Need for protection Nature and causes of faults Types of faults Fault current calculation using symmetrical components Zones of protection Primary and back up protection Essential qualities of protection Typical protection schemes.

1. DINESH SHARMA
B.TECH.(EEE)
SHARDA UNIVERSITY

2.  Need for protection
 Nature and causes of faults
 Types of faults
 Fault current calculation using symmetrical
components
 Power system earthing
 Zones of protection
 Primary and back up protection
 Essential qualities of protection
 Typical protection schemes.

3.  What is the power system ?

6.  Heavy currents associated with short circuits is to
cause for damage
 If short circuit presents on a system for longer
time , it cause to damage the some of sections in
system
Fire exists
System voltage reduces to low level and generators in
power station may lose synchronism
By un cleared fault to cause for failure of the system
 To protect the system from faults need to use
automatic protective devices like
Circuit breakers
Protection relays ,isolate the faulty element

7.  C.B can disconnect the faulty element of the system
 Protective relay is to detect and locate a fault and
trip C.B
 Protective relay senses abnormal conditions in
system and gives alarm signal
 Those abnormal conditions are
 short circuits
 Over speed of generators & motors
 Over voltage
 Under frequency
 Loss of excitation
 Over heating of stator& rotor of an alternator

9.  Causes of fault on over head lines
 Birds bodies touch the phases
 Direct lighting strokes
 Snakes
 Ice
 Snow loading
 Earth quakes
 Causes of faults in case of cables,
transformers, generators
 Failure of solid insulation
 Flash over due to over voltages

12. AVERAGE 400 KV LINE FAULT FREQUENCY STATISTICS BY
FAULT CAUSE PER 100 KM PERYEAR

14.  Unsymmetrical faults
 Single phase to ground fault(L-G)
 Line to line fault(L-L)
 Double line to ground fault(L-L-G)
 Open circuited phases fault
 Winding faults
 Symmetrical faults
 3phases to ground fault(LLLG)
 3phase fault

15.  All 3 phases are short circuited
 In case of symmetrical fault current is equally
shared by the 3phases
 In symmetrical to analysis fault by one phase
only
 Only +Ve sequence component exist in
symmetrical
 Remaining two components always zero.

16.  Caused by a break in the conducting path
 Occurs when
 one or more phase conductors break
 cable joint or a joint on the over head lines fails
 Fault rises when C.Bs or isolators open but fail
to close one or more phases

17.  Occurs on the alternator, motor &
transformer windings

19.  L-G 70-80%
 L-L 17-10%
 LL-G 10-8%
 3 PHASE 3-2%

20.  Heavy short circuit current cause to damage to
equipment
 Arcs associated with short circuits may cause
fire
 Reduction in the supply voltage of the healthy
feeders
 Loss of industrial loads
 Heating rotating machines due to reduction in
the supplyV&I
 Loss of stability

21.  Separate protective schemes for each piece of
equipment or element of the power system.
 dependence on the equipment divided in to a
number of the zones
 Each zone covers one or a the most two elements
of the power systems
 The protective zone plan to cover the entire power
system.
 Adjacent protective zones must over lap each
other
 A relatively low extent of over lap reduces the
probability of faults in that region.
 Tripping of too many breakers does not occurs
frequently.

26.  Every zone have a suitable protection scheme
 If fault occurs in a particular zone, it is duty of
primary relays of the zone to isolate the
faulty element.
 If its fails, back up protection scheme is to
clear the fault.
 The protection schemes improves the
system performance

27.  It operates after a time delay to give the
primary relay sufficient time to operate.
 When a back up relay operates a larger time
of the power system is disconnected from the
power sources.
 Types of back up relays are
 Remote back up
 Relay back up
 Breaker back up

28.  It is cheapest and simplest
 It is widely used for back up protection of
transmission lines
 Most desirable
 It will not fail due to the factors causing the
failure of the primary protection

29.  Additional relay is provided for protection.
 It trips the same CB if the primary relay fails
and this operation takes place with out delay.
 Costly
 Used where back up is not possible.

30.  Bus bar fault :
When protective relay operates in
response to a fault but the C.B fails to trip,
the fault is treated as a bus bar fault.
 used for bus bar system , where a number of
C.B’S are connected to it.
 At time of bus bar fault , necessary that all
other C.Bs on the bus bar should trip.

31. A
C D
E
Breaker 5
Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F

32.  The basic requirements of a protective
system are
 Selectivity or discrimination
 Reliability
 Sensitivity
 Stability
 Fast operation

33. The selectivity of protective system dependence
on
 quality of a protective relay
▪ It is able to discriminate between in the protected section &
normal section.
▪ It should be able to distinguish whether a fault lies with in its
zones of protection or outside the zone.
 The relay able to discriminate between a fault
& transient conditions
 power surges
 inrush of a transformer’s magnetizing current.

