detecting a state of the insulator of the stator winding of the generator using a theory of the cross capacitance; and

carrying out the water absorption test of the insulator of the stator winding of the generator using the state detected in the detecting wherein the detecting comprises measuring the capacitance and the dielectric tangent of the insulator of the stator winding of the generator according to thickness; wherein the measuring, when a sensor includes a single electrode, a double electrode, and a quadruple electrode, while the electrodes are arranged in the planar types and the measurement for the insulator of the stator winding of the power generator is carried out according to the thickness, firstly the sensor becomes the single electrode and measures the capacitance corresponding a thickness caused by the single electrode, next the sensor becomes the double electrode and measures the capacitance corresponding to a thickness caused by the double electrode, and finally the sensor becomes the quadruple electrode and measures the capacitance corresponding to a thickness caused by the quadruple electrode, so that the state of the insulator of the stator winding of the generator is detected.