Mechanism And Countermeasures Study On Ultra-Low Frequency Oscillation Of Yunnan Power Grid After Asynchronous Interconnection

After a long-term development,China Southern Power Grid(CSG)has formed a main structure with AC and DC parallel operation.The risk of blackouts in CSG is growing as a result of the interaction between AC and DC.In order to enhance the intrinsic safety performance of power grid,Yunnan power grid is asynchronously interconnected with the main grid of CSG.The risk of transient power angle instability has been greatly alieviated.However,a new problem of frequency instability has begun to emerge.On 28 March 2016,the frequency of Yunnan power grid has a continuous oscilaltion of twenty seconds,which is extremely rare in modern large power grids.This dissertation focuses on this issue,and the work is summarized as following:(1)This dissertation compares and analyzes power adjustment characteristics of speed regulating systems of hydro-electric generating set and thermal power unit.By analyzing damping effect of the speed regulating system on frequency oscillation of the power grid,it reveals mechanism of frequency oscillation of the power grid with high-proportion hydro-electric generating set.Corresponding conclusion indicates that in power grids with high-proportion hydro-electric generating set,the primary cause for frequency oscillation is negative damping effect of the speed regulating system of hydro-electric generating set due to influence of water hammer effect,and because of lack of positive damping support of thermal power unit,continuous frequency oscillation may happen to the power grid.(2)According to the analysis of PID control characteristics of governor,the overall configuration strategy of PID control parameters in power grid with high-proportion hydro-electric generating set is proposed.And the optimization scheme of governor parameters for hydropower units in Yunnan power grid is put forward.The frequency stability of power grid optimized by governor parameters is verified by simulation based on actual data in PSD-BPA.(3)The proposed optimization scheme has been tested and verified in the third practical asynchronous interconnection experiment.The experimental results under small and large disturbance show that the optimization scheme is effective.The scheme has been applied to the actual operation of Yunnan power grid,effectively ensuring the safe and stable operation of the power gird.