Accuracy and Reliability of Switching Transients Measurement with Open-Air Capacitive Sensors

Abstract

Contactless capacitive (open-air) sensors are applied to monitor overvoltages near overhead line terminations at a substation or at the transition from underground cables to overhead lines. It is shown that these sensors, applied in a differentiating/integrating measuring concept, can result in excellent characteristics in terms of electromagnetic compatibility. The inherent cross-coupling from open-air sensors to other phases is dealt with. The paper describes a method to calibrate the sensor to line coupling matrix based on assumed 50 Hz symmetric phase voltages and in particular focuses on uncertainty analysis of assumptions made. Network simulation shows that predicted maximum overvoltages agree within typically 7% compared to reconstructed values from measurement, also with significant cross-coupling. Transient voltages from energization of an (extra-)high voltage connection can cause large and steep rising ground currents near the line terminations. Comparison with results obtained by a capacitive divider confirms the intrinsic capability in interference rejection by the differentiating/integrating measurement methodology.

Methodology and accuracy for non‐invasive detection of switching transient overvoltages from compensation coils connected to power transformers

“…• Verification of existing dividers: Such devices can be prone to disturbance, e.g. at connections with cable-to-line transitions [2]. Upon a switching transient large ground, currents arise, causing signal disturbance via unintended coupling routes [16].…”

“…They all assume symmetric power frequency voltages, but employ different ways to complete the information needed to construct the full coupling matrix between the three-phase voltages and the three sensors situated near the phases. The uncertainty analysis developed in [2] is employed in Section 4 to compare the considered decoupling methods. Section 5 discusses the results and summarises the conclusions.…”

“…Such an approach inevitably introduces uncertainty in the analysis. In the previous research concerning 380 kV terminations in a line-cable-line connection [1,2] the method provided accurate results when comparing them to power system simulation (with PSCAD, [21]). There, the cross-coupling was relatively small (with a value up to about 10% with respect to the direct coupling strength).…”

“…However, imprecise positioning and metal structures in the vicinity may violate perfect symmetry, which must be accounted for. In [2] the following matrix is proposed for a similar symmetry:…”

“…As suggested by the capacitance matrix (3) this value is about equal to M 12 (=M 4 ) and M 32 (=M 5 ) and therefore the average value is taken. The solution of (4) using (5) as coupling matrix is provided in [2]. To account for uncertainty, margins of ±10% are assumed for having M 21 and M 23 equal (M 6 ), and for taking them equal to the average of M 4 and M 5 .…”