Abstract: The safe and stable operation of ultra-high voltage direct current (UHVDC) transmission system can be significantly affected by open-circuit faults of thyristor-based synchronous condenser excitation system. To improve the reliability of synchronous condenser excitation system, the open-circuit faults should be detected timely and the faulty switches should be located accurately. Hence, an error-current-based diagnosis method was proposed for the open-circuit faults in thyristors of the synchronous condenser excitation system. In the proposed diagnosis method, the branch error currents were utilized, which has the advantages of fast detection speed and strong robustness. First, the branch error currents were calculated. Then, the fault detection variables were obtained by the error currents to detect the fault. Finally, the fault switches were judged by the fault location flags. This method can diagnose all 78 faults in single or double switches. The effectiveness of the proposed diagnosis method is verified by the experimental results.