Research On Operational Flexibility Of AC-DC Power Systems Considering The Uncertainty Of High Penetration Renewable Energy

With the increasing penetration of large-scale of Renewable Energy Resource(RES),its natural uncertainty brings challenges to the operation and dispatch for modern power systems.Meanwhile,High Voltage Direct Current(HVDC)system is adopted to accommodate the bulk power of renewable energy with long distance.As a result,the low inertia and the complex operational mechanism induced by the formation of AC-DC framework also bring more risks to the power systems.Therefore,it is necessary to fully consider the regulation characteristics of the HVDC system,to device a more adaptable and foresighted method to depict the output distribution of RES,and to construct an efficient,complementary and flexible scheduling strategy through the coordination between dispatch resources in DC sides and AC sides.It is of great significance for the operators to realize the RES accommodation and ensure the stability when pursuing the economic benefits of power systems.As a result,in this paper,the output distribution characteristics of RES is concentrated and the novel structure of operational schedule for AC-DC system is designed,when fully focusing the controllability of AC-DC dispatch framework,the flexible and fast HVDC regulation is exploited;the trade-off between model robustness and economic benefits of dispatch schedule is optimized.The research concentrations of this paper are listed as follow:(1)To overcome the insufficient ramping rates of conventional synchronous generators,and relieve the contradiction between model reliability and economic benefits,a novel two-stage unit commitment model for AC-DC system is proposed.The model utilizes the flexible power transmission capabilities of HVDC system and enhance the operational flexibility of dispatch model.Different from the idea of using the operating cost as an indicator to determine extreme scenarios in the traditional model,this model starts from the basic prediction scenario(base case),combines the real-time cross-regional power regulation function of HVDC,and returns the response strategy of the AC-DC system under the current extreme scenario to the first stage.The model robustness constraints are progressively enhanced during the feedback.The model makes full use of the real-time modulation characteristics of HVDC,and optimally allocates the adjustment tasks undertaken by day-ahead scheduling and realtime dispatch.Therefore,the optimal trade-off between system reliability and economic benefits is obtained by the proposed model.(2)In order to ensure the frequency stability of the AC/DC system under extreme faults,and at the same time maintain the economic benefits of the system operation,this paper proposes a grid resilience dispatch model considering frequency coordination control.The model firstly designs a frequency emergency control strategy combining the short-term overload capacity of the HVDC lines;then,considering the constraints of the power decrease rate when the HVDC system exits the overload state,the frequency response model of the AC-DC system is deduced on the basis of the coordination of the AC-DC frequency response resources.After system re-entering the steady state,the model economically dispatches the optimal operating point of the ACDC system,reducing the impact on the economic operation as much as possible on the basis of ensuring sufficient dispatching resources.The model is divided into three stages: day-ahead scheduling,emergency frequency control,and steady-state economic rescheduling.Considering the constraints characteristics in this model,a method combining Column-and-constraint generation(C&CG)and Benders Decomposition(BD)algorithm is developed to solve the problem.The method involves a three-layer structure and is solved iteratively.By comparing with the conventional model,the results of the case study prove that the frequency coordination scheduling model improves the operational flexibility based on the HVDC regulation,and reduces the impact on the economic operation of the system on the basis of ensuring the frequency stability of the system.(3)Considering the problem of power flow congestion caused by the large-scale fluctuation of RES,in order to improve the adaptability of the uncertainty set in the dispatch model to the real-time RES output scenarios,this paper proposes a robust dispatch model for managing power flow congestion.The model obtains the possible power flow blocking conditions and the characteristics of the corresponding AC-DC modulation strategy through the test scene set,and then corrects the upper and lower bounds of uncertainty set,and specifically eliminates the low-probability extreme scenarios from it.Therefore,this model builds a more adaptive new energy output characterization method,and adjusts the ability of the day-ahead plan to manage network congestion in the real-time scheduling process by modifying the uncertainty set.Applying this model to the improved AC-DC hybrid system,it can be proved by analyzing the operation results that in the actual operation process,the low probability extreme uncertainty scenario will block the backup transmission channel,and this model solves the problem through the flexible HVDC real-time modulation function.This kind of problem can be solved,so the economic cost of operation can be reduced on the basis of ensuring the stability of the system operation.(4)In order to handle the over-conservativeness of traditional robust models in describing the output range of new energy,and to study the quantification method of system operation risk,this paper proposes an adjustable uncertainty set dispatch model based on wind power out-of-limit risk.On the basis of ensuring the strict feasibility of the model,this paper quantifies the operation risk of the dispatch strategy and construct a rapid deployment strategy,in combination with the out-of-limit distribution characteristics of renewable energy.Through the analysis of the HVDC interconnected system,it can be proved that the model can adjust the upper and lower bounds of the uncertainty set based on the distribution characteristics of RES,so as to control the over-conservativeness of the robust dispatching strategy;The HVDC regulation improves the adjustment margin of dispatching resources in the process of model solving.It can be proved from the relationship between the uncertainty set and the probability distribution of renewable energy that the HVDC regulation is activated in extreme scenarios with small probability,which ensures the model operational flexibility.