Power Swing

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Brief Introduction for Power Swing Detection Methods & Testing

Kamin DaveApplication Engineer (Protective Relaying)

DOBLE ENGINEERING COMPANYDoble Conference - 2007

Basic definitions:

Power Swing: A variation in three phase power flow which occurs when the generator rotor angles are advancing or retarding relative to each other in response to changes in load magnitude and direction, line switching, loss of generation, faults, and other system disturbances.

Pole Slip: A condition whereby a generator, or group of generators, terminal voltage angles (or phases) go past 180 degrees with respect to the rest of the connected power system.

Stable Power Swing: A power swing is considered stable if the generators do not slip poles and the system reaches a new state of equilibrium, i.e. an acceptable operating condition.

Unstable Power Swing: A power swing that will result in a generator or group of generators experiencing pole slipping for which some corrective action must be taken.

Power swing due to;Power system faults (System more affected on 3-ph fault, so power gain is very less)Sudden changes to electrical power due to rejection of load or generatorsThese type of disturbances causes oscillation in machine rotor angles (Load Angle = 900 or 1200) & can result in sever power flow swings. Power swings are variations in power flow that occur when the internal voltages of generators at different network locations of the power system slip relative to each other.

Vector diagram: two machines

ΙL ⋅ ZS1 ΙL ⋅ ZL ΙL ⋅ ZS2

E2 = E'2E1UA UB

Ι 'L ΙL

ϑ' ϑE'1

E1 E2ZS1 ZS2ZL

Two Machine Problem

If the Load angle becomes too large, the system stability can be lost

Pmax = {(E1*E2)/X}*SIN(theta)

Figure:-1

Power diagram:

If the Load angle becomes too large, the system stability can be lost

Pmax = {(E1*E2)/X}*SIN(theta)

Figure:-1A

Critical Angle

Rotor Angle

Stability limit

Power Diagram

0 50 100 150 200

Angle in Degree

Stability region

Power swing detection methods:Conventional “Rate of Change of Impedance” (Dz/Dt) Methods:Concentric characteristic schemes.Continuous Impedance calculation schemes.Blinder Schemes:1) Single Blinder2) Double Blinder

Conventional PSB detection methods:It measures the Positive sequence impedanceDuring a system swing, the measured impedance moves slowly on the impedance plane, and the rate of impedance change is determined by the slip frequencyo f a n e q u i v a l e n t t w o - s o u r c e s y s t e mTravel Time (Delta-t) from one point to next point is to differentiate between fault & swing. Travel time is more than set value then, distance relay sense as swing. Travel time less than set value then, it sense as fault

Conventional PSB detection methods:Concentric circle characteristic schemes:It is simplest method to detect “rate of change of impedance”Two impedance zones. Inner zone & outer zoneFigure-3 shows, Relay indicates PSB when transition time of impedance from outer zone to inner zone is more than set value (impedance locus move slowly).PSB: Blocks the zones when swing is stable (Figure:-3)OST: Trip zones when swing is un stable (Figure-4)Major drawback of this characteristic is load encroachment.

Figure:-3

Swing Locus

∆t = time for transition of Z from outer to inner zone

Stable Swing

Set “Zin” & “Zout” on line angle

Concentric characteristic scheme: MHO Circle

Figure:-4

∆t Un stable Swing

Figure:-5

∆t •R & X reach are dependent on Z value

•Difficult to set on Load Encroachment

Set “Zin” & “Zout” on line angle

Concentric characteristic scheme: Lens

Avoid mal operation during load encroachment

Figure:-6

Conventional PSB detection methods:

Concentric characteristic schemes:Polygon Characteristic concept is much more better than concentric circle characteristic.Rset & Xset for out zone can able to set independently.Rset of outer zone can not reach in to Load region

Concentric characteristic scheme: PolygonCharacteristic in relay type REL511

RZload

Dz1/Dt1Dz2/Dt2

Dz3/Dt3

Dz4/Dt4

Inner boundary = Zin Outer boundary = Zout

Polygon Characteristic for Power swing detection: Set R & X reach independently

Figure:-7

Set “R” reach

Set “X” reach

∆t = time for transition of Z from outer to inner zoneR reach reduce for Load encroachment

Concentric characteristic scheme: PolygonCharacteristic in relay type 7SA610

Polygon Characteristic: Set “R” reach according to Load Encroachment

Figure:-8

Load Angle = 120o or 90o

Angle = 45o

E1 = E2

E1 > E2

E1 < E2

ϑϑ' load point

Figure:-9

Concentric characteristic scheme: PolygonCharacteristic in relay type 7SA610

Concentric characteristic scheme: Quadrilateral

Quadrilateral Characteristic: Set “R” reach according to Load Encroachment

Figure:-10

Line Angle

Concentric characteristic scheme: PolygonCharacteristic in relay type RELZ 100

Stable Swing

Unstable Swing

Quad + Polygon

Zone 3 limit

Zout boundary

• Timer unit count to starts travel time when Power swing locus crosses Zout boundary • Travel time (Delta T) more than set time, relay detect as a Power swing

