The thyristor-controlled reactor (TCR) is used for AC voltage control on transmission lines and industrial busbars. It has a relatively simple main circuit and its performance depends very much on its control system. The phaselocked oscillator type of control system is now universally used for HVDC convertors, and is being introduced for the TCR. The form of this control described here is of the ‘zero-delay’ type, without control filters or other lags, and gives freedom from harmonic instability and a good combination of stability at other frequencies, with fast response. Examples of applications are described briefly.