Abstract: There is a trip on 30 MW turbo-generator caused by loss of excitation for over excitation limiter failure in a thermal power plant.Firstly, the fault process is defined through the primary analysis of fault recorded data of the excitation regulator.Secondly, combining with field inspection, testing and comparison, it conducts further analysis from the aspects of over excitation limiter the causes of over excitation limiter action, limit effect, switching of channel and operation mode, coordination between low excitation limiter and loss of excitation protection.It is pointed out that although the fault is mainly caused by the short circuit between windings of the generator rotor, it also exposes the deficiencies in the design and operation management of the excitation regulator.Finally, some improvement measures has been proposed which provide a reference for optimizing the auxiliary limiting control strategy of the excitation regulator and perfecting the coordination between the auxiliary control ring and the voltage control main ring, and also has a reference significance for the operation management of the regulator.