Transcript Document

May, 8th, 2007
S. Bozhko , G. M. Asher, J. C. Clare, L. Yao, and M. Bazargan
Grid Integration of Large Offshore Wind
Farms Using STATCOM-Controlled HVDC
Power Transmission:
Control and Engineering Issues
Reporter:
Dr S. Bozhko
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Introduction
• World electricity demand – to be covered for up to 12% by 2020
• Offshore: wind conditions are better, planning restrictions are reduced
• HVDC vs HVAC
• VSC HVDC vs LCC HVDC
• SG vs DFIG
• DFIG + STATCOM + LCC HVDC: well studied as separate components
• Existing studies consider the overall system concept and possible
control paradigms – no detailed study or rigorous design procedure
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
The power system studied:
Technological
Platform (island)
Collection Bus
Coast
Line
AC filters
Local loads
Submarine
AC cable
Rectifier HVDC
TC
10…20km
Submarine
DC cable
Inverter HVDC
TI
80…150km
TS
STATCOM
Onshore AC grid
TWF
CS
Total wind farm power: 1GW (set of DFIG-based WTG 3.3MVA each)
Collection Bus Voltage: 33kV; Offshore Bus Voltage: 132kV;
Onshore Grid: 400kV @ SCR=2,5;
HVDC Link: 1GW (2kA@500kV)
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Control system should provide:
• optimal tracking of collected wind power and its transfer
into the HVDC link
• control of voltage and frequency of the offshore grid
Main steps of our investigation include :
• Detailed mathematical study of the system
• The controlled plant model appropriate for rigorous control
design and understanding of the power system interactions
• Engineering studies of the designed control system
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
System model development for control design:
• aggregation of multiple WTG into a single one with similar
DFIG control strategy
• aggregated DFIG as a controlled current source
• harmonic filters are represented by their low-frequency
capacitive properties
• no power losses in the STATCOM and HVDC rectifier
• HVDC inverter is in voltage-control mode and can be
replaced by an equivalent DC voltage source
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Control Approach
Simplified diagram of the studied system:
IG
VS
TS
IS
VG
IC
TC
+
CS
L0
VC
ES
R0
E0
V0
_
Cf
V*S ABC
STATCOM
AOR (α)
AC F
HVDC
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Control Approach
Reduced plant of control
Proposed control structure
x2
dV
Cf Gd  I*Sd  I Cd  I Gd
dt
Cf
CS
dVGq
dt
dE S20
dt
 I*Sq
 I Cq  I Gq  Cf VGd
E*S DC
PI
V*Gd
I*Sd
PI
V*Gq
 K R I*0  K V I Gd
ES DC
I*0
PI
I*Sq
Controlled
Plant
VGd
VGq
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Detailed block-diagram of the proposed control structure
controllers
VGd
controlled plant
PI
Isd*
PI
Isq
*
x2
V*Sd
PI
*
VGq*
(= 0)
V*Sα
ejθ
V*Sq
PI
CS
ES
V*Sβ 2/3
V*S ABC
ω e*
ωLS
ωCf
ωCf
θ
ωLS
ISd
ISq
e-jθ
VGd
VGq
e-jθ VGβ 3/2
I0*
PI
e-jθ
L
IS
ISα
ISβ 3/2
VGα
ICd
ICq
(ES20 )*
RS
2π50
S
IG
VG
Cf
IC
ICα
ICβ 3/2
AOR (α)
PI
_
I0
V0 +
I0
3VGd0/CS
IGd
IGq
IGα
e-jθ
IGβ 3/2
L0
R0
E0
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Control Approach
PSCAD/EMTDC simulations of the proposed control system
