Joint Optimization Of Active-reactive Power In Island Weak AC Grid With VSC-MTDC

Restricted by geographical conditions,island AC grids are usually weak,and face various challenges in operation.For example,the network structure is relatively vulnerable,and the coupling degree of active power and reactive power is relatively high,leading to limited means to adjust the operation mode.The island load are interconnected by AC submarine cables.The reactive charging power on the submarine cable is large,which leads to a large risk of over-voltage and decreases the stability of power system.The fluctuation characteristics of the load on the island are usually obvious,resulting in frequent switching of compensation equipment.The loss and depreciation of equipment cannot be ignored.The unique characteristics of island power grid create a typical scenario for the application of VSC-MTDC.However,for many AC/DC hybrid island grids in operation,the power allocation strategy is not reasonable enough to make full use of the flexible adjustment ability of VSC-MTDC.Meanwhile,the access of wind power and other new energy with random output characteristics gives probability characteristics to the distribution of power flow,affects the operation margin of VSC-MTDC to a large extent.VSC-MTDC has the ability to flexibly adjust the active and reactive power exchanged between AC and DC systems,which provides strong support for the parallel island weak AC,improves the power flow distribution,voltage level and the reliability of power supply of AC grid.It is possible to relieve the operation pressure of traditional island grid with VSC-MTDC.Combining with the special operation challenges faced by island grid,this paper discusses the optimization strategy of island weak AC grid with the participation of VSC-MTDC.Based on the joint control of active power and reactive power,a multi-objective optimization model considering the operation economy,the stability and the operation margin of VSC-MTDC is established.The main contents of this paper are as follows:(1)Joint optimization of active-reactive power in island weak AC grid with VSC-MTDC has been studied,which focuses on the positive role of active/reactive power provided by VSC-MTDC to AC grid.(2)The optimization index related to economic cost has been discussed.Aiming at the specific economic problems in island grid,such as large peak-valley difference of load and non-ignorable depreciation of compensation equipment,this paper has established the objective function considering both the network loss and the extra economic cost caused by the frequent switching of operation mode.(3)The optimization index related to system stability has been discussed.The minimum singular value of convergent power flow Jacobin matrix J,which can evaluate angle stability/voltage stability from the mechanism,has been selected as the evaluation index of stability,to reflect the distance between the current operating point and the stability limit point.(4)The impact of uncertain energy sources such as wind power on power flow has been considered.An index to measure the operating margin of VSC-HVDC has been proposed.Meanwhile,the positive effect of taking the droop control slope of converter station into optimization variable on the overall operation optimization of AC/DC grid has been analyzed.(5)The Pareto multi-objective optimization model has been established in this paper,to avoid subjective weight judgment when single objective optimization model is used to evaluate multiple indexes of different dimensions.