Control of HVDC systems based on diode rectifier for offshore wind farm applications

The transfer of energy from offshore wind farms using HVDC technology is highly dependant on the converter technology used. The conversion through HVDC has conventionally been done by Line Commutated Converters (LCC) or Voltage Source Converters (VSC), by using thyristor rectifiers or IGBTs, respectively. However, 12-pulse diode rectifier at the offshore converter station can be a promising option due to lower conduction losses, reduced installation costs and converter size, and higher reliability. The main drawback is that the overall control strategy needs to adapt to the uncontrollable diode rectifier. This paper describes the system topology and control solutions proposed with the use of diode rectifier. In addition the paper proposes a shifted voltage and frequency droop control strategy related to the Phase Locked Loop (PLL) to improve the control system as an original contribution. The control strategies are validated in Matlab/Simulink.