Electronic circuit for measuring the current slope (dI/dt) and the post-arc current in gas-filled circuit breakers.

We have developed an electronic circuit for measuring the slope of the current near the current-zero crossing of an alternating current (dI/dt) and the post-arc current in gas-filled circuit breakers. The dI/dt and the post-arc current are measured in the current injection loop of a Weil-Dobke circuit (a synthetic test circuit that consists of a loop to generate a high short-circuit current at a relatively low voltage and a separate loop to generate high voltage after the short-circuit current is interrupted). The arc voltage and the dI/dt can be used to calculate the arc resistance of a breaker; the arc resistance and the post-arc current of the breaker provide important information about its interruption performance. The dI/dt and the post-arc current are measured with a commercially available high frequency coaxial shunt. The output voltage of the shunt is limited by using an electronic circuit to achieve better accuracy in the low current regime. The limited voltage signal is amplified with an operational amplifier that acts as a line driver. The prototype has a bandwidth of 12 MHz (+0.005 dB/-0.1 dB) and a propagation delay time of 10 ns. The nominal input voltage is ±50 V, and the maximum dU/dt (slope of the input voltage) that can be measured is 700 mV/μs with a linear range of ±0.6 V. Post-arc currents down to 200 mA can be measured.