Because the capacity of AC grid at the sending-end of DC transmission system is small and the proportion of wind power is relatively high, there is a problem of poor frequency control ability. Therefore, it is of practical significance to use the hybrid DC system with wind farm access to control the frequency of AC system at the sending end. In the paper, a frequency control method is proposed, which makes the hybrid DC system and wind farm jointly support the frequency of the sending-end power grid. Aiming at the problem that the hybrid cascaded DC system reduces the transmission DC current due to the modular multilevel converter (MMC) connected to the power grid operates at full load, an improved frequency control of hybrid cascaded DC system under the heavy load operation of converter is proposed. Finally, the corresponding simulation model is established on PSCAD / EMTDC simulation platform to verify the effectiveness of the proposed frequency control method.

DC transmission system, wind power, hybrid cascaded direct current system, frequency control