The Impacts of UHV AC Transmission Lines on Traditional Line Differential Protection Functions

University essay from KTH/Skolan för elektroteknik och datavetenskap (EECS)

Abstract: UHV transmission lines have several advantages when it comes to transferring of bulk amount ofpower in long distances. Although the technology is available from 1980’s, the number of UHVtransmission lines around the world has been increased in the beginning of this century in orderto supply the high demand of electricity. The number is going to increase even more in the nextfew years. Thus, it is important to study the UHV line characteristics and update the powersystem equipment as well as the operation procedure accordingly.UHV transmission lines exhibits some distinct phenomena which are not present in thetransmission lines with lower voltage levels e.g. high amount of charging current, non-linearincrease of apparent fault impedance with the increase in fault distance, longer time constant forthe DC component in fault current etc. Some of these are very important for different protectionschemes of the transmission line. The aim of this thesis work is to study the impact of UHV linecharacteristics on line differential protection and propose solutions to overcome them.Line differential protection is popular for its good selectivity and simplicity as long as there is adependable reliable communication system between the two ends of the line. High amount ofcapacitive charging current and large phase shift between sending and receiving end currents aretwo important characteristics of UHV lines which have severe impact on the line differentialprotection. It becomes very critical for the protection scheme to maintain good sensitivity andsecurity at the same time. As a result, compensation of the charging current becomes essential.The fixed compensation method is used to compensate the charging current of long lines withlower voltage levels. However, it cannot satisfy the sensitivity requirements for line differentialprotection scheme on UHV lines. Hence, it is necessary to search for other compensationmethods.In this thesis, a solution related with charging current compensation method is proposed with theuse of adaptive phase shift compensation. Several simulations have been done to examine thecharacteristics of the developed method in the worst case scenarios. It is found that the methodexhibits very good sensitivity as well as security for line differential protection on UHV lines.

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