Calibration setup for traceable measurements of very fast transients

Wideband sensors like D-dot probes or capacitive probes are used to measure very fast transient overvoltages, but there is no recognized traceability for this kind of measurements. In the present version of IEC 61869-1, the test conditions for both GIS and air-insulated systems are given, but there is no guidance on the measurement methods that are used to assess the uncertainty of measurements because of a lack of traceability for HV transients. A calibration SF₆ set-up has been developed to establish the European traceability in this field. This paper presents this VFT calibration setup composed by a step voltage generator up to 100 kV with a rise time less than 10 ns, and a reference measuring system. The calibration setup of 7 m length is based on a cylindrical SF₆ configuration, which uses a stainless-steel duct with an outside diameter of 500 mm and an internal aluminium tube of 200 mm diameter as a high voltage conductor to have characteristic impedance around 50Ω, as a representative figure of the SF₆ GIS. A spark-gap of SF₆ at 2 bars is used to achieve a pulse with a rise time of less than 10 ns. Conical elements allow a good impedance matching between different sections of the calibration setup. The probe to be calibrated is either installed in a duct window in the case of a capacitive probe, or mounted in an intermediate flange between two duct elements in the case of a D-dot probe. A fast resistive reference measuring system is installed at the end of the metallic duct. A wideband digital recorder of 400 MHz of bandwidth, 14-bit of vertical resolution and 4.5 ns settling time is used to acquire and analyse the generated VFT pulses. The paper describes the design of the calibration setup and its characterization.