A System for Recording Thyristor and Rectifier Current Waveforms

To faithfully reproduce waveforms of both high-forward current and high-speed reverse recovery current in a high-power three-phase bridge converter is a difficult task. Ordinary dc shunts, or fuses which are sometimes used as shunts, are too inductive to satisfactorily reproduce the reverse recovery current. Noninductive shunts are available but suffer from several serious disadvantages which are explained in the paper. A system is described which overcomes all of these difficulties. It uses specially designed miniature through-type CTs that have good frequency response from dc through several hundred kHz. Saturation by the dc component is avoided by a core resetting scheme. A circuit is also described which relieves the CT of its burden thereby permitting reproduction of short-circuit currents without CT saturation.