A power conversion system is capable of distinguishing easily and clearly between being interconnected to the power system and being in an isolated operation. The power conversion system converts DC power to AC power to interconnect with a power system and injects reactive power into a grid node for determining an isolated operation, including: a voltage detector that detects an output voltage at the grid node; a current detector that detects an output current at the grid node; a frequency measurement unit that measures a frequency of the output voltage at the grid node; a reactive-power measurement unit that measures reactive power at the grid node; and an isolated-operation determination unit that determines whether or not the power conversion system is in an isolated operation as disconnected from the power system. The isolated-operation determination unit includes a reactive-power-fluctuation determination unit that performs a first isolated-operation determination from the reactive power.