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Differential protection : (protective relaying by
Blackburn)Differential principle is applicable to all part of the
power system: generation, motor, buses, transformer, line
capacitor, reactor and some times combination of these. During
normal condition or external fault the sum of the current flowing
into the relays circuit is almost zero .However in internal fault
condition the net current into the relay circuit is not zero. For
example IP is the primary current is shown in fig (6.1)To reduce
this voltage a non linear resistor should be imposed, this resistor
increase the absorbance of current 32 times, if the voltage is
doubled, eg. Voltage across relay current in non-linear resistor
120 0.01 240 0.32 480 10.24 600 30
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Relay Types ( GEC textbook )Plain Impedance Relay ( non-unit )
The relay impedance characteristic is a circle when it plotted on
R-X diagram with its center at the origin, became the relay does
not take into account the phase angle between voltage and current
applied to it Fig(11.5) it is a non directional so that it can
operate for all faults a long the vector GL & GM . it has three
main disadvantages It is non directional and it requires a
directional element It is affected by arc resistance It is highly
affected by power swings because of the large area covered by the
impedance circle
Mho Relay:The characteristic of this relay is a circle, whose
circumference passes through the origin, when it is plotted on R/X
diagram fig (11.10.b) showing the relay is inherently directional
and will operate only for a fault in the forward direction. The
relay is adjusted be setting Zreach (Zn) and , the angle of
displacement of the diameter from Raxis. Angle is known as relay
characteristic angle (RCA). The relay operate for Zf within the
circle. The mho relay characteristic can be obtained by using a
phase comparator circuit which compares input signals S2 and S1 and
operates whenever S2 lags S1 by between 90 and 270 [ ( s1 s2 )
between 90 and 270 ]The two input signals are:S2 = V I * ZnS1 =
VWhere: V = fault voltage from VT secondary winding. I = fault
current from CT secondary winding. Zn = impedance setting of
distance relay.
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REACTANCE RELAY :Reactance relay measures only the line
reactance and does not vary with the presence of arc resistance.
Fig (11-12) shows that any increase in arc resistance will not
change the value of the reactance. reach as the relay continue to
measure the value of reactance X. Distance Protection ( Electrical
Power System ) :The basic idea of using distance protection , is to
eliminate the pilot . Since the pilot can be used up to 30 km in
route length . for a feeder shown in Fig (101) , the line (A-B) is
to be protected by Distance relay , which located at bus (A) .
Distance relay compares system voltage to system current presented
to it to find out the voltage as the VT primary side :
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example : A distance relay , which located at A ( fig below) ,
protects line AB and BC . the minimum source MVA (3- fault level )
at A is 1500 MVA reglect R . The relay characteristic angle 60 .
calculated the setting of the relay to give a reach of 80% at stage
1 of AB .
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Solution : Maximum Xs = ((132)/(3)^(1/3)) /
(1500/((3)^(1/3)*132))=132^2/1500 = j 11.62 ohm / ph (primary)Reach
of stage 1 = 0.8 * 0.435 * 96.5 = 33.6L70 ohm / ph (primary) =
11.48+j31.6 ohm / ph Total fault impedance = 11.48+j31.6+j11.26 =
44.6L71.8 ohm ohm/phMinimum fault current = (132/(3)^(1/3))/44.6 =
1710 A (primary) = 1710*0/600 = 2.8 A(sec)Line voltage at relaying
point = ((3)^(1/3)) * 1710 * 33.6 = 99516 V = 99516 *
(110/132000)=82.9 v (sec) Reach of compensated impedance= ((32.9) /
(((3)^(1/3))*2.85) = 16.8 ohm/ph (scenery) ORReach of compensated
impedance 33.6 * CT ratio/VT ratio = 16.8ohm/phRelay Setting =
relay reach / cost(70-60) = 17ohm/phase (Typical relay setting
range from 320 ohm)
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Figure 11.8 Plain impedance relay characteristic
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Phase sequence
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Electrical power system by guile and pates Quadrilateral
Relay:Quadrilateral relay combines the advantage of reactance with
directional relay and resistive reach control. Characteristic It is
applied for earth fault protection of short and medium lines where
high fault resistance tolerance is required. Fig(11.13) Transformer
Protection Scheme:Transformer are costly units. Thus, protection is
essential to prevent damages might be caused due to internal or
external faults. The types of protective relays are summarized
below.
