IMPACT OF EXCITATION SYSTEM ON POWER SYSTEM STABILITY

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Impact of excitation system on power system stability

1. INTRODUCTION

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Impact of excitation system on power system stability HV SYSTEM

THE POWER STATION STEP UP TRANSFORMER

LV SWITCHGEAR AC & DC AUXILIARY SYSTEMS

HVHV- BREAK BREAKER ER

AUX. TRANSF .

GOVERNOR

CONTROL SYSTEMS

PROTECTION 1

GENERATOR BREAKER

1

SYNCHRONIZING

PT’s & CT’s

EXCITATIO NSYSTEM

SYNCHRONOUS GENERATOR

TURBINE

STAR POINT CUBICLE

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CONTROL ROOM

EXCITATION TRANSFORMER

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Impact of excitation system on power system stability HV SYSTEM

THE POWER STATION STEP UP TRANSFORMER

LV SWITCHGEAR AC & DC AUXILIARY SYSTEMS

HVHV- BREAK BREAKER ER

AUX. TRANSF .

GOVERNOR

CONTROL SYSTEMS

PROTECTION 1

GENERATOR BREAKER

1

SYNCHRONIZING

PT’s & CT’s

EXCITATIO NSYSTEM

SYNCHRONOUS GENERATOR

TURBINE

STAR POINT CUBICLE

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CONTROL ROOM

EXCITATION TRANSFORMER

ABB

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If

Synchronous Machine

Ug

Grid

Excitation System Controller The automatic voltage control system

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~ SM

E

1 a 200 A Rotating exciter

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= 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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Operational Application

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Main Components of an UNITROL 5000 System

AVR

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Impact of excitation system on power system stability System overview

AVR 2 AVR 1 g n i r u s a e M

CONVERTER N

Protection

CONVERTER 2 CONVERTER 1

Monitoring AVR + FCR

Converter Control

Logic Control

Field flashing Field suppression

Fieldbreaker AVR = Autom. Voltage Reg. FCR = Field Current Reg.

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Impact of excitation system on power system stability Main Functions of the AVR with Field Current Controller UG, IG UG

n o i s r e U , v G n o c D / A d n U , a G g n i r u s U , a G e M

IG, If

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

PID _ Σ

+

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

Overexcit. limiter IG, If IG

-Max field current -Stator current lag

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Σ

Σ

To pulse generation

Power System Stabilizer PSS

PI Man setpoint

If

y t i r o i r p e u l a v . x a m / . n i M

_ +

Σ

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Impact of excitation system on power system stability 2. IMPACT ON POWER SYSTEM STABILITY

• Voltage Control Dynamics ( controller, power stage and power source) • Limiters • Power System Stabilizer

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

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Impact of excitation system on power system stability Computer representation of the AVR

from under-excitation Limiters UEL

Generator voltage [p.u.]

UT

1 1+sTR

HV Gate

+

UT setpoint [p.u.] Generator QT Reactive Power [p.u.] Generator PT Active Power [p.u.]

+

1+sTR

KIR

1 1+sTR

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+ +

1

KIA

from over-excitation Limiters OEL

LV Gate

UP KR

+

1+sTC2 1+sTB2

+ UST Stabilizing signal from PSS

UP KR Parameter TR Ts KIR KIA KR Kc TB1 TB2 TC1 TC2 Up+ Up-

TB1 TC 1

UP KR

TB1 TC1

Up+ UT–Kc If 1

1+sTC1 1+sTB1

UP KR

KR

Uf [p.u.]

1+sTs Up-

UT

Description Measuring filter time constant Gate control unit and converter time constant Reactive power compensation factor active power compensation factor Steady state gain Voltage drop to commutations and impedances Controller first lag time constant Controller second lag time constant Controller first lead time constant Controller second lead time constant AVR output positive ceiling value AVR output negative ceiling value

Unit s s p.u. p.u. p.u. p.u.

Range 0.020 0.004 -0.20...+0.2 -0.20...+0.2 10...1000 Acc. Transf.

s s s s p.u. p.u.

