01 -intro - protection
TRANSCRIPT
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> Basic Protection Philosophy - January 2004
Basic Protection Philosophy
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> Basic Protection Philosophy - January 2004
Protec tion - Why Is It Needed?
PROTECTION IS INSTALLED TO :
Detect fault occurrence and isolate the faultedequipment.
SO THAT :
Damage to the faulted equipment is limited;
Disruption of supplies to adjacent unfaultedequipment is minimised.
PROTECTION IS EFFECTIVELY AN INSURANCE POLICY- AN INVESTMENT AGAINST DAMAGE FROM FUTUREFAULTS.
All Power Systems may experience faults at some
time.
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Protec tion - Why Is It Needed?
FAULTS ON POWER SYSTEMS RISK :
Severe damage to the faulted equipment :
Excessive current may flow;
Causes burning of conductors or equipment
windings;Arcing - energy dissipation;
Risk of explosions for oil - filled switchgear, or whenin hazardous environments.
Damage to adjacent plant :
As the fault evolves, if not cleared quickly;
Due to the voltage depression / loss of supply.
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Protec tion - Why Is It Needed?
FAULTS ON POWER SYSTEMS RISK :
Danger to staff or the public :
Risk of shock from direct contact with the faultedequipment;
Danger of potential (voltage) rises in exposedmetalwork accessible to touch;
Fumes released by burning insulation;
Burns etc.
Disruption to adjacent plant :
Prolonged voltage dips cause motors to stall;
Loss of synchronism for synchronous generators /motors.
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Protec tion - Why Is It Needed?
SUMMARY :
Protection must :
Detect faults and abnormal operating conditions;
Isolate the faulted equipment.
So as to :
Limit damage caused by fault energy;
Limit effect on rest of system.
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Impo rtant Cons iderat ions When Applying
Protect ion
Types of fault and abnormal conditions to be protected against
Quantities available for measurement
Types of protection available
Speed
Fault position discrimination
Dependability / Reliability Security / Stability
Overlap of protections
Phase discrimination / Selectivity
CTs and VTs
Auxiliary supplies Back-up protection
Cost
Duplication of protection
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Faults Are Mainly Caused By Insulat ion Fai lure
Underground Cables
DiggersOverloading
Oil Leakage
Ageing
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Faults Are Mainly Caused By Insulat ion Fai lure
Overhead Lines
Lightning
Kites
Trees
Moisture
Salt
Birds
Broken Conductors
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Faults Are Mainly Caused By Insulat ion Fai lure
Machines
Mechanical Damage
Unbalanced Load
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Types o f Fault
a
b
c
e
/E
//E
/
e
a
b
c
3
3/E
e
a
b
c
a
b
c
a
b
c
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Types o f Fault
CROSSCOUNTRYFAULT
a
bc
e
a'
b'c'
e
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Types o f Fault
OPEN
CIRCUIT+ /E
FAULTBETWEEN
ADJACENTPARALLELLINES
a
b
c
e
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Types o f Fault
CHANGINGFAULT INCABLE
a
b c
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Types Of Protect ion
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Types of Protect ion
Fuses
For : LV Systems, Distribution Feeders andTransformers, VTs, Auxiliary Supplies
Direct Acting AC Trip
For : LV Systems, Pole Mounted Reclosers
Overcurrent and Earthfault
Widely used in all Power Systems
Non-DirectionalVoltage Dependant
Directional
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Types of Protect ion
Differential
For : Feeders, Busbars, Transformers, Generators,
etc.
High ImpedanceRestricted E/F
Biased (or low-impedance)
Pilot Wire
Digital
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Types of Protect ion
DistanceFor : Distribution Feeders and Transmissionand Sub-Transmission Circuits
Also used as Back-up Protection for
Transformers and Generators
Phase Comparison
For : Transmission Lines
Directional Comparison
For : Transmission Lines
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Types of Protect ion
MiscellaneousUnder and Over Voltage
Under and Over Frequency
Special Relays for Generators,Transformers, Motors, etc.
Control Relays
Auto-Reclose, Tap Change Control, etc.
Tripping and Auxiliary Relays
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Overcu rrent Protect ion Direct Ac t ing AC Trip
AC series trip
common for electromechanical O/C relays
51
IF
Trip Coil
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Overcu rrent Protect ion DC Shunt Trip
IF'
IF
DC
BATTERYSHUNT
TRIP COIL
51
Requires secure DC auxiliary
No trip if DC fails
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Overcu rrent Protect ion Co-ordinat ion Principle
Relay closest to faultmust operate first
Other relays must haveadequate additional
operating time toprevent them operating
Current setting chosento allow FLC
Consider worst case
conditions, operatingmodes and currentflows
T
IS1IS2 MaximumFaultLevel
I
R2R1
IF1
Diff t i l P t t i P i i l (1)
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Differential Protect io n Princ iple (1)
Protected
Circuit
R
Diff t i l P t t i P i i l (2)
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Differential Protect io n Princ iple (2)
Protected
Circuit
R
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Basic Principle of Distance Protect ion
LOADLR
RR ZZVZmeasuredImpedance
RelayPT.
