Wind farms with HVDC delivery
in Load Frequency Control
Lingling Fan
April 22, 2010
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 Pm=Pe
• 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 Pm and Pe 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.
HVDC control –conventional &
proposed
Frequency change
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.
Simulation results
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