basic protection and relaying
TRANSCRIPT
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BASIC PROTECTION ANDRELAYING SCHEMES
Submitted by-
Somali ajal Das
0901106068
Guided by- Dr. Abhimanyu
Mohapatra Dr. Ranjan Ku. Jena
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Agenda
Why protection is needed Principles and elements of the protection
system Basic protection schemes Digital relay advantages and enhancements
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Disturbances: Light or Severe The power system must maintain acceptable
operation 24 hours a day Voltage and frequency must stay within certain limits
Small disturbancesThe control system can handle theseExample: variation in transformer or generator load
Severe disturbances require a protection systemThey can jeopardize the entire power systemThey cannot be overcome by a control system
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Power System ProtectionOperation during severe disturbances:
System element protectionSystem protectionAutomatic reclosingAutomatic transfer to alternate power suppliesAutomatic synchronization
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Electric Power System Exposure to External Agents
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Damage to Main Equipment
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Protection System
A series of devices whose main purpose is to protect persons and primary electric
power equipment from the effects of faults
The “Sentinels”
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Blackouts
Loss of service in a large area or population region
Hazard to human life May result in
enormous economic losses
Overreaction of the protection system
Bad design of the protection system
Characteristics Main Causes
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Short Circuits Produce High Currents
FaultSubstation
abc
I
IWire
Three-Phase Line
Thousands of Amps
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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
when in 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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Mechanical Damage DuringShort Circuits Very destructive in busbars, isolators,
supports, transformers, and machines Damage is instantaneous
i1
i2
f1 f2
Rigid Conductors f1(t) = k i1(t) i2(t)
Mechanical Forces
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The Fuse
Fuse
Transformer
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Essential qualities of protection:
Reliability Selectivity-
Absolute or relative Fastness Discrimination
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Protection System Elements Protective relays Circuit breakers Current and voltage transducers Communications channels DC supply system Control cables
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Protective relays:
A device which detect intolerable or unwanted conditions within the assigned area.
* A watchman or watchdog for the equipment/area
* Silent sentinels to power system.
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How relays are differentiated? Can be differentiated based on: * Functional categories * Input quantities *Operating Principles * Performance Characteristics.
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What are various design criteria? * Dependability/Reliability * Security * Selectivity *Speed * Simplicity/flexibility *Stability *Performance Vs. Economy
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What are various technique used? * Electromechanical *Solid state/Static * Microprocessor/Numerical
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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.
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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.
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Types of relays
As per function: Main Auxiliary Signal
As per actuating quantity Overrelays Underrelays
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Types…
As per connection Primary Secondary(common)
As per action on CB Direct acting Indirect acting
As per construction Electromagnetic
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Types..
Static Numerical
As per comparator types Single input comparator Two input comparator Multiple input comparator
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Methods of disciminations: To locate fault
by time
by current grading
by time and direction
by distance
by time, current and distance
by current balance
by power direction comparison Type of fault
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Three-Phase Diagram of the Protection Team
CTs
VTs
Relay
CB
Control
Protected Equipment
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DC Tripping Circuit
SI
52TC
DC StationBattery
SIRelay
Contact
Relay
CircuitBreaker
52a
+
–
RedLamp
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Circuit Breakers
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Current Transformers
Very High Voltage CTMedium-Voltage CT
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Voltage Transformers
Medium Voltage
High Voltage
Note: Voltage transformers are also known as potential transformers
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Protective Relays
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Examples of Relay Panels
Old Electromechanical
Microprocessor-Based Relay
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How Do Relays Detect Faults? When a fault takes place, the current,