34.  When a transformer is first energized, a
transient current up to 10 to 50 times larger than
the rated transformer current can flow for
several cycles
 inrush happens when the primary winding is
connected at an instant around the zero-crossing
of the primary voltage (which for a pure
inductance would be the current maximum in the
AC cycle).
 In the absence of any magnetic remanance from
a preceding half cycle, the effective magnetizing
force is doubled compared to the steady state
condition

35.  The typical value of reliability of protective system is
95%
 reliability dependence on
 Robustness
 Simplicity
 In case of relay
▪ Contact pressure
▪ Contact material of relay
 To achieve a high degree of reliability
 Design
 Installation
 Maintenance
 Testing of the various element of the protective system.

36.  It dependences on pick up current value
 Pick –up current:-
a protective relay should
operate when the magnitude of the
current exceeds the present value.
 A relay should be sensitive to operate
when the operating current just exceeds
its pick up value.

37.  Protective system should stable for
 Large current due to fault
 Internal fault
 External fault

38. It means
 Isolate the faulty element quickly.
 To minimize damage to the equipment .
 To maintain system stability
 To avoid the loss of synchronism
 The operating time of the protective system should
not exceed the critical clearing time.
Critical clearing time:- the minimum time taken by
protective relay to clear the fault.
 By fast operation of protection system to avoid
 Burning due to heavy fault current
 Interruption of supply to consumers
 Fall in system voltage it results Loss of industrial loads
 Operating time is usually in one cycle or half
cycle.

39.  A protective scheme is used to protect an
equipment or a section of the line
 The protective schemes are
 Over current protection
 Distance protection
 Carrier current protection
 Differential protection

40.  This protection includes one or more over
current relays.
 over current relay operates when the current
exceeds its pick up value.
 It is used for the protection of
 Distribution lines
 Large motors

41.  It includes number of distance relays of
same or different.
 Distance relay measures the distance
between the relay location and the point of
fault in terms of impedance, reactance
 RelayType Measurement
1. Impedance relay impedance
2. Reactance relay reactance
3. Mho relay admittance
 This protection scheme is used for the
protection of
 transmission lines
 Sub- transmission lines

42.  In this used relays are distance type.
 Their tripping operation is controlled by the
carrier signal.
 A transmitter and receiver are installed at each
end of transmission line to be protected.
 Information of direction of fault current is
transmitted from one end of the line section to
the other by carrier signal.
 Relays placed at end
 It trips if the fault lies with in their protected
section.
 Do not trip for external fault.
 It is used for the protection of EHV &UHV line
(132kv above)

43.  C.T’S are placed on both side of each windings of a machine.
 The outputs of their secondaries are applied to the relay
coils.
 The relay compare the current entering & leaving of a
machine winding
 This difference in the current actuates the relay.
 In case of internal fault , the current values are not equal.
 The relay inoperative under normal condition and external
fault.
 It is used for the protection of
 Generators
 Transformers
 Motors of various size
 Bus zones
 In bus zone protection, C.T’S are placed on the both sides of
the bus bar.

46.  Relations of voltage components in matrix
form
 Relations of current components in matrix
form

59.  No grounding
 Solid grounding
 Neutral impedance grounding
 delta

64.  The performance of the system in terms of the
short circuits, protection etc. is greatly affected
by the state of the neutral.
 There are various method of grounding the
neutral of the system.
 Solid grounding
 Resistance grounding
 Reactance grounding
 Voltage transformer grounding
 Zig-zag transformer grounding

65.  In case of L-G fault at phase C
 charging current = 3*(per phase charging current)
 The voltages of the healthy phases rises to
0.866Vph
 These voltages can be eliminated by connecting
an inductance of suitable value between the
neutral and the ground or Arcing ground.

66.  The value of the inductive reactance is such
that the fault current balances exactly the
charging current .
 Resonant grounding is also called ground
fault neutralizer or Peterson coil.
 The resonant grounding will reduce the line
interruption due to transient line to ground
fault.

67.  The neutral is connected directly to the
ground with out any intentional impendence
between neutral and the ground.
 For low voltages up to 600volts and above 33
kV.

68.  The value of the resistance commonly used is quite
high (in order to limit power loss in resistor during
LG-fault) as compared with the system reactance.
 It is normally used where the charging current is
small i.e., for low voltage short length over head
lines.
 It reduces the ground hazards.
 It has helped in improving the stability of the
system during ground fault by replacing the power
dropped as a result of low voltage, with an
approximating equal power loss in the resistor.
 To limit the stator short current .
 Generators are normally provide with resistance
grounding.

69.  Reactance grounding system (Xo/X1) >3
 Solid grounding system (Xo/X1) < 3
 Reactance grounding may be used for grounding
the neutral of synchronous motors &
synchronous capacitors and also for circuits
having large charging current
 For medium voltages 3.3kV & 33KV resistance or
reactance grounding is used.

70.  It is used , if neutral point is requied which
other wise is not available (eg. Delta
connection, bus bar points)

71.  Voltages of the phases are limited to phase to
ground voltages
 The high voltages due to arcing grounding or
transient line to ground faults are eliminated
 The over voltages due to lighting are
discharged to ground

72.  It is possible o maintain the supply with a
fault on one line
 Interference with communication lines is
reduced because of the absence of Zero
sequence currents.