Digital Implementation of Method for Discontinuous Current Mode Compensation Of High-Performance Line-Commutated Converters

Many high-performance power converters at CERN are implemented with thyristors, mostly in CCM (Continuous Conduction Mode) due to their better dynamic response. In DCM (Discontinuous Conduction Mode), the converter reduces its small-signal gain, producing a degraded time response which can lead to instability. Several of these converters use digital regulation, allowing the implementation of complex non- linear control techniques. Therefore, a strategy to control the converters in DCM (Discontinuous Conduction Mode) can be implemented, bearing in mind that their standard operation (sampling time, measured signals, acquisition system) can not be drastically changed. This report presents the implementation of a new method to control line-commutated converters in discontinuous conduction mode (DCM). The method is based on the modfication of the thyristor's ring angle _ using a linear approximation, yielding nearly the same dynamic performance as in continuous conduction mode. The _ring angle is corrected by adding a supplementary angle, calculated by estimating the average output current. The proposed technique signficantly reduces computational burden, being suitable for on-line calculation. Both, simulations and experimental results are presented on a 1kW laboratory prototype based on CERN GEREG crate.