Power conversion apparatus that judges system power failure based on system frequency and voltage

A power conversion apparatus comprising:

• a power supply;
• an inverter configured to convert DC power supplied from the power supply into AC power and AC power supplied from a power system into DC power;
• a switch configured to switch an electrical connection state of an AC line between an AC terminal of the inverter and an input terminal from the power system;
• a first voltage detector configured to detect an output voltage of the inverter;
• a second voltage detector configured to detect a system voltage of the power system;
• and
• a control circuit configured to control the inverter and the switch,
• wherein the control circuit:
• judges whether or not the power system has a power failure, based on values of the system voltage and a frequency of the power system,
• when the power system has recovered from the power failure,
• calculates a phase difference between a phase of the output voltage of the inverter and a phase of the system voltage and generate, by using the phase difference, a first output frequency pattern for changing a frequency of the output voltage of the inverter, and controls the inverter to change the frequency of the output voltage of the inverter using the first output frequency pattern,
• re-calculates the phase difference when it is judged that the phase difference is larger than the predetermined threshold after controlling the inverter using the first output frequency pattern, and generates, by using the re-calculated phase difference, a second output frequency pattern for changing the frequency of the output voltage of the inverter, and controls the inverter to change the frequency of the output voltage of the inverter using the second output frequency pattern, and
• closes the switch when the phase difference becomes smaller than or equal to a threshold.