TB1≥TB2 0
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Impact of excitation system on power system stability IR UR

IDR

UT If IT

Uf

IDT

IDS

IS

US

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Synchronous machine three-phase representation

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Impact of excitation system on power system stability D axis ra Id Ud

Ψd

rdD

Stator Ψ dD

IdD

δ

ω

rf Uf

Ψ f Ψ Q1

If

I Q 1

Rotor

Ψq

Ψ Q2

rQ1

I Q 2

Q axis

rQ2

ra Iq

Uq

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d-q decomposition

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Impact of excitation system on power system stability Torque

TM

PM speed

TE

US Ep sin Xq XT XE

TM

Motion equation:

TM 0o

45o

TE

180o

The characteristic Dynamic Equation ABB Industrie AG

2 H

d dt

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Impact of excitation system on power system stability AVR - Voltage control dynamic • System type (Static excitation, Brush-less, DC Exciter) • Settings of control algorithm • Response time • Ceiling capabilities

“Excitation system nominal response” IEEE 421.1 NR=

ce-ao (ao)(oe)

c UF ceiling

UF nominal

a

o

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b

d

e=0.5s

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Impact of excitation system on power system stability Comparison of Excitation System Types DC-Exciter

AC-Exciter

AC-Exciter

Stat. Exciter

stat. diodes

rot. diodes

stat. thyristors

yes

yes

yes

main machine

- Size of exciter

power and speed

power and speed

power and speed

power

- Sliprings

yes

yes

no

yes

Electrical - Rectifier

not required

static external

static rotary

static

- Direct measuring of field current

possible

possible

not possible

possible

- Fast field

possible

possible

not possible

possible

Mechanical - Conversion of mech. to electr. Energy

suppression

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Impact of excitation system on power system stability Comparison of Exciter Characteristics DC-Exciter

AC-Exciter stat. diodes

AC-Exciter rot. diodes

Stat. Exciter stat. thyristors

Td’L+TE

Td’L+TE

Td’L+TE

Td’L

- Ceiling factor

limited

not limited

limited

not limited

- Negative field current

possible

not possible

not possible

possible

Reliability (MTBF ) - Machine, Transformer

good

good

good

better

- Converter

good

better

better

better

Maintenance - Machine, Transf.

at standstill

at standstill

at standstill

at standstill

- Converter

at standstill

during operation

at standstill

during operation

Dynamic Performance - Major time constant of control circuit

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Ceiling limit

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2.2 LIMITERS

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Impact of excitation system on power system stability Setpoint building Automatic channel

Follow up Automatic Ug AC

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

Power system stabilizer (PSS)

+

Transducer and filter

Ug actual

Ucmax

-

Σ

Gain

HV Gate

Under excitation limiters (UEL)

+

LV Gate

+

+

DC Gain

HF gain

Σ

+

P gain

Σ

Uc AVR Uc α

Over excitation limiters (OEL)

1/TA

Ucmin

1/TB

ω

Ucmax

Gain

IF

Transducer and filter

DC Gain

-

Σ

Uc FCR P gain

+ 1/TA

Manual set point Follow up Manual Follow up Automatic

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Uc AVR

Follow up control

Ucmin

1/TB

ω

AVR FCR

Uc FCR

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Impact of excitation system on power system stability Theorectical stability limit

Practical stability limit

Declared rated operation point of generating unit

P [p.u.]

Turbine ouput power limit

Thermal limit of stator

F I

≈

LEADING (underexcited)

-1.0 p.u.

U2 U2 Xq Xd

n r e S w r o p ϕn w e n o o i t p a t t i n x c E re a p p a d te a R

Thermal limit of rotor

LAGGING (overexcited)

1.0

Minimum Field current limit

Q [p.u.]

Safe operation range

The Power Chart of a solid pole synchronous machine ABB Industrie AG

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Synchronous machine operation Limits Max. field current limiter Min. field current limiter Stator current limiter Under excitation P,Q limiter

P

Theoretical stability limit

-Q

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1/Xd

+Q

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Main duty of the limiters: Keep the synchronous machine operating within the safe and stable operation limits, avoiding the action of protection devices that may trip the unit.

