專利摘要：
A phase-locked loop method for use in utility electricity parallel-connection system is introduced. The phase-locked loop method comprises a conversion signal generating step, an error calculating step, a frequency correction signal obtaining step, an angle signal obtaining step, and a synchronous signal generating step to not only calculate an error value by detecting a utility electricity voltage, but also reduce or return-to-zero the error value by proportional integral adjustment. With the steps, the phase-locked loop method achieves synchrony precisely by eliminating input utility electricity voltage distortion and frequency variation. Furthermore, the phase-locked loop method advantageously features quick response and a wide frequency range and therefore is effective in tracking power generation facilities, such as a diesel generator, and expanding inverters.