Research On New Strategies For Control And Protection Of AC/DC Hybrid Distribution Network

With the extensive development and application of renewable energy,the proportion of new energy sources such as wind and solar energy in the energy structure has continued to increase.On the load side,there are more and more DC loads such as electric vehicles and inverter air conditioners,which has led to a new form of the load structure of the power system.Under the above background,the traditional distribution network structure is gradually moving from AC distribution network to AC/DC hybrid distribution network.The AC distribution network side retains the technical advantages of the traditional distribution network,while the DC distribution network side has the advantages of high transmission efficiency and facilitates distributed generation(DG)access,which has become an important development direction of the future power grid.In order to ensure the stability and safety of the AC/DC hybrid distribution network,research on its control strategy and line protection technology under faults is the current focus of the industry.The AC distribution network and the DC distribution network are connected through a converter station.When a fault occurs on the AC distribution network side,the converter station works in the static var generator(SVG)mode during the voltage drop on the AC network side.Inject reactive power to the AC side.Due to the limitation of its own capacity,it cannot take into account the power balance demand in the DC distribution network.At this time,it is necessary to switch the operation mode of each unit in the DC distribution network.These mode switching processes will inevitably bring about economic losses.How to balance the stability and economy of the system at the same time is an unsolved problem.When a fault occurs on the DC distribution network side,the DC side line protection should have high reliability and sensitivity.However,the current development of DC distribution line protection technology is still immature,and there are often contradictions in selectivity and sensitivity.In response to the above discussion,the specific work and achievements of this article are as follows:First,the AC-DC hybrid distribution network structure and equipment parameters studied in this paper are introduced,and a transient analysis model of the AC-DC hybrid distribution network is built based on the PSCAD/EMTDC simulation platform.On this basis,the principle of reactive power compensation strategy in SVG mode of converter station is introduced,and the adaptability of traditional low voltage ride through control strategy in this distribution network,and the sensitivity of the traditional differential under-voltage protection and high frequency energy protection on the distribution line are analyzed.Secondly,aiming at the contradiction between supporting the AC side voltage and participating in the active power balance of the DC side in the SVG mode of the grid-connected converter station,a dual-objective optimization control strategy based on NSGA-Ⅱ is proposed.This strategy first quantitatively analyzes the reactive power support efficiency of the converter station and the overall loss of the DC distribution network during the AC side failure,and then combines the operation principle of the DC distribution network system and the constraint conditions of the working limits of each component to establish the dual-objective optimization model considering both stability and economy.The NSGA-Ⅱ multi-objective genetic algorithm is used to solve this problem.The analysis results verify the rationality and effectiveness of the optimization model.Finally,aiming at the problem of poor sensitivity of existing protection methods for flexible DC distribution networks to high-resistance faults,a highly sensitive fault identification strategy for flexible DC distribution networks based on Deep Belief Networks(DBN)is proposed.This strategy uses the feature extraction capability of the DBN network to extract features from the time-domain waveform data of the positive and negative voltage transients for fault identification.Use simulation to collect sample data to train the DBN network,obtain the optimal parameters of the model,and form a DBN model suitable for DC distribution network fault identification.The model performance is tested based on the PSCAD/EMTDC simulation platform.The test results show that the proposed strategy has good sensitivity in high-impedance fault identification and has strong generalization ability.