Group hydropower plants may form an isolated grid with the outgoing of their transmission line. In this special situation, the characteristics of high-frequency and over-voltage of the isolated hydropower group have great influence on the self-excitation boundary conditions and transient control of the hydropower generators. This paper studies the self-excitation characteristics of generators in isolated hydropower group. The emphasis is placed on the great influence on self-excitation boundary conditions and transient process by the characteristics of high-frequency and over-voltage of the isolated hydropower group. To avoid self-excitation, two practical criterions are derived respectively in the form of impedance and capacity. Furthermore, simulation of the self-excitation transient process is carried out in an isolated hydropower group in Sichuan Power Grid. The results validate the conclusions of the research.
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