Fast acting autonomous and rugged shunt protection against open circuit faults in high power conversion

Very high voltage and high power converters inserted in transmission and distribution grids have a strategic role in the operation of the power system of which they are part. Their complete failure is simply not acceptable and, also for this reason, their realizations are highly modular in nature. The exceptionally large number of discrete components characterizing such converters implies that the unpredictable localized failures of few composing sub-units, be them single components or sub-converters / sub-systems, is an almost certain probabilistic event during the lifetime. In spite of this reality such converters must guarantee extremely high availability and continuity of grid operation for decades, with the only exclusion of their planned maintenance periods, which recur yearly at most, biennially at present custom. The endeavour is complicated by the fact that even some locally confined and non-propagating faults, having character of open circuit, could be unacceptable for the entire converter. This occurs when the circuital graph of the converter bestows on such faults the capability of severely altering, or ideally even interrupting, the normal grid line currents or the currents in the important converter branches where they materialize. In reality a temporary, yet greatly harmful, electric arc may emerge. In approaching this scenario, the paper proposes a fully autonomous, fast acting shunt device aimed at protecting against open circuit faults. The circuit is devised to be as simple and rugged as possible by employing thyristors together with rugged passive components only and by avoiding any controller, as well as any external supply. Owing to a proposed unconventional use of fuses, arresters and surge protectors in the thyristors triggering circuit, the shunt intervenes by relying only on the grid current whose flowing must be guaranteed. With the support of simulations the paper concerns the behaviour and design of a shunt rated at 1400 A RMS and capabl- - e of protecting converter sub-units employing 6.5 kV high power IGBTs.