Frequency Regulation Control Method Of Receiving Converter Of Mtdc System Based On Virtual Synchronous Generator

Modular multi-level converter based multi-terminal high voltage direct current system(MMC-MTDC)can flexibly adjust power flow control,and has multi-source and multi-infeed power supply and other advantages.However,power converter in the MMC-MTDC system intrinsically has no rotational inertia,which will undoubtedly cause the overall inertia of the AC-DC hybrid system to decrease.When the system disturbance leads to the change of DC transmission power,the AC power grid connected to the receiving converter may have short-term frequency changes.In order to enable the converter to actively participate in the frequency regulation of the AC power grid,this paper designs reasonable MMC-MTDC control strategies,which makes the grid-connected converter provide inertia and damping when the transmission power fluctuates,while participating in the frequency regulation of the AC power grid,further improving the coordinated control ability of the AC-DC hybrid system.Firstly,this paper analyzes the topology of MMC and its working mode in detail,and designs MMC inner and outer loop controllers and phase-to-phase circulating current suppression controllers.Based on the Opal-RT simulation platform,a five-terminal 31-level MMC-MTDC transmission system simulation model including a cluster wind field is built,which provides simulation verification for the theoretical analysis later.Secondly,To address this issue that zero inertia characteristic of MMC,which can not improve the inertia response of the system,a control strategy of adding virtual synchronous generator(VSG)to the control of MMC receiving converter is proposed.This control strategy adds a first-order inertia link to the power outer loop control of MMC to provide inertia parameters and damping parameters for the system,which makes the converter similar to the synchronous generator in the external characteristics of the operation.At the same time,establishing the mathematical model of VSG to analyze the dynamic response of inertia parameters and damping parameters.Based on Opal-RT real-time simulation,The validity and correctness of the proposed control strategy are verified by the 5-terminal MMC-MTDC system.The simulation comparison fully display that the control strategy can inhibit the frequency deviation of AC network without relying on complex communication system when the power of AC/DC system fluctuates.The proposed VSG control strategy can effectively provide inertia and damping for the converter,so that the converter can reasonably cooperate with the AC synchronous generator to perform the primary frequency regulation of the AC grid.Finally,To address this issue that the frequency regulation of the AC system by conventional virtual synchronous generator(CVSG)belongs to differential frequency regulation.an improved virtual synchronous generator(IVSG)converter control strategy is proposed.Based on retaining the inertia and damping parameters in CVSG control,frequency deviation is introduced into the integral controller to make the converter participate in the secondary frequency regulation of AC system,and realize deviation-free frequency control of AC system through coordinating with other converters.Based on Opal-RT simulator,a model of 5-terminal 31-level MMC-MTDC system interconnected with wind farm is used to simulate and compare the proposed control strategies under different conditions of MMC-MTDC system.Results show that the proposed control strategy can reasonably distribute the received power,increase the inertia of converter and participate in secondary frequency regulation of AC system when the power of DC system fluctuates.