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7/29/2019 Protecion Philosphy
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General Protect ion Phi losophy
By:A.K.SAHU, Sr.Engineer(T&C)
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General Protect ion Phi losophy
FOLLOWING DEFINITIONS ARE GENERALLY USED IN POWER SYSTEM PROTECTION:
1. PROTECTION SYSTEM: A COMPLETE ARRANGEMENT OF PROTECTION
EQUIPMENT AND OTHER DEVICES REQUIRED TO ACHIEVE A
SPECIFIED FUNCTION BASED ON A PROTECTION PRINCIPLE.
2. PROTECTION EQUIPMENT: A COLLECTION OF PROTECTION DEVICES
(RELAYS, FUSE ETC.) EXCLUDED ARE THE DEVICES SUCH AS CTs, PTs,
CBs etc.
3. PROTECTION SCHEMES: A COLLECTION OF PROTECTION EQUIPMENT
PROVIDING A DEFINED FUNCTION AND INCLUDING ALL EQUIPMENT
REQUIRED TO SCHEME WORK.
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General Protect ion Phi losophy
Objective of relay protection
Protect persons and equipment in
the surrounding of the power system
Protect apparatus in the power
system
Separate faulty parts from the rest of
the power system to facilitate the
operation of the healthy part of the
system
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General Protect ion Phi losophy
REQUIREMENT OF PROTECTION SYSTEM:1. RELIABILITY
2. SELECTIVITY
3. STABILITY
4. SPEED
5. SENSITIVITY
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General Protect ion Phi losophy
1. RELIABILITY:THE PROTECTION SYSTEM PROVIDED REQUIRED TO BE HIGHLY
RELIABLE FOR MAXIMUM UTILIZATION OF SYSTEM. TODAYS
SYSTEMS ARE VERY HUGE AND IN SUCH A HUGE SYSTEM, THE
RISK OF OCCURRING OF FAULT CAN NOT BE IGNORED. THE
PROTECTION SYSTEM MUST OPERATE TO ISOLATE THE FAULT
RELIABLY IS UTMOST IMPORTANCE. ANY NON OPERATION MAY
LEAD TO CATASTROPHIC CONSEQUENCE FOR POWER SYSTEM
AND MAY LEAD TO MULTIPLE FAILURE. THUS PROTECTION
SYSTEM MUST BE RELIABLE.
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General Protect ion Phi losophy
2. SELECTIVITY:THE PROTECTION IS GENERALLY ARRANGED IN ZONES WHICH
SHOULD COVER THE POWER SYSTEM COMPLETELY, LEAVING NO
PART UNPROTECTED. WHEN A FAULT OCCURS THE PROTECTION
IS REQUIRED TO SELECT AND TRIP ONLY THE NEAREST CIRCUITBREAKERS TO AFFECT MINIMUM POSSIBLE OUTAGE. THIS
PROPERTY OF SELECTIVITY IS ALSO CALLED DISCRIMINATION
AND IS ACHIEVED BY TWO GENERAL METHODS:
a. TIME GRADED SYSTEM
b. UNIT SYSTEM
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General Protect ion Phi losophy
3. STABILITY:THIS TERM APPLIED TO PROTECTION AS DISTINCT FROM THE
POWER SYSTEM, REFERS TO THE ABILITY OF THIS PROTECTION
SYSTEM TO REMAIN INERT TO ALL LOAD CONDITIONS AND FAULT
EXTERNAL (THROUGH FAULT) TO THE RELEVANT ZONE. THE
SYSTEM MUST REMAIN STABLE IN CASE OF FAULTS OCCURRING
OUTSIDE THE PROTECTED ZONE AND SHOULD NOT TRIP.
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General Protect ion Phi losophy
4. SPEED:THE FUNCTION OF PROTECTION IS TO ISOLATE FAULTS FROM
THE POWER SYSTEM IN MINIMUM POSSIBLE TIME TO LIMIT THE
DAMAGE TO POWER SYSTEM EQUIPMENT THAT MAY BE
GENERATORS, TRANSMISSION LINES, REACTORS,
TRANSFORMER, BUS BARS ETC. THE OBJECT IS TO SAFEGUARD
CONTINUITY OF SUPPLY BY REMOVING EACH DISTURBANCE
BEFORE IT LEADS TO WIDESPREAD AFFECT IN THE POWER
SYSTEM CAUSING MULTIPLE OUTAGES.
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General Protect ion Phi losophy
5. SENSITIVITY:SENSITIVITY IS A TERM FREQUENTLY USED WHEN REFERRING TO
THE MINIMUM OPERATING QUANTITY THAT MAY BE CURRENT,
VOLTAGE OF A COMPLETE PROTECTION SYSTEM. THE
PROTECTION SYSTEM MUST BE ABLE TO SENSE THE FAULT IN
MINIMUM POSSIBLE VOLTAGE/CURRENT TO SUCCESSFULLY
IDENTIFY THE FAULT AND ISOLATE THE SAME.
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General Protect ion Phi losophy
EQUIPMENT TO BE PROTECTED
TRANSMISSION LINES
TRANSFORMERS
REACTORS
BUS BARS
CAPACITOR BANKS
ETC.
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General Protect ion Phi losophy
Electrical faults in the power system Transmission lines 85%
Busbar 12%
Transformer/ Generator 3%
Total 100%
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General Protect ion Phi losophy
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General Protect ion Phi losophy
Line Fault Statistics Single phase to earth 80%
Two phases to earth 10%
Phase to phase faults 5%
Three phase faults 5%
The probability of line faults caused by lightnings are0,2-3 faults/ 100 km and year
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General Protect ion Phi losophy
Fault types Transient faults
are common on transmission lines,
approximately 80-85%
lightnings are the most common reason
can also be caused by birds, falling trees,swinging lines etc.
will disappear after a short dead interval
Persistent faults can be caused by a broken conductor fallen
down
can be a tree falling on a line
must be located and repaired before normalservice
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Measuring Principles Overcurrent protection
Differential protection
Phase comparison
Directional- wave
protection Distance protection
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General Protect ion Phi losophy
Overcurrent Protection Over current protection is mainly provided with distribution
feeders. It is a simply current protection which operates on
the basis of current settings. To discriminate between the
faults on a network normally an IDMT characteristics areselected.
In distribution network the main protection provided on the
network is Over current protection with IDMT
characteristics. In EHV network, over current protection in the form of
directional back up over current earth fault protection is
used. This is basically important protection to clear high
resistance earth fault which is otherwise not detected by
distance protection.
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General Protect ion Phi losophy
Differential Protection Differential protection is mainly provided with distribution
feeders. It is a simply current differential protection which
operates on the basis of differential current settings. This
protection is a unit protection and requires the informationfrom remote end as well.
This is a very fast and secure protection and is successful
for very complex networks as well.
Its utility is well proven in case of series compensated linesas well which otherwise is difficult to set.
In EHV network also, some utilities are using this protection
as Main-II world wide such as DEWA.
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General Protect ion Phi losophy
Phase Comparison and Directional Wave
Detection Protection
This is somewhat similar to differential protection.
It works basically on the basis of difference of phasor angle
of both sides. This is a very fast and secure protection and is successful
for very complex networks as well.
Its utility is well proven in case of series compensated lines
as well which otherwise is difficult to set.
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The Principle of Distance Protection
Z