Theoretical analysis of harmonic instability in a.c.-d.c. convertors

There have been reports of difficulties encountered in the operation of high-voltage a.c.-d.c. conversion control schemes, particularly with reference to the so-called harmonic-instability phenomena. In the paper, an attempt is made to model the conventional convertor timing-loop control scheme using previously published concepts developed by the authors. This essentially enables the Z transformation to be used in assessing stability considerations. It is shown that this approach indicates likely troubles with conventional control schemes. A numerical example is given to show how the existence of a stability problem may be determined at the design stage, and a comparison of theoretical results and reported behaviour shows a good agreement.