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Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power System Protection
S.A.Soman
Department of Electrical EngineeringIIT Bombay
Power Swings and Distance Relaying
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
1 Introduction to Power Swings
2 Analysis of Two Area System
3 Determination of Power Swing Locus
4 Electrical Center
5 Three Stepped Distance Protection
6 Summary
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Power Swings
It refers to oscillation in active and reactive power flowson a transmission line.
It is a consequence of large system disturbance like afault.
The post fault power swings may encroach the relaycharacteristics.
This will be seen by the relay as an impedance swingon the R-X plane.
If the impedance trajectory stays in the relay zone forsufficiently long time the relay will issue trip decision.
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Analysis of Two Area System
During steady state power flows from A to B andassume that the system is purely reactive.
Pm0 is the output power and δ0 is the rotor angle understeady state condition.
δ0 = sin−1(
Pm0Pmax
)During fault, Pe drops to zero and rotor accelerates toδ1.
As per equal area criteria, the rotor will swing up tomaximum rotor angle δmax so thatAccelerating Area (A1) = Decelerating Area (A2).
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Rotor angle corresponding to fault clearing time tcr can becomputed by swing equation,
2Hd2δ
ωsdt2 = Pm0
It can be derived that,
Pm0(δ1 − δ0) = Pmax(cosδ1 − cosδmax)− Pm0(δmax − δ1)
cosδmax = cosδ1 −Pm0
Pmax(δmax − δ0)
δmax = f (Pm0, tcr )
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Variation of δmax versus Pm0 for different values of tcr
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Determination of Power Swing LocusThe impedance seen by the relay is given by,
Zseen(relay) =Vrelay
Irelay=
Es∠δ − IrelayZs
Irelay
= −Zs +ZT
2− j
ZT
2cot
δ
2At δ = 180◦,
Zseen = −Zs +ZT
2
The vector component −Zs + ZT2 is a constant in R-X plane.
The component −j ZT2 cot δ
2 lies on a straight lineperpendicular to ZT
2 .
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Thus it can be seen thatThe trajectory of the impedance measured by relayduring the power swing is straight line.The angle subtended by a point in the locus on S and Rend points is the angle δ.The swing intersects the line AB, when δ = 180.This point of intersection of swing impedance trajectoryon the impedance line is known as electrical center ofthe swing.At the electrical center, angle between two sources is180◦.The existence of the electrical center is an indication ofsystem instability. i.e, the two generators are now out ofstep.If the post fault system is stable, the power swingretraces its path at δmax .
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
If ESER
= k 6= 1 then the power swing locus is an arc of thecircle.
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
It can be shown that,
ES
ES − ER=
k(cosδ + jsinδ)
k(cosδ + jsinδ)− 1=
k [(k − cosδ)− jsinδ]
(k − cosδ)2 + sin2δ
Then,
Zseen = −ZS +k [(k − cosδ)− jsinδ]
(k − cosδ)2 + sin2δZT
The location of electrical center depends upon |ES ||ER |
ratio.
Appearance of electrical center on a transmission lineis a transient phenomenon.
The electrical center vanishes after some time.
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
At the electrical center voltage is zero and the relays atboth ends will trip the line.
Thus, existence of electrical center indicates systeminstability and nuisance tripping of the distance relay.
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Power Swings and Distance Relaying
Let δZ1, δZ2 and δZ3 are the rotor angles when swing justenters the 3 zones, then,
If δmax < δZ3, then the swing will not enter the relaycharacteristics.
If δZ3 ≤ δmax ≤ δZ2, then the swing will enter in zone 3.
If δZ2 ≤ δmax ≤ δZ1, then the swing will enter in bothzone 2 and 3.
If δmax ≥ δZ1, then the swing will enter in all the 3 zonesand the relay will trip on zone 1.
Power SystemProtection
S.A.Soman
Introduction toPower Swings
Analysis ofTwo AreaSystem
Determinationof PowerSwing Locus
ElectricalCenter
ThreeSteppedDistanceProtection
Summary
Setting of Distance Relays
Summary
Introduction to power swings
Distance relay perspective of power swings
Swing locus seen by distance relay
Electrical center
Three stepped distance protection
Possibility of distance relay tripping on power swings
