Research On Coordination Of Ground Distance Protection Of Specially-structured Transmission Lines

Grounding distance protection is one of the most widely used backup protection of thetransmission lines. It has such significant advantages: less influenced by system operationmodes, provided with relatively fixed protection scope and relatively steady relay settingvalue. With the development of microcomputer protection technology, and simplificationof zero sequence current protection, ground distance protection becomes the main backupprotection for ground fault. Setting reasonable ground distance protection value hasimportant significance for improving the performance of ground distance protection andensuring the safety and stability of the power system. There are many specially-structuredtransmission lines in the power system. Coordination of ground distance protection forthese specially-structured transmission lines can often be complicated. And it is faced withmore problems and challenges. So it is necessary to go into more detail on this subject.This paper is dedicated to the study of coordination of ground distance protection forspecially-structured transmission lines, including effects of zero sequence parameters oncoordination of zone1ground distance protection based on detective impedance, settingcoordination method of ground distance protection for transmission lines withdouble-branched-structure, effects of series capacitor compensation on distance protection.Some of the research result has been applied in practical project. It provides theoreticalbasis for calculation of relay protection.Firstly, this paper explores the effects of zero sequence parameters on the coordinationof zone1ground distance protection based on detective impedance. It has been analyzedtheoretically that the effects of zero sequence parameters on the coordination of zone1ground distance protection based on detective impedance under different operation modes.And then, application to a practical power system verifies the conclusion which gets fromtheoretical analysis above. Study on this subject provides principles for the zone1grounddistance protection coordination.Secondly, coordination of ground distance protection for transmission lines with double-branched-structure has been discussed. A model for double-branched lines has beenestablished based on its structure, and on the basis, effect of the double branched structureto selectivity of ground distance protection has been discussed. The analysis shows that, fortransmission lines with double-branched structure, it only needs to calculate zero-sequencecompensating coefficient with the correct actual measured value of the newly structuredlines or the adjusted parameters, that can get reasonable setting values in the light of theoriginal setting calculation method for usual lines.Finally, analyze the distance protection performance in transmission lines with seriescapacitors compensation of power grid. The total work including the following threeaspects: introduce reverse characteristic of voltage、reverse characteristic of current,characteristic of measured impedance in the power grid with series capacitorscompensation; analyze the influence of series capacitors compensation for distanceprotection; make suggestions to set calculation of distance protection for the seriescompensated line and its adjacent lines.