Simulation and optimization of load shedding scheme for islanded power system

The effect on system stability of load shedding has been a popular research topic for many years, especially in an islanded power system. But seldom studies gave full considerations to the actions of Primary Frequency Regulations (PFRs) and Overspeed Protection Control (OPC) systems of the generation units, which may have great impacts on the system responses against disturbances or emergency situations. When accidents occurred in the islanded power system, the power between generator units and load will be unbalanced. The rotating speed of steam turbine will clearly drop (rise) to a lower (higher) level. That may cause the action of load shedding control system (OPC of the generation units) to ensure the stability of the whole system. In this paper, a frequency response model incorporating an under-frequency load-shedding scheme is presented for islanded power system simulations and it is applied for a practical power system with both hydro and fossil fired power generators. Load shedding controls and OPCs are involved in the mathematic model. Some useful conclusions are obtained from the simulation results, based on which the mathematic model of the power system has been optimized in latter sections. Reasonable agreements are observed between the simulation results and the accident records. The simulation results also show that an optimized load shedding scheme may lead to better system stability which means the frequency oscillation will be reduced. When the PFR of hydropower isn't operated during the speed governing process, the frequency response of the whole system will be much better. It also suggests that if more electric power is generated by fossil fired turbine units, the system could get an advantageous frequency response.

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