Research On Segregated Current Differential Protection Based On Accurate Parameter Estimation For Transmission Lines

On Sept. 26^th 2005, the national transmission line with highest voltage, whichstarts from Guanting Qinghai Province to Lanzhou Shanxi Province with its voltageclass up to 750 kV, was bringed to service formally. It is a tremendous achievementfor the development of the national power industry. The EHV or UHV transmissionline is the framework to link power transmission system or to link remote powertransmissionsystem. Thereforeit is veryimportant toimprovethequalityoftherelayprotectionforit.The Segregated Current Differential Protection is an ideal protection principle,which has many advantages such as its absolute selectivity, its bearing effect ofvibration of power system and so on. As the popularization of optical fiber grid andthe optical commumication being matured, the Segregated Current DifferentialProtection will have been the preferred longitudinal protection for the EHV or UHVtransmissionline.Distributed capacitance current of EHV or UHV transmission line is always amajor factor to effect the quality of the Segregated Current Differential Protection.And getting the accurate line parameters is the key to compensate the capacitancecurrent exactly. The line parameters of UHV transmission line are obtained fromoff-line calculation or actual measurement. But because of the capricious location,climate and resistivity of land, especially in the abominoable paramos thetransmission line with cryoconite cover, the line parameters derive from the settingvalue obviously. The paper gives the Newton-Raphson method to estimate the lineparameter on-line, and the emulation and analysis demonstrates that it can completeon-line parameters estimation accurately for transmission line with shunt reactor ornone.In addition, the paper analyses the two methods of Segregated CurrentDifferential Protection: the method of instantaneous value and the method of phasor,the implementation approach of which are also given. And with the software ofEMTDC, the merits and drawbacks of the two methods will be compared in thevarious operational aspects. Finally, a programme which can integraed reuse the twomethodswillbegiven.