Study On Faults Protection And Optimal Coordinated Control Of MMC-MTDC

With the shortage of fossil energy and increasing requirement on carbon emission reduction,the utilization of renewable energy has developed rapidly over the past ten years.Compared with conventional DC,the multi-terminal flexible DC system(VSC-MTDC)or flexible DC grid has more technical benefits of power supply and voltage control,and thus having a better application prospect in large-scale and long-distance delivery of renewable energy.However,the development of VSC-MTDC is subj ect to technique challenges,such as coordinated control strategies,fast DC-fault identification and isolation technologies,especially heavy dependence on communication systems.Based on the above background,this thesis focus on how to consider the economy issue in control strategy design and how to construct a DC-grid protection scheme for fast fault detection.The main works are summarized as follows:1)For meshed VSC-MTDC system,a single-end transient protection scheme based on some current-limiting hybrid DC circuit breakers(HDCCB)is designed,in which the boundary and directional characteristics of VSC-MTDC installing HDCCBs is considered.The proposed transient characteristic differences under different faults are jointly applied,thus internal and external faults can be quickly distinguished according to the transient energy extracted by Hilbert-Huang transform,and the backward faults are eliminated by selecting proper the threshold of DC-reactor voltage-drop.The scheme can protect both transmission lines and bus bar quickly with local signals.Pole-to-pole DC faults could be identified and isolated in several milliseconds without any communication.2)For offshore wind farms integrated VSC-MTDC system,a self-adaptive and hierarchical control scheme is proposed to consider the optimal allocation of imbalanced power when the connected wind power outputs fluctuate frequently.The equivalent extra cost function in the form of active/reactive power and voltage on the valve side is deduced firstly in details.Based on the common DC bus voltage and the incremental cost curve of extra generation,the scheme can locally make unbalanced power shared economically without any centralized processing layer,having similar effects to the equal increment principle.Simulation results show that the proposed scheme can reduce the extra generation cost effectively under normal conditions.