Coordinated Control For Hybrid AC/DC Microgrids With Multiple Subgrids

With the social and economic development,the power system is undergoing profound reforms.Since there are plenty of distributed generations(DGs),the conventional unidirectional generation-transmission-distribution power consumption has been changed,especially on the distribution side,the bidirectional power flow and the coexistence of loads and generations form new requirements for the existing regulation,relay protection and so on.At the same time,large numbers of dc DGs and do loads cause some new challenges for the traditional pure ac power supply.The hybrid ac/dc microgrid can integrate ac/dc subgrids effectively,in which the ac/dc DGs and loads can be allocated orderly.In this way,it can exploit the prominent features of both ac and dc power supply,and then has high conversion efficiency,flexibility and reliability.Therefore,the hybrid ac/dc microgird is an effective way to integrate ac/dc DGs and loads for the modern power system.However,the conventional hybrid ac/dc microgrid with single ac subgrid and single dc subgrid only has the unique dc voltage and ac frequency,which cannot integrate various loads and DGs effectively.At the same time,the ac/dc subgrids cannot be split divisionally when there are faults in the system.These drawbacks will greatly limit the application of hybrid ac/dc microgrids.To solve these problems,this thesis proposes a hybrid ac/dc microgrid with multiple subgrids and focuses on its primary level control.According to the clues from system stability,to critical devices and finally to system regulation,the comprehensive and in-depth research is conducted from the stability analysis of the grid-connected DGs,circulating currents suppression among interlinking converters and system power coordinated control these three aspects,which include major parts about the primary level control for the hybrid ac/dc microgrids with multiple subgrids.In field of the control for the hybrid ac/dc microgrids,we have obtained some original research results.Furthermore,some of these technologies are applied to the practical national demonstration project.First,focusing on the stability issue of the grid-connected DGs,this paper proposes a stability analysis method based on the describing function to analyze the stablizing and destablizing mechnaism of the grid-connected DGs,which can provide effective guidance for the design of DGs and ensure the system stability.When the DGs are connected to the weak grid,the power oscillation is easy to occur,which greatly influences the utilization of renewable energy sources.The conventional stability analysis is based on the small-signal model,which is a kind of pure linear analysis methods.However,the pure linear analysis methods cannot consider some essential nonlinear elements(like amplitude limiting,hysteresis and so on),have difficulty to analyze the critical stability,cannot evaluate the system stability based on the stable margin for the non-minimum phase system and so on.Hence,the obtained results are incomplete and inaccurate,which cannot provide accurate guidance for the paramter design.For this problem,this paper adopts describing function method to analyze the stability of the grid-connected DGs.Compared to the conventional pure linear analysis methods,the desccribing function method can take the essential nonlinear elements into consideration,then some nonlinear links like the perturbation&observation based power control loop can be considered.In this way,the completeness of the system model can be enhanced.In addition,through the describing function method,the stabilty evaluation does not depend on the stable margin,the oscillation amplitude and frequency can be calculated out accurately.Hence,the accuracy about the stability analysis can be improved.Based on the more complete and accurate results,the instability mechanism when DGs are connected to the weak grid can be well explained,which can provide better guidance for the paramter design.Second,focusing on the circulating currents among interlinking converters that are the critical devices in the hybrid ac/dc microgrids with multiple subgrids,this paper proposes a two-degrees-of-freedom suppression strategy,which can suppress the circulating currents effectively and ensure the reliable operation of interlinking convertes.Different ac and dc subgrids are connected with each other through the interlinking converters,thus the stable operation of interlinking converters is critical for the whole system.Nevertheless,there are circulating currents among the multiple interlinking converters connected in the input-parallel output-parallel(IPOP)form.The circulating currents will influence the stable operation of the system.For this problem,this paper systematically analyzes circulating currents and their generation mechanism among the IPOP non-isolated bidirectional DC/DC converters,single-phase DC/AC converters and three-phase DC/AC converters.It is found that there are various types of circulating currents among the IPOP non-isolated interlinking converters and the circulating currents are related to the asymmetric parameters.The influence of asymmetric parameters on system dynamics,circulating currents,harmonics and so on are carefully studied.Based on the generation mechanism,from the points of the control algorithm,topology,isolation configuration,a two-degrees-of-freedom suppression strategy is proposed,which can be applied to different interlinking converters.The proposed suppression strategy can suppress the different kinds of circulating currents in the decoupled way,then all the circulating currents can be eliminated effectively.Consequently,the parallel interlinking converters can share the output power,which enhances the system reliability when subgrids are connected as a whole.Third,focusing on the regulation among mutiple sources and the output fluctuation of renewable energy sources,this paper proposes a decentralized multi-time scale network-generation-storage-load coordination method to ensure the stable operation of the system and enhance the local absorption ability for the renewable energy sources.There are multiple subgrids in the hybrid ac/dc microgrid,when the system suffers from disturbances like the power fluctuation,these subgrids should support each other to overcome the disturbances in coordination.However,these heterogeneous subgrids are coupled with each other strongly,thus their interaction is complicated.Hence,the coordinated control for the hybrid ac/dc subgrid with multiple subgrids is a major challenge.For this problem,this paper explores the mechanism of power interaction and coupling among the subgrids.After this,a decentralized power control strategy based on the ac frequency and dc voltage is designed.Combining generation-load coordination,SOC control and so on,a decentralized multi-time scale network-generation-storage-load coordination method is formed.Then,the disturbances are suppressed both within the single subgrid and among the multiple subgrids.In addition,taking the capacities and load types of subgrids into consideration,the proposed coordinated control strategy can ensure that the subgrids with high proportion of critical loads have the priority of power supply.Furthermore,the proposed coordinated control strategy is embedded into the primary control and does not need communication,thus it can balance the power in time,which enhances the system stability and enrich the primary control at the same time.