Figure:-11

Z1(t1)Z2(t2)

Z3(t3)

Z4(t4)

Concentric characteristic scheme: PolygonCharacteristic in relay type P442

Line Angle

Figure:-12

Continuous Impedance calculation for PSB detection methods:

Scheme determine Power swing condition based on a continuous impedance calculation. i.e for each 2ms or 5ms step an impedance calculation is performed & compared with the impedance calculation of previous 2ms or 5ms.This step value is depend on sampling freq. rate.The next impedance that should be calculated 2ms or 5ms later is predicted based on impedance difference of the previous measured impedances.Not required any delta impedance settingsRequire delta time settingsDrawback is; if prediction is correct, then only distance prot. consider as a traveling impedance & a power swing condition is detected. For security reason additional predictive calculation is required

Figure:-13

Continuous Impedance calculation for PSB detection methods:

Continuous Power calculation for PSB detection methods: Polygon characteristic in relay type P437

Stable Swing

Unstable Swing

Zpos(t)

Setting Parameters:•Rset•Xset•Line Angle

Detect as Fault:Zpos(t1) = 0 ms Zpos(t2) < 40 ms

Detect as Swing:Zpos(t1) = 0 ms Zpos(t2) > 40 ms

Figure:-14

Relay measures the rate of change of Positive sequence Apparent Power:

Delta-SPOS = (SPOS2 – SPOS1) / (SPOS2)

Let,SPOS1 = Positive sequence Apparent Power at time (t1)SPOS2 = Positive sequence Apparent Power at time (t1 + 40 ms)

Continuous Power calculation for PSB detection methods: Polygon characteristic in relay type P437

Pole slip detection method: Relay type REM54

• Scheme is applicable for Out of Step tripping of Generator • Pole slip detection = OMHO characteristic + Counter• Counter count number of operation• Reset time required to set properly other wise it will detect loss of field function• Normally counter setting is 3 or 4

X’d / 21

Offset mho characteristic

Counter

Operating Region

Counter Reset

Figure:-15

Pole slip detection method: Relay type REG 316

Pole-Slip Characteristic

-7 -5 -3 -1 1 3 5 7

R in Ohm

ZAWarning Angle

Tripping Angle

Figure:-16

Trip When Load angle isMore than Trip Angle

System Load angle is more than set value with calculated slip freq., relay will detect pole slip (Power swing)Relay calculating Freq slip based on T-Slip time (say 55 ms)Tripping Angle setting should be based on critical angle of machineWithout system study, recommended angle setting is 120o

Pole slip detection method: Relay type REG 316

Blinder Schemes: Single BlinderBlinder schemes:Single Blinder (one pair) scheme (Figure:-17):One sets of blinders: Resistive reach inner (RRI) & Resistive reach outer (RRO)It can be used to restrict tripping of the distance relay for loads outside of the blinders.

DR/DT schemes:Single Blinder scheme (Figure:-17):One sets of blinders: It can be used to restrict tripping of the distance relay for loads outside of the blinders.If travel time is more than set time & it crosses blinder-1, a power swing situation is detected further it crosses blinder-2 then it gives a tripping. Scheme is more suitable for out of step protection for generators.Correct settings of blinder-2 & RRR are very difficult to set, again it’s require high skilled expert to do grid analysis & effect on machines.

Blinder Schemes: Single Blinder

DR/DT scheme:How to set timer?T = {(Critical load angle – Rotor angle)*Fn}/{(360)*(slip frequency)}

Where;Critical load angle = 150 degreeRotor angle = 90 degreeFn = 50Hzslip freq. = 4Hz

T = {(150 – 90)*(50)} / {(360)*(4)}= 2.0833 Cycles

T = 2.0833 * 20 = 41.66ms

Blinder-1Blinder-2

Figure:-17

Single Blinder

Un stable swingStable swing

RRR = Reverse Resistive Reach in Ohms

OST-REGION

OST = Out of Step trip

Relay Type: ZTO 11

Blinder-1Blinder-2

Single Blinder

Un stable swingStable swing

MHO supervising unit

Mho supervising unit will restrict mal-operation of relay

Rset is not adjustable in forwarddirection

Figure:-18

CEB + CEH Relays

Blinder 1

Line Angle

MHO supervising control

Load Angle = 90o or 120o

Swing Centre = {X’d + (XT + XS)} / 2

Figure:-19

Swing Locus

Blinder schemes:Double Blinder (Two Pair) scheme (Figure:-20):Two sets of blinders: Resistive reach inner (RRI) & Resistive reach outer (RRO)Time Counter starts when the impedance vector crosses outer limit (RRO) & stops when it crosses inner limit (RRI)If travel time is more than set time, power swing situation is detectedScheme is applicable for independent of the distance zonesCorrect setting for Blinders are very difficult & it requires good expert who can able to do power grid analysis.

