New device for measuring post-arc currents in circuit breakers

Abstract

This paper describes a new technique for measuring post-arc currents in circuit breakers. The measurements are performed with a specially designed current monitor. In contrast to commonly used current monitors saturation of the core of the transformer at high pulses of current is prevented by shunting the turns of the transformer by antiparallel fast-recovery diodes. This new device offers several advantages in comparison to other known techniques for post-arc measurements: there is no galvanic coupling to the network, the system is easy to handle, and the costs of the components are low. The two post-arc-current monitors (PACM) described have a sensitivity of 0.1 V/A and 10 mV/A, and are linear within 1% in the range of ±4 and ±100 A, respectively. The first PACM has an upper cutoff frequency of 18 MHz, the second one 3.8 MHz and are designed for sinusoidal fault currents of 6.3 kA and 56 kA, respectively, at a frequency of 50 Hz. Examples of applications to axially blown arcs in SF6 and to vacuum circuit breakers are presented.