| As UHVDC technology is widely used in large capacity and long distance power transmission project,the grid space truss structure gradually change,the emergence of asynchronous networking and dc island operation mode,effectively avoid the original in the process of power grid operation caused by HVDC commutation failure or blocking system instability risk,but lead to hydropower accounts for high than ultra-low frequency oscillation accidents occurred frequently in the power grid,Serious threats to the safe and stable operation of power grid and dc mode arrangement.Hydropower installed capacity in the country’s energy structure reform,the background of increasing the energy and water and electricity transmission ratio increasing,so this article in view of the low frequency oscillation phenomenon,producing mechanism and restraining measures from two aspects carries on the thorough analysis,the results of the study to guarantee the safe and stable operation of power grid and hydro turbine regulation performance is of great significance.The main research contents and achievements of this paper can be summarized into the following three aspects:(1)according to the characteristic line method to quantify the unit pipeline characteristics,establish the refinement of hydro turbine model can accurately calculate flow rate and pressure of each node in the filling and emptying system,combining the actual running state of each boundary water and turbine characteristic curve,through the flow rate,rotational speed,contact head as the coupling parameter hydraulic system and mechanical properties.According to the data of a power station in China,the simulation results show that the average error of speed is 2.32%,and the average error of volute pressure is 0.43%,which meet the precision requirements of engineering and lay the model foundation for the accurate description of ultra-low frequency oscillation characteristics.(2)The variation mechanism of ultra-low frequency oscillation characteristics of the system with the pipeline characteristics of the unit,the initial operating conditions,the load distribution scheme of the two-unit system and the change of the operating head is revealed.The results show that the amplitude of ultra-low frequency oscillation increases with the decrease of initial output at 0.95 times of rated head,and the lower the risk of system instability is when the initial output is closer to the rated output at higher head.Secondly,when the load difference between the two units is greater than 20%,the unit with smaller load distribution will appear a amplitude greater than 3 times or even more than the speed deviation of the large oscillation,beyond the engineering specification requirements of the output oscillation limit;In addition,the damping coefficient of the system increases with the decrease of the total planned output of the unit when the unit adopts only shared tail-water piping arrangement,and the piping arrangement with shared diversion and tail-water has the greatest influence on the amplitude of ultra-low frequency oscillation.(3)The influence mechanism of dead zone limit and governor parameters as restraining measures on ultra-low frequency oscillation was explored,and a multi-objective optimization strategy based on particle swarm optimization was proposed to improve the response quality of the system.The results show that the system damping can be increased when the dead zone limiting link is added to the unit.Secondly,the system damping coefficient increases with the increase of the unit governor parameters,which can suppress the ultra-low frequency oscillation of the system to a certain extent.In addition,the multiobjective optimization strategy proposed in this paper increases the speed regulation time by41.8s and reduces the oscillation times by 2 times,which can improve the damping characteristics of the speed regulation system and improve the dynamic response performance of primary frequency modulation. |
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