Study On Current Differential Protection Arithmetic Of UHV Transmission Lines

Ultra-High-Voltage has a economic transmission cost,a simple grid structure,less power transmission corridors,etc.According to the characteristics of China's energy structure and the expanded scale of the power system,China has decided to build UHV transmission system,the Jindongnan-Nanyang-Jingmen 1000kV UHV AC Transmission Project was put into operation in January 2009.The current differential protection based on KCL current theory is one of the main protection of UHV transmission lines.UHV transmission lines has higher performance requirements of relay protection,and its special structure and characteristics have a great impact to the current differential protection.The larger distributed capacitance current,the seriously transient process, the larger fault resistance and the probable accession of series capacitor or paraller reactor will all have an impact on line current differential protection.This development of UHV transmission about domestic and international is summarized,the present research situation of EHV/UHV line current differential protection is analysed and the key issues of UHV line current differential protection is pointed out.Several typical line current differential protection principles and action criteria is analysed.Based on the power system electromagnetic transient simulation software PSCAD/EMTDC,typical 1000kV,750kV and 500kV transimission line model is simulated.The distributed capacitance current of the models is compared and its impact on the current differential protection is analysed.With analysis of the shortage of steady-state capacitance current compensation,an improved self-adaptive capacitance current compensation method and its simulation in 1000kV line model is carried out. Based on the differential eguation of transmission line,the transient current expression of no-load closing and line fault is deduced and validated.The characteristics of transient current and its effect on current differential protection is analysed.The performance of several typical current differential protection in no-load closing,external fault,internal different fault point and resistance is studied.Based on the result,a suitable UHV transmission line improved current differential protection criterion is brought forward.Based on the 1000kV line model,the action characteristics of the improved protection criterion in no-load closing,external fault,internal different fault conditions is simulated and analysed,the results proved the correctness and effectiveness of the improved criterion.