| Important electrical equipments are usually equipped with an emergency power supply,among which diesel generators and battery energy storage systems are the most common.Diesel generators have the advantages of large capacity and sustainable power supply for a long time,but it needs adjusting time from start-up to stable power generation and their response speed is slow,so they can not complete the task of uninterrupted power supply.Battery energy storage system has no mechanical components and has fast response speed.But the capacity is very limited,so it can not provide long-term emergency power supply for electrical equipments;combining the two to form a composite emergency power supply system can complement each other's strengths,and provide long-term uninterrupted power supply for electrical load.In Beijing "coal to electricity" emergency power supply support technology research project,diesel generators are used as the emergency power supply,which can not achieve uninterrupted power supply.Therefore,this thesis proposes a diesel generators and energy storage hybrid emergency power supply system.In the process of putting into operation,the hybrid emergency power supply system needs to fonnulate four working modes for battery energy storage system,which are floating charging mode,constant voltage and constant frequency discharging mode,power control mode and constant current and constant voltage charging mode.Floating charging mode is applied to the normal operation of municipal electricity to keep the battery in a good state of charge;constant voltage and constant frequency discharging mode provides short-term emergency power for the load;power control mode is applied to diesel generating units to ensure that there is only one constant voltage and constant frequency power supply in the system,so as to avoid double voltage sources affecting the stable operation of the system;constant current and constant voltage charging mode is commonly used in storage batteries charging schemes.Three kinds of power supply disturbances may occur in the diesel generators and the energy storage hybrid emergency power supply system under the influence of control strategy.(1)At the moment of power failure,the diesel generator set starts and the battery energy storage system is switched from floating charging mode to constant voltage and constant frequency discharging mode.If the controlled voltage is inconsistent with that before power failure,the power supply disturbance will occur when the electric equipment is transferred from the municipal power supply to the battery energy storage system.(2)When no-load diesel generators are incorporated into emergency power supply system,if the voltage amplitude or frequency of diesel generators and battery energy storage system are different,and the battery energy storage system is not switched to power control mode in time,power supply disturbance will occur when the power supply of electric equipment is transferred from battery energy storage system to dual power supply with diesel generators as the main power source.(3)When the battery energy storage system is switched to power control mode,if the power supply load of the electrical equipment is not transferred to the diesel generators,the power supply disturbance will occur when the battery energy storage system is switched from discharging mode to charging mode.For the first kind of power supply disturbance,the voltage amplitude and phase before power loss can be obtained by the detection elements,and the voltage amplitude can be taken as the reference value of the outer loop,and the voltage and current double closed-loop constant voltage and constant frequency control can be established with the reference frequency of 50Hz to ensure uninterrupted power supply.For the second kind of power supply disturbance,the diesel generators can immediately control the battery energy storage system to work in power when no load is connected to the grid.For the third kind of power supply disturbance,when the diesel generator is connected to the grid without load,the variable power control is adopted to control the battery energy storage system.After the power supply load is smoothly transferred to the diesel generator,the battery energy storage system is switched from discharging to charging mode.The simulation model is built on the platform of MATLAB/Simulink,and the simulation results are analyzed.It is verified that the proposed hybrid emergency power supply system and its control strategy can realize the non-impact input of diesel generators and energy storage hybrid emergency power supply system,thus providing large-capacity uninterrupted emergency power supply for electrical equipment. |
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