protecion philosphy

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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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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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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REQUIREMENT OF PROTECTION SYSTEM:1. RELIABILITY

2. SELECTIVITY

3. STABILITY

4. SPEED

5. SENSITIVITY


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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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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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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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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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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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EQUIPMENT TO BE PROTECTED

TRANSMISSION LINES

TRANSFORMERS

REACTORS

BUS BARS

CAPACITOR BANKS

ETC.


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Electrical faults in the power system Transmission lines 85%

Busbar 12%

Transformer/ Generator 3%

Total 100%


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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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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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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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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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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

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