voltage, frequency, and other electrical variables behave in a peculiar way. For example:Current suddenly increasesVoltage suddenly decreases
Relays can measure the currents and the voltages and detect that there is an overcurrent, or an undervoltage, or a combination of both
Many other detection principles determine the design of protective relays
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Primary Protection
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Primary Protection Zone Overlapping
ProtectionZone B
ProtectionZone A
To Zone BRelays
To Zone ARelays
52 ProtectionZone B
ProtectionZone A
To Zone BRelays
To Zone ARelays
52
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Backup Protection
AC D
E
Breaker 5Fails
1 2 5 6 11 12
T
3 4 7 8 9 10
B F
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Typical Short-Circuit Type Distribution
Single-Phase-Ground: 70–80%
Phase-Phase-Ground: 17–10%
Phase-Phase: 10–8%
Three-Phase: 3–2%
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Balanced vs. Unbalanced Conditions
Balanced System Unbalanced System
cI
aI
bI
aI
cI
bI
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Decomposition of an Unbalanced System
Positive-Sequence
Balanced BalancedNegative-Sequence
1bI
1cI1aI
2bI
2aI
2cI
0aI
0bI
0cI
aI
cI
bI
Zero-Sequence
Single-Phase
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Power Line Protection Principles
Overcurrent (50, 51, 50N, 51N) Directional Overcurrent (67, 67N) Distance (21, 21N) Differential (87)
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Characteristics of overcurrent relays: Definite time IDMT- inverse definite minimum time Very inverse Extremely inverse
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Application of Inverse-Type Relays
tRelay Operation Time
I
Fault Load
Radial Line
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Distance
Distance
t
I
T
Inverse-Time Relay Coordination
T T
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50/51 Relay Coordination
Distance
Distance
t
I
T T T
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Directional Overcurrent ProtectionBasic Applications
K
L
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Distance Relay Principle
Three-Phase Solid Fault
d
L
RadialLine21
Suppose Relay Is Designed to Operate When:
||||)8.0(|| 1 aLa IZV
cba III ,,
cba VVV ,,
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The Impedance Relay Characteristic
21
22rZXR
R
X Plain Impedance RelayOperation Zone
Zr1
Radius Zr11rZZ
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Need for Directionality
1 2 3 4 5 6
F1F2
R
XRELAY 3Operation Zone
F1
F2Nonselective Relay Operation
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Three-Zone Distance Protection
1 2 3 4 5 6
Zone 1
Zone 2Zone 3
Time
TimeZone 1 Is Instantaneous
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Circular Distance Relay Characteristics
MHO
OFFSETMHO (1)
PLAIN IMPEDANCE
R
X
R
X
R
X
OFFSETMHO (2)
R
X
LENS(RESTRICTED MHO 1)
TOMATO(RESTRICTED MHO 2)
R
X
R
X
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Differential Protection Principle
No Relay Operation if CTs Are Considered Ideal
ExternalFault
IDIF = 0
CT CT
50
Balanced CT Ratio
ProtectedEquipment
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Differential Protection Principle
InternalFault
IDIF > ISETTING
CTR CTR
50
Relay Operates
ProtectedEquipment
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Problem of Unequal CT Performance
False differential current can occur if a CT saturates during a through-fault
Use some measure of through-current to desensitize the relay when high currents are present
ExternalFault
ProtectedEquipment
IDIF 0
CT CT
50
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Possible Scheme – Percentage Differential Protection Principle
ProtectedEquipment
ĪRĪS
CTR CTR
Compares:
Relay(87)
OP S RI I I
| | | |
2S R
RT
I Ik I k
ĪRPĪSP
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Differential Protection Applications Bus protection Transformer protection Generator protection Line protection Large motor protection Reactor protection Capacitor bank protection Compound equipment protection
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Differential ProtectionSummary The overcurrent differential scheme is simple
and economical, but it does not respond well to unequal current transformer performance
The percentage differential scheme responds better to CT saturation
Percentage differential protection can be analyzed in the relay and the alpha plane
Differential protection is the best alternative selectivity/speed with present technology
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Advantages of Digital Relays
MultifunctionalCompatibility withdigital integrated
systems
Low maintenance(self-supervision)
Highly sensitive,secure, and
selectiveAdaptive
Highly reliable(self-supervision)
Reduced burden on
CTs and VTs
ProgrammableVersatile
Low Cost
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Why study this protection scheme?? Protection scheme plays a vital & important role
for the normal operation or the steady state operation of different components of power system network, which must be reliable, fast and efficient.
In order to achieve all these features, it is essential that these should be proper care in designing and choosing an appropriate and efficient protection scheme.
The protective relays functions as the brain behind the whole schemes…
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THANK YOU