Abstract: To improve the accuracy of protection action analysis and maintenance response after excitation system faults in a 350 MW combined heat and power unit. Method: Based on GB/T 14285—2023, fault recording sequences, setting checks, switching plates and secondary circuits were integrated into a process of fault signal, protection criterion, trip logic and verification. Result: Identification rules for loss of excitation, over-excitation and excitation channel abnormalities were formed. Conclusion: The method is applicable to generator-transformer protection testing during maintenance and rapid handling of abnormal operation.