Self-excitation of generators may occur due to the large capacitive currents of the long distance ultra-high voltage（UHV） transmission lines connected to a generator. It may cause severe overvoltage and consequently damage electric power equipments and insulators of the transmission lines. As such， its original cause， mechanism of development and methods of suppression must be carefully studied. In literature， generator self-excitation is generally considered as parametric resonance due to the cyclical variations of the parameters of the generator. This paper denotes that the original cause of the generator self-excitation is the positive feedback action due to the additive magnetizing effect of the capacitive current on the dynamic system consisting of generators， transformers and transmission lines. This standpoint is proved by using Routh stability criterion and digital simulation of an ideal non-salient pole generator， whose xd= xq ， with excitation circuit opened and the same damper windings on the direct and quadrature axis. The self-excitation also occurs in such conditions although there are no cyclical variations of the parameters of the generator. Besides， the theoretical study and digital simulation of self-excitation overvoltage due to load rejection at the end of the actual generator-UHV transmission-line system are also given. The feasibility of overvoltage suppression by inserting metal oxide arrester（MOA） at the moment of load rejection is proven as well.