Normal
Load
IR ZLZS
VRVS ZLOAD
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Basic Principle of Distance Protect ion
Fault
IRZ
S
VRVS ZLOAD
ZL
ZF
Impedance Measured ZR= VR/IR= ZF
Relay Operates if ZF < Z where Z = setting
Increasing VRhas a Restraining Effect VRcalled Restraining Voltage
Increasing IRhas an Operating Effect
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Plain Impedance Characterist ic
jX ZL
R
TRIP STABLE
Impedance Seen AtMeasuring LocationFor Line Faults
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Impedance Character ist ic Generat ion
Operate
IF
VF
Restrain
Spring
Trip
zF
Ampere Turns : VF IZ
Trip Conditions : VF < IFZ
jIX
IZV1V2
V3
IR
TRIP STABLE
Voltage to Relay = VCurrent to Relay = IReplica Impedance = Z
Trip Condition : S2 < S1
where : S1 =IZ Z
S2 = V ZF
Buchho lz Relay Ins tal lat ion
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Buchho lz Relay Ins tal lat ion
5 x internal pipe
diameter (minimum)
3 x internal pipe
diameter (minimum)
Transformer
3 minimum
Oil conservator
Conservator
Auto reclose Benef i ts (1)
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Auto reclose Benef i ts (1)
Improved continuity of supply
Supply restoration is automatic (does not requirehuman intervention)
Shorter duration interruptions
Less consumer hours lost
Use of instantaneous protection for faster fault clearance(NB: some healthy circuits may also be tripped)
Less damage
Less pre-heating of circuit breaker contacts (reduced
maintenance?) Less chance of transient fault becoming permanent
Auto reclose Benef i ts (2)
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Auto reclose Benef i ts (2)
Less frequent visits to substations
More unmanned substations
Reduced operating costs
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Defin i t ions & Considerat ions
C f
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Classes of Protect ion
Non-Unit, or Unrestricted Protection :
No specific point downstream up to which
protection will protect
Will operate for faults on the protected
equipment;
May also operate for faults on downstream
equipment, which has its own protection;
Need for discrimination with downstream
protection, usually by means of time grading.
Cl f P t t i
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Classes of Protect ion
Unit, or Restricted Protection :
Has an accurately defined zone of protection
An item of power system plant is protected as
a unit;
Will not operate for out of zone faults, thus no
back-up protection for downstream faults.
C di t i
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Co-ordinat ion
LOAD
LOAD
LOAD
LOAD F2 F3F1
SOURCE
Co-ordinate protection so that relay nearest tofault operates first minimises amount ofsystem disconnection.
ANSI R f N b
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ANSI Reference Numbers
2 Time Delay
21 Distance
25 Synchronising Check
27 Undervoltage
30 Annunciator
32 Directional Power
37 Undercurrent or Under Power
40 Field Failure
46 Negative Sequence
49 Thermal
50 Instantaneous Overcurrent
51 Time Delayed Overcurrent
51N Time Delayed Earthfault
52 Circuit Breaker
52a Auxiliary Switch - Normally Open
52b Auxiliary Switch - Normally Closed
59 Overvoltage
60 Voltage or Current Balance
64 Instantaneous Earth Fault (High Impedance)
67 Directional Overcurrent
67N Directional Earthfault
74 Alarm
79 Auto-Reclose
81 Frequency
85 Signal Receive86 Lock-Out
85 Signal Receive
86 Lock-Out
87 Differential
I t t C id t i Wh A l i
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Impo rtant Cons iderat ions When Applying
Protect ion
Speed
Fast operation :
Minimises damage and danger
Very fast operation :Minimises system instability
Discrimination and security can be costly toachieve as it generally involves additionalsignaling / communications equipment.
