Self-synchronous Active Support Technology At The Receiving-end Of New Energy Through MMC-HVDC System

Under the strategic goal of "dual carbon",China’s offshore wind power will usher in a new generation of development characterized by "large unit","large-scale",and "far-reaching sea".With the continuous maturity of flexible DC transmission technology,transmission through flexible DC power grid will become an important way for large-scale offshore wind power grid connection.With the increasing capacity of offshore wind power,the integration of traditional controlled VSC-HVDC systems into the grid lacks spontaneous and active support for the inertia,frequency,and voltage of the receiving-end power grid,which can seriously threaten the security and stability of the onshore AC power grid.The grid-forming control of the receiving-end converters station of the VSC-HVDC system will be an important direction to solve this challenge,but issues such as how to achieve self-synchronous active support control at the receiving-end converters,and how to provide the energy source required for active power control at the receiving end need to be addressed urgently.In this paper,a self-synchronous grid-connection control structure for offshore wind power transmission systems is established,and adaptive control parameters are designed to optimize the rational allocation of active power at the sending-end converters side.A grid structured active support control strategy based on VSG control for the receiving-end converters is proposed.The main research results obtained are as follows:Firstly,the mathematical models of offshore wind farms and MMC-HVDC systems are established and their typical control structures are summarized.The basic model and control framework for self-synchronous grid connection of offshore wind power through VSC-HVDC transmission systems without energy storage and communication are designed.An electromagnetic transient simulation model is built to verify the feasibility of the control structure.Secondly,an adaptive droop control for the sending-end converters considering power margin and DC voltage deviation is designed to address the issue of active power support for the receiving-end of the network.Starting from the response process of the active power support at the sending-end,the droop characteristics between the output active power at the sending-end and the DC voltage of the VSC-HVDC system are analyzed.Aiming at the problem of active power support from multiple sending-ends to receiving-ends,considering the impact of active power margin and DC voltage deviation,an adaptive droop coefficient is designed.The two factors are involved in the adjustment of the droop coefficient in the form of proportional factor and exponential factor,respectively,to achieve a reasonable allocation of active power in multiple sending-end converters and improve the DC voltage deviation of VSC-HVDC systems.A simulation model is built to compare and analyze the control effects of the proposed adaptive droop coefficient and fixed droop coefficient,verifying the effectiveness and superiority of the proposed control strategy.Finally,a virtual synchronization control strategy based on a constant impedance model is proposed for the grid connection control of the receiving-end converters to achieve steady and transient state active support control.The voltage source impedance model is adopted for the receiving-end converters,and a power-current dual loop control structure is established.The outer loop controls the receiving-end converters to participate in power grid frequency regulation and voltage regulation through VSG and AVR controls;Instead of using a voltage loop,the input reference value of the current loop is calculated using an impedance model.On this basis,based on the idea of constant impedance,a voltage ride through control technology is proposed,which mainly includes low traversal control technology based on virtual forced excitation and high traversal control technology based on virtual under-excitation.The implementation methods of virtual forced excitation and virtual under-excitation are given.After that,based on the operational requirements and constraints of the receiving-end converters under various operating conditions,the design and setting principles of important parameters in impedance model and fault ride through control are given.Finally,a simulation model is built to analyze the actual control effect of the control strategy,verifying the effectiveness and superiority of the proposed control strategy.