The Phase-Locked Oscillator - A New Control System for Controlled Static Convertors

Static convertors for high-voltage direct-current transmission links, whether based on mercury-arc valves or thyristors, are controlled by accurately timed firing pulses from a control system. With conventional control systems it has not so far been practicable to operate a convertor on a very weak (high impedance) ac system; one principal reason is inherent harmonic instability caused by the direct dependence of the control system on ac line voltage and its distortion.