Research On The Stability Of Asynchronous Interconnected Sending End Network

The uneven distribution of energy bases and load demands in China requires bulk-capacity long-distance power.If the sending-end network and the receiving-end network are only connected by dc lines,it is called asynchronous interconnection power grid.If the asynchronous interconnection mode is adopted,the sending-end netwoek and the receiving-end grid will no longer belong to a synchronous power grid.There will be no power angle stability between the end power grid and the receiving power grid.However,after the asynchronous interconnection,there are ultra-low frequency oscillation,frequency stability and low frequency oscillation in the sending end grid,which affect the overall safety and stability of the power grid.This paper mainly focuses on the large-scale DC asynchronous interconnected power grid,and studies some stability problems in the sending-end netwoek and puts forward the corresponding improvement measures.The main research contents are as follows:This paper establishes the stability analysis model of hydropower generating units,and analyzes the characteristic equation of the transfer function of the model.The results show that the unreasonable setting of the governor control parameters will lead to instability of the power grid.If the ratio of the proportional parameter to the integral parameter of the governor is too small,the ultra-low frequency oscillation will occur.The conclusion is verified by time domain simulation.At last,the critical parameter method and the pole assignment method are proposed to suppress the ultra-low-frequency oscillation.The power angle stability of the large area power grid is improved through the HVDC asynchronous connection.But on the side of the sending-end,the DC fault will cause the high frequency.The frequency deviation peak is an important index in the study of the power grid frequency stability.The method of calculating the frequency deviation peak based on the rate of the frequency deviation is obtained from the single machine model.Then,the method is expanded to the actual power system and the application of the method in the HVDC additional control and generator-tripping is studied.Finally,a single machine model and Yunnan Power Grid are simulated to verify the method.When the DC blocking fault occurs in the system,the problem of high frequency in the sending end network can be suppressed by using the frequency limit controller(FLC).The paper introduces the dead zone selection method of FLC,and the parameters optimization is proposed to avoid FLC actions again when the FLC exits.After asynchronous interconnection,the main low-frequency oscillation mode in the sending end network is changed.The HVDC damping controller is used to increase the damping level of the sending-end netwoek and suppress the low-frequency oscillation.Taking Yunnan power grid as the research object,the result shows that the HVDC damping controller can improve the damping level of the power grid.