MMC-HVDC Converter Station Control And Low Voltage Ride Through Strategy With Dc Bus Voltage Support Capability

High voltage direct current(HVDC)transmission has the advantages of large transmission capacity,high efficiency and low loss,and has been widely used in the field of trans-regional long-distance transmission.However,due to the complex control of the HVDC transmission system and the long time to restart,the converter station is required to have the ability of low voltage crossing when the voltage sag of the AC network causes the voltage fluctuation of the HVDC bus.At present,less attention has been paid to the problem of DC voltage sag when the grid voltage sag on the rectifier side occurs,because the converter station on the rectifier side cannot transfer enough power to the DC side,resulting in the DC voltage sag.Based on this,this paper adopts the low voltage crossing control method with the DC bus voltage support control ability.When the DC voltage sag is caused by the grid voltage sag on the rectifier side,the converter station on the inverter side can automatically switch the control algorithm to support the DC voltage according to the change of the DC voltage and the inverter current.In order to verify the proposed control strategy,the effectiveness of the proposed control strategy is verified in the case of symmetrical three-voltage sag and asymmetric voltage sag in the rectifying side of the grid.The specific work is as follows:(1)This paper summarizes the development background and research significance of MMC-HVDC system,focuses on the research status of system low voltage through-crossing technology,and establishes the idea of this paper to study the low voltage through-crossing control strategy of voltage sways on the rectifier side of the grid.(2)In this paper,the topological structure of MMC is introduced,and the mathematical model of MMC is obtained.According to the circulation generation mechanism in MMC converter station,the circulation suppression controller is designed.Based on the CPS-PWM modulation strategy,the capacitive voltage equalization control strategy is designed for the carrier phase shift modulation strategy.(3)Aiming at the problem of voltage sag of DC bus caused by three-phase sag of grid voltage on the rectifier side,this paper proposes a low voltage crossing control strategy with the supporting ability of DC bus voltage,which can restore the DC voltage to the set value in the fault period and achieve the purpose of stabilizing the DC voltage.(4)In order to solve the problems of double frequency AC harmonics and DC voltage sags in the power grid voltage imbalance fault,the T/4 delay method and three-phase-locked loop method are adopted in this paper,and the current loop method of decoupling in the dual synchronous coordinate system is used to eliminate the negative sequence current.The DC bus voltage support control strategy is designed.According to the change of DC voltage and inverter current,the control algorithm is automatically switched to support the DC voltage.(5)In the MATLAB/SIMULINK environment,a system simulation platform based on MMC was built,and all the research results were simulated and verified.