Patent ID: 8553436

Claim:
A synchronization detecting circuit comprising: a detection unit that detects a value relating to a frequency difference between an external single-phase AC voltage source that is a parallelization target of a single-phase voltage-type DC-to-AC converting device performing an autonomous parallel operation based on a 1st-axis voltage command value, which is used for adjusting an amplitude of an output single-phase AC voltage, and a 2nd-axis voltage command value, which is used for adjusting a frequency, and the single-phase voltage-type DC-to-AC converting device and a value relating to an effective value of a voltage of the external single-phase AC voltage source, wherein the detection unit comprises: a sampler that detects a target voltage waveform of the external single-phase AC voltage source; a delay circuit that generates a waveform of a delayed voltage that is delayed from a waveform of the target voltage detected by the sampler by time that is longer than (m−1)π/ω co and shorter than mπ/ω co (here, m is a natural number and ω co is a reference angular frequency); and a calculation unit that calculates a frequency difference cosine signal and a frequency difference sine signal that have an angular frequency corresponding to a difference between a target angular frequency of the waveform of the target voltage and a given target-source angular frequency of the single phase voltage-type DC-to-AC converting device, using the waveform of the target voltage, which is detected by the sampler, the waveform of the delayed voltage, which is generated by the delay circuit, and the given target-source angular frequency, wherein the value relating to the frequency difference and the value relating to the effective value of the voltage are detected based on the frequency difference cosine signal and the frequency difference sine signal that are calculated by the calculation unit.