SYSTEM INTERCONNECTION SYSTEM

PROBLEM TO BE SOLVED: To provide a system interconnection system in which erroneous detection of power interruption of a distribution system is prevented. SOLUTION: The system interconnection system comprises an inverter 2, an interconnection switch 3, an inverter control circuit 6, an inverter output current phase variation command circuit 7a, a circuit 7b' for measuring the period of system voltage, and a power interruption decision circuit 7c for computing |T1-T2|, where T1 is a first voltage period measured before a phase variation command has been delivered and T2 is a second voltage period measured after a phase variation command has been delivered, and making a decision that power supply of a distribution system is interrupted when the operated difference is longer than a reference time for deciding phase variation, determined by expressing the phase angle in terms of time. The system interconnection system further comprises a circuit for operating the first voltage period by calculating the mean value of a plurality of voltage periods measured before a phase variation command has been delivered, and a circuit 7d' for setting the reference time based on the mean value thus operated.