Research On The Primary Frequency Regulation Ability Of The Sending Grid Of Asynchronous System

Asynchronous operation mode can effectively solve the overload risk of power flow transfer in AC-DC hybrid power grid.However,there is no standard to measure whether asynchronous operation is needed in AC-DC hybrid power grid.Based on the consideration of economy and stability,this paper puts forward the condition of AC-DC hybrid power grid transforming to asynchronous power grid,that is,when the utilization ratio of AC tie line is too low and the short-circuit current at the tie line landing point is too high,the AC-DC hybrid power grid is suitable for decoupled asynchronous operation,so as to eliminate the risk of power flow transfer.After asynchronous operation,the size of sending grid becomes smaller and the system stability decreases.In order to evaluate the change of frequency regulation capability before and after asynchronous operation of sending grid,a quantitative calculation method of primary frequency regulation capability of sending grid is proposed based on the theory of stochastic process.The results obtained by this method show that the ability of primary frequency modulation is obviously reduced and the frequency fluctuation is increased after being disturbed.In order to improve the frequency response of the sending grid,DC frequency modulation can be introduced as a new frequency modulation resource of the sending grid.Based on the calculation of the primary frequency regulation ability of the sending grid of the interconnected system of the last section,the mathematical model of the DC frequency modulation link is established,and the nonlinear link such as the amplitude limiting of the DC modulation device is considered.The analytical expression for calculating the primary frequency regulation capacity is further optimized,so that it can better reflect the primary frequency regulation ability of the sending grid after introducing the DC frequency modulation.The results show that the DC frequency modulation improves the primary frequency regulation ability of sending grid.PID controller is used in traditional DC frequency modulation,which needs a lot of parameter tuning work to optimize the control effect when the load changes suddenly.The model predictive control can predict the output of the system at the next moment according to the existing state and input of the system,and omit the parameter setting link.In this paper,the model predictive control is used to replace the traditional PID controller.Under the condition of DC block and steep load rise,the control effect of PID and model predictive is compared.The results show that the model predictive control response is faster and the frequency fluctuation amplitude is smaller.