| Abstract |
Very high voltage and high power converters inserted in transmission and distribution grids have a strategicrole in the operation of the power system of which they are part. Their complete failure is simply notacceptable and, also for this reason, their realizations are highly modular in nature. The exceptionallylarge number of discrete components characterizing such converters implies that the unpredictable localizedfailures 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 mustguarantee extremely high availability and continuity of grid operation for decades, with the only exclusionof 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 thecircuital graph of the converter bestows on such faults the capability of severely altering, or ideally eveninterrupting, the normal grid line currents or the currents in the important converter branches where theymaterialize. In reality a temporary, yet greatly harmful, electric arc may emerge. In approaching thisscenario, the paper proposes a fully autonomous, fast acting shunt device aimed at protecting againstopen circuit faults. The circuit is devised to be as simple and rugged as possible by employing thyristorstogether with rugged passive components only and by avoiding any controller, as well as any externalsupply. Owing to a proposed unconventional use of fuses, arresters and surge protectors in the thyristorstriggering 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 at1400 A RMS and capable of protecting converter sub-units employing 6.5 kV high power IGBTs. |
