Switch gear and protection

DINESH SHARMA
B.TECH.(EEE)
SHARDA UNIVERSITY

 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.

 What is the power system ?

 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

 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

 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

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

 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

 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.

 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

 Occurs on the alternator, motor &
transformer windings

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

 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

 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.

 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

 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

 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

 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.

 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.

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

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

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.

 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

 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.

 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.

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

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.

 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

 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

 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

 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)

 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.

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

 No grounding
 Solid grounding
 Neutral impedance grounding
 delta

 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

 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.

 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.

 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.

 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.

 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.

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

 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

 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.

Switch gear and protection

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