Research On Primary Frequency Regulation Control Strategy Of Coal-fired Unit Assisted By Flywheel Array Energy Storage System

The proposal of the "dual carbon" strategic goal has promoted the optimization and upgrading of the energy structure,and the penetration rate of new energy sources such as wind power and photovoltaics has continued to increase,whose randomness and volatility seriously threaten the stability of the power grid frequency.Therefore,in order to improve the grid frequency instability problem caused by large-scale intermittent new energy grid connection,it is necessary to introduce new auxiliary frequency regulation means to improve the overall frequency regulation capability of the grid.As a high-quality frequency regulation resource,flywheel energy storage has significant advantages in assisting coal-fired units in primary frequency regulation by virtue of its fast response and large instantaneous throughput power,and has been applied in practical engineering.The flywheel array energy storage system adopts modular management,which has the advantages of strong scalability and flexible configuration.Using flywheel array energy storage system can not only improve the overall frequency regulation performance of coal-fired units,but also suppress the auxiliary power voltage sag problem caused by short-circuit faults or large motor startup.Therefore,the study of flywheel array energy storage system operation control strategy has important theoretical value and practical engineering significance.The main research content of this paper is as follows:Firstly,the modeling and charge-discharge control strategy of the flywheel array energy.storage system are studied.A charge-discharge control strategy for flywheel array energy storage system based on layered architecture is proposed.The upper controller is mainly responsible for the power distribution of each flywheel energy storage unit to ensure the state of charge(SOC)consistency of each flywheel energy storage unit,while the lower controller is mainly responsible for responding to the upper controller to control the charge-discharge of each flywheel unit.A dual-mode charge-discharge control strategy for the flywheel unit is designed for the lower controller.According to the SOC,the charge-discharge modes of the flywheel unit are divided into normal mode and emergency mode,which avoids overcharging or overdischarging of the flywheel unit.An adaptive grouping equal time power allocation strategy is proposed for the upper controller,which can adjust the charge-discharge power of each flywheel unit in real time according to the SOC,so that the remaining energy of the flywheel unit can be kept in balance during the charge-discharge process.Secondly,the control strategy of the flywheel array energy storage system participating in the primary frequency regulation is studied,and a primary frequency regulation self-adaptive comprehensive control strategy taking into account SOC self-recovery is proposed.The strategy adopts the combination of positive virtual inertia control,negative virtual inertia control and droop control,and introduces an adaptive frequency deviation coefficient,so that it can adaptively adjust the inertia coefficient and droop coefficient according to the frequency regulation depth and SOC.In the SOC self-recovery stage,the SOC self-recovery output strategy of the flywheel array energy storage system is proposed,which improves the bidirectional adjustment performance of the flywheel array energy storage system.Then,a coordinated control strategy for the joint frequency regulation of the coal-fired unit-flywheel array energy storage system is designed.The coordinating controller issues the total energy storage output command according to the system frequency deviation and the overall SOC of the flywheel array energy storage system,and the flywheel array energy storage system sends the received total output command value to the upper controller.The upper controller distributes the total power according to the current SOC state of each flywheel unit and obtains the power instruction value of each flywheel unit,and then controls each flywheel unit through the lower controller,so that the flywheel array energy storage system can assist the coal-fired unit to carry out primary frequency regulation.Finally,in order to improve the auxiliary power voltage quality problems caused by short-circuit faults or large motor startup,on the basis of the original joint frequency regulation system,a set of series transformers was added,and a dynamic voltage restorer model based on flywheel array energy storage system is established.When the flywheel array energy storage system is responsible for frequency regulation,the power conversion system adopts a voltage-oriented vector control strategy to ensure the stability of the DC bus voltage.When a voltage sag occurs,the power conversion system adopts a compound control strategy and performs voltage compensation through series transformer.