Research On Control Strategy Of Grid-connected Inverter When The Power Grid Of Hybrid Energy Ships Is Unbalanced

The need for energy conservation and emission reduction of ships and the development of renewable energy grid-connected technology have promoted the development of hybrid energy ships.In the process of hybrid energy ships generating electricity and connecting to the grid,due to the existence of single-phase loads in the grid,it is easy to cause grid voltage imbalance faults,and even generate harmonic components.In response to this fault,it is necessary to adopt an appropriate phaselocked loop and grid-connected control strategy.At present,the decoupling doublesynchronous coordinate system phase-locked loop method is mostly used under the grid voltage unbalance fault,but this phase-locked loop contains harmonic components.Under the condition of unbalanced voltage,the phase locking is inaccurate and the positive sequence component in the coordinate system will fluctuate.Therefore,an improved decoupled dual-synchronous coordinate system phase-locked loop is proposed.In order to improve the quality of grid-connected current when the ship grid voltage is unbalanced,this thesis studies the hybrid energy ship inverter grid-connected system,the LCL inverter,the phase-locked loop technology in the case of grid unbalance and the control algorithm when the ship grid voltage is unbalanced Carry out analysis and research,use Matlab/Simulink simulation platform to carry out simulation experiments,and analyze the results of simulation experiments.Firstly,according to the structural characteristics of the hybrid energy ship gridconnected inverter,the mathematical model of the grid-connected inverter under ideal grid conditions and grid voltage imbalance conditions is established,and the threephase static coordinate system,two-phase static coordinate system and two-phase static coordinate system are established.The mathematical model of the grid-connected inverter is analyzed in the phase rotation coordinate system to solve the system instability problem of the LCL filter during operation,and the damping control of the LCL filter is carried out.Secondly,in view of the problems of unbalanced ship grid voltage and the difficulty of separating positive and negative sequence components under the condition of containing specific harmonic components,considering the widely used phase-locked loop and second-order generalized integrator in the decoupled double synchronous coordinate system Filter function,design a phase-locked loop of decoupling double synchronous coordinate system based on improved biquad generalized integrator,when the grid voltage is unbalanced,the positive sequence component is separated and phaselocked control;build simulation through Matlab/Simulink The control effect is verified by the model,and the simulation results show that the improved phase-locked loop has good positive-sequence component separation effect and phase-locking performance.Then,the calculation method of the reference current command value when the ship grid voltage is unbalanced is designed.For the grid-connected current control method,the control method combining grid-side current feedback and capacitor current feedback is adopted here;and the designed grid-connected current control method is carried out.Stability analysis,verify the stability of the grid-connected current control system,and then design the controller parameters.Finally,the influence of the grid voltage on the grid-connected current is studied,the grid voltage full feedforward control strategy is adopted,and the simulation model of the hybrid energy ship inverter grid-connected system is built in Matlab/Simulink.The simulation experiment of the grid-connected system is carried out,and the results show that the grid voltage full feedforward control can eliminate the disturbance of grid voltage to grid-connected current;The full feedforward control of the grid voltage of the loop is used for comparative simulation experiments.The simulation results show that the improved DDSRF-PLL can realize the three-phase balance and low harmonic content of the grid-connected current when the grid voltage is unbalanced and contains harmonics.Incorporated into the ship’s power grid.