A study on the on-line measurement of transmission line impedances for improved relaying protection

This paper proposed an on-line measurement of transmission line impedance in order to improve the accuracy of distance relays and analysis of power system. Distance relays are generally set by the estimated impedance of the protected lines, which involves a certain amount of error in itself. Especially, the zero sequence impedance has more erroneous terms associated with mutual effects in parallel line as well as uncertainty of ground return path. The proposed method is measuring impedance either during normal operation or during faults, and can be applied on-line correction of the setting value of distance relays. This on-line measured positive impedance may be used to monitor the transmission line condition during operation and more accurate impedance data may also improve performance of power system analysis. For verification of the proposed method, several simulations have been executed and shown satisfactory results.