Research On Power Coordinated Control Strategy Of Multi-terminal DC Transmission System

Voltage source converter based multi-terminal DC transmission system has obvious advantages in large scale new energy integration and asynchronous grid interconnection because of its flexible control,no commutation failure,and power supply to islands or weak AC grid.However,with the more extensive application of MTDC system,the topology will become more complex and the number of converter stations will increase.Therefore,the control strategy of MTDC system is still a research hotspot at present.This paper takes the multi-terminal DC transmission system based on MMC as the main research object,and conducts detailed research on the power coordinated control strategy of the system.The specific research contents include the following aspects:This paper introduces the topology structure,working principle and control modes of MMC,and summarizes the basic MTDC mathematical model.Then the overall control strategy of MMCMTDC is analyzed and the basic operation mode of MMC-MTDC is determined.The fourterminal MTDC system simulation model is built on PLECS software,which provides simulation analysis and simulation platform for proposed control strategy.Due to the droop coefficient is fixed in the traditional droop control.When the power fluctuates greatly,it is easy to lead to the overload of the converter station,and the DC line resistance will affect the active power-sharing of converter.To solve this problem,an adaptive droop control method considering the influence of line resistance is proposed.First,the equivalent circuit of MTDC under droop control is analyzed,and the droop coefficient setting method considering the influence of line resistance is deduced.Then the fixed power ratio is replaced by the power margin ratio of the converter station in the setting method to realize the adaptive adjustment of the droop coefficient.The control strategy can not only reasonably distribute power to avoid the overload of converter stations,but also accurately distribute power between converter stations considering the influence of line resistance after the system is disturbed.The effectiveness of the proposed control strategy is verified by simulation analysis.At present,many renewable energy is connected to the power grid and the inertia level of system is reduced.The frequency of the AC power grid is prone to large deviation after disturbance,especially for the weak power grid,which seriously affects its stable operation.Owing to the fast power regulation and asynchronous grid interconnection ability of MTDC,a novel frequency droop control strategy is proposed.Firstly,the power allocation relationship of traditional frequency droop control strategy is analyzed.Secondly,frequency margin and power margin are introduced in DC voltage droop coefficient.After the system is disturbed,even when the MTDC system is connected to the weak power grid,the droop control terminal can reasonably allocate the unbalanced power according to the proportion of the frequency margin and power margin,which can greatly reduce the frequency deviation of the weak power grid and improve the frequency stability of the entire AC grid.Besides,the control strategy can make full use of the converter station capacity and improve the power transmission capacity of converter station.