Analysis and control on ultra low frequency oscillation at seeding end of UHVDC power system

When disturbance occurs in UHVDC (Ultra High Voltage Direct Current) isolated system, the frequency control is very difficult and frequency fluctuation will cause great security risk to the isolated system. The typical DC isolated system is set up and the frequency stability is studied. In this system, turbine governor has a great effect on system dynamic. It can cause ultra-low-frequency oscillation (ULFO) at UHVDC seeding end system. This phenomenon is verified by both filed tests and time domain simulation. Small signal eigenvalue analysis shows ULFO is strongly correlated with the turbine governor. The governor operation model and parameter can significantly change the eigenvalues´ position in complex plane, and then affect the damping ratio. The coordination and optimization of governor can improve the level of power system stability effectively. Further the impact of other factor is studied, such as the load characteristics, HVDC (High Voltage Direct Current) modulation and so on. An important conclusion is that, the parameters of HVDC modulation should coordinate with the primary frequency control (PFC) of turbine governor.