Research On Multi-level Control Technology Of AC/DC Hybrid Microgrid Cluster

Driven by the "30·60" strategic goal,the penetration rate of distributed power generations represented by photovoltaics and wind power continues to increase.Because of its flexible operation mode,microgrid can effectively play the complementary advantages of multiple types of generations,loads and storage resources,and has become an important way for distributed power grid-connected consumption.However,a single microgrid has disadvantages such as limited working capacity and weak anti-disturbance ability,which can only meet the access requirements of distributed power generations in local areas.If multiple AC and DC microgrids are interconnected to form an AC/DC hybrid microgrid cluster,a large number of small-scale assets can be aggregated to form a large number of product-use-storage integration aggregates,so as to achieve hierarchical management and distributed collaboration of distributed resources.Hence,AC/DC hybrid microgrid cluster is an important direction of microgrid development.However,the AC/DC hybrid microgrid cluster has a complex system structure and numerous control objects,and its stable operation highly depends on the normal operation of converter devices,it also faces the power balance problem caused by random fluctuation of distributed power generation,which brings many challenges to its operation and control.In view of this,this paper starts with the control technology of microgrid converter,the source-load interaction technology of microgrid,and the collaborative control technology of AC/DC hybrid microgrid cluster,so as to carry out the research on multi-level control technology of AC/DC hybrid microgrid cluster.The research can provide theoretical and technical support for the stable operation and economic control of DC hybrid microgrid clusters.The main contents are as follows:(1)For the converter control problem when the distributed power generation is connected through parallel converters,a microgrid parallel AC/DC converter operation characteristic analysis method and compensation control strategy is proposed,which provides support for grid-connected consumption of large-scale distributed power supply.Firstly,the closed-loop path and distribution characteristics of zero-sequence circulation are analyzed according to the switch function model of parallel converter.Secondly,the characteristics of zero-sequence circulation and dq disturbance current of parallel converter with unbalanced input resistance at DC side and unbalanced output impedance at AC side were deduced respectively.According to the generation mechanism,the disturbance caused by unbalanced conditions was divided into primary disturbance and secondary disturbance.Finally,a disturbance compensation control strategy is proposed to effectively eliminate the zero-sequence circulating current and DC side current ripple caused by dq disturbance current.Simulation results verify the effectiveness of the proposed zero-sequence circulation analysis method and compensation control strategy.(2)For the converter control problem when AC and DC sub-microgrids are interconnected through AC/DC converters,a model predictive control strategy for interlinking converters is proposed.Firstly,according to the mathematical model of interlinking converter,the operation characteristics and voltage control equations under the influence of unbalanced grid conditions and asymmetric filter inductance are derived.Secondly,considering asymmetric filter inductance and disturbance power compensation,a model predictive direct power control strategy was proposed to achieve accurate power control of the bidirectional converter,and effectively suppress the DC current ripple caused by the double frequency disturbance power while maintaining the good sinusoidal characteristics of the AC current.Finally,in order to improve the adaptability of control strategy to mismatched filter inductance,an online identification method of asymmetric filter inductance is proposed to provide accurate model parameters for the model predictive control.Simulation results verify the effectiveness of the proposed control strategy and the online identification method of filter inductance.(3)For the power fluctuation problem of microgrid caused by the distributed power generations,a sub-microgrid power fluctuation suppression method considering the participation of thermostatically controlled load was proposed to provide support for the power balance of AC/DC hybrid microgrid cluster.Firstly,taking inverter air conditioner as an example,the equivalent generation model based on temperature regulation optimization and the equivalent energy storage model based on direct power control are constructed.Secondly,in terms of temperature control,the dynamic process of inverter air conditioners responding to target temperature was considered,and the optimal setting of temperature command was realized by means of cluster optimization.On this basis,the sequential operation strategy of temperature command was further studied to alleviate the power impact caused by the simultaneous operation of a large number of inverter air conditioners.Then,in terms of direct power control,considering the state difference between each unit,the fast power control of inverter air conditioners cluster is realized by cluster optimization and independent response.Finally,by combining the advantages of these two control modes,the multi-time scale control of inverter air conditioner cluster is realized,and the control strategy is applied to suppress the power fluctuation of microgrid.Simulation results verify the correctness and effectiveness of the proposed control strategy.(4)For the autonomous operation and cooperative control of AC and DC submicrogrids,a hierarchical distributed control strategy of AC/DC hybrid microgrid cluster is proposed,which can realize the autonomous economic control of AC/DC hybrid microgrid cluster and "plug and play" of distributed power supply.Firstly,the control system of AC/DC hybrid microgrid cluster is divided into sub-microgrid control and microgrid cluster control based on "two-level and three-layer control architecture".Secondly,at the level of sub-micro grid control,by introducing the secondary adjustment items of cost,frequency,voltage and reactive power distribution into the traditional economic droop control,the distributed control principle is adopted to realize the autonomous stability and economic power distribution of the sub-microgrid.Then,at the level of microgrid cluster control of microgrid,the local control strategy of interlinking converters based on the deviation of cost increment is constructed,and the second adjustment term based on discrete consistency is introduced to realize the economic power distribution between different sub-microgrids.Finally,the hierarchical and distributed autonomous economic control of AC/DC hybrid microgrid cluster is realized through the coordination of these two control strategies.Based on the simulation results of the established AC/DC hybrid microgrid cluster model,the effectiveness of the proposed control strategy is verified.