| Cascaded H-Bridge Battery Energy Storage System(CHB-BESS),as a new form of energy storage technology combined with cascade converter technology and electrochemical energy storage technology,is directly connected to medium-high voltage power grid without transformer,effectively improving the energy conversion efficiency and supporting power grid capacity of the energy storage system.Due to the modular cascade structure of the system and the corresponding optimization control strategy,the electric energy is evenly distributed into each power module,which can improve the effective capacity and utilization rate of the system,reduce the system output harmonics,and improve the power quality.CHB-BESS has shown strong technical advantages and broad application prospects in the fields of large-scale new energy consumption,supporting stable operation of power grids,and black start of regional power grids.This paper is taking CHB-BESS as the research object to carry out research on its key technologies.(1)In view of the relatively complex electrical topology of CHB-BESS,the mathematical model of its continuous domain equivalent switch is established,and the relationship between its external variables(grid voltage,current,etc.)and its internal state variables(power module capacitance voltage,battery voltage,battery SOC,etc.)is derived.Aiming at the requirements of system access voltage level,power conversion rate,second harmonic wave suppression and other requirements,a joint design method of key parameters is proposed to reduce the deviation between the expected value of operation objectives and engineering design parameters,laying a theoretical foundation for subsequent research.(2)The traditional vector control method used in CHB-BESS system has the problems that multiple control objectives cannot be considered and multiple control loops are nested.In this paper,a fast voltage predictive control method based on security constraints for CHB-BESS is proposed,which can effectively improve the dynamic response of the system and realize multiobjective parallel optimal control.An active support control method of power grid frequency for direct-mounted energy storage system is proposed,which can realize its fast and active support to the power grid under the premise of taking into account the security constraints of SOC terminal charging and discharging.A redundant control strategy based on model prediction is proposed to realize the fast switching of redundant modules and effectively improve the fault traversal capability of the system.(3)In order to solve the problem of CHB-BESS battery dispersion and energy balance,this paper sets the energy balance control system into three control levels.In this paper,a phaseto-phase balance control strategy of variable coefficient direct-mounted energy storage system based on zero-sequence voltage injection is proposed to achieve the energy balance between three phases under the premise of ensuring the output quality of the AC side of the system.A SOC equalization control method based on the bias voltage component is proposed to achieve independent control of each battery cluster.This paper proposes a dynamic correction method for the SOC of the cell for CHB-BESS to improve the estimation accuracy of the state of charge of the battery system.(4)Based on the above research contents,a demonstration project of direct-mounted energy storage system with voltage level of 10 kV,rated power of 2 MW and rated capacity of 1 MWh was built.The modular design of power unit and battery pack in the system is carried out.Redundant power module is designed,which has online switching and online maintenance functions.The system adopts the integrated mode of container preset,and has carried out targeted design of 10 kV voltage level in terms of system insulation and voltage resistance,temperature control,etc.The key technologies mentioned in this paper are applied and verified in the demonstration project. |
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