The present invention relates to islanding detection apparatus and a method using partial phase window monitoring scheme, which obtain state information on common power by measuring a variations in a grid-connected grid voltage, thereby preventing an islanding operation. To this end, the apparatus of the present invention comprises the following: a grid voltage sensor for obtaining waveform information of a grid voltage by sensing a grid-connected voltage of grid-connected common power; an A/D converter for converting signals of the measured grid voltage into digital data while monitoring an anti-islanding state of the common power by using partial phase window monitoring scheme on the grid voltage; a grid-connected inverter proportional integral (PI) current control unit; a grid-connected PWM inverter; and a grid-connected filter unit. Also, the apparatus calculates a difference between the grid voltage and a reference voltage to compare with a threshold value, and, if the difference is greater than or equal to the threshold value, increases an islanding counter value by one. If the islanding counter value is greater than 15, it is determined that an islanding operation occurs. When the islanding operation occurs, the apparatus stops an operation of the inverter by setting an islanding operation flag and setting a PWM duty cycle as zero, and stops a grid-connected operation by resetting an islanding counter variable to zero.
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