Mixed type three-pole direct current power transmission system and control method thereof

The invention discloses a mixed type three-pole direct current power transmission system and a control method thereof. The system comprises a rectifying converter station and an inverting converter station, wherein the rectifying converter station and the inverting converter station respectively comprise an alternating current bus and a three-pole converting system, each alternating current bus is connected with a corresponding alternating current power grid, each three-pole converting system is hung on each alternating current bus through a converting transformer, and a converting unit H3 of each three-pole converting system adopts an MMC (modular multilevel converter). The system and the method have the advantages that the decoupling control capability of the active and inactive parts is realized, the power can be transmitted to an inactive network, the number of inactive compensation equipment is greatly reduced, the balance of the direct current among the three poles is maintained by introducing the MCC (modified current control), the current of a grounding pole is inhibited, the service life of the grounding pole is effectively prolonged, the on-site balance of the inactive power is realized by introducing the AVC (alternating voltage control), and the voltage of the connected AC (alternating current) system is maintained to solve the overvoltage or undervoltage problem.