Research On Fast Frequency Response Control Strategy Of Power Grid Considering Emergency Dc Power Support

With the gradual increase in the proportion of new energy sources in grids and the increasing number of HVDC transmission projects,the low inertia characteristics of grids are beginning to appear.When a large disturbance event occurs,the system frequency changes rapidly,and it is prone to problems such as frequency instability or collapse leading to large-scale power outages.The fast frequency response resource can quickly provide active power support for the system and improve the frequency response characteristics of the system under large disturbances.Therefore,this thesis uses the fast frequency response resource of HVDC transmission to study the method of controlling the steady-state frequency after disturbance to the target value for the system connected to multiple VSC-HVDC,and minimize the impact on interconnected grids;For asynchronous interconnected power grids,the method of controlling the frequency nadir and steady-state frequency after disturbance to target values is studied to use the reserve capacity of interconnected areas effectively.The main research contents of this paper are as follows:This paper improves the existing steady-state frequency prediction method in grid connected to VSC-HVDC.From the perspective of power flow and node power characteristics,respectively,list the power increment equations at the moment after disturbance and steady state,and introduce mechanical damping into the generator node power increment equation.Comparing the simulation results with the existing research,it is verified that the method can improve the prediction accuracy of the steady-state frequency after the system is disturbed.Aiming at system connected to multiple VSC-HVDC,based on the steady-state frequency prediction method that takes into account the multi-HVDC emergency power support,an optimization model of the multi-HVDC emergency power support is established.The objective function of the model can more reasonably weigh the impact of power support on different interconnection areas,and the constraint conditions can take into account the control objective of the steady-state frequency of the disturbance area.Solving the model by Differential Evolution algorithm can solve the problem of supporting power distribution on multiple HVDC transmission lines.Compared with the objective function of existing research,it is verified that this strategy can restore the steady-state frequency to the target value,and the amount of support borne by the interconnected area is more even.For the HVDC asynchronous interconnected power grid,a short-term constant power support optimization model is proposed.The model takes the minimum support energy as the optimization goal,and takes the frequency nadir control to the target value as the main constraint.Through the Differential Evolution algorithm,the optimal set of short-term power support amount,support start time and support duration can be obtained.As a comparison,this thesis establishes an optimal control model for variable power support.Under the same objective function,it is solved by Gaussian pseudo-spectrum method to obtain the optimal power support curve.Analyzing two different control strategies,it is believed that short-term constant power emergency power support can replace variable power optimal control,which can effectively use the energy provided by the interconnected grid to control the frequency nadir of the disturbed grid to the set target value.Integrating the steady-state frequency control strategy and the short-term constant power emergency power support control strategy,a long short-term emergency power support coordinated control strategy is proposed.This thesis designs the frequency stability analysis and control process after disturbance,using long-term emergency power support to control the steady-state frequency to the target value,and using short-term constant power emergency power support to control the frequency nadir to the target value.By comparing with only long-term emergency power support,the superiority of the proposed long short-term coordination strategy is verified.While realizing emergency frequency control,it reduces energy consumption and reduce the impact on interconnected regional power grids.