Merits and challenges of a differentiating-integrating measurement methodology with air capacitors for high-frequency transients

Increased urbanization as well as political and environmental aspects are leading to an increasing utilization of HVAC underground power cables instead of overhead lines (OHLs) in transmission grids. The insertion of mixed OHL-cable connections can give rise to technical challenges related to the occurrence of electromagnetic transients. More specifically, during switching operations high overvoltages can arise, in particular at the transition points between OHL and cable, and knowledge of
their magnitudes is important information for safe operation of the connection. This paper discusses measurements, performed for the Dutch transmission system operator, based on open air capacitances
picking up the electric fields from the line. For the open air sensors, the result at the end of the measuring cable is a differentiated voltage signal and the original waveform is restored by means of integration.
Since the transported signal is integrated at the input of the measuring cabinet, interference pick-up is significantly reduced. The system is simple in terms of installation, does not require special HV components and has a large bandwidth with excellent electromagnetic compatibility (EMC)
performance. However, since the sensors are open air, they also intercept field lines from the other phases, and possibly even from neighbouring circuits. Measurement decoupling is necessary, so methods to restore single-phase waveforms from switching events are discussed. Results from measurements, using the Differentiating/Integrating (D/I) technique, at a 380 kV mixed overhead line underground power cable connection obtained at a transition point and at a substation are presented and
compared with PSCAD simulations. For switching transients the measurements confirm simulated overvoltage magnitudes to be reliable and within the safe margin of 2 pu.