High Voltage Topologies for Very Fast Transient Measurements

High voltage equipment is subjected to several types of electrical stress during operation, why testing by transient wave shapes is routinely performed. Measurement systems are being developed by National Metrology Institutes (NMI) for traceable calibration of sensors used in Gas Insulated Switchgear (GIS) where front times below 10 ns are common. We refer to front times below this as Very Fast Transients (VFT). This paper presents advantages and disadvantages of various divider designs to be used for VFT measurement systems. Many of these are a heritage from lightning impulse measurement systems. Several divider designs are studied to investigate the contribution on uncertainties in both amplitude and time parameters in a measurement system. Four resistive designs based on ceramic bulk resistors, bifilar wire winding, a resistive liquid and thick film SMD resistors have been characterized and compared with a small damped capacitive divider. Step response measurements have been used to characterize several dividers in different geometrical configurations, looking at the transient response out of a wave propagation point of view with analysis of loop sizes, field distributions and proximity effects. The findings are that and minimizing skin effects and inductance in the dividers are essential for a fast response and settling. Convolution of the step responses with ideal reference curves showed that more development is needed to eliminate oscillations in the step response. Further improvement of the geometries and impedance matching is needed to achieve acceptable errors for VFT with front times T1 below 10 ns.