Accuracy of switching transients measurement with open-air capacitive sensors near overhead lines

Application of open-air sensors is an attractive option for detecting switching over-voltages as their installation does not interfere with operation of the lines. Drawback is that coupling strengths are not known beforehand and cross-coupling between a sensor near one phase and other phase conductors or even to other circuits affects the recordings. Calibration of the coupling matrix between phase voltages and sensor signals can be realized by relating the power frequency component in the recorded signals to a priori assumed base of symmetric phase voltages. This paper establishes a method to quantify the different causes for measurement uncertainties. The coupling matrix cannot be completely reconstructed from measurements, enforcing model assumptions which introduce uncertainties. Uncertainties arising from not perfect symmetric phase voltages and measurement equipment are investigated as well, but play a minor role when judging over-voltages on a per unit base. The methodology is presented for measurements at an overhead line to underground cable transition point and to a substation. At transition points, the sensors could be positioned such that the cross-coupling remains low and the resulting uncertainty in the determination of maximum per unit over-voltages is typically 2%. Despite the installation at the substation resulting in relatively strong cross-coupling, per unit over-voltages could still be determined with an accuracy within 10%.