Research On Reactive Power And Voltage Coordinated Control Technology Under Uhvdc Hierarchical Connection Mode

The large scale global clean energy configuration must be realized by future Global Energy Internet, which is decided by the converse distribution characteristics of global resources endowment and load centers. In order to realize the large scale and high efficiency energy configuration, Ultra High Voltage Direct Current (UHVDC) which has the ability of large capacity and long distance power transmission would be the technical foundation. With the formation of multi-infeed HVDC system, many serious challenges are brought to the reactive power support capability and power flow of receiving-end system, which will affect the safe and stable operation of the grid. In order to solve the pressing problem, a new pattern called UHVDC Hierarchical Connection (HC) to AC grid was put forward by Chinese researchers from the perspective of grid structure, that is, the double high-end and low-end inverters are respectively connected to 500/1 000kV systems via the converter transformers, and can be applied in the actual projects. In order to ensure the safe and stable operation of AC/DC system, the characteristics of receiving-end hybrid system are in need of comprehensive understanding and research urgently with the upcoming operation.Based on the actual UHVDC HC project, this paper analyzed the new connection pattern from three parts.At first, the paper established the equivalent model of AC/DC system under HC mode, and then presented the improved definition of Hierarchical Connection Short Circuit Ratio (HCSCR) under the analysis of influencing factors, furthermore, how absorbing ability of receiving system affected by HCSCR was analyzed, the criteria of the strength of receiving system under HC mode was given by calculating the Hierarchical Connection Critical Short Circuit Ratio(HCCSCR) and Hierarchical Connection Boundary Short Circuit Ratio(HCBSCR).Secondly, the interaction mechanism of reactive power and voltage was analyzed by taking both improved HC interaction factor and coupling relationship between high-end and low-end converters into consideration. Then, coupling characteristics influenced by interactions of AC and DC sides were verified and perfected with steady and dynamic time-domain simulations using equivalent model in PSCAD, and a method was presented to judge possibility of commutation failure of both high-end and low-end inverters.Finally, the interaction mechanism and influence factors of reactive power of hybrid system were analyzed; the advantages and disadvantages of different control modes were compared and analyzed by taking the differences of influence factors and fluctuation of reactive power into consideration, and a mixed coordinated control mode in narrow sense which is suitable for weak AC system with strong DC power input was put forward, simulations in PSCAD showed that the operation stability and reliability of AC/DC receiving end system were improved by the mode. At last, a kind of reactive power and voltage prevention-emergency coordinated control scheme in broad sense which is suitable for large capacity AC/DC hybrid grid was put forward.The research work of this paper can provide technical references for the safe and stable operation of actual projects.