Research Of Distributed State Estimation And Power Flow Calculation For Interconnected AC/DC Power Grid

The distribution of power resources and load demand in China is reverse,so long-distance and cross-region power transmission are needed to make rational use of power resources.Due to many technical advantages in long-distance power transmission,high voltage direct current(HVDC)technology has become an important choice for cross-region power transmission.In recent years,the HVDC projects which have been put into operation in succession have deepened the degree of grid interconnection and formed a large-scale AC/DC hybrid power grid.The AC/DC power grid is physically interconnected as a whole,but its control and management are hierarchical and decentralized,there are contradictions between them.At present,the energy management system(EMS)of each regional power grid mainly adopts the independent calculation mode,so it is unable to obtain the accurate information of the external network,and the correctness of analysis and calculation of the power system executed by the EMS cannot be guaranteed.If the distributed computing mode is adopted for computing,each subsystem can exchange a small amount of information and get the same results as the centralized calculation,which is an ideal choice.Taking state estimation and power flow calculation,the important application software of EMS system,as an example,this paper studies the distributed calculation method of AC/DC interconnected power grid with LCC(line commutated converter)-HVDC as tie-line.(1)The distributed state estimation algorithm of the AC/DC system based on equivalent information exchange is realized.the algorithm consists of two main parts:the distributed state estimation of the AC system and the state estimation of the subsystem’s DC system.Firstly,based on the bilinear state estimation,a calculation coordination mechanism based on equivalent information exchange is established.Each subsystem calculates and uploads its equivalent information to the coordinator,and receives boundary states from the coordinator to calculate internal state estimates.When the distributed state estimation of the AC system is completed,each subsystem performs DC system state estimation based on PCC voltage magnitude estimates.The AC system state estimation and DC system state estimation are carried out alternately,and the results are consistent with centralized calculation.For the problem of bad data processing,based on the principle of equivalent weight,the robust estimation model of the exponential objective function is transformed into the least square estimation with variable weight to be solved,and the weight information is condensed and transferred in the equivalent information to realize the distributed robust estimation algorithm.(2)Based on the previous work,the sequential solution of the AC/DC system and the fixed-point iteration of boundary bus states are effectively combined,and the fixed-point iteration scheme of the AC/DC system is constructed to realize the distributed power flow calculation.The proposed algorithm does not need complex combination calculation of discrete variables of DC system,so it is convenient to realize converter transformer tap adjustment and control mode conversion of DC system.Also,the unbalanced power distribution method is improved.The improved unbalanced power distribution method is more in line with the reality,and because the number of subsystems participating in the unbalanced power distribution is decreased,the method has better convergence than the previous work.