• Detailed PSCAD/EMTDC simulation model is used
Local Offshore
Bus
Wind Farm 1000MVA
1kV
TW
Main Onshore
Grid Connection
SCR=2.5
33kV
TG
132kV
L0/2 R0/2
TC1
+
StatCom
CS
10kV
TC2
R0/2 L0/2
C0
TI1
+
V0
E0
_
_
TS
AC Filters
HVDC
Rectifier
TI2
400kV
HVDC
Inverter
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Control Approach
Simulation results:
• Confirm high
performance in both
normal conditions and
during a severe fault
•Raise engineering
concerns regarding
STATCOM rating (1.3pu in
order to handle the fault)
• Also raise concerns
regarding STATCOM
capacitor overvoltage
(1.92pu)
Wind Power Active (1) and Reactive (2) Powers, pu
Wind Power Active (1) and Reactive (2) Powers, pu
1
1
1
1
0.5
0.5
2
2
0
0
STATCOM Active (1) and Reactive (2) Powers, pu
0.2
STATCOM Active (1) and Reactive (2) Powers, pu
1
1
0
-0.2
1
-0.4
-1
HVDC DC-Link Current, kA
HVDC DC-Link Current, kA
2
4
1
2
0
0
Offshore Grid Voltage, kV
140
120
100
Offshore Grid Frequency, Hz
50.2
Offshore Grid Voltage, kV
150
140
130
120
110
100
Offshore Grid Frequency, Hz
50.2
50.1
50
50
49.9
49.8
STATCOM DC-Link Voltage, kV
38
STATCOM DC-Link Voltage, kV
70
60
36
• Some measures must
be undertaken to improve
the system practicality
2
0
2
50
34
40
32
Rectifier Firing Angle, deg
60
Rectifier Firing Angle, deg
100
40
50
20
0
0
0.5
1
1.5
2
2.5
3
3.5
Time, s
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
Time, s
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
STATCOM DC-link capacitor sizing
• Energy stored in this capacitor:
t
2
CSESdc
e( t )  e0   (PG  PC  Pl )dt 
2
t
0
PG0
τd
tt
PL0
Generator’s power
tt
Losses
PC0
t t
• Overvoltage factor:
k V  E Sdc max / E Sdc 0
Power to HVDC link
• Power balancing equation:
2
CSESdc
0
2
(k 2V  1)  PG 0 d  PG 0 G (1  e

t  d
G
)  PL0 t  PC0 C (1  e

t
C
)
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
STATCOM DC-link capacitor sizing
• Can be used to derive a criterion
for the STATCOM capacitor
sizing in order to guarantee that
the capacitor overvoltage during
a fault will not exceed the
acceptable level:
0.10
CS min, F
0.08
0.06
n=3
n=2
0.04
n=1
0.02
0
0
0.005
0.01
0.015
0.02
Communication delay τd, s
CS MIN = F(tf, τd, τG, τC, kV, PG0, PC0, PL0)
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Power system operation during a fault
Wind Farm
TW
33kV
TG
Local Offshore
Grid 132kV
Q*wf
PI
P*wf
PI
I*rd
I*fd
I*rq
HVDC
Rectifier
L0/2 R0/2
V0
Efdc=E*fdc
I*fq
TC1
R0/2 L0/2
C0
Q*f=0
TC2
I*rq=0
HVDC
Inverter
E0
Main Onshore
Grid Connection
TI1
SCR=2.5
TI2
400kV
STATCOM
10kV
CS
TS
AC Filters
PI
I0*
ES2 *
τd
Fault
Detected
0 pu
0.25
pu
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Influence of communication delay τd on STATCOM rating
• If assume ideal performance of HVDC
current control loop during a fault:
• then the behavior of AOA can be found as:
I0 (t )  (I0ini  I0fin )et  I0fin
   (t )
• and HVDC rectifier AC-side currents then can be derived as follows:
I Cd
2
2 I 02ini ( R0  γL0 )  2 γt
  kI 0 cos  
e
3
3
VGd
I Cq
2 2 γt
k r2 kT2VGd
e
2
2 I 02ini ( R0  γL0 ) 2γt
   kI 0 sin   
e
1
3
3
VGd
I 02ini ( R0  L0 ) 2
The dynamics of HVDC rectifier AC currents is twice as faster than the
dynamics of HVDC DC-link current loop!
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
STATCOM rating issue (continued)
• STATCOM rating can be reduced substantially only if no communication delay or if
it is very small compare to HVDC DC-link current control time constant
• If communication delay exceeds some value, the STATCOM apparent power demand
during faults can reach the value of wind farm delivered apparent power
STATCOM S, P and Q vs communication delay
1.0
2.65ms 2.65ms
Sst
1.0
q
Pst
0.8
d
0.2kA
7.95ms
7.95ms
0.6
0.6
0.4
d
IC0
Sst
0.6kA
Pst
0.8
0.6kA
VG
IC0
IS0
0.2kA
0.4
7.95ms
Qst
0.2
Qst
0.2
0.6kA
0
IC0q
0
IG0
2.65ms
0.2kA
2
3
4
5
6
7
8
9
10
-3
x 10
2
3
4
5
6
7
8
9
10
-3
x 10
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Power system operation during a fault
Wind Farm
TW
33kV
TG
Local Offshore
Grid 132kV
Q*wf
PI
P*wf
PI
I*rd
I*fd
I*rq
HVDC
Rectifier
L0/2 R0/2
V0
Efdc=E*fdc
I*fq
TC1
I*rq=0
TC2
3/2
R0/2 L0/2
C0
Q*f=0
ICFq
ICFd
HVDC
Inverter
E0
Main Onshore
Grid Connection
TI1
SCR=2.5
TI2
400kV
STATCOM
10kV
CS
TS
AC Filters
PI
I0*
ES2 *
τd
Fault
Detected
0.25pu
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Reduction of the STATCOM rating can be achieved by:
• Suppression of STATCOM DC-link voltage control: fault detection scheme can set
the HVDC current demand I0* to some value I0fin in order to absorb the AC filters
reactive power by HVDC link, not by STATCOM;
• Reduction of wind farm output power via fast DFIG current control loops;
• Communication delay τd due to distant location if WTGs: should be lowered
• Reactive power capabilities of DFIGs front-end converters: the reactive current
reference as a function of reactive current component at HVDC input;
• Active power support through rotor q-current controls: the q-current reference
as a function of active current component at HVDC/filters input;
• Improvement of HVDC DC-link current control: need adaptation to fault conditions;
• Lowering the bandwidth of offshore grid voltage and frequency controls;
• Hard Limits on STATCOM currents.
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Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Simulation of fault in the enhanced system
DFIG shaft speed, pu
Rectifier firing angle, deg
150
1.5
100
1.4
50
1.3
0
Wind farm active (1) and reactive (2) pow ers, pu
HVDC DC-link current, kA
3
1
0.5
1
2
2
1
0
0
Offshore grid voltage, kV
160
140
STATCOM DC-link voltage, kV
45
120
STATCOM apparent power
during fault development
40
1
Fault
100
80
35
0.8
60
Td=10ms
Offshore grid frequency, Hz
1
Td=6ms
0.6
STATCOM active (1) and reactive (2) pow ers, pu
0.4
51
Td=6ms
1
50
Td=2ms
2
49
0
0
-1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Time, s
Td=4ms
0.2
0
0.998
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1
1.002
1.004
1.006
1.008
1.01
1.012
Time, s
• STATCOM active and reactive power demand is significantly lowered
• STATCOM DC-link overvoltage is reduced from 94% to 25%
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Time,s
Grid Integration of Large Offshore Wind Farms Using
STATCOM-Controlled HVDC Power Transmission:
Control and Engineering Issues
Conclusions
• A large offshore wind farm with a LCC HVDC connection to the main onshore
grid is considered
• The proposed control system is proven to provide high performance control of
the offshore grid and wind power transfer to onshore
• Engineering issues related to the STATCOM sizing is considered
• Recommendations for control system enhancement are given
• The proposed system can be a satisfactory solution for integrating large
offshore DFIG-based wind farms into existing AC networks
Acknowledgement
Authors would like to express their appreciation for the partial funding support from the New and
Renewable Energy Programme of the DTI, UK under the contract K/KL/00340/00/00.
THANK YOU!
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