Stability of Hydro Power Plants in Island Operation
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- Institutt for elkraftteknikk 
In this thesis, the stability of hydro power plant in island operation is studied. The thesis is conducted in three prospective. First, the hydro power plant power system model is built.hydraulic turbine and governing system, exciting system and synchronous generator are studied in a theoretical way. There are two types of hydraulic turbine introducing in the thesis.One is simple linear turbine and the other is nonlinear turbine. The comparison between these two turbine is made. The results shows that nonlinear turbine has better performance. Further the mechanical-hydraulic governor and PID governor are presented in the thesis.The working principle of automatic voltage regulator and IEEE PSS2A stabilizer is illustrated. Second order generator model and third order generator model are shown in the thesis and the detailed derivation process is presented. Based on the theory above, hydro power plant power system model is built in PowerFactory and simulink. There are four types of hydro power plant model: simple hydraulic power plant model in simulink, complex hydraulic power plant model in simulink, single generation power system in PowerFactory, two generation power system in PowerFactory. Through comparison between the simulink models and PowerFactory model, the simulink model is validated. It proves that simple hydraulic power plant model is not suitable applying in island operation. The complex hydraulic power plant model predicts much precise dynamics than simple hydraulic power plant model. Second, characteristics of hydraulic governor are analysed in the island operation. Sensitivity analysis regarding the temporary droop, reset time and permanent droop is executed. It concludes that increment of the temporary droop and reset time will eliminate the oscillations, however, the response time of the system will be slow.In addition, the governor characteristics in both transient state and steady state are explored. The results shows that the governor characteristic are both linear in terms of the island operation. Third, the effect of power system stabilizer on power system stability are studied. In order to explore the influence of the PSS, a three-phase short circuit at a bus which is close to the load is defined. First, a three phase short circuit test is applied in grid-connected operation, which aims to illustrates that power system stabilizer has positive effect on the generator in response to the disturbances. After validation, a three phase short circuit is made at the same bus with the grid-connected operation in island operation. There are ten cases being analyzed in this thesis. By analyzing the power system stabilizer in different cases, it is concluded that the power system stabilizer has positive effect on both grid-connected mode and island mode. Furthermore, the parameters of the PSS has huge impact on the behavior of the power system. Inappropriate parameters could worsen the performance of the power system regarding to the small disturbances.