Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Power System Protection
S.A.Soman
Department of Electrical EngineeringIIT Bombay
Desirable Attributes of Protection
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Dependability
A relay is said to be dependable if it trips only when it isexpected to trip.
Dependability is the degree of certainty that the relaywill operate correctly.
Dependability can be improved by increasing thesensitivity of the relaying scheme.
%Dependability =No. of correct trippings × 100Total No. of desired trippings
SensitivityIt is the ability of the relay to pick up even on smallestpossible faults.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Security
Security is a property used to characterize falsetrippings.
A relay is said to be secure if it does not trip when it isnot expected to trip.
It is the degree of certainty that the relay will notoperate incorrectly.
Security can be improved by improving selectivity of therelaying scheme.
%Security =No. of correct trippings × 100
Total No. of trippings
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Selectivity
It is the ability to correctly locate and classify the fault.A relay should be able to discriminate whether the faultis in its jurisdiction or not.This jurisdiction of a relay is called zone of protection.Protection zones are classified into primary and backupzones.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Selectivity is usually provided by
Time discrimination. Overcurrent and distance relaysuse this principle.
Applying differential protection principle. In the case ofdifferential protection, the CT location provides crispdemarcation of zone of protection.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attribute of Protection
Reliability
It is the ability to ’not to fail’ in its function.
It can be achieved by redundancy.
Redundancy in protection depends on the criticality ofthe apparatus.
Reliability can be improved by providing backupprotection.
% Reliability =No. of correct trips × 100
No. of desired trips + No. of incorrect trips
Number of desired tripping can be greater than or equal tonumber of correct trippings.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Zone of Protection
A relay’s zone of protection is a region which defines itsjurisdiction.
It is shown by demarcating the boundary.
It is essential that primary zones of protection shouldalways overlap so that no portion of the system everremain unprotected.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Necessity of Speed in Relaying
A fault should be cleared as quickly as possible in orderto
Maximize safety
Minimize equipment damageMinimize system instability.
The relay should quickly arrive at a decision and circuitbreakers should be fast enough.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Necessity of Speed in Relaying
A fault should be cleared as quickly as possible in orderto
Maximize safetyMinimize equipment damage
Minimize system instability.
The relay should quickly arrive at a decision and circuitbreakers should be fast enough.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Necessity of Speed in Relaying
A fault should be cleared as quickly as possible in orderto
Maximize safetyMinimize equipment damageMinimize system instability.
The relay should quickly arrive at a decision and circuitbreakers should be fast enough.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Necessity of Speed in Relaying
A fault should be cleared as quickly as possible in orderto
Maximize safetyMinimize equipment damageMinimize system instability.
The relay should quickly arrive at a decision and circuitbreakers should be fast enough.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
For a single machine infinite bus system,
2H d2δdt2 = Pm − Pe
Occurrence of fault reducesPe to zero and δ increases.
At point D, Pe > Pm andthe machine decelerates.
At point E, dδdt = 0 and
Pe = Pm.
As δ reduces, generatorstarts accelerating and theswing reverses.
Finally system stabilises atpoint O.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
For a single machine infinite bus system,
2H d2δdt2 = Pm − Pe
Occurrence of fault reducesPe to zero and δ increases.
At point D, Pe > Pm andthe machine decelerates.
At point E, dδdt = 0 and
Pe = Pm.
As δ reduces, generatorstarts accelerating and theswing reverses.
Finally system stabilises atpoint O.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
For a single machine infinite bus system,
2H d2δdt2 = Pm − Pe
Occurrence of fault reducesPe to zero and δ increases.
At point D, Pe > Pm andthe machine decelerates.
At point E, dδdt = 0 and
Pe = Pm.
As δ reduces, generatorstarts accelerating and theswing reverses.
Finally system stabilises atpoint O.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
For a single machine infinite bus system,
2H d2δdt2 = Pm − Pe
Occurrence of fault reducesPe to zero and δ increases.
At point D, Pe > Pm andthe machine decelerates.
At point E, dδdt = 0 and
Pe = Pm.
As δ reduces, generatorstarts accelerating and theswing reverses.
Finally system stabilises atpoint O.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
For a single machine infinite bus system,
2H d2δdt2 = Pm − Pe
Occurrence of fault reducesPe to zero and δ increases.
At point D, Pe > Pm andthe machine decelerates.
At point E, dδdt = 0 and
Pe = Pm.
As δ reduces, generatorstarts accelerating and theswing reverses.
Finally system stabilises atpoint O.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
For a single machine infinite bus system,
2H d2δdt2 = Pm − Pe
Occurrence of fault reducesPe to zero and δ increases.
At point D, Pe > Pm andthe machine decelerates.
At point E, dδdt = 0 and
Pe = Pm.
As δ reduces, generatorstarts accelerating and theswing reverses.
Finally system stabilises atpoint O.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
As per equal area criteria, the post fault systemreaches stable equilibrium if accelerating area is equalto the decelerating area.If a machine goes out-of-step with the rest of thesystem, it has to be islanded to a sub-system.As the fault clearing time increases, the stability marginreduces.The fault clearing time at which the stability marginreduces zero is known as critical clearing time.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
1 Desirable AttributesDependabilitySecuritySelectivityReliability
2 Speed in RelayingNecessity of Speed in RelayingSpeed Vs. Accuracy Conflict
3 Summary
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Speed Vs. Accuracy ConflictThe consequences of quick tripping decisions are
Nuisance tripping or tripping when there is no fault.Tripping for faults outside the relay jurisdiction.High-speed system tend to be less accurate because oflesser amount of information.
Power SystemProtection
S.A.Soman
DesirableAttributesDependability
Security
Selectivity
Reliability
Speed inRelayingNecessity of Speedin Relaying
Speed Vs. AccuracyConflict
Summary
Desirable Attributes of Protection
Summary
Desirable Attributes of a Protection SystemDependabilitySecuritySensitivitySelectivityReliability
Speed Vs Accuracy Conflict
