-Dot Probe for Fast-Front High-Voltage Measurement

This paper presents the operating principle, design, and testing of a coaxial D-dot (the time derivative of electric flux density) probe to measure fast-front high voltages, e.g., the residual voltages of surge arresters (SAs). This probe consists of three identical copper toroids placed around a high-voltage electrode, where all are coaxially assembled in a large earthed cylinder. The probe was first simulated by a finite-element package to optimize the assembly and reduce the electric field inside it. This was confirmed by an alternating current test to ensure a corona-free design. Simultaneous impulse voltage measurements were done using the designed D-dot probe - two commercial mixed resistive-capacitive (RC) probes and a damped capacitive voltage divider. The linearity of the D-dot probe was checked under unloaded and loaded conditions. Results reveal that the larger the toroid separation and/or the lower the attenuator capacitance is, the higher the measured voltage from the middle “signal” toroid will be. The residual voltage waveforms for an 11-kV SA, measured by two commercial mixed RC probes and the damped capacitive voltage divider, showed an initial inductive overshoot superimposed on the waveform and a significant decay, even before the current peak instant. On the contrary, the voltage measured by the designed D-dot probe gave a voltage waveform that looked like that of the current and slightly led the latter. For the damped capacitive voltage divider and the two commercial mixed RC probes, neither the peak voltage nor the voltage at peak current gave the correct current-voltage characteristics. This confirms the contradiction of some published SA models in the high-conduction regime because most models were based on measurements done by different and large-impulse capacitive or resistive voltage dividers with improper compensation.