| With the development of new energy generation technology,a large number of distributed power sources need to be connected to the microgrid,and then incorporated into the grid.The microgrid presents weak grid characteristics,such as voltage imbalance,grid frequency fluctuation,grid impedance change and so on.When the distributed power is connected to the grid,it is necessary to ensure that the grid-connected current can track the change of the grid voltage in order to reduce the pollution to the grid,so it is particularly important to study the current control technology of the grid-connected inverter.In this paper,the control strategy of photovoltaic grid-connected inverter applied in microgrid is studied.There are mainly three parts of related control strategies,such as feedforward decoupling control strategy in three-phase unