Thyristor current source with an improved power factor

This paper describes the steady state and dynamic characteristics of a microprocessor controlled current source. The source is based on an 8-thyristor phase commutated bridge which gives an improved power factor and decreased harmonic distortion. Limiting curves for the tridge continuous conduction are presented together with line and thyristor currents in a Δ-y input transformer bank. System´s salient characteristics, such as an equidistant firing scheme, a line variation compensator and the elimination of a filter in the current feedback loop are described. The transfer function of the current source, with the thyristor switching delays included, is derived and is verified experimentally. It is practically demonstrated that a very fast response is attainable even with microprocessor controlled inner current loop.