1. Control and Engineering Issues
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

2. 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

3. The power system studied:
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
The power system studied:
AC filters
Local loads
STATCOM C S TS T
Rectifier HVDC
Inverter HVDC
Submarine
DC cable I
Collection Bus
Onshore AC grid
Technological
Platform (island)
Coast
Line
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: SCR=2,5; HVDC Link: 1GW

4. Control system should provide:
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
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 :

5. System model development for control design:
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

6. 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 L0 R0 TS + TC CS IS IC E0 VS VG VC ES V0 _ Cf
V*S ABC
AOR (α)
STATCOM
AC F
HVDC

7. Proposed control structure
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
Control Approach
Reduced plant of control
Controlled Plant x 2 PI
I*0
I*Sd
I*Sq
ES DC
VGd
VGq
E*S DC
V*Gd
V*Gq
Proposed control structure

8. 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
Detailed block-diagram of the proposed control structure
controllers
controlled plant ES CS x2
Isd* PI PI
V*Sd
ejθ
V*Sα
V*S ABC
VGd*
V*Sβ
Isq*
2/3 PI
V*Sq PI
VGq*
(= 0)
ωe* RS
ωLS
ωLS θ ∫
2π50 LS
ISd
ISα
ωCf IS
ISq
e-jθ
ISβ
3/2
ωCf
VGd
VGα IG VG
VGq
e-jθ
VGβ
3/2 Cf
ICd
ICα IC
ICq
e-jθ
ICβ
3/2
I0* PI PI
AOR (α) _ I0 V0 + L0 I0
3VGd0/CS R0
IGd
IGα E0
IGq
e-jθ
IGβ
3/2

9. 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

10. 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)
Some measures must be undertaken to improve the system practicality

11. 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:
Generator’s power
PG0 τd t
Power to HVDC link
PC0 t
Losses
PL0 t
Overvoltage factor:
Power balancing equation:

12. Communication delay τd, s
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:
Communication delay τd, s
CS min, F
n=1
n=2
n=3
0.005
0.01
0.015
0.02
0.04
0.06
0.08
0.10
CS MIN = F(tf, τd, τG, τC, kV, PG0, PC0, PL0)

13. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
Power system operation during a fault PI
132kV
Local Offshore
Grid
I0*
AC Filters T S C
STATCOM
10kV /2 R L E I1 I2
400kV
Main Onshore
Grid Connection
SCR=2.5
HVDC
Inverter C1 C2 V
Rectifier
ES2 * G
33kV W
Wind Farm
I*rd
I*rq
Q*wf
P*wf
I*fd
I*fq
Q*f=0
Efdc=E*fdc
I*rq=0
Fault Detected
0 pu
0.25 pu τd

14. 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:
and HVDC rectifier AC-side currents then can be derived as follows:
The dynamics of HVDC rectifier AC currents is twice as faster than the dynamics of HVDC DC-link current loop!

15. STATCOM S, P and Q vs communication delay
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 d q VG
IC0d
IC0q
IC0
IS0
IG0

16. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
Power system operation during a fault PI
132kV
Local Offshore
Grid
I0*
AC Filters T S C
STATCOM
10kV /2 R L E I1 I2
400kV
Main Onshore
Grid Connection
SCR=2.5
HVDC
Inverter C1 C2 V
Rectifier
ES2 * G
33kV W
Wind Farm
I*rd
I*rq
Q*wf
P*wf
I*fd
I*fq
Q*f=0
Efdc=E*fdc
I*rq=0
3/2
ICFq
ICFd
Fault Detected
0.25pu τd

17. 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.

18. STATCOM apparent power during fault development
Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues
Simulation of fault in the enhanced system
STATCOM apparent power during fault development
STATCOM active and reactive power demand is significantly lowered
STATCOM DC-link overvoltage is reduced from 94% to 25%

19. 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!