I t t C id t i Wh A l i
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Impo rtant Cons iderat ions When Applying
Protect ion
Fault Position Discrimination
Power system divided into PROTECTED ZONESMust isolate only the faulty equipment or section
Z f P t t i
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Zones of Protect ion
BUSBAR
ZONE
TRANSF-
ORMER
ZONE
BUSBAR
ZONE
FEEDER
ZONE
GENERATION ZONE
BUSBAR
ZONE FEEDER
ZONE
I t t C id t i Wh A l i
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Impo rtant Cons iderat ions When Applying
Protect ion
Overlap of Protections
No blind spots
Where possible use overlapping CTs
Protect ion Overlap
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Protect ion Overlap
BBP
1
BBP
2
JH
Z
LG LPJ
LP
HK
LP
L
LP
K
Impo rtant Cons iderat ions When Applying
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Impo rtant Cons iderat ions When Applying
Protect ion
Dependability / Reliability
Protection must operate when required to
Failure to operate can be extremely damagingand disruptive
Faults are rare. Protection must operate evenafter years of inactivity
Improved by use of: Back-up protection andduplicate protection
Impo rtant Cons iderat ions When Applying
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Impo rtant Cons iderat ions When Applying
Protect ion
Security / Stability
Protection must not operate when not required to,e.g. due to : Load switchingFaults on other parts of the systemRecoverable power swings
Impo rtant Cons iderat ions When Applying
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Impo rtant Cons iderat ions When Applying
Protect ion
Phase Discrimination
Correct indication of phases involved in the fault
Important for single phase tripping and auto-reclosing applications
Cost
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Cost
The cost of protection is equivalent to an insurancepolicy against damage to plant, and loss of supplyand customer goodwill.
Acceptable cost is based on a balance of economics
and technical factors. Cost of protection should bebalanced against the cost of potential hazards.
There is an economic limit on what can be spent.
MINIMUM COST : Must ensure that all faultyequipment is isolated byprotection.
Cost
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Cost
TOTAL COST should take account of :
Relays, schemes and associated panels andpanel wiring
Setting studies Commissioning
CTs and VTs
Maintenance and repairs to relays
Damage repair if protection fails to operate Lost revenue if protection operates
unnecessarily
Cost
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Cost
DISTRIBUTION SYSTEMS
Large numbers of switching and distributionpoints, transformers and feeders
Economics often overrides technical issues
Protection may be the minimum consistent
with statutory safety regulations Speed less important than on transmission
systems
Back-up protection can be simple and is often
inherent in the main protection Although important, the consequences of
maloperation or failure to operate is lessserious than for transmission systems
Cost
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Cost
TRANSMISSION SYSTEMS
Emphasis is on technical considerations ratherthan economics
Economics cannot be ignored but is ofsecondary importance compared with the needfor highly reliable, fully discriminative high speed
protection Higher protection costs justifiable by high capital
cost of power system elements protected
Risk of security of supply should be reduced tolowest practical levels
High speed protection requires unit protection
Duplicate protections used to improve reliability
Single phase tripping and auto-reclose may berequired to maintain system stability
Impo rtant Considerat ions When Applying
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Impo rtant Considerat ions When Applying
Protect ion
Current and Voltage Transformers
These are an essential part of the protection schemeto reduce primary current and volts to a low levelsuitable to input to relay.
They must be suitably specified to meet therequirements of the protective relays.
Correct connection of CTs and VTs to the protection isimportant. In particular for directional, distance, phase
comparison and differential protections. VTs may be electromagnetic or capacitor types.
Busbar VTs : Special consideration needed whenused for line protection.
Current Transfo rmer Circui ts
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Current Transfo rmer Circui ts
Never open circuit a CT secondary circuit, so :
Never fuse CT circuits;
VTs must be fused or protected by MCB.
Do wire test blocks in circuit (both VT and CT) toallow commissioning and periodic injection testing of
relays.
Earth CT and VT circuits at one point only;
Wire gauge > 2.5mm2recommended for mechanicalstrength.
Auxi l iary Supp l ies
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Auxi l iary Supp l ies
Required for : TRIPPING CIRCUIT BREAKERS
CLOSING CIRCUIT BREAKERS
PROTECTION and TRIP RELAYS
AC AUXILIARY SUPPLIES are only used on LV and MV systems.
DC AUXILIARY SUPPLIES are more secure than AC supplies.
SEPARATELY FUSED SUPPLIES used for each protection.DUPLICATE BATTERIES are occasionally provided for extrasecurity.
MODERN PROTECTION RELAYS need a continuous auxiliarysupply.
During unoperated (healthy) conditions, they draw a smallQUIESCENTload to keep relay circuits energised.
During operation, they draw a larger current which increases due tooperation of output elements.
Relay Outputs
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Relay Outputs
TRIP OUTPUT CONTACTS :
Check that these are rated sufficiently to make and
carry the circuit breaker trip coil current. If not, aheavier duty tripping relay will be needed.
Use a circuit breaker normally open (52a) contact to
interrupt trip coil current. This extends the life of the
protection relay trip contacts.TYPE OF CONTACTS :
Can be break before make (BBM)
or make before break (MBB).
Changeover (C/O)
Close when de-energised.Break (B) /
Normally Closed (NC)
Close when energised, typically
used for tripping.
Make (M) /
Normally Open (NO)
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