<p>As a reactive power compensation device, fault diagnosis of synchronous condensers is of great significance for ensuring the stable operation of UHV DC transmission systems. Traditional fault diagnosis methods based on vibration signals have limitations, and existing signal decomposition methods also have their respective shortcomings. This paper proposes a wide-domain feature decomposition (WDFD) method, which first performs preliminary mode decomposition of signals according to spectral trends, then calculates the wide-domain feature indicator of each mode to quantify fault information in both time and frequency domains, and finally uses envelope demodulation for fault diagnosis. Verifications through simulated signal and experimental signal of synchronous condenser show that WDFD can effectively extract the modulation pulse characteristics in the fault signal of condenser. Compared with traditional methods, it can more accurately realize fault diagnosis of synchronous condensers under complex interferences, which provides a new effective approach for fault diagnosis of synchronous condensers.</p>