Research On Control Strategy Of Flexible Power Transformer That Supports New Energy Connecting Power Grid Friendly

With the rising proportion of new energy installations in China,the randomness and volatility of new energy has caused the phenomenon of high and low voltage crossing limits in the grid,which restricts the grid connection of new energy.The flexible power transformer is an excellent solution to the voltage regulation problem in the new energy scenario because it has the advantages of continuous regulation,low cost and low loss by connecting the fractionally rated voltage source converter in series between the neutral points of the star windings of the electromagnetic transformer.At present,the research of flexible power transformer control strategy mainly focuses on the load voltage control of traditional AC distribution network and AC power flow control,but there is no research on the voltage regulation strategy of various new energy grid-connected modes.Therefore,in this paper,we study the control strategy of flexible power transformer to support the new energy-friendly grid connection for the new energy grid connection.The main work is summarized as follows:(1)The basic working principle of the flexible power transformer is analyzed,and the difference between the active and passive parts is clarified;its control functions that can be realized in the conventional AC network are analyzed;a cascaded H-bridge type flexible power transformer topology with isolation transformer energy extraction is proposed,and two topologies that the active converter part should be coupled to its primary and secondary sides based on the wiring of the main transformer are proposed,as a way to establish respectively The mathematical model is established.(2)The voltage regulation control strategy of the flexible power transformer to support the friendly grid connection of wind farms is proposed.Firstly,the device level control strategy of the flexible power transformer is designed based on the mathematical model of the flexible power transformer;then the temporary and steady-state operation modes of the active converter part are proposed according to the difference of the wind farm grid connection mode,and the control strategies under different operation modes are designed;the strategy to suppress the circulating reactive power between the converters on both sides is proposed;finally,based on the two operation modes proposed in this paper,the difference between the active part coupled to the primary and secondary sides of the main transformer is analyzed theoretically.(3)A simulation model of wind farm grid connection based on flexible power transformer is established in PSCAD,and several operating conditions such as centralized grid connection,local grid connection and grid side fault are designed to verify the correctness of the strategy proposed in this paper.Under the centralized grid-connected condition,the flexible power transformer can improve the wind farm short-circuit ratio to 1.42 and control the voltage fluctuation at the point of connection of wind farm within 1%,which is more stable than the traditional STATCOM;under the local grid-connected condition,the flexible power transformer can control the voltage fluctuation at point of connection of wind farm within 1%;under the transient condition of network-side fault,the flexible power transformers have the ability to enhance the fault ride-through of wind farms,which can support wind farms to be grid-friendly.