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VOLTAGE REGULATOR WITH LIMITERS

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Impact of excitation system on power system stability Supply Voltage setpoint

-

~

+

= + Ifth setpoint

E T A G . L A V R E W O L

∫ I dt max 2

Ifact +

=/~ =

SM

-

∫

I2 dt

Ifmax COMP / #

U

= ~ THE OPERATING PHILOSOPHY OF Ifmax LIMITER

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Ceiling limit N F I x t n e r r u c d l e i F

thermal limit setpoint switch time

Time [s]

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2.3 Power system Stabilizer

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Impact of excitation system on power system stability Xq(s)

Ep

I

XT

XE

UG

US XT+XE

Infinite Bus

Ep

= I. Xq

Ep-S

Ep’ H

=I.Xq’ UG

X q(s )

Xq

1 1

sTq' sTqo'

1 1

sTq'' sTqo''

= I. (XE+XT) US

Stationary and transient air gap voltages ABB Industrie AG

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Impact of excitation system on power system stability Ep

Pe

UG

US

90o

phasor rotation

Transient behavior of ABB Industrie AG

and PE

PE

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Impact of excitation system on power system stability A)

Infinite Bus

H XT+XE

B)

H1

H2

Equivalent Inertia

Heq

XT+XE

taking

TM

TE

d 2 H dt

For small oscillations keeping the driving torque constant the dynamic equation linearized can be written as:

TM

2 H d2 n dt 2 Inertia. Ang. Acc.

TE

D d n dt Damping torque

Dynamic equation of synchronous machine+grid for small oscillations ABB Industrie AG

2 H

K1

H1 H2 H1 H2

d2 dt 2

0

Synchronizing Torque

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Impact of excitation system on power system stability Oscillation frequency

K1 n rad/s 2 H

phasor rotation

Negative Damping (Unstable Region)

UG

Positive Damping (Stable Region)

D/ws*

Damping axis

K1=synchronizing coefficient

Torque

operating point K1

Te=K1.

slope

T

Ep'. Us Xq' XE

XT

cos o'

Resulting Torque Synchronizing Axis

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0o

o 90o

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

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Impact of excitation system on power system stability 2 H d2 n dt 2

D d n dt

Inertia.Ang. Acc.

Damping torque

K1

K2

Synchron. Torque

Ep'

0

Add .Torque from exc. system

phasor rotation UG

Negative Damping (Unstable Region) K2. Ep’

D/ws*

Positive Damping (Stable Region)

- UG Resulting torque component without excitation system Resulting torque component with excitation system

Te=K1. Synchronizing Axis

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

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Impact of excitation system on power system stability FINAL CTRL ELEMENT ALTERNATIVE

~ AVR

U

PSS

G

M

P ,Q

Fig3: PSS in the excitation system ABB Industrie AG

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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 the highest possible rotor oscillation frequency. 2) A positive torque component on synchronizing axis for partial compensation of generator terminal voltage variations even for the highest possible oscillation frequency

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Impact of excitation system on power system stability PSS2A acc. IEEE 421.5 1992

PM 1 + s.2.H PA 1 + s.2.H

s.TW1 1 + s.TW1

+

s.TW2

(1 s T 8) (1 s T 9)M

1 + s.TW2

N

+

+ Ks1

VSTmax

VSTmax

1+s.T1

1+s.T3

1+s.T2

1+s.T4

VST

VSTmin

VSTmin

Ks3

PE

s.TW3

s.TW4

Ks2

1 + s.TW3

1 + s.TW4

1 + s.T7 PE 1 + s.2.H

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Impact of excitation system on power system stability Adaptive PSS (APSS) – Alternative to PSS2A Driving Power Pa Uref

+

AVR

Uf

+ +

Synchronous Generator

UG , IG

GRID

+ -

Measurement UG

APSS

Filter 3rd Order System Model Estimator

White noise

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P

+

P

Regulator +

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IMPACT OF EXCITATION SYSTEM ON POWER SYSTEM STABILITY

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