This paper presents a self-adaptive compensated differential protection of AC synchronous generators， a particular type of high-voltage graded insulated cable-wound generator， Powerformer. The phase voltages at the terminals are utilized to estimate the capacitive currents when the generator is synchronized and no fault is detected. With the calculated capacitive current the“suppositional” capacitance distribution coefficient along with the winding of the generator can be determined. This capacitance will be equivalently arranged“suppositionally” on the terminal and neutral of the generator respectively. Then， the capacitive current in the case of normal operation， external fault and internal fault can be calculated using the terminal voltage and the determined capacitance. It can be compensated to improve the reliability of the differential protection. The feasibility and effectiveness of the proposed scheme is demonstrated with the simulation test results.