SIEMENS - Out of Step Protection

Out of Step Protection

Basics to the Static Stability Two Machine Problem, Replica, Vectors Xd

XN

I

U‘P

UG

U‘N

Rotor voltage Voltage on generator terminals Reference voltage of network Synchronous direct-axis reactance Reactance of network

Ohmic Load

Inductive Load U‘P

U‘P UG U‘N Xd XN

with: X:=Xd + XN The active power is:

U‘P

jX I

jX I

δ

U‘N

I

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U‘N

δ I

U 'N U 'P P= sinδ X Siemens. Innovation for generations.

Basics to the Static Stability Power Angle Characteristic Point A: Deflection of rotor angle from point A Psupply = Pm + ∆P > Pm

Threshold angle of static stability

P

∆P is taken from the stored energy in the

Pm

A

∆P

Psupply

centrifugal mass

B

Rotor will be slow down and the machine returns to point A ∆P

3

δA ∆δ 90°

δB ∆δ

Pm mechanical Power of Turbine (Calculation is done per Phase)

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Point B: Deflection of rotor angle from point B Psupply < Pm

δ

Rotor will be accelerate (∆P is going into rotor) and the machine falls out of step

Stability is always fulfilled, if the inclination of supplied power is positive Siemens. Innovation for generations.

Basics to the Transient Stability Discussion of the Areas U 'N U ' P P= sinδ X'

Stiff system U= const. Tm infinite

Pbefore

P




3phase SC: Pm > Pduring (SC) Rotor is accelerated




After the fault clearance: Pafter >Pm Rotor is braked

Pafter V

Pm 3

B

Pduring δperm.

δbefore δafter

t SC perm =

Transient stability is fulfilled if area V ≥ area B δLimit

δ

Is during the SC δ < δperml. , than the transient stability is fulfilled (can be guaranteed) Is δ > δLimit than the machine falls out of step

2 Tm S N (δ perm − δ before ) ω∆P

3 ∆P = Pm − Pduring (δbefore ) − Pduring (δperm ) 2

[

]

Tm - mechanical time constant Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Measures for Improvement of Transient Stability (Warranty of Dynamic Stability)
U‘p should be large (overexcited operation at synchronous

machines)
U‘N should be large (high voltage level in the grid)
U‘p should be large in the case of a fault (field forcing)
X should be small (high degree of meshed networks)
δ perm. should be small (fast fault clearing time, especially at

faults close to the power plant; dead time should be so optimised, that the changing of the rotor angle is very small during an unsuccessful AR at the critical machine for the stability)
Pmechanical decreasing during short circuit (total or partly closing

of valves (fast valving))
P increasing with additional load impedance during a short

circuit (e.g. concrete resistor) Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Why is a Out of Step Protection necessary ? Xd

XTr XN

Stiff system U= const. Tm infinite

Protection 1. If the generator supplies too long to the short circuit, the rotor will be accelerates and the permissible angle δ perm. passes the limit. 2. Active power swings appears after the clearing of the short circuit, because the generator was fallen out of step. 3. When the out of step stress passes into the generator region, an inadmissible mechanical (torsion vibration) and thermal stress of the system (generator, turbine) is possible. Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Example of a Power Swing Gemessene Leiterströme Currents

Strom in A

5

iL1

0

i

5

400

600

800

1000

1200

1400

1600

1200

1400

1600

ta

Gemessene Phase to Leiter-Erde-Spannungen earth voltagesZeit in msi 100

Spannung in V

50

uL1

0

i

50

100

400

600

800

1000 ta

i Zeit in ms

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Siemens. Innovation for generations.

Equivalent Circuit for the Description of the Process X’d

Measuring point Protection

R

ZG U’p

XT

XN

ZT

ZN

UR

U‘N Measured Impedance:

U 'P Z ZR = ' - ZG ' UP − UN ZG Generator impedance ZT Transformer impedance ZN Network impedance

Copyright © Siemens Australia & NZ 2007. All rights reserved.

'

δ

Z = ZG + ZT + Z N

Z ZR = - ZG ' U N - jδ 1− ' e UP

U N U 'N - jδ = ' e ' UP UP

ZR = f(δ)

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Principle Impedance Trajectory during a Out of Step Condition (Power Swing)

ZR =

jX ' jX d U 'N - jδ 1− ' e UP

Example: X = 10 p.u X‘d = 3 p.u.

0° 70

50

180°

Im( Z1( δ ) ) Im( Z2( δ ) )

7 3

0

Im( Z3( δ ) )

Power swing centre: Un/Up = 1 and X/2 -3 + 5 = +2

X

50

0° 70 60 70

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40

Un/Up =.85 Un/Up = 1,15 Un/Up = 1

20

0

20

Re( Z1( δ ) ) , Re( Z2( δ ) ) , Re( Z3( δ ) )

40

60 70

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Impedance Curves during a Power Swing

127.761

150

Impedanz in Ohm

100

Z1 R1

i

50

i

X1 i

10 10

0

50

72.53

100 300

400

600

800

1000

1200 ta i

1400

1600

1800 1.75 .10

3

Zeit in ms Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Impedance Trajectory during a Power Swing 126.856

150 10

10

100

X1 i

50

Short circuit 10 0

10

Load 8.156

50

80 72.53

Copyright © Siemens Australia & NZ 2007. All rights reserved.

