Synchronization detecting circuit and automatic synchronous parallelization apparatus

Disclosed is an automatic synchronous parallelization apparatus capable of suppressing an inrush current by automatically adjusting the magnitude, the frequency, and the phase of the voltage when a single-phase voltage-type DC-to-AC converting device performing an autonomous parallel operation is connected in parallel with an external single-phase AC voltage source. The automatic synchronous parallelization apparatus 17 according to the present invention includes a synchronization detecting circuit 130, a voltage amplitude command value generating circuit 180, and a frequency command value generating circuit 190. The voltage amplitude command value generating circuit 180 generates the 1st-axis voltage command value, and the frequency command value generating circuit 190 generates the 2nd-axis voltage command value, based on a value relating to a frequency difference between an external single-phase AC voltage source 12 and a single-phase voltage-type DC-to-AC converting device 11 and an effective value of a voltage of the external single-phase AC voltage source 12 that are detected by synchronization detecting circuit 130. By using such command values, an output of the single-phase voltage-type DC-to-AC converting device 11 can be adjusted to as to coincide with a voltage waveform of the external single-phase AC voltage source 12.