages.
5-More flexible and reduced size devices.
6-More accurate in setting process.
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7-Their characteristics can be shaped by adjusting logic
elements as the fixed characteristics of induction relays.
8-Solid-state relays are not affected by vibration or dust
and require less mounting space.
Disadvantages:-
1- Limited capability to high temperature and humidity,
over voltages and over currents .
2-It requires independent power supplies .
3-Solid-state relays are designed, assembled and tested
as a system which puts the overall responsibility for
proper operation on the manufacturer.
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 Types:-
 Solid-state relay circuits may be divided into tow categories:
a) Analog circuits that are either fault- sensing or measuring
circuits.
b) Digital logic circuits for operation on logical variables.
These circuits may arrange to provide desired relay characteristics
such as :
a) Solid – state instantaneous over current relays .
b) Solid – state distance relays .
We shall consider these as a circuit configuration and its principle
of operation and characteristics as this are in use .
a) Solid – state instantaneous overcurrent relays :
Consider the circuit shown below . The input current I is passed
through the resistive shunt R , full – wave rectified by the bridge
rectifier , filtered to remove the ripple by the R-C filter , and
applied to a high – gain summing amplifier A .
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The other input of the summing amplifier is supplied with an adjustable
reference voltage er . when the input an the positive input of the summing
amplifier exceeds the reference setting, the amplifier output goes high
and this step change is delayed by a time-delay circuit in order to provide
immunity against spurious transient signals in the input circuit. wave
forms at various points in this circuit are shown in figure far an assumed
input fault current of a magnitude above the pickup setting er of the relay.
By making the time-delay circuit adjustable, and by making the mount of
delay depend upon the magnitude of the input current, a time-delay
overcurrent relay character is tic can be obtained
b)Solid –state distance relays :-
The type of relays are designed to be used as impedance, mho relays for
the application of transmission lines protection. It measures the voltage
and current at the location of the relay at on end of T.L. then the
impedance, reactance or the distance to fault location is measured. The
impendence characteristics is represented as R-X circle and the
measured impedance is projected on this diagram to detect the fault and
producing the trip or block signals. An analog circuit may be designed to
measure the angle between the input current and voltage and then these
is supplied to a logic circuit to get the tripping signal for the internal fault
.
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2.5 computer relays :-
With the advent of rugged, high performance micro process it is obvious that
a digital computer can perform the same function as other techniques for
detecting and decision. Since the usual relay input consist of power system
voltages and currents, it is necessary to obtain a digital representation of
these parameters. This is done by sampling the analog signals, and using an
appropriate
Computer algorithm to create suitable digital representation of the signals.
This is done by a digital filter algorithm. The functional blocks shown
represent a possible configuration far a digital relay(computer relay) the
current and voltage signals from the power system are processed by signal
conditioners consisting of analog circuits such as transducers, surge
suppression circuits, anti-aliasing fitters before being sampled and converted
to digital form by the analog to digital converter. The sampling clock provides
pulses at sampling frequencies (8-32)time the power system frequency. The
relaying algorithm processes the sample data to produce a digital output.
Computer relays will play a very important role in the protection ,control, and
monitoring of power system it has many advantages such as :
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1. Its ability to diagnose a complex cases without great effort,
cost, and complexity as for analog relay.
2. It provides a communication capability that allows it to warn
system operation when it is not functioning properly.
3. Permits remote diagnostics, and possible correction, and
provides local and remote readout of its settings and operation
.
4. Another dimension was added to the reliability of the
protection system.
5. The ability to adapt itself in real time to changing system
conditions, and setting, and other characteristics.
6. Data sharing abilities of microprocessors with the central
control computer or other protection terminals.
One of important disadvantages is the need to the interface unit for
mixing of digital and analog relays within a common overall
protection system.
Indicating light are used for targeting and trace the tripping
sequence and other information in the system.
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Numerical relays :-
The can viewed as natural development of digital relays as a result
of advances in technology, they use a specialized digital signal
processor (DSP) as the computational hardware together with the
associated software tools.
The input analogue signals are converted into digital
representation and processed using mathematical algorithm.
Processing is carried out using a specialized microprocessor that is
optimized for signal processing application (DSP). Single item of
hardware is used to provide a rang of functions.
Advantages :-
1-several setting groups 2-wider range of parameter
3- remote communication built in 4-internal fault diagnosis
5-power system measurements available , 6-distance to fault
locator 7- disturbance recorder 8- CB monitoring (state,
condition) 9- backup protection function in – built
11/26/2014 43

Relay operation principles

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