60

40

20

0 R1

i

20

40

60 52.638

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Impedance Trajectory during a Synchronous Power Swing Ortskurve: Circle diagram 140 10 120

100

80 X1

i 60

40

20 10 0

0

20

40

60 R1

80

100

120

i

System goes back into the stability operation Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Rate of Impedance Change during a Power Swing jX Z= - jX 'd ' U 1 − N' e - jδ UP

Z=

X π fP dZ (t ) dR(t) ≈ = 2 sin 2 ( π f P t) dt dt =

X π fP Ω in δ 2 sin 2 ( ) s 2

300

jX U 'N - j2π fpt 1− ' e UP

- jX 'd

fp = 1Hz; X=10Ω

300

250

200

dZ( δ ) 150

100

fp Power swing frequency 50

The power swing has in principle a different rate of impedance change. It has at π (180°) the minimum; (that means at the minimum of calculated Z) Copyright © Siemens Australia & NZ 2007. All rights reserved.

0

0

0

0.573

50

100

150

200

250

. 180

δ π Winkel in Grad

300

350

400 359.817

Rate of impedance change Änderungsgeschwundigkeit in Ohm/s

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Measuring algorithm

1. Filtering and vector calculation

Connection: Xd

XTr

iL uL

Fourier filter

IL = IrL + j IiL UL = UrL + j UiL

2. Positive and negative components

Protection

IL UL

Symmetrical components

I2 I1 U1

3. Calculation of the positive-sequence impedance

I1 U1

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Positive-seq. impedance

Z1= R + jX

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Characteristics and Settings Im(Z)

Zd

Power swing in the network

Setting hints: Zb: transient direct-axis reactance X’d

Ch 2

Zc: 0,7-0,9 Transformer reactance XT

Alarm

Zd-Zc: Reactance of the network + rest transformer

Zc ϕp

Power swing in generator and unit transformer

Trip

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Ch 1

Zb

Re(Z)

Za =

Za

Z sum / 2 tan(δ / 2)

with δ = 120 o

Zsum = Zb + Zc

Power swing angle between generator and transformer

Zsum = Zb + Zd

Power swing angle between generator and network

Inclination angle consider R-part; and the infeed of more than one generator

If Ch1 and Ch 2 is used

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Basic Structure of the Protection Function

Ch.1 I1 >

&

Alarm Ch. 1

Release

I2 <

Counter 1

n > n1 Measuring Z1

TRIP Ch. 1

Ch.2 Alarm Ch. 2 Counter 2

n > n2

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TRIP Ch. 2

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How are the Counters increased? Im(Z)

Ch.2 (n2)

Rules: 1. The impedance vector must enter into the power swing polygon (Ch.1 or Ch.2)

+1(n2) +1(n1)

0 Re(Z)

+1(n1)

Ch.1 (n1)

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0

2. The crossing of the middle line (red) is the criterion of the counter increasing 3. Increasing the counter of characteristic in which the middle line was crossed 4. The increasing of the counter is active, if the vector goes out of the power swing polygon

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Settings in DIGSI Secondary values

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Siemens. Innovation for generations.

Characteristics of the Competitors (ABB)

Im(Z)

Direction element

Network Trip angle e.g. 120° Transformer

Generator

Zone2 Zone1

Measuring method: U cos ϕ (Voltage, which is in phase with the current - Voltage at zero crossing of the currents)

Re(Z)

Alarm angle (e.g. 60°)

Alarm signal, possibility for control application ???

Alarm: Siemens renounced on that - this is a part of the control

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Siemens. Innovation for generations.

Characteristics of the Competitors (IEEE) Single and Double Blinder Elements

Im(Z)

Blinders Direction Element

System

Gen

Re(Z)

Gen (X‘d)

∆t

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Supervision of the impedance vector if he enters and leaves the blinders - logic and timers are than active. The double blinder principle is used in the case of a faster decision between short circuit and power swing (indirect speed measurement via the time (∆t) in which the vector is between the internal and external blinder) The superimposed impedance characteristic is a MHO - circle. Siemens. Innovation for generations.

Dynamic Test of 7UM62 with Real Time Simulator (RTDS) Test Configuration Impedance protection zones Z2 Z1 ALF1´

Load

ALF2´ ≠

G 7UM62 ALF: Actual Accuracy limiting factor of CT

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Z< t=0,4 s

Network protection trips with a delay; after than a power swing occurs

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Dynamic Test of 7UM62 with RTDS Instantaneous Fault Record (RMS Curves)

Fault

Trip Network Protection

Trip Out of Step

Power Swing

Power swing trip event Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Dynamic Test of 7UM62 with RTDS RMS Fault Record (Fault Duration 300ms)

Positive Sequence Current Positive Sequence Voltage Resistance

Reactance

After switching off of the short circuit a swing swing occurs. The out of step protection trips, because the power swing centre was in the generator unit.

Power swing trip event Copyright © Siemens Australia & NZ 2007. All rights reserved.

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Dynamic Test of 7UM62 with RTDS Vectors in the R, X - Diagram (primary impedance)

1

1 0.15

Primary ReactanceininOhm Ohm Primärwiderstand

0.5

0.15

Load point

Short circuit

0.17

X1 i

0 .21

Power swing through the characteristic 0.5

1

1

1 1

Copyright © Siemens Australia & NZ 2007. All rights reserved.

0.5

0

0.5 R1

1

i Primärwiderstand in Ohm Primary Resistance in Ohm

1.5 1.5

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