IEEE IMPACT OF EXCITATION SYSTEM ON POWER SYSTEM STABILITY · PDF fileImpact of excitation...

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Impact of excitation system on power system stability ABB Industrie AG -1- ABB 1. INTRODUCTION

Transcript of IEEE IMPACT OF EXCITATION SYSTEM ON POWER SYSTEM STABILITY · PDF fileImpact of excitation...

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ABB

1. INTRODUCTION

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ABB

THE POWER STATION

GOVERNOR

AC & DCAUXILIARY SYSTEMS

PROTECTION

PT’s&CT’s

LV SWITCHGEAR

GENERATORBREAKER

STEP UP TRANSFORMER

HV- BREAKER

HV SYSTEM

CONTROL ROOM

1

1

CONTROL SYSTEMS

EXCITATIONSYSTEM

AUX.TRANSF

.

SYNCHRONOUSGENERATOR

STARPOINT

CUBICLE

TURBINE

EXCITATIONTRANSFORMER

SYNCHRONIZING

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ABB

The automatic voltage control system

SynchronousMachine

Excitation System

GridIf Ug

Controller

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ABB

~=

SM E

1 a 200 A

Rotating exciter

~=SM

100 a 10000 A

Static excitation

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Basic Requirements• Excitation Current up to 10’000 amps• Input frequency range from 16 Hz to 400 Hz• Adaptable to different redundancy requirements for controls and

converters• State of the art man machine interface• Compatibility with most applied power plant control systems• Remote diagnostics• Comfortable commissioning tools

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ABB

Operational Application

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ABB

Main Components of an UNITROL 5000 System

AVR

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ABB

System overview

Mea

surin

g Protection

Monitoring

AVR + FCR

Logic Control

FieldbreakerField flashingField suppression

AVR 1AVR 2

ConverterControl

CONVERTER 1CONVERTER 2

CONVERTER N

AVR = Autom. Voltage Reg.

FCR = Field Current Reg.

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Main Functions of the AVR with Field Current Controller

+

_

+

Σ

Σ Σ

AVR Setpoint-V/Hz limiter- Soft start- IQ Compensation- IP Compensation

Underexcit. limiter-Q=F(P,U,UG)-Stator current lead-Min. field current

Overexcit. limiter-Max field current-Stator current lag

Power SystemStabilizer PSS

Man setpoint

Mea

surin

g an

d A

/D c

onve

rsio

n

Min

. / m

ax. v

alue

prio

rity

PID

PI

To pulse generation

UG, IG, If

UG, IG

UG

UG, IG, If

UG, IG

If

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2. IMPACT ON POWER SYSTEM STABILITY

• Voltage Control Dynamics ( controller, power stage and power source)

• Limiters

• Power System Stabilizer

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ABB

2.1 Voltage Control Dynamics ( controller, power stage and power source)

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ABB

Computer representation of the AVR

Parameter Description Unit RangeTR Measuring filter time constant s 0.020Ts Gate control unit and converter time constant s 0.004KIR Reactive power compensation factor p.u. -0.20...+0.20KIA active power compensation factor p.u. -0.20...+0.20KR Steady state gain p.u. 10...1000Kc Voltage drop to commutations and

impedancesp.u. Acc. Transf.

TB1 Controller first lag time constant s TB1≥TB2TB2 Controller second lag time constant s 0<TB2≤TC2TC1 Controller first lead time constant s 0.01...10TC2 Controller second lead time constant s 0.01...2Up+ AVR output positive ceiling value p.u. FixedUp- AVR output negative ceiling value p.u. Fixed

Uf [p.u.]

Generatorvoltage [p.u.]

GeneratorReactivePower [p.u.]

11+sTR

HVGate

LVGate 1+sTB2

1+sTC2

UPKR

TBTC

+ 11

UPKR

TBTC

− 11

1+sTB1

1+sTC1

UPKR

+

UPKR

KR1

1+sTs

Up+∗ UT–Kc∗ If

Up- ∗ UT

from under-excitationLimiters UEL

from over-excitationLimiters OEL

UT -

UT setpoint [p.u.]

Σ

+

1

1+sTR

1

1+sTR

KIR

KIA

QT

PT

+

+

+

USTStabilizingsignal from PSS

GeneratorActivePower [p.u.]

