| Modular Multilevel Converter(MMC)has the advantages of low loss,low harmonic content,and high scalability,which is the mainstream choice of converters for VSC-HVDC systems.Most of the existing MMCs use the vector control,but the MMC under the vector control exhibits low inertia and low damping characteristics,and does not have the ability to participate in frequency and voltage regulation.And it is difficult to ensure its frequency and voltage quality when connected to a weak AC system.In addition,studies have shown that weak grid conditions will affect the dq-axis decoupling characteristics and phase-locked loop performance of the vector control,thereby affecting the stability of system operation.Therefore,this paper proposes the virtual synchronous generator(VSG)control method of MMC,and studies the weak grid adaptability of MMC under the VSG control on this basis.The specific work is as follows:First of all,the working principle of MMC is analyzed,the mathematical model of MMC is established,and the basic control methods of MMC such as the vector control and the interphase circulation suppression control are studied.And this is used as the underlying control system of MMC.Based on the MATLAB/Simulink platform,a 313-level MMC-HVDC system model was built.The simulation verified that the built model has a good output effect,and laid the foundation for the subsequent design of the VSG control strategy of MMC.Secondly,the VSG ontology control algorithm of MMC is proposed.On this basis,the active-frequency control and reactive-voltage control are continued to be designed,and the main control parameters are designed by deriving the small signal model of the VSG control.Based on the VSG ontology algorithm,the droop relationship between active power and frequency is introduced into the rotor motion equation,which can realize the primary frequency modulation of the receiving end power grid without relying on the phase-locked loop,and the integral control of frequency deviation is introduced to achieve the secondary frequency modulation.The reactive power adjustment coefficient and the voltage adjustment coefficient are introduced into the stator electrical equation to realize the voltage adjustment of the receiving end power grid.Comparison and simulation with the vector control show that MMC under the VSG control has inertia and damping characteristics,and provides certain frequency and voltage support for the receiving end power grid under interference.Finally,in order to analyze the weak power grid adaptability of MMC under the VSG control,the small disturbance analysis method is used to derive the dq-axis output impedance model of MMC under the VSG control,and the correctness of the impedance model is verified by frequency scanning in the simulation software.According to the generalized Nyquist stability criterion,the weak grid adaptability of MMC under the VSG control is studied through the analysis of the characteristic trajectory of the return ratio matrix.For the receiving-end power grids connected with different power grid strengths,a time-domain comparison simulation was carried out on the MMC using the vector control and the VSG control.Impedance theory and simulation show that the MMC under the VSG control can simulate the external characteristics of synchronous generators and has a good ability to operate stably on weak power grids. |
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