The traditional method for analyzing the mechanism of stator winding inter turn short-circuit faults of synchronous condensers usually assumes that the stator current of the motor is close to three-phase symmetry, and based on this, a mathematical representation of the fault current of synchronous condensers is established. However, once a stator winding inter turn short-circuit fault occurs, the symmetry of the three-phase stator current of the synchronous condenser will be disrupted, making the mathematical representation established by traditional fault mechanism analysis methods unable to accurately reflect the changes in the internal electrical quantities of the motor. A mathematical model is established for the instantaneous active and reactive power of a synchronous condenser after a fault by introducing the symmetrical component method. Using the second harmonic in the instantaneous active and reactive power for stator winding inter turn short-circuit fault diagnosis is proposed. The simulation and experimental results indicate that compared to the traditional method of diagnosing faults using the ratio of the third harmonic of stator current to the amplitude of the fundamental wave, the proposed method can improve the diagnostic sensitivity by at least seven times under mild fault conditions, making it easy to complete early fault diagnosis. At the same time, the fault feature quantities in the proposed method are not affected by the synchronous condenser operating conditions and fault locations, and also have strong robustness.