Research On Operation Characteristics And Unity Power Factor Control Of Evolutional Shunt Capacitor Commutated Converter

China has a vast territory,energy resources and load centers are geographically reversed,and due to the continuous emergence of energy shortage as well as environmental pollution and other problems,the scale of new energy utilization such as wind power,nuclear energy and solar energy continues to expand,and its inherent dispersion,low energy density,far from the load center,large volatility and other characteristics,make AC transmission technology appear inefficient and uneconomical.Line Commutated Converter-based High Voltage Direct Current(LCC-HVDC)with its advantages of large transmission capacity,long transmission distance,low construction cost,fast and controllable power,etc.,is very suitable for the current scenarios of cross-regional longdistance power transmission and new energy transmission in China,which is widely used in strategic projects such as "West-East Electricity Transmission Project","National Grid Interconnection" in China’s power grid.However,with the expansion of large-scale energy optimization configuration,large-scale new energy power generation grid-connected and other needs,the requirements for the stable operation of converters are becoming more and more stringent.LCC uses semi-controlled thyristors as switching devices,which relies on grid voltage for commutation,when the AC bus voltage of inverter is weak or faulty,it is easy to have commutation failure problems,moreover,LCC needs to install a large number of filtering and reactive power compensation devices on the AC side,which leads to problems such as a large footprint.In order to improve the above problems,an Evolutional Shunt Capacitor Commutated Converter(ESCCC)and its derivative topologies and control strategies are proposed.Firstly,based on the topology of ESCCC,its mathematical model and working mechanism are derived,the reactive power characteristics and commutation characteristics of ESCCC are analyzed in detail,its improvements with LCC and other topologies are compared.The optimized design is made for the main circuit parameters,and the simulation verifies that the ESCCC with optimized parameters has good AC/DC harmonic characteristics,the ESCCC structure can effectively reduce the risk of commutation failure of the system.Secondly,based on ESCCC,a unity power factor control combined with the capacitors that can be switched on the AC bus is further proposed.The principle of unity power factor control is analyzed,the methods of main circuit parameter design and capacitor switching of the new topology are studied,and the proposed control strategy effectively realizes the unity power factor operation to reduce DC line loss under all working conditions,and system has good steady-state and transient characteristics,the footprint of the converter is greatly reduced.Finally,a Shunt LC Commutated Converter(SLCCC)and its control strategy are proposed,the mathematical model of SLCCC is derived,its commutation characteristics are analyzed in detail,and the calculation method of commutation angle is obtained.The main circuit parameters are designed by the multi-objective planning method,and a closedloop unity power factor control strategy that can realize active and reactive power decoupling is designed.The simulation verifies that the new parameters and control strategies can effectively improve the system operation performance.