Research On Dynamic Model Of The Hydropower Unit For Fast Frequency Response

In recent years,China's power grid has vigorously developed UHV transmission,and the voltage level and scale of the power grid have increased year by year.While realizing large-scale,long-distance transmission and large-scale optimal allocation of energy,DC blocking faults often occur,resulting in high-power loss accidents,which can lead to a sharp reduction in system frequency and increase the probability of low-frequency load shedding accidents.As the first line of defense for frequency control,fast frequency response is the key measure to prevent the frequency from dropping sharply in the absence of high power.However,a large number of new energy sources such as wind power in the system are lack of frequency response ability,and occupy the generating space of conventional units,which results in the decline of frequency response ability of the system.Therefore,it is necessary to rationally supplement and allocate the fast frequency response resources of the system.The primary problem of allocating system fast frequency response resources is to accurately analyze the operation status of conventional units and their frequency modulation capability.In the previous analysis of unit frequency response capability,the static viewpoint is usually used to describe the state and FM capability of the unit,that is to say,the frequency characteristic coefficient of the unit is a fixed value and does not change with the external working conditions.However,under different operating conditions,the unit's real frequency response capability is deviated from that described in its static viewpoint,which results in inaccurate description of the system's frequency response capability in the static viewpoint.This inaccuracy is acceptable when the frequency response capability margin of the system is large.But when the margin is critical,the deviation will cause a bigger problem.That is,when the frequency response ability of the system is tense in the absence of high power,it is difficult to describe the frequency response ability of the unit accurately from a static point of view,which is not conducive to the accurate estimation of the frequency response ability of the system,and will also affect the reasonable allocation of the frequency response reserve of the system.Therefore,it is necessary to describe the frequency response capability of the unit from a dynamic point of view.Based on the analysis of the existing static model of hydropower units,this paper studies the full parameter model of hydropower units,and points out that the calculation of transfer coefficient is complex and difficult to obtain,which is not suitable for the study of dynamic frequency response characteristics.Aiming at the requirement of fast frequency response of hydropower units under high power loss,an improved model of hydropower units reflecting the dynamic effects of head height and load size is established by deducing relevant formulas,starting from some simplified conditions of operation characteristics of turbines and considering the influence of external parameters of turbines.The model can reflect the dynamic frequency response characteristics of hydropower units relatively quickly and accurately,and is suitable for dynamic analysis of fast frequency response of hydropower units.On this basis,the analytic expression of dynamic frequency response characteristic coefficient of hydropower units is derived,which can directly describe the dynamic change of transient frequency stability value of fast frequency response of hydropower units with external conditions.The simulation example shows that the dynamic model can not only avoid the complex problem of calculating the transfer coefficient of the full parameter model,but also describe the dynamic characteristics of the fast frequency response of hydropower units relatively accurately.Applying this model to the related simulation calculation of fast frequency response,the dynamic frequency response capability of hydropower units can be accurately estimated according to the different external parameters of the units,which provides a basis for the rational allocation of frequency response resources.