Research On Coordination Control Strategy Of Multi-terminal Hybrid AC And DC Transmission System Based On Voltage Source Converter

With the continuous development of social economy, the urban electric load increases rapidly and the demands for power supply and power quality are much higher than before.The shortage of the existing traditional ac transmission system seriously restrict the development of transmission technology. Compared with the traditional transmission mode, multi-terminal direct current transmission systembased on voltage source converter has many advantages:such as fast independent control of active and reactive power, dynamicreactive power compensationability,power energy quality improving ability, passive network power supply capacity, unaltered voltage polarity when flow is turned, easy to form multi-terminal dc (MTDC) network, etc. the transmission capacity of lines are greatly improved for the same transmission corridor, the cables can be laid underground using the existing feasible transmission corridor to reduce the radiation and protect environment, the stations canbe installed easily and much land resources can be saved by using modular and compact design; due to the adoption of IGBT switch device, the active and reactive power can be controlled independently with great flexibility and fast response.The above advantages make VSC-MTDC technology have a good application prospect in city power grid, but the current researches of VSC-MTDC are basically focused on wind power integration and supplying the island.So it is of great significanceto research the hybrid system of VSC-MTDC with urban city AC grid.The multi-terminal flexible direct current transmission (VSC-MTDC) system is developed on the basis of VSC-HVDC system with multiple voltage source converters (VSC), it not only has the characteristics of the two-terminal VSC-HVDC system, but also can be used to solve the transmission problem of multiple power supply or multiple power consumption, the power flow between the converter stations can be coordinated, and the power system can be more economic and flexible.The above excellent technical performance makes VSC-MTDC technology have great application prospects in urban poer grid.Therefore, now it’s dramatically adapted to renewable generating integration, power supply of urban grids and island networks, asynchronous interconnection of ac system, etc. However, compared with the traditional AC transmission system, the mutual interaction will arise in AC/DC hybrid system, and the difficulty of coordination and control is further increased. Therefore, it is necessary to study the coordination control strategy and AC/DC interaction on mixed system of VSC-MTDC and AC system.Firstly, the hierarchical control principle of the VSC-HVDC system is introduced and the mathematical model of the three-phase two-level VSC system is established in PSCAD/EMTDC. The vector control anddecouple are adopted in the control of converter station, and the inner loop current controller and outer loop voltage controller are designedrespectively.Then, in view of the deficiency of the existing control strategy of the VSC-MTDC system, the characteristic curve of the fixed active power control is improved in this thesis. The additional active power calculation method is introduced and the adjustable capacity margin of active power converter in sharing unbalanced power is also considered, and then the coordinated control strategy of VSC-MTDC system based on additional active power signal is proposed. Then theworking principle of the strategy is analyzed, and the system is simulated in PSCAD/EMTDC.Finally, in view of the problem when VSC-MTDC system is fedinto AC system, in the base of AC system equivalent model, a hybrid model of the four terminal flexible DC system and AC system is built in PSCAD,thecoordinated control strategy of VSC-MTDC System for urban power supply is designed.The controllersof inverter side.converter are improved and the reasonable distribution of power transmission consideringthe requirement of different AC systems and the output power of the seeding sides is realized so that the transmission power will be used effectively in the receiving AC systems.