+

Σ

Σ

+

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ABBSynchronous machinethree-phase representation

URIR

US

IS

UT

IT

UfIf

IDR

IDT

IDS

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ABBd-q decomposition

UdId

Uf If

IdD

Uq

Iq

IQ2

IQ1

Ψd

ΨfΨq

ΨdD

ΨQ1 ΨQ2

D axis

Q axis

δω

ra

ra

rdD

rf

rQ1 rQ2

Rotor

Stator

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ABB

TM PMspeed

TE US EpXq XT XE

=

= ⋅ ⋅+ +

sinδ

Motion equation:

TM TE H ddt

− = ⋅ ⋅2 ω

δ45o0o

TM

Torque

180o

The characteristic Dynamic Equation

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ABB

AVR - Voltage control dynamic

• Response time

• Ceiling capabilities

• Settings of control algorithm

“Excitation system nominalresponse”IEEE 421.1

• System type (Static excitation, Brush-less, DC Exciter)

UF ceiling

e=0.5so

a

b

d

c

UF nominal

NR= ce-ao

(ao)(oe)

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Comparison of Excitation System Types

Mechanical- Conversion of

mech. to electr.Energy

- Size of exciter

- Sliprings

Electrical- Rectifier

- Direct measuringof field current

- Fast fieldsuppression

DC-Exciter

yes

power and speed

yes

not required

possible

possible

AC-Exciterstat. diodes

yes

power and speed

yes

static external

possible

possible

AC-Exciterrot. diodes

yes

power and speed

no

static rotary

not possible

not possible

Stat. Exciterstat. thyristors

main machine

power

yes

static

possible

possible

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Comparison of Exciter Characteristics

DynamicPerformance- Major time

constant of control circuit

- Ceiling factor

- Negative field current

Reliability (MTBF)- Machine, Transformer

- Converter

Maintenance- Machine, Transf.

- Converter

DC-Exciter

Td’L+TE

limited

possible

good

good

at standstill

at standstill

AC-Exciterstat. diodes

Td’L+TE

not limited

not possible

good

better

at standstill

duringoperation

AC-Exciterrot. diodes

Td’L+TE

limited

not possible

good

better

at standstill

at standstill

Stat. Exciterstat. thyristors

Td’L

not limited

possible

better

better

at standstill

duringoperation

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ABB

Ceiling limit

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ABB

2.2 LIMITERS

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ABB

ΣTransducerand filter

-Setpoint -Soft start -V/Hz limiter -Q influence -P influence

HVGate

ΣLVGate

Power systemstabilizer

(PSS)

ΣUnder excitationlimiters (UEL)

Ug actual

effective setpoint

UgAC

Setpoint buildingAutomaticchannel

Over excitationlimiters (OEL)

DCGain

P gain

HF gain

1/TA 1/TB

Gain

ω

Ucmin

Ucmax

+

+

+

DCGain

P gain

1/TA 1/TB

Gain

ω

Ucmin

Ucmax

Transducerand filter

IF

+

-

Σ+

-

Manual set point

Follow upcontrol

Uc

α

Follow up Manual

Follow upAutomatic

Uc AVR

Uc FCR

Follow upAutomatic

Uc FCR

Uc AVR

+

FCR

AVR

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ABB

The Power Chart of a solid polesynchronous machine

LAGGING(overexcited)

LEADING(underexcited)

XdU2

XqU2 1.0-1.0 p.u. Q [p.u.]

P [p.u.]

Thermal limit ofstator

Thermal limitof rotor

ϕn

Rat

ed a

ppar

ent p

ower

Sn

Theorecticalstability limit

Practical stabilitylimit

Turbine ouputpower limit

Excit

ation

pow

er ≈ I

F

Declared ratedoperation point ofgenerating unit

Minimum Fieldcurrent limit

Safe operation range

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Synchronous machine operation Limits

Max. field current limiterMax. field current limiterMin. field current limiterMin. field current limiterStator current limiterStator current limiterUnder excitation P,Q limiterUnder excitation P,Q limiter P

+Q-Q 1/Xd

Theoreticalstability limit

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ABB

Main duty of the limiters:

Keep the synchronous machine operating withinthe safe and stable operation limits, avoidingthe action of protection devices that may trip the unit.

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ABBVOLTAGE REGULATOR WITH LIMITERS

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ABB

~=

SM =

Voltagesetpoint

== / ~

Supply

~

Ifth setpoint

Ifact -+ I dt2∫ COMPU

/ #

I dt2∫ maxIfmax

LOW

ER V

AL.

GA

TE

-

+

+-

THE OPERATING PHILOSOPHY OF Ifmax LIMITER

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ABB

Fiel

d cu

rrent

x IF

N

Time [s]

thermal limit

Ceiling limit

setpointswitch time

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ABB

y=KVHZ.x+U_fn-KVHZ

V / Hz Limiter

LVGate

U_fn

Max. field current limiter

Σ

IFth1

x2

Σ KIFmax+

-

+

-

KCFKHF

x>0

π

1s

0x>yx

EmaxF y

IFmax1IFmax2

Setpoint 2 forIF max limiter

(option)

IFth2

Machinefrequency

fT [p.u]

Machine fieldcurrent

IF [p.u.]

Stator current limiter1

1+ sTR

Σ

ITth

x2

Σ KITind+

-

+

-

KCSKHS

x>0

π

1s

0x>yx

EmaxS y

ITmax

StatorCurrentIT [p.u.]

