Switchgear

1
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
2

• 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 Ø, the angle of displacement of
the diameter from Raxis. Angle Ø 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
Zn S1 V Where V fault
voltage from VT secondary winding.
I fault current from CT
secondary winding. Zn impedance setting
of distance relay.
3
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
4
example  A distance relay , which located at
A ( fig below) , protects line AB and BC . the
minimum source MVA (3-ø 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
.
5

•  
• Solution
•  
• Maximum Xs ((132)/(3)(1/3)) /
  (1500/((3)(1/3)132))1322/1500
• j 11.62 ohm / ph (primary)
• Reach of stage 1
• 0.8 0.435 96.5 33.6L70 ohm / ph (primary)
  11.48j31.6 ohm / ph
•  
• Total fault impedance 11.48j31.6j11.26
  44.6L71.8 ohm ohm/ph
•  
• Minimum fault current (132/(3)(1/3))/44.6
  1710 A (primary) 17100/600 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)
•  
• OR
• Reach of compensated impedance 33.6 CT ratio/VT
  ratio 16.8ohm/ph
• Relay Setting relay reach / cost(70-60)
• 17ohm/phase (Typical relay setting range from
  3?20 ohm)

6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
Figure 11.8 Plain impedance relay characteristic
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
Phase sequence
17
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.

18

• 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

19

• Directional over-current relay
• This type of over-current is usually mounted at
  the secondary side of the transformer
• Highest instantaneous relay
• It 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 relay
• Usually 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.

20

• 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

21
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.
22
Generator Protection SchemeGenerator protection
requires immediate disconnection due to
insulation failures. Other faults may be allowed
for some time due to unsatis factory operating
conditions.

• Insulation Failure

23

• Stator Protection
• High impedance differential relay is usual for
  stator protection and is applied on phase by
  phase basis

24

• 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 .

25

• 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.

26

• 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.

27

• 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.

28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
?? ????? ???? ????? ???? ????????? ??? ???????
???? ???????? ???? ?????? ?????????
???? ???? ??????? ????????? ???????
????? ??????? ????????? ???????