| The hybrid multi-terminal HVDC system based on LCC and MMC is a significant technology in large-scale renewable energy transmission,flexible dc power distribution,and ac network stability improvement.It is important to study the strategy of uninterrupted operation under various conditions for the applications in the fields of long-distance,bulk-capacity,UH-voltage,and overhead transmission and so on.A hybrid three-terminal HVDC system is proposed in this thesis.The uninterrupted operation associated with control strategies is studied to achieve the stable operation of the system without blocking the converters,quick recovery and prevent power shortage under certain conditions.The main research contents and results are as follows:(1)A control strategy of the system under dc fault is proposed.The relationships among ac voltage,dc voltage,and arm voltage are deduced.The de-coupling controllability between the ac and dc electrical variables of Hybrid MMC is then analyzed.And the feasibility of active control of dc current during the dc fault is obtained.Therefore the fault current can be self-cleared by converters.The self-cleaning and recovery under the transient dc fault of the system are verified through simulations.(2)A control strategy for the system under ac fault is proposed.The expression of Hybrid MMC during ac symmetric/asymmetric fault is deduced.The effects of ac current imbalance,circulating arm current,power fluctuation and capacitor overvoltage caused by ac fault are analyzed.In case to the instability of the converter and in order to limit the impact on the dc side,the ANF-PLL,negative-sequence current suppression strategy,low voltage ride-through strategy,and circulating current suppression strategy are introduced.The limitation of impact and stable operation of the system are verified through simulations.(3)In view of the application of bulk-capacity and UH-voltage,a structure of series converter groups is proposed.To solve the voltage imbalance under steady-state operation,the strategy of voltage balancing is designed.Considering the capacity and cost of bypassing breakers,the technical requirement of LCC and Hybrid MMC are designed respectively.Low-voltage and zero-current cut-off of the breakers is achieved.The voltage and power during inserting/bypassing can be controlled smoothly.The flexible switch between the single converter and double converters are verified through simulations. |
