Pilot Protection Of Ultra-High-Voltage Transmission Lines With Directional Comparision

China State Grid Corporation is beginning to build a Ultra-high voltagetransmission line from southeast shan-xi province to Jing-men of Hu-bei province.This project is a new milestone in history of china power grid development. It willserve as strongbackboneof Chinapower system andhas veryimportant significancein enhancing the reliability and stability of system operation. Cautious selection oftheprotectionschemeforUHVtransmissionlines is animportant task ofcolleagueworkinginfieldof protectiverelaying.UHV transmission lines have large distributed capacitance, decaying slowlynon-periodic component and rich high frequency components. Based on thecharacteristics of UHV transmission lines, pilot protection with directionalcomparision for UHV transmission lines and a main protection scheme forUHVtransmissionlines were studiedinthis paper.Against negative sequence directional components, logic block diagram ofultra-high voltage transmission line protection and under various fault conditions thelogic of protection action are given. Digital Second-order RC low-pass filter withphaselet-based algorithm and phase-mode transformation, negative sequence voltage,current and power can be obtained quickly in a few milliseconds. This negativesequence directional protection using the proposed algorithm do not operate whenfault is in reverse direction, but it can reliablyact duringvarious internal asymmetricand symmetric faults. This algorithm can capture the beginning asymmetry of threephase short-circuit if the asymmetry only lasts 5ms. Simulations for the 1000 kVUHV transmission lines under construction in China proved the correctness of thisalgorithm .Based on the transmission line model with distribution parameters, a newcompensation algorithm was proposed in this paper. The compensation algorithmeliminates influence of the line distributed capacitance on the negative sequencedirectional protection. The compensation algorithm has the same effect both in thetransient process and steady-state process after faults occurrence.Segregated phase compensator type directional relay based on the faultcomponent was proposed. The relay can be applied to UHV transmission lines andhas the function of phase selection. It can respond to various fault under three phaseoperation conditionand twophaseoperation condition. Aformula which describes the transient process from beginning traveling waveto steadytraveling wave was deduced and the concept of initial traveling wave stagefirst was defined and its duration was given. It leads to a conclusion that reflectionfactor is a time dependent function, and variation range and reflection factor atdifferent stage were presented .Based on unified traveling wave theory andtransmission line model of reflective terminal with energy storage element , powerpolarity of initial traveling wave was analysed with both physical and mathematicalmethod.A new directional relay principle for parallel line on same towers has beenproposed inthis paper. Parallel lineonsametowers is regardedas oneprotectedunit.The directional relay is based on same directional negative sequencecomponents .Because of influence of mutual inductance, directional protection onsingle line has to be latched-up during cross line fault between different phases, thisshortcomingis eliminatedinthis newprinciple.