SWITCH GEAR AND PROTECTION BY RAMU SRIKAKULAPU ASSISTANT PROFESSOR SHARDA UNIVERSITY
SWITCH GEAR AND PROTECTION
BY
RAMU SRIKAKULAPU
ASSISTANT PROFESSOR
SHARDA UNIVERSITY
UNIT -1 INTRODUCTION TO POWER
SYSTEMS
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.
UNIT 2: Relay Operating Principles and Constructions
UNIT 3: Protection of Power Apparatus
UNIT4: Modern Relays
UNIT 5: Theory of Circuit Interruption
UNIT 6: Circuit Breakers
No of lectures
Assignments
Class test
Subject presentation
TEXT BOOKS Badri ram , D.N.Vishwakarma, ‘power system
protection & switchgear’ , Tata McGraw –hill publishing company ltd, New Delhi.
C.L Wadhwa , ‘ electrical power systems’ , New Age International (p) limited.
B. Ravindranath, and N. Chander, ‘Power System Protection & Switchgear’, Wiley Eastern Ltd., 1977.
Sunil S. Rao, ‘Switchgear and Protection’, Khanna publishers, New Delhi, 1986 .
Need for protective systems What is the power system ?
POWER FLOW
NEED FOR PROTECTION 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
PROTECTION SCHEME OPERATION
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 FAULTS 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
DIRECT LIGHTING STROKES
Practical analysis for
causes of fault
AVERAGE 400 KV LINE FAULT FREQUENCY STATISTICS BY FAULT
CAUSE PER 100 KM PER YEAR
TRANSMISSION LINE FAULT CAUSES – 12,229 FAULTS FROM 132 KV TO 765
KV
TYPES OF FAULTS 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
SYMMETRICAL FAULTS
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.
OPEN CIRCUITED PHASES FAULT
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
WINDING FAULTS
Occurs on the alternator, motor & transformer windings
SIMULTANEOUS FAULTS
PERCENTAGES OF FAULT IN POWER SYSTEM
L-G 70-80%
L-L 17-10%
LL-G 10-8%
3 PHASE 3-2%
EFFECTS OF FAULTS
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 supply V&I Loss of stability
ZONES OF PROTECTION 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.
EXAMPLE
PRIMARY & BACK UP PROTECTION
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
BACK UP RELAY 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
REMOTE 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
RELAY BACK UP
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.
BREAKER BACK UP 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.
BACK UP PROTECTION
AC D
E
Breaker 5Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F
QUALITIES OF PROTECTION
The basic requirements of a protective system are
Selectivity or discriminationReliabilitySensitivityStabilityFast operation
SELECTIVITY
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.
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
RELIABILITY 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.
SENSITIVITY
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.
STABILITY
Protective system should stable for Large current due to fault Internal fault External fault
FAST OPERATIONIt 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.
PROTECTIVE SCHEME CLASSIFICATIONS A protective scheme is used to protect an
equipment or a section of the line
The protective schemes areOver current protectionDistance protectionCarrier current protectionDifferential protection
OVER CURRENT 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
DISTANCE PROTECTION 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
Relay Type Measurement1. Impedance relay impedance2. Reactance relay reactance3. Mho relay admittance
This protection scheme is used for the protection of transmission lines Sub- transmission lines
CARRIER CURRENT PROTECTION
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)
DIFFERENTIAL PROTECTION 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.
FAULT CURRENT CALCULATION
SEQUENCE COMPONENTS
UNSYMMETRICAL VECTOR FROM SYMMETRICAL COMPONENTS
Relations of voltage components in matrix form
Relations of current components in matrix form
SYMMETRICAL VECTOR FROM
UNSYMMETRICAL VECTOR
SEQUENCE IMPEDANCE
SYMMETRICAL COMPONENTS OF GENERATOR
POSITIVE SEQUENCE IMPEDANCE
NEGATIVE SEQUENCE IMPEDANCE
ZERO SEQUENCE IMPEDANCE
SYMMETRICAL COMPONENTS OF
TRANSFORMER (ZERO SEQUENCE)
1
2
3
4
5
ZERO SEQUENCE IMPEDANCE OF 3Ф LOAD
No grounding Solid grounding Neutral impedance grounding delta
L-G FAULT
L-L FAULT
LL-G FAULT
NEUTRAL GROUNDING 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
UNGROUNDED SYSTEM
In case of L-G fault at phase Ccharging current = 3*(per phase
charging current)The voltages of the healthy phases
rises to 0.866VphThese voltages can be eliminated by
connecting an inductance of suitable value between the neutral and the ground or Arcing ground.
RESONANT GROUNDING
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.
SOLID GROUNDING 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.
RESISTANCE GROUNDING 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
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.
ZIG-ZAG TRANSFORMER GROUNDING
It is used , if neutral point is requied which other wise is not available (eg. Delta connection, bus bar points)
ADVANTAGES OF NEUTRAL GROUNDING
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
ADVANTAGES OF ISOLATED NEUTRAL
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.