Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission :

1. May, 8th, 2007 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. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues Technological Platform (island) Collection Bus Coast Line AC filters Local loads Inverter HVDC Submarine TWF Rectifier HVDC Submarine AC cable T T DC cable C I 10…20km 80…150km TS Onshore AC grid STATCOM C S The power system studied: 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)

4. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues 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 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

5. 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 E0 IC CS IS VS VG VC ES V0 _ Cf V*S ABC AOR (α) STATCOM HVDC AC F

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

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

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 PG0 t t τd Generator’s power PC0 Losses t t PL0 t t • Overvoltage factor: Power to HVDC link • Power balancing equation: STATCOM DC-link capacitor sizing • Energy stored in this capacitor:

12. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues 0.10 CSmin, 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 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)

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

14. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues • 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: 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!

15. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues d STATCOM S, P and Q vs communication delay VG IC0d q IC0 IS0 IG0 IC0q 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

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

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. Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues STATCOM apparent power during fault development 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%

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!