Phase-comparison methods in automatic synchronising

The success or otherwise of connecting two alternating sources in parallel depends very largely upon securing small and preferably diminishing deviations in the magnitude, phase and frequency of the two sources, at the instant at which electrical connection is made between them. Checking these deviations, and controlling the two sources until conditions are favourable for closure, have for long been completed by manual methods, but the main electrical conditions required for successful synchronisation may be ensured, and synchronisation finally effected, by entirely automatic methods. The purpose of the paper is to provide a comprehensive account of automatic synchronising techniques in which frequency and phase-displacement deviations are assessed using methods of phase comparison. The derivation of slip-dependent control signals using a balanced bridge-ring modulator is described, and the arrangement of orthodox logical elements for checking and control purposes is developed. Two methods of anticipating the instant of zero phase displacement to allow for the operating time of the connecting circuit breaker are described: one interprets the required anticipatory period as an angle deviation and is strictly accurate at one slip frequency only, whilst the advance-time features of the second method more accurately match the fixed delay of the circuit breaker, at least over the frequency spectrum for which synchronisation is to be permitted. The performance of an experimental system using the methods described is summarised from the results of trial synchronisations of a small laboratory test machine.