Frequency Characteristics Research Of Send-endding Power Grid Under Asynchronous Networking Conditions

While HVDC links were widely supplied in transmitting huge power over long distance,asynchronous operation of power grids in different areas became realizable.DC transmission has better economics in long-distance and high-power transmission,and the rapid controllability and overload capacity of DC transmission lines provide reliable power support for both ends of the grid.In addition,the DC transmission line isolates the AC grid which greatly reduces the possibility of generator rotor angle instability and improves power system operation reliability.However,the generator inertia of the regional power grid is reduced after asynchronous interconnected and the DC faults often cause massive active power imbalance,that makes frequency stability problem becomes an important factor affecting the safe and stable operation of the power grid instead of the generator rotor angle stability problem,especially the high frequency problem of the weak generator inertia end power system has caused widespread concern in recent years.In the actual project,the high-frequency phenomenon at the send-ending grid is suppressed with the application of DC additional frequency control,which is certainly effective.In this paper,the mathematical algebraic differential equations of components related to hydropower generation power system and two FLC(Frequency Limit Control)logic frameworks are introduced.On this basis,equivalent mathematical model of the hydroelectric send-ending in asynchronous interconnected grid is built.The analytic formula of the power system frequency response under the single machine model is derived,and a brief analysis of the frequency characteristics of the send-ending system is done.Then,based on direct current loadflow,the analytical expression of frequency response in asynchronous interconnected power system is deduced in consideration of the impact of hydro-turbine governor and HVDC frequency controller,the impact of grid structure changing and the addition of DC additional frequency modulation on the frequency response of the both ending power grids is analyzed.In the end,and the results of the modified 4-machine power system simulation model are used to verify these conclusions.After discussing the influence of network structure changing and DC additional frequency modulation on the frequency of send-ending network,this paper further discusses the influence of the modulation parameters on the damping ratio and overshoot of the frequency modulation system.Firstly,the analysis formula that states intrinsic relationship between the modulation parameters and the damping ratio of the send-ending power system is obtained after simplifying the transfer function block diagram of the control system.Next,The effects of frequency modulation parameters on the stability performance of the send-ending power system are investigated by using the Rolls criterion and the root locus method,which help to figure out reasonable scope of modulation parameters.Finally,the effects of various parameters and modulation methods on the frequency response are verified in the modified 10-machine power system model and the actual Yunnan power grid simulation model.This thesis researches the impact on frequency characteristics when the DC frequency modulation is involved in frequency modulation of hydroelectric generating set in send-ending system in the form of analyzing,it provides a valid method to study the frequency stability of send-ending where DC frequency modulation is involved in frequency modulation of hydroelectric generating set.