Harmonic instability between controlled static convertors and a.c. networks

A controlled mercury-arc or thyristor convertor with a conventional firing-pulse control system can both generate and magnify a.c. harmonics additional to the fifth, seventh etc. of classical convertor theory. This effect is normally of little importance if the impedance of the a.c. system is low, but it is shown by analysis that, if this impedance is relatively high, as may occur on the larger sizes of convertor in h.v. d.c. transmission, this type of harmonic magnification may become excessive and may even prevent stable operation of the convertors. The analysis is confirmed by model tests and appears to account for several reported instances of excessive abnormal harmonics associated with firing-angle unbalance in h.v. d.c. convertor installations. Generation of abnormal harmonics can be substantially reduced and harmonic magnification largely eliminated by a new control system in which firing pulses are timed by a phase-locked oscillator, controlled to satisfy the usual constant-current or extinction-angle requirements.