To OEL limitergate of AVR

1

1+ sTR

1

1+ sTR

Min. field current limiterΣ KIFmin+

-

IFmin

HVGate

KPQΣ-1

1+ sTR

KITcapΣ+

-

1

1+ sTRπ

Look-upTableyn = f (xn)

Generatorterminalvoltage

UT [p.u.]

Generatoractive power

PT [p.u.]

Generatorreactive power

QT [p.u.]

+

P/Q limiter

To UEL limitergate of AVR

Σ+

-x>0

MachineterminalvoltageUT [p.u]

DelayT_V_Hz

UTMAX

UTMAX

0U_V/HZVolt/Hz

influencesignalto AVR[p.u.]

KVHZ

x y

x2

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ABB

2.3 Power system Stabilizer

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ABB

Ep

Xq(s)

InfiniteBus

XEXT

UGXT+XE

US

I

X q s X q s T qs T q o '

s T qs T q o '

( ) ' ' ''

= ⋅ ++

⋅ ++

11

11

Stationary and transient air gap voltages

δEp-S

δ

Ep

Ep’

UG

US

=I.Xq’

= I. (XE+XT)

= I. Xq

H

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ABB

USUG

Ep

∆PE

∆ω

phasorrotation

Transient behavior of ∆ω and ∆PE

90o

Pe ω

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ABBDynamic equation of synchronousmachine+grid for small oscillations

XT+XE

Infinite BusHA)

B)XT+XE

H1 H2 Equivalent Inertia Heq H HH H

= ⋅+

1 21 2

For small oscillations keeping the drivingtorque constant the dynamic equationlinearized can be written as:

TM TE H ddt

− = ⋅ ⋅2 ωtaking TM TE H d

dt− = ⋅ ⋅2

2

2 1 02

2⋅ ⋅ + ⋅ + =Hn

ddt

Dn

ddt

δω

δ ∆δ

Dampingtorque

SynchronizingTorque

Inertia. Ang. Acc.

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ABB

Ω ≅ ⋅⋅

K nH

12

ωOscillation frequency Ω

rad/s

δ90o0o

Torque

operating point

K slope T Ep UsXq' XE XT

o1 = = =+ +

⋅∆∆δ

'. cos 'δ

K1=synchronizing coefficient

δo

Phasor diagram of machine+grid for small oscillations ( without AVR)

∆ω

D/ws*∆ω

phasor rotation

∆Te=K1.∆δ

∆δ

∆UG Positive Damping (Stable Region)

Negative Damping (Unstable Region)

Synchronizing Axis

Damping axis

ResultingTorque

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ABB

2 1 02

2⋅ ⋅ + ⋅ + ⋅ + ⋅ =Hn

ddt

Dn

ddt

K K2 Epω

δω

δ ∆δ ∆ '

Dampingtorque

Synchron.Torque

Inertia.Ang. Acc. Add .Torquefrom exc. system

K2.∆Ep’

Phasor diagram of machine+grid+excitation system for small oscillations

∆ω

D/ws*∆ω

phasor rotation

∆Te=K1.∆δ

∆UGPositive Damping (Stable Region)

Negative Damping (Unstable Region)

Synchronizing Axis

-∆UG

Resulting torque componentwithout excitation system

Resulting torque componentwith excitation system

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ABB

P,Q

M

UG

ALTERNATIVE

FINALCTRLELEMENT

~

AVR PSS

Fig3: PSS in the excitation system

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ABB

Main Target of PSS:

• It provides an additional torque component in order to get:

1) A positive resulting torque component on damping axis, even for thehighest possible rotor oscillation frequency.

2) A positive torque component on synchronizing axis for partialcompensation of generator terminal voltage variations even forthe highest possible oscillation frequency

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ABB

PSS2A acc. IEEE 421.5 1992

Σω

PE

+

s.TW11 + s.TW1

s.TW3 1 + s.TW3

∆PM1 + s.2.H

Ks21 + s.T7

s.TW21 + s.TW2

s.TW4 1 + s.TW4

( )( )

1 81 9

+ ⋅+ ⋅

s Ts T M

N

Ks3

Σ

++

-

1+s.T1 1+s.T2Ks1

1+s.T31+s.T4

VST

VSTmax VSTmax

VSTmin VSTmin

∆PE1 + s.2.H

∆PA1 + s.2.H

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ABB

Adaptive PSS (APSS) – Alternative to PSS2A

AVRSynchronous Generator

Uf

Driving Power Pa

Measurement

Filter

PUG

UG , IG

APSS

∆PEstimator

3rd Order System Model

RegulatorWhitenoise

+

+ -

+

+

Uref +

+GRID

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ABB

Σ

OutputLimit To

AVRω∆

FB1(s)

FB2(s)

FI1(s)

FI2(s)

FH1(s)

FH2(s)

K b

K i

K h

Σ

Σ

Σ

+

-

-

-

+

+

+

+

Multi Band PSS (MBPSS)