impact of excitation system on power system stability
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Impact of excitation system on power system stability
ABB Industrie AG
- 1 -
ABB
1. INTRODUCTION
Impact of excitation system on power system stability
ABB Industrie AG
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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
Impact of excitation system on power system stability
ABB Industrie AG
- 3 -
ABB
The automatic voltage control system
SynchronousMachine
Excitation System
GridIf Ug
Controller
Impact of excitation system on power system stability
ABB Industrie AG
- 4 -
ABB
~=
SM E
1 a 200 A
Rotating exciter
~=SM
100 a 10000 A
Static excitation
Impact of excitation system on power system stability
ABB Industrie AG
- 5 -
ABB
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
Impact of excitation system on power system stability
ABB Industrie AG
- 6 -
ABB
Operational Application
Impact of excitation system on power system stability
ABB Industrie AG
- 7 -
ABB
Main Components of an UNITROL 5000 System
AVR
Impact of excitation system on power system stability
ABB Industrie AG
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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.
Impact of excitation system on power system stability
ABB Industrie AG
- 9 -
ABB
Main Functions of the AVR with Field Current Controller
+
_
+
_Σ
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
Impact of excitation system on power system stability
ABB Industrie AG
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ABB
2. IMPACT ON POWER SYSTEM STABILITY
• Voltage Control Dynamics ( controller, power stage and power source)
• Limiters
• Power System Stabilizer
Impact of excitation system on power system stability
ABB Industrie AG
- 11 -
ABB
2.1 Voltage Control Dynamics ( controller, power stage and power source)
Impact of excitation system on power system stability
ABB Industrie AG
- 12 -
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
+
Impact of excitation system on power system stability
ABB Industrie AG
- 13 -
ABBSynchronous machinethree-phase representation
URIR
US
IS
UT
IT
UfIf
IDR
IDT
IDS
Impact of excitation system on power system stability
ABB Industrie AG
- 14 -
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
Impact of excitation system on power system stability
ABB Industrie AG
- 15 -
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
Impact of excitation system on power system stability
ABB Industrie AG
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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)
Impact of excitation system on power system stability
ABB Industrie AG
- 17 -
ABB
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
Impact of excitation system on power system stability
ABB Industrie AG
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ABB
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
Impact of excitation system on power system stability
ABB Industrie AG
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ABB
Ceiling limit
Impact of excitation system on power system stability
ABB Industrie AG
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ABB
2.2 LIMITERS
Impact of excitation system on power system stability
ABB Industrie AG
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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
Impact of excitation system on power system stability
ABB Industrie AG
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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
Impact of excitation system on power system stability
ABB Industrie AG
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ABB
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
Impact of excitation system on power system stability
ABB Industrie AG
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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.
Impact of excitation system on power system stability
ABB Industrie AG
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ABBVOLTAGE REGULATOR WITH LIMITERS
Impact of excitation system on power system stability
ABB Industrie AG
- 26 -
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
Impact of excitation system on power system stability
ABB Industrie AG
- 27 -
ABB
Fiel
d cu
rrent
x IF
N
Time [s]
thermal limit
Ceiling limit
setpointswitch time
Impact of excitation system on power system stability
ABB Industrie AG
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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
Impact of excitation system on power system stability
ABB Industrie AG
- 29 -
ABB
2.3 Power system Stabilizer
Impact of excitation system on power system stability
ABB Industrie AG
- 30 -
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
Impact of excitation system on power system stability
ABB Industrie AG
- 31 -
ABB
USUG
Ep
∆PE
∆ω
phasorrotation
Transient behavior of ∆ω and ∆PE
90o
Pe ω
Impact of excitation system on power system stability
ABB Industrie AG
- 32 -
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
2δ
2 1 02
2⋅ ⋅ + ⋅ + =Hn
ddt
Dn
ddt
Kω
δω
δ ∆δ
Dampingtorque
SynchronizingTorque
Inertia. Ang. Acc.
Impact of excitation system on power system stability
ABB Industrie AG
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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
Impact of excitation system on power system stability
ABB Industrie AG
- 34 -
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
Impact of excitation system on power system stability
ABB Industrie AG
- 35 -
ABB
P,Q
M
UG
ALTERNATIVE
FINALCTRLELEMENT
~
AVR PSS
Fig3: PSS in the excitation system
Impact of excitation system on power system stability
ABB Industrie AG
- 36 -
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
Impact of excitation system on power system stability
ABB Industrie AG
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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
Impact of excitation system on power system stability
ABB Industrie AG
- 38 -
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
Impact of excitation system on power system stability
ABB Industrie AG
- 39 -
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)
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