Measurement Uncertainty Evaluation of Transmission Line Parameters

Transmission line parameters vary with environmental and operating conditions, thus introducing uncertainties in power flow studies and protection zone settings of relay protection equipment. Variations in line parameters are fundamental for the determination of static and dynamic line power ratings. Usually, nominal values are given without an associated confidence interval, and therefore an important piece of information is missing. With the proliferation of phasor measurement units (PMUs) that allows for synchronized measurements at different geographical positions arouse the possibility for simple and efficient measurements of line parameters. This article analyzes uncertainties associated with the estimation of line parameters, based on the equivalent pi model, valid for short- and medium-length transmission lines. This problem has been addressed in literature, but only from a theoretical perspective and the obtained results were verified with simulations only. Here, measurements conducted on two 220-kV transmission lines are analyzed. It was found that the correlation between voltage and current measurements must be taken into consideration when analyzing uncertainty propagation law. Only then statistical analysis of estimations matches the predictions obtained by applying the theoretical model. Accuracy limits were analyzed as well, but in this case some results were within the calculated limits, while others did not follow predictions. Improvements in accuracy limits' determination require the separation of environmental and operating condition variations from deviations due to measurement equipment, thus requiring additional information.