| As the crucial support of renewable energy source(RES)development,the energy storage(ES)industry and energy storage technology cover the needs of power supply side,power grid side,users side,residents side and socialized functional ES facilities.Distributed energy storage(DES)unit can transfer the energy in the time the space domains,features flexibility in the installation location,and plays an important role for promoting RES consumption and improving the stability of hybrid power grid.Thus,DES is an important form and developing trend of ES resources in the future grid.However,as an emerging technology and resource,there are still some problems in the utilization of DES,such as low utilization efficiency,high vacancy rate and single functional scene.The ac/dc hybrid power grid has the dual advantages of ac power grid and dc power grid,which meets the demand of distributed energy utilization and load efficient access.However,there are some problems to be solved,such as difficulty in power transfer and control in ac/dc interface,low inertial characteristic and intermittent output power of RES,which bring about issues to be solved for the system operation and the power coordination control.In this thesis,driven by the demands of flexible and stable operation of the ac/dc power grid,and with the target of high efficient utilization of DES unit,we hope to build a bridge between the hybrid gird's demands and the high-quality solution of DES,meet the power grid demand and meanwhile use the potential of efficient utilization of DES.The energy flexible control,inertia control and multi-bus dc voltage stability of the ac/dc hybrid power grid are the basic problems for the coordinated operation of the hybrid grid,the solutions of which can provide the basic guarantee for the stable and reliable operation of the power grid and serve as the foundation of long-term stable operation.In this thesis,with the background of the hybrid microgrid and the dc distribution network,we have conducted the researches on high-efficiency utilization of DES,which are summarized as foll ows:(1)Different from the traditional normalization method,the coupling relation between ac and dc subnet is studied derived from the power coupling relationship,and then the characteristic equation of ac frequency-dc voltage coupling is constructed.The coupling relationship reveals the intrinsic relation between dc voltage and ac frequency when the bi-directional power flow is transferred in the ac/dc interface.Through the virtual inertia and virtual capacitance parameters,the coupling degree can be quantitatively described,which has a clear physical meaning,and thus lays a theoretical foundation for establishing the connection between ac and dc electrical variables.(2)A robust power control strategy for ac/dc interface of hybrid microgrid with DES is proposed.Based on the novel droop characteristics derived from the ac /dc coupling relationship,the power control strategy containing the dynamic operation characteristics of ac and dc sub-grids is established,so that DES can simultaneously respond to the dynamic power demand of local and adjacent grids,and can acquire the physical characteristics of different power grids.The IC can simulate the response characteristics of synchronous generator and dc capacitor so that the cross-grid power compensation has better dynamic performance and robustness.The proposed strategy enables DES not only to respond to local power demand,but also to acquire the ability to cross-grid power demand,which is not limited by the topology and features good feasibility in practice.(3)The unified inertia is defined as the index to evaluate the global inertial level of the hybrid grid,which is beneficial for the quantitative estimation of inertia.The operation characteristics of different distribution generations(DGs)are analyzed,which provides the foundation for the inertia response control of DG s,and the coordinated control of DGs and DES is accordingly designed.The proposed strategy includes two parts: local inertia support and cross-grid inertia support,which can reduce the risk of shutdown or second power disturbance after participating in inertia support.While providing inertia support,the output power is designed according to the ramp rate characteristics of different ES components in hybrid energy storage(HES),which improves the operational efficiency and service life of ES equipment.With the help of DES,the proposed method can make full use of the inertia response capability of the DGs and achieve the improvement of the overall inertia level and operation stability of the hybrid micro grid.(4)A flexible voltage regulation strategy for the dc distribution network with DES is proposed.The disadvantages of the conventional voltage control method and the operational modes of the connected microgrid are analyzed.Then,based on the voltage and frequency coupling relationship of the ac/dc interface and the cascading droop characteristics of bi-directional dc interface,the flexible voltage regulation control strategy of different interfaces is designed with the involvement of DES.The characteristics of basic control parameters are analyzed and the value determination process is given.With the proposed strategy,the voltage fluctuation of the dc distribution network bus and its influence on the ac power g rid can be effectively reduced when the power disturbance occurs,and the system's robustness and fault crossing ability can be improved,which makes full use of the power regulation margin of DES.The proposed strategy provides a novel method for voltage control of multi-terminal multi-node dc distribution network and the system-level control of DES.(5)Aiming at the demands of maintaining state of charge(So C)and the economic operation of DES,a two-layer optimal voltage regulation strategy is proposed for the dc distribution network.The operational cost characteristics of the ES unit are analyzed,and the quantitative estimation method of operating loss of ES is accordingly derived.Based on the principle of dynamic equal incremental cost(EIC),the proposed method can adjust the output power of DES without changing the droop characteristics,which is achieved by innovatively tuning the outer loop reference.The proposed strategy considers both the voltage regulation performance and the economic optimization of DES operation,which provides a trade-off solution for solving the contradiction between improving the DES utilization efficiency and reducing the cost of DES operation.With the background of hybrid ac/dc microgrid and dc distribution network,th is thesis performs an in-depth exploration and research on the efficient utilization of DES in the hybrid power grid.The research is based on the flexible operation demands of power grid,expands the application scenarios and improves the utilization efficiency of DES,which provides a useful reference for the application of ES resources in power grid,confirms to the overall development trend of ES industry and energy industry,and has important practical significance. |
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