PURPOSE: To bring a device into a parallel operation with a system synchronously and exactly in a short time, by maintaining the speed of a starting motor at a value slightly higher or lower than that of system frequency via a liquid resistor using a liquid resistance controller.

CONSTITUTION: When a pumped-storage operation or a phase modifying opera tion is selected, then a starting breaker 6 is switched on, and a starting motor 1 is started. First, the operation is accelerated slowly by the primary current constant-control of the starting motor 1, and when the speed of a generator- motor 2 comes to 90% or more, then a field breaker is switched on, and when the speed comes to 95% or more, then automatic synchronizers 13, 14 are operat ed. In this case, by a liquid controller 4, a liquid resistor 5 is controlled and the speed of the generator-motor 2 is held at a value higher than that of system frequency by f. By an automatic voltage balancer 14, the voltage of the genera tor motor 2 is balanced with system voltage, and by the automatic synchronizer 13, whether phase difference and frequency difference are within tolerance is discriminated, and a parallel breaker 7 is switched on.