Research On The Control Strategies Of Cascaded Energy Storage System Under Non-ideal Grid

With the increasing development of new energy technology,its randomness and volatility bring challenges to the large-scale grid connection of new energy sources,and energy storage technology is considered an effective means to solve this problem.Cascade Power Conversion System(PCS)has received widespread attention because of its modular structure,high voltage and power,redundancy,and direct connection to the medium and high voltage grid.In actual grid operation,the three-phase grid voltage may be unbalanced due to single-phase fault,load unbalance and other factors,resulting in negative sequence current in the output current of PCS operation and power imbalance among the three phases,thus causing SOC imbalance among the three phases.It is of theoretical value and practical significance to study the control strategy of cascaded energy storage PCS under unbalanced working conditions.This study focuses on cascaded power storage PCS as the research subject and delves into its four-quadrant working principle.To achieve this,the single-pole frequency multiplier carrier phase shift modulation method is selected,and a mathematical model of PCS is established,considering both balanced and unbalanced operating conditions.For balanced operating conditions,a hierarchical control approach is employed to investigate the control strategy of the PCS.At the inner power control layer,feedforward decoupling control based on a PI regulator is adopted.This control scheme effectively modulates the output waveform of each power module,ensuring the fundamental function of energy storage charging/discharging within the PCS.The outer layer control consists of SOC compensation control and fault-tolerant control.This control mechanism generates regulation modulation waveforms for each module,which are superimposed on the power control output modulation waveform,thereby ensuring safe and stable system operation.To address the SOC imbalance issue arising from battery characteristics and the initial state of each module,a comprehensive analysis of SOC unbalance control between PCS modules is conducted.The introduction of an adaptive equalization coefficient allows for the adjustment of the maximum zero-sequence voltage.Furthermore,the injection of the third harmonic expands the adjustment range and enhances the equalization speed,ensuring the equalization of inter-phase SOC.Additionally,the reference voltage amplitude adjustment method is employed to equalize the power output of each module within a phase,thereby achieving intra-phase SOC equalization.This approach leads to battery SOC convergence during the charging and discharging process,promoting stable system operation and prolonging battery life.In the event of a module failure during PCS operation,a fault-tolerant strategy involving the injection of zero-sequence voltage to shift the neutral point is implemented.This strategy effectively prevents system failure and ensures continuous operation.To address the phenomenon of negative sequence current in the PCS output current under unbalanced conditions,based on the positive and negative sequence mathematical model of PCS,a method based on double second-order generalised integrator(SOGI-QSG)is used to achieve positive and negative sequence separation of grid voltage,and a method based on decoupled double synchronous coordinate system is used to achieve positive and negative sequence separation of current,and a double current loop control strategy based on positive and negative sequence rotating coordinate system is established.to ensure that the PCS output current is balanced.The theoretical analysis of the factors affecting the output power under an unbalanced grid,and the design of injecting zero-sequence voltage to regulate the three-phase output power of the PCS to ensure balanced output power and avoid unbalanced SOC among the three phases of the PCS.A simulation model is built using Matlab/Simulink software to verify the rationality and effectiveness of the control strategy used in the project.A hardware-inthe-loop experimental platform and a small power prototype experimental platform will be built to experimentally analyse the power control,balancing control and faulttolerant control of the PCS to verify the rationality of the control strategy used.