How a Measurement You May Not Be Doing Can Improve Relay Operations

Orlando Utilities Commission (OUC) discovered that even one seemingly small error could produce an 18.5% deviation in calculated zero sequence impedance value. The utility began measuring transmission line impedance to analyze different scenarios including underground cable sections, mutually coupled lines, and neighboring utility distribution lines. By comparing values from the collected field-tested impedance data, OUC and Leidos improved the 18.5% zero sequence impedance error between OUC's CAPE model and actual transmission line field measurements. The error was corrected when the neutral conductor on an offset distribution line near the 230kV transmission line was modeled. This paper discusses how utilities can identify and measure assets that can impact their zero sequence impedance the most. Accurate data results in more effective relay operation, and overall improved relay settings. A conference attendee will take away a better understanding of the impact different parameters have on calculating zero sequence impedance and how to get an accurate zero sequence impedance in the models. Topics include how engineers can calculate zero sequence line impedance, determine the highest priority parameters, and using transmission modeling software to model a variety of parameters including standard tower designs, conductor types, temperature, and distribution lines. Many times, the distribution system is not accounted for even though it has a significant impact on zero sequence impedance.