Wind farm + HVDC Why use HVDC to deliver wind power? – Off-shore wind farm, dc cable is preferred for undersea use (no need of shunt reactive power compensation) – Large-scale wind power, the existing ac system is already congested Injecting large wind power at any point could cause congestion in other lines – we cannot control flows in ac lines (KCL, KVL). We like point to point transmission (from generation to load)--- HVDC.
Problem identified Wind farms are not traditional synchronous generators – Doubly fed induction generator – Permanent magnet synchronous generator – Interconnected with the grid partially or fully by converters The output power level follows maximum wind power (Maximum Power Point Tracking) What about load sharing or primary frequency response? No existing research addressed such problem.
Frequency response Primary frequency response – If there is increase in loads in the grid, rotating masses of synchronous generators will release kinetic energy, the grid frequency will drop. – In turn, each turbine will increase its output – mechanical power until P m =P e How can wind farms with HVDC delivery participate in frequency response?
Things to be considered There should be spinning reserve in wind farms to participate load sharing. Wind farms cannot operate at maximum power point tracking mode. Need 20% margin. HVDC delivering power is controllable through firing angle of the converters. – We can design a feedback control to force HVDC delivering power responding to frequency change – We can then think about how P m and P e get balanced in wind farms.
Study case G2 (600 MW) will be tripped. The system frequency will decrease. G1, G3 and G4 will increase their output power levels to make up 600 MW. Wind power with HVDC delivery is planned to participate in load sharing.
Coordination in wind farms Wind turbine has pitch controllers to change pitch angle to change wind power extracted. Pitch controllers measure the rotating speed. If it is too high, reduce wind power by increasing pitch angle. Too low, increase wind power by reducing pitch angle If the electric power increases, the rotating speed decreases. Pitch angle decreases, the mechanical power increases to match the electric power.
Conclusions Wind farms with HVDC delivery can participate in load sharing by introducing a feedback control loop Pitch controllers help balance the powers in wind farms. Operating margin should be left for primary frequency response by wind farms