02_basic protection technology
TRANSCRIPT
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Power System Structure
Generation Transmission / Sub transmission Distribution
Extra High Voltage 765 kV
400 kV
220 kV
High Voltage 132 kV
110 kV
66 kV
Medium Voltage 33 kV
22 kV
11 kV
Medium 24 kV
Voltage 21 kV
15 kV
13.8 kV
The purpose of an electrical power system is to generate and supply electrical energy toconsumers. The system should be designed and managed to deliver this energy to the
utilisation points with both reliability and economy.
The purpose of an electrical power system is to generate and supply electrical energy toconsumers. The system should be designed and managed to deliver this energy to the
utilisation points with both reliability and economy.
Many items of equipment are very expensive, and so the complete power system represents avery large capital investment.
.
Many items of equipment are very expensive, and so the complete power system represents avery large capital investment.
.
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System Disturbances
Short Circuits in earthed systems
Symmetrical (3 phase)
Phase to Phase (and Earth)
Phase to Earth
Earth Faults in non effectively
earthed systems
Overload Conditions
Underfrequency/Undervoltage
Overvoltage
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Protective Relaying is the most important feature
of power system design aimed at minimising the
damage to equipment and interruption to service in
the event of faults. It is therefore a co-factor
among other factors resorted to improve reliability
of power system.
Protective Relaying
Role of Protection
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The Purpose of Protection
But it can:Limit the damage caused by shortcircuits
While:Protecting people and plant fromdamage
Selectively clearing faults inmiliseconds
Protecting plant from overloadconditions
The protection can not prevent system faults,
Power system must operate in a safe manner at all times.
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Causes and Probability of System Disturbances
Causes
Operator Mistakes
Pollution/Condensation
Equipment failures, e.g. P.T.'s, Isolators
Transient OvervoltagesProbability
System faults (220/400 kV): 3p.a. and 100 km
10-20 kV metal clad switchgear: 10-3 p.a. and feeder
GIS switchgear: 5-10-2 p.a. and bus
outdoor switchgear: 110/132 kV 7*10-2 p.a. and bus220/275 kV 10-1 p.a. and bus
400 kV 2*10-1 p.a. and bus
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Since protective relaying comes into action at the time of
equipment distress, a certain safeguard is necessary in
the unlikely event of its failure to act at the hour of need.
Hence, two groups of protective schemes are generally
employed -
a) Primary Protection
b) Back-up Protection
Primary Protection is the first line of defense, whereas
back-up relaying takes over the protection of equipment,
should the primary protection fail.
Principles of Relaying
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The Primary Protection has following characteristic features -
1. It has always a defined zone of operation.
2. It should operate before any back-up protection
could operate, therefore, it should be faster inoperation.
3. It should be able to completely isolate the fault
from all the current feeding sources.
4. It should be stable for all operating conditions.
Primary Protection
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1. Back-up protection should provide sufficient time
for the primary protection to perform its duty.
2. Back-up protection covers a wider zone of
protection. Therefore, there is always a possibility
of large scale disturbance, when back-up relays
operate.
3. Under primary protection failure, several back-up
relays may operate for complete isolation of fault.
Back-up Protection
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Primary protections failure could be due to any of the
following reasons -
1. Current or Potential Transformer failure
2. Loss of Auxiliary Control Voltage3. Defective Primary Relays
4. Open Circuits in Control & Trip Coil
5. Failure of Breaker
It is therefore logical that back-up relays should not
utilise any of the above items as common with primary
relays.
Reasons of Primary Protection Failure
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Protection Concept
The system is only as strong as the weakest link!
DISTANCE RELAY
Circuit BreakerCT / VT
Protection Battery
Cabling
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Basic Protection Requirements
Reliability dependability (availability)
high dependability = low risk of failure to trip
Security stable for all operating conditions ,
high security = low risk of over-trip
Speed high speed minimizes damage
high speed reduces stability problems
Selectivity trip the minimum number of circuit breakers
Sensitivity notice smallest fault value
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Zones of Protection
To limit the extent of the power system that is disconnected when a fault
occurs, protection is arranged in zones
Zones of protection should overlap, so that no part of the power system is left
unprotected
Location of the CT connection to the protection usually defines the zone Unit type protections have clear zones reach e.g Diff. Relay, REF relay
Zone reach depends on measurement of the system quantities e.g OC , EF,
distance relays . The start will be defined but the extent (or reach) is subject
to variation, owing to changes in system conditions and measurement errors.
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Protection - One Out of Two Principle
System1
TripCoil
1
Battery 1
System2
TripCoil
2
Battery 2
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Redundancy Concept of DC Circuits
Battery 1
Battery 2
Main Protection
87T
TR
TC 1
L-
Back-up Protection
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TR
TC 2
Busbar Protection
87BB
TR
BF
L-
Trip remote infeed
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Factors that influence fault current magnitude
G
Short circuit power of the infeed Voltage level Line impedance Fault resistance (arc) Treatment of star point
Infeed Consumer Line
Estimate of short circuit currents:
Medium Voltage (10 kV upto 30kV) ISCmin > ILmaxHigh Voltage (110 kV) I
SCmin>= I
LmaxExtra High Voltage (220kV + ) ISCmin = 0,25 ILmax
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Earth faults: Star-point configuration
R L
earthed system Peterson Coil isolated neutral
Earth fault = short circuitis recognised by normalover-current protection.
With low impedance earthingthe residual current detectionmust be more sensitive.
Earth faults = no short circuit Supply is not disrupted Earth fault must be alarmed and removedas fast as possible
Earth fault location is achieved withwattmetric earth fault detection
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Protection Criterion - Current
The overcurrent condition is evaluated I>
Suitable for:
Additional criterion - Time(to ensure selectivity)
Protection: Fuses
inverse time protection (IDMT)
definite time protection (DT)
I
ISCminILmax
I>
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Protection Criterion - Current Difference
Evaluation of node I1 + I2 + I3 + ... In = 0; if the equation is notsatisfied the fault is internal
Security is increased by stabilisation |I1|+|I2|+ ... |In| = Istab Characteristic:
definite distinction internal / external faults (no back-up)
Protection: Line differential protection Generator-, motor-, transformer differential protection Busbar protection
Trip
Istab
Idiff
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Protection Criterion - Impedance
From the voltage and current signals the
impedance is calculated
The impedance is proportional to the fault distance
Characteristic:
Additional criterion - Time
(Required for selectivity and back-up protection)Protection: Multiple stage distance protection
R
X
Z