Research On Search Method And Its Application For Cascading Failures In AC/DC Hybrid System

Blackouts are mostly caused by cascading failures.With the ever-increasing large-scale HVDC and HVAC transmission systems in China,the interconnection between regions has been strengthened and many regional grids show the AC-DC hybrid pattern,which makes the operating characteristics change significantly.Especially on the condition that the strength of AC system can not match the scale of DC transmission power,large-scale cascading failures are prone to occur.Compared with the pure AC system,the risks of cascading failures in AC/DC hybrid system increase dramatically.Aiming at the AC/DC hybrid system,this thesis studies event chain search model,correlation analysis,blocking control and expansion planning to explore the propagation mechanism,blackout model and prevention and control measures.The interaction and coupling between AC and DC,sending and receiving end is introduced.The operation risks in AC/DC hybrid system are analyzed and the evolutionary form of cascading failures is summarized.In addition,the optimal load shedding model after DC blocking considering transient security is proposed.Taking the DC systems in Shandong grid as examples,the security control measures after DC blocking are formulated with this model to provide the basis for loss quantification in the search process of cascading failures.An event chain search model is presented for fast contingency screening in AC/DC hybrid system,which includes alternating AC faults search module and DC faults judgement module.In the AC faults search module,the probability index of branch outage,the influence index on operation of DC systems and the structural importance index are defined and the search strategy which is based on outage risk index after preliminary screening by outage probability index is put forward.In the DC faults judgement module,short-circuit faults and overload trips in the AC system are set apart to consider the interactions with DC systems respectively,which can cover the evolutionary patterns of DC blocking more comprehensively.Two modules work alternately to form the complete event chain search process.Simulation results on Shandong grid demonstrate that the event chain search model can search the fault paths effectively in which DC systems are out of operation and reduce the omission of high-risk cascading failures,which is suitable for contingency screening.An Apriori algorithm considering item order and transaction weight is proposed to analyze the correlation between failures,with which we can mine strong association rules that threaten system security from the macro perspective.For the key association rule,a multi-resource coordination blocking control model is established by taking control cost,comprehensive index of power flow in AC system and weighted multi-infeed short circuit ratio as objective functions and taking DC power emergency modulation,output adjustment of generators and load shedding as control means.Simulation results on Shandong grid demonstrate that the correlation analysis method can identify the strong association rules which endanger system security,and the blocking control model can realize the coordination control between AC and DC systems and improve operation status,which prevents failures from spreading further.A multi-stage joint expansion planning method for generation and transmission considering the risk of cascading failures is put forward to reduce the risk of cascading failures in the medium-long term from the planning perspective.Taking life cycle cost and average risk values of cascading failures in the extreme operating modes as economic and reliability objective functions and taking each year in the research cycle as each stage,this thesis establishes a multi-objective and multi-stage joint expansion planning model,which is sovled by NSGA-II algorithm.Simulation results on Shandong grid demonstrate that by solving the model we can get Pareto solution set and the recommended solution.The structure of planning system is strong and reasonable,which can effectively improve the ability to resist cascading failures in the medium-long term under the premise of ensuring economic efficiency.