The present invention relates to an apparatus and method for isolated operation detection using a partial phase window monitoring technique, and is capable of measuring isolated system operation voltage fluctuations to obtain state information of a commercial power supply to prevent isolated operation. In order to achieve this, a system voltage sensor for sensing the grid voltage of commercial power supply voltage and obtaining the waveform information of the grid voltage is provided. In addition, the grid voltage partial window monitoring technique is implemented to monitor the anti- And includes an A / D converter for converting the measured grid voltage signal into digital data, a PI (Propotional Integral) current controller for grid-connected inverter, a grid-connected PWM inverter, and a grid link filter unit. Also, the deviation between the grid voltage and the reference voltage is calculated and compared with the threshold value. If the value is greater than the threshold value, the isolated operation counter value is incremented by 1. If the isolation operation counter value is greater than 15, In the case of the isolated operation occurrence state, the isolated operation flag is set and the PWM duty cycle is set to 0 to stop the inverter operation, and the isolated counter variable is set to zero to stop the grid connection operation.
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