Blinder Schemes: Double Blinder

Blinder-1Blinder-2

Figure:-20

Double Blinders

Blinder-1Blinder-2

Blinder Schemes: Double Blinder

Testing methods with F6150:

Asynchronous method by using Doble advance transient creatorDz/Dt simulation by using State simulation module

Asynchronous method by using doble advance transient creatorTwo machines

Power Swing Testing: Asynchronous method

Select “Swing”

Source-1 parametersSource-2 parameters

Slip freq. = 2Hz

Line parameters

Save datc file

Asynchronous method: Power swing Waveform

Save comtrade file for test

Power Swing Testing: Dz/Dt Method

Dz/Dt simulation by using State simulation moduleTotal 25 nos. of statesEach state having independent magnitude, phase angle & frequency controlledIndividual time setting for each stateEach state having option to enter individual value for “L/R” constantPhasor diagram for each state to show voltage & current vectors

Power Swing Blocking Testing: REL100

Figure: 11 shows relay characteristicModule Type: State SimulationTest Sequence:

Load condition: State 1Apply 3-phase Fault in PSB locus(Z1): State 2Apply 3-phase Fault in Zone 3: State 3Travelling time between State 2 & State 3 should be more than time out setting of relay (Say: 70 ms if relay setting = 50ms)Remove fault: State 4

State Simulation

Travel TimeZ1 Z2 Z3

Phasor

State: 2 State: 3 State: 4

Excel Spread sheet Control:

Micro Soft Excel Spread sheet for Custom ApplicationEasy to useState Simulation up to 20 statesEasy to simulate any Power System EventsMore flexibility to calculate numerical functionsInterface with F6150 via serial communicationEngineers’ choice

REL100: PSB Testing

REL100:Relay settings:ZONE-3:Rin = 7.97 Ohms, Xin = 43.97 OhmsPSB settings (Polygon char.):Rout = 7.97*1.2 = 9.56 OhmsXout = 43.97*1.2 = 52.76 Ohms

PSB settings

Power Swing Testing: REL100

State: 1/2/3/4/5

Plot “Dz/Dt” locus

PSB Polygon

Run Test

Select Com Port

Auxiliary DC

Assign Trigger

REL521: PSB Testing

Dz/Dt simulation by using State simulation moduleREL521:PSB settings:Inner zone settings (Polygon char.):RIN = 3.0 Ohms, XIN = 3.0 OhmsOuter zone settings:KR = 125%, KX = 125%RIN = 3*1.25 = 3.75 OhmsXIN = 3.*1.25 = 3.75 Ohms

REL521: PSB Testing

PSB settings

Enter values in states

Enter travel time in states

REL521: PSB Testing

Dz/Dt locusZin Zout

Pole Slip Relay Testing with F6150 & Excel Spread sheet control

Basic definitions: Pole Slip

Pole Slip: A condition whereby a generator, or group of generators, terminal voltage angles (or phases) go past 180 degrees with respect to the rest of the connected power system.High level Transient torque develop in machine shaft can break the shaftHigh Stator Iron flux can over heat generator stator coreProtection is also known as a out of step Protection of machineProtection is applicable for synchronous machines

Basic definitions: Pole Slip

X’d = 25%

XT + Xs = 18%

Electrical Centre

Theta = Line Angle

Rotor Angle

XTX’d

Relay Details:MAKE : GEC ALSTHOMMODEL : ZTO 11

Relay Settings:K1 = 1K2 = 1K3 = 8R = K1*K2*k3 = 8 OhmsBlinder 1 = 65 DegreeBlinder 2 = 65 DegreeTime = 55ms

Relay Characteristic: Blinder Scheme

Traveling Time (Dt) should be more than 55 ms from one point to next point

Power swing Locus

65o 65o

A’’

B’’

ZTO 11

Relay Characteristic: Blinder Scheme

Traveling Time (Dt) should be more than 55 ms from one point to next point

Power swing Locus

65o 65o

A’’

B’’

Optional Scheme:ZTO 11 + YTGM15

Mho Characteristic to reduce the mal operation of relay

Pole Slip Protection: ZTO11

Four ElementsOver current starterBlinder 1 Unit (A-Unit)Timer Unit (T-Unit)Blinder 2 Unit (B-Unit)

Alarm: Operation of A & T units with O/C starterFault condition: Operation of A & B units with O/C starterPole Slip condition: Operation of A, T & B units in sequence with O/C starter

ZTO11: Stable Swing

Enter Time out Time more thanSet value

Swing Locus

ZTO11: Pole Slip Condition

Enter Time out Time more thanSet value

Swing Locus

VTest & ITest values set for Test

ZTO11: Fault Condition

Enter Time out Time less than set value

Swing Locus

VTest & ITest values set for Test

ZTO11: Reverse Resistive Reach Test

Enter Time out Time less than set value

Swing Locus

DOBLE ENGINEERING PVT. LTD305-SAKAR, OLD PADRA ROAD,VADODARA-390 015, INDIA.Cell: +91 94267 47545PH: (+91)(265) 655 77 15 & Fax: (+91)(265) 235 62 85E-MAIL: kdave@doble.com Website: www.doble.com

Power Swing

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