| One hydroelectric power plant of Yunnan power network which has direct heavy load had been wired up, Result in temporarily form the "large units, small network" which is isolated network mode of the special operation, Ultra-low frequency oscillation phenomenon occurs when orphaned network is running. The active and reactive power of generator fluctuates greatly and threatens the safe and stable operation of the orphaned network.Aimed at the phenomenon of low frequency oscillation occurring in the operation of the orphaned network, The mathematical model of hydraulic, mechanical and electrical system of hydropower station is analyzed. Considering the interaction between them, we establish a water motor simulation analysis model which contains a model turbine, governor model, model of generator, excitation system model, PSS models and load models. Through MATLAB simulation, the cause of periodic fluctuation of generator active power and reactive power are analyzed. Through analysis, when the orphaned network operate, hydraulic system and generator interactions are main causes of ultra-low frequency oscillation, The operating condition of the long line and light load makes the generator under-excitation operation and also is a factor of the system damping weakening. The simulation results show that the problem of the ultra-low frequency oscillation in the orphaned network can be effectively suppressed by setting the PID parameter reasonably. |
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