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Differential Protection: This type of protection is only applied
to large transformers.
Buchholz Protection:The Buchholz relay is the best device
available for detecting incipient (primary) faults and is specially
sensitive to interterm faults. This relay will operate for one of
the reason:1 The introduction of air during filling or because of
mechanical failure of the oil system.2 Gas produced by the
breakdown of the oil.3 Gas produced by the breakdown of the solid
insulation.The gases produced are mainly hydrogen and carbon
monoxide. If the quantity is greater than 1% then sort of action
must be taken. The analysis of the gases may be as follow;
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Directional over-current relay This type of over-current is
usually mounted at the secondary side of the transformer Highest
instantaneous relayIt comes with IDMT over-current relay and they
mounted at the primary side of the transformer. This relay detects
and operates for the primary faults only since the relay setting is
above the maximum of secondary fault level.Restricted earth fault
relayUsually in transformers, earth fault current is limited by the
inclusion of an earthing resistor. The effective operation level of
a restricted earth fault relay is normally less than 25% of the
resistor rating.
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Standby earth fault relay:
This relay has a long time delay and it regarded as a last line
of defense and intend to trip only in the event of a sustained
earth fault condition.
Overload protection:
This type of protection is used for large transformers fitted
with oil and winding temperature indicators 1- If the gas is mainly
hydrogen with less than 2 % carbon monoxide then the fault is
likely involve only the insulation oil.2- If the gas is hydrogen
with about 20 % carbon monoxide then the fault is concerned with
both solid insulation and insulating oil
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Balanced Earth Fault Relay:-
For secondary earth fault on secondary side the current on delta
side is equal and opposite in two phases and therefore the output
to the relay will be zero. Thus, the relay will operate only for
single earth fault on the delta side.
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Generator Protection Scheme
Generator protection requires immediate disconnection due to
insulation failures. Other faults may be allowed for some time due
to unsatis factory operating conditions.
Insulation Failure:
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Stator Protection:High impedance differential relay is usual for
stator protection and is applied on phase by phase basis
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Earth Fault Protection :Earth fault protection by an over
current relay is essential to compliment the differential
protection scheme and to provide a back-up protection for the
differential relay. When the generator is directly connected to the
power system i.e- without generator transformer, it provides a
back-up protection for the bus bars and the whole system. In this
case it should have a very long time delay and should be thought of
as the last line of defense.
Rotor Earth Fault Protection :Earth fault an the rotor will not
cause any current to flow to earth and does not , therefore ,
constitute a dangerous condition . It a second fault happened , a
portion of the field winding will be short circuited and resulting
in an unbalanced magnetic pull on the rotor. Thus , earth fault
protection is essential.
Unsatisfactory Operation Condition :The conditions in general
does not require immediate disconnection .
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Unbalanced loading :Unbalanced loading of the generator phases
results in the production of negative phase sequence ( NPS )
currents.These currents will have a phase rotation in the opposite
direction to the normal phase rotation, produces a magnetic field
which induces currents in the rotor at twice the system frequency.
Time will result in considerable heating in the rotor and would
cause damage if allowed to persist.
Overload:Overload protection can be achieved by embedded a
thermometer in the stator winding. Overload relay operates over
hundreds to thousands range where an over current relay operates in
the one-ten second range.
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Failure of prime mover:In the failure of prime mover, the
generator continues to run, but as a synchronous motor and this can
cause dangerous condition in the prime mover. To prevent this, a
reverse power relay should be applied.
Loss of Field:Failure of the field system results in
acceleration of the rotor to above synchronous speed where it
continue to generate power as an induction generator. This
condition can be protected by undercurrent relay.
Over speed:The rotor speed is controlled by the governor and
steam valve. A sensitive under power relay is used to detect when
rotor is over speed.
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Overvoltage:Voltage is generally controlled by a high-speed
voltage regulator. An instantaneous relay set to 150% is used to
cater for defective operation of voltage regulator.
Protection of Generator/Transformer units:This can be achieved
biased differential protection.
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Dr. Adnan H. Kaki*
Dr. Adnan H. Kaki*