| With the development of the shipping industry,the new energy utilization of ships has become an inevitable trend.There are a large number of inductive loads and power electronic devices in the marine new energy grid-connected system,which need sufficient reactive power to ensure the power quality of the marine power grid.At present,reactive power compensation devices are mostly used to compensate reactive power,but there are shortcomings of high cost and large occupation of land for this method.In order to solve this problem,the working principle of reactive power compensation device was studied.It was found that the structure of grid-connected inverters is similar to that of reactive power compensation devices,and the grid-connected inverters directly affect the quality of grid-connected power.Therefore,it was proposed to use the redundant capacity of grid-connected inverters to compensate reactive power,while transmitting active and reactive power,so as to meet the needs of ship load for reactive power while saving costs.In order to improve the control performance of grid-connected inverters,it is necessary to study its power control strategy to make its output meet the requirements.Firstly,PI controller and direct current method were used to control grid-connected inverters to achieve the control target by tracking the target current instantaneously,which has high dynamic and control precision.The instantaneous reactive power theory was used to detect the load reactive current,which was used as the grid-connected current instruction to control the output reactive power of the inverter.The simulation results showed that the control strategy has good control effect in ideal power grid,but the control performance becomes worse when the load changes suddenly and the grid voltage is unbalanced,which needs further optimization.Secondly,in order to improve the anti-disturbance ability of grid-connected inverters when the load changes suddenly,considering the non-linearity and complexity of the ship power system,a sliding mode variable structure control strategy with non-linearity and invariance was adopted.The control strategy has a fixed sliding mode,which can ensure the system to run in a fixed state and improve the robustness of the system when external disturbances occur.Aiming at the buffeting caused by discontinuity of sliding mode control,adaptive compensation term was used to weaken the buffeting,and an adaptive sliding mode controller was designed.The simulation results under abrupt load conditions showed that the designed adaptive sliding mode controller improved the anti-disturbance performance of the ship grid-connected system,and the buffeting phenomenon was not obvious,so it has good robustness and stability.Finally,in order to ensure the effect of reactive power compensation for grid-connected inverters,how to eliminate the voltage disturbance was studied,and the full feed-forward control strategy of grid voltage was determined.Then the instantaneous power characteristics of grid-connected inverters were analyzed,and the relationship between output current and output power was studied.Finally,a method of coordinated control of grid-connected current,active power and reactive power of inverters was proposed by changing the proportion of negative sequence components in grid-connected current instructions and utilizing different instruction currents.The simulation results showed that the proposed coordinated control strategy can maintain the grid-connected current,active and reactive power balance and achieve coordinated control in the case of unbalanced voltage. |
