Operation Characteristics And Improvements Of DC Island Sending System

The traditional high voltage direct current transmission technology plays an important role in the transmission project of electricity from the west to the east,and with the intensive exploitation of large-scale hydropower and thermal power bases,there is possilbility that the the sending end ac system at the power bases may operate under island mode.Lacking the support of the main grid,the short circuit ratio and the equivalent inertia constant of a dc island sending system are significantly smaller than those when connected with the main grid,so the ac/dc interaction effect is quite obvious and the operation and control of the dc island sending system are quite difficult.The existing studies about weak ac system mainly aim at the receiving end system,which means the conclusions only have limited applicability at the dc island sending system.What's more,the existing studies about dc island sending systems are short of systematic theoretical modeling and analysis.Therefore,this thesis does specific research to the equivalent model,analysis approach and description indice of the dc island sending system,and analyzes the influence factors and improvement measures of the dc island sending system's power transmission capacity and temporate overvoltage amplitude.The thesis is organized as follows:(1)The equivalent model is established and the analysis approach is introduced.First,based on the system characteristic differences between the receiving end system and dc island sending system,this thesis establishes the periodical equivalent model of the dc island sending system specificly.Second,this thesis details the theoretical analytical approach based on the periodical equivalent model and the simulation analysis approach based on the PSCAD/EMTDC simulation program.Last,this thesis utilizes the two analytical approaches to analyze the temporate overvoltage amplitude and power transmission capacity of a same dc island sending system.The results of the two analytical approaches are basically in accordance,demonstrating the validity of the periodical equivalent model.(2)Three indexes for describing the dc island sending system are summarized and proposed,which are ac system transient impedance,ac system steady impedance and equivalent inertia constant.The ac system transient impedance qualitatively describes the temporary overvoltage amplitude of the dc island sending system following the dc bipolar block.The ac system steady impedance exactly describes the power transmission capacity of the dc island sending system.The equivalent inertia constant describes the transient frequency performance of the dc island sending system following a disturbance within a short time.The simulation analysis of Yunnan-Guangdong,Nuozhadu and Xiluodu UHVDC project operating under dc island mode verifies the applicability of the three indexes of describing the dc island sending system.(3)The influence factors and enhancement measures of the dc island sending system's power transmission capacity are studied.First,based on theoretical analysis and time domain simulation,the thesis discusses the influence of excitation system parameters on the power transmission capacity and transient process.Second,based on theoretical analysis,the thesis analyzes the sensitivity of the power transmission capacity to the change of the converter transformer tap position,the reactive power compensation,the ac system steady impedance and the generator terminal voltage.Last,the thesis applies the conclusions to a specific ac system fault at the sending end,explains the mechanism of instability and proposes two measures to improve the system transient response characteristics,which are dc current limiter and generator excitation booster.The sample system and Yunnan-Guangdong UHVDC project both verify the validity of the two improvement measures.(4)The influence factors and restraint measures of temporate overvoltage amplitude of the dc island sending system following the dc bipolar block are studied.First,based on theoretical calculation method,the thesis quantitatively analyzes the influence of the ac system transient impedance,the converter transformer leakage reactance,the steady-state firing angle and the reactive power undercompensation on the temporate overvoltage amplitude.Second,based on time domain simulation method,the thesis qualitatively analyzes the influence of the light-load line capacitance effect,the converter transformer saturation effect,the ac surge arresters and the converter blocking process on the temporate overvoltage amplitude.Last,based on the analysis results,the thesis proposes two measures to restrain the temporate overvoltage amplitude,which are reactive power undercompensation and generator excitation reduction.The sample system and Yunnan-Guangdong UHVDC project both verify the validity of the two restraint measures.