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Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC
Power Transmission:
Control and Engineering Issues
S. Bozhko , G. M. Asher, J. C. Clare, L. Yao, and M. Bazargan
Reporter: Dr S. Bozhko
May, 8th, 2007
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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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AC filters
Local loads
STATCOM
CS
TS
TC Rectifier HVDC Inverter HVDCSubmarine
DC cable TI
Collection Bus
Ons
hore
AC
gri
d
TechnologicalPlatform (island)
CoastLine
TWF Submarine AC cable
10…20km 80…150km
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)
The power system studied:
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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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
• 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
Main steps of our investigation include :
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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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
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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Control Approach
Simplified diagram of the studied system:
VGIC
IG
ISTC
TSCS
ES
L0 R0
E0
V0
+
_
V*S ABC AOR (α)
VS VC
STATCOM HVDC
Cf
AC F
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
7
Control Approach
Reduced plant of control
GdV*0R
20S
S
GdfGqCq*Sq
Gqf
GdCd*Sd
Gdf
IKIKdt
dEC
VCIIIdt
dVC
IIIdt
dVC
Controlled Plant
x2
PI
PI
PI
I*0
I*Sd
I*Sq
ES DC
VGd
VGq
E*S DC
V*Gd
V*Gq
Proposed control structure
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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∫
CS
VG
2π50
PI
IC
IG
ISLS
RS
ES
L0
R0
E0
I0 V0+
_
2/3
V*Sα
VGβ
ejθ
3/2
3/2
3/2
Cf
V*S ABC
VGα
V*Sβ
ωe*
θ
ICβ
VGd
e-jθ
x2
ISq
AOR (α)
ISd
ICd
ICq
ISβ
ISα
I0
IGd
IGq
ωLS
3/2
ICα
IGβ
IGα
V*Sd
V*Sq
VGq e-jθ
e-jθ
ωLS
e-jθ
PI
PI
controlled plant
VGd*
ωCf
*20S )E(
ωCf
VGq*
(= 0)
3VGd0/CS
PI
PI
PI
controllers
Isd*
I0*
Isq*
Detailed block-diagram of the proposed control structure
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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PSCAD/EMTDC simulations of the proposed control system
Control Approach
• Detailed PSCAD/EMTDC simulation model is used
TG
TC1
TC2 TS
CS
AC Filters
V0
_
+
L0/2 R0/2
HVDC Rectifier
L0/2 R0/2
C0
HVDC Inverter
StatCom
33kV
132kV
Local Offshore Bus TW 1kV
Wind Farm 1000MVA
10kV E0
_
+ TI1
TI2 400kV
Main Onshore Grid Connection
SCR=2.5
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
10
Simulation results:
Control Approach
0
0.5
1
-0.4
-0.2
0
0.2
0
1
2
100
120
140
49.8
50
50.2
32
34
36
38
0 0.5 1 1.5 2 2.5 3 3.50
20
40
60
0
0.5
1Wind Power Active (1) and Reactive (2) Powers, pu
-1
0
1
STATCOM Active (1) and Reactive (2) Powers, pu
0
2
4
HVDC DC-Link Current, kA
100110120130140150
Offshore Grid Voltage, kV
49.9
50
50.1
50.2Offshore Grid Frequency, Hz
40
50
60
70STATCOM DC-Link Voltage, kV
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.10
50
100Rectifier Firing Angle, deg
Time, s
1
2
1
2
1
2 1
2
Wind Power Active (1) and Reactive (2) Powers, pu
STATCOM Active (1) and Reactive (2) Powers, pu
HVDC DC-Link Current, kA
Offshore Grid Voltage, kV
Offshore Grid Frequency, Hz
STATCOM DC-Link Voltage, kV
Rectifier Firing Angle, deg
Time, s
• 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)
• Some measures must be undertaken to improve the system practicality
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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STATCOM DC-link capacitor sizing
• Energy stored in this capacitor:2
ECdt)PPP(e)t(e
2SdcS
t
tlCG0
0
• Overvoltage factor:
0 Sdcmax SdcV E/Ek
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
Power to HVDC link
PC0
t t
Generator’s power
PG0
τd
tt
Losses
PL0 tt
• Power balancing equation:
)e1(PtP)e1(PP)1k(2
ECCG
d t
C0C0L
t
G0Gd0G2V
20SdcS
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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:
CS MIN = F(tf, τd, τG, τC, kV, PG0, PC0, PL0)
Communication delay τd, s
CS m
in,
F
n=1
n=2
n=3
0 0.005 0.01 0.015 0.020
0.02
0.04
0.06
0.08
0.10
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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Power system operation during a fault
PI
132kVLocal Offshore Grid
I0*AC Filters PI
TSCS
STATCOM
10kV
0/2 R0/2 L0/2 R0/2
C0
E0
TI1
TI2 400kV
Main Onshore Grid Connection
SCR=2.5
HVDC Inverter
TC1
TC2
V0 L
HVDC Rectifier
ES2 *
TG33kVTW
Wind Farm
PI I*rd
I*rq
Q*wf
P*wf
I*fd
I*fq
Q*f=0
Efdc=E*fdc
Fault Detected
0 pu
I*rq=0
τd
0.25 pu
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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Influence of communication delay τd on STATCOM rating
The dynamics of HVDC rectifier AC currents is twice as faster than the dynamics of HVDC DC-link current loop!
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
fin0t
fin0ini00 Ie)II()t(I • If assume ideal performance of HVDC current control loop during a fault:
)(t • then the behavior of AOA can be found as:
• and HVDC rectifier AC-side currents then can be derived as follows:
1)(
)γ(
3
2sin
3
22
0020
γ2222γ200
20
0
LRI
eVkke
V
LRIkII
ini
tGdTrt
Gd
iniCq
t
Gd
iniCd e
V
LRIkII γ200
20
0)γ(
3
2cos
3
2
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STATCOM rating issue (continued)
• If communication delay exceeds some value, the STATCOM apparent power demand during faults can reach the value of wind farm delivered apparent power
d
q
VG
IC0d
IC0q
IC0
IS0
IG0
• 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
STATCOM S, P and Q vs communication delay
2 3 4 5 6 7 8 9 10
x 10-3
0
0.2
0.4
0.6
0.8
1.0
0.6kA
0.2kA
0.2kA
0.6kA
0.2kA
0.6kA Sst
Pst
Qst
2 3 4 5 6 7 8 9 10
x 10-3
0
0.2
0.4
0.6
0.8
1.0
7.95ms
2.65ms
2.65ms
7.95ms
2.65ms
7.95ms
Sst
Pst
Qst
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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Power system operation during a fault
PI
132kVLocal Offshore Grid
I0*AC Filters PI
TSCS
STATCOM
10kV
0/2 R0/2 L0/2 R0/2
C0
E0
TI1
TI2 400kV
Main Onshore Grid Connection
SCR=2.5
HVDC Inverter
TC1
TC2
V0 L
HVDC Rectifier
ES2 *
TG33kVTW
Wind Farm
PI I*rd
I*rq
Q*wf
P*wf
I*fd
I*fq
Q*f=0
Efdc=E*fdc
Fault Detected
0.25pu
I*rq=0
τd
3/2ICFq
ICFd
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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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.
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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0
0.5
1
Wind farm active (1) and reactive (2) pow ers, pu
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1
0
1STATCOM active (1) and reactive (2) pow ers, pu
0
1
2
3
HVDC DC-link current, kA
60
80
100
120
140
160Offshore grid voltage, kV
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
49
50
51
Offshore grid frequency, Hz
Time, s
35
40
45STATCOM DC-link voltage, kV
0
50
100
150Rectif ier f iring angle, deg
1.3
1.4
1.5
DFIG shaft speed, pu
1
2
1
2
Time, s
Simulation of fault in the enhanced system
• STATCOM active and reactive power demand is significantly lowered
• STATCOM DC-link overvoltage is reduced from 94% to 25%
0.998 1 1.002 1.004 1.006 1.008 1.01 1.012 Time,s
0
0.2
0.4
0.6
0.8
1
Td=2ms
Td=4ms
Td=6ms
Td=6ms Td=10ms
Fault
STATCOM apparent power during fault development
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
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Conclusions
Acknowledgement
THANK YOU!
• 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
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.
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues