Research On MMC Control Strategy Applied To Retired Power Battery Energy Storage Station

The application of retired power battery in energy storage station is an important solution for power battery recycling.Modular Multilevel Converter based Battery Energy Storage System(MMC-BESS)has the advantages of flexible control and high modularity.The consistency and safety of retired power batteries are low,and the MMC control strategy is studied for this problem.The working principle and modulation method of MMC-BESS are studied,and the mathematical model of the system is established.By analyzing the characteristics of retired power battery,it is proposed to set the working current limit for each battery according to the aging degree,and then avoid the battery working current to exceed the limit for a long time to improve the safety of retired power battery application and slow down its capacity decay.Grid-connected control and state of charge equalization are the basis of MMC-BESS application.The effect of multi-output PID neural network decoupling instead of traditional PI feedforward decoupling on the system dq-axis grid-connected current control is investigated.A model predictive control method is used to control the system three-phase grid-connected current,thus avoiding PI parameter rectification in the grid-connected current control link.The system output power is controlled on the basis of current control.The battery state of charge is estimated using a convolutional neural network-gated cyclic unit.The sequencing method is combined with nearest level modulation to equalize the state of charge within the bridge arm.By analyzing the relationship between the circulating current and the internal power flow of the system,the system upper and lower bridge arms and inter-phase state of charge equalization strategies are investigated.The system sub-module retired power battery operating current is affected by both the output current and the circulating current,and the output current depends on the output power.Therefore,the output power and circulating current control strategy is proposed to take into account the battery operating current.The strategy first makes a difference between the battery operating current and its limit value,and then adjusts the output power and circulating current limit value in real time by the proportional-integral controller,so as to avoid the battery operating current from exceeding the limit for a long time.In order to avoid the battery thermal runaway caused by sub-module failure,fault diagnosis is performed for IGBT and battery which are more prone to failure in sub-module.Among them,the battery fault diagnosis mainly focuses on the single battery consistency fault of the sub-module battery pack.The system fault-tolerant operation control strategy is studied to realize the redundancy of sub-module hot standby and the fault-tolerant operation of the system after the redundancy is exhausted.In the fault-tolerant operation after redundancy depletion,the system is firstly adjusted to ensure stable operation by adjusting the nearest level modulation method.Then,a triple frequency zero-sequence sinusoidal signal is superimposed on the three-phase modulated waveform to reduce the voltage and current distortion rate of the system output.