Circulating Unbalanced Currents of EHV/UHV Untransposed Double-Circuit Lines and Their Influence on Pilot Protection

This paper elaborates on the reasons for current unbalance in extremely high voltage/ultra-high voltage (UHV) double-circuit transmission-line systems based on analyses of a typical tower structure and feasible phase arrangements. In addition to the studies of the system under normal operation condition, the unbalanced currents of the double-circuit line under the condition of external faults are first investigated. The frequency-dependent model of UHV double-circuit lines is built using the ElectroMagnetic Transients Program including DC (EMTDC) software package. Simulation results and real data obtained from the power utility show that the circulating unbalanced currents in both cases of normal operation and external fault conditions cannot be ignored due to their high magnitude. The significant influence of the circulating unbalanced currents on pilot directional protection of double-circuit lines is proven and verified by various simulation tests and real field data. Consequently, an improved scheme is proposed to overcome the influence of circulating unbalanced current and significantly improve the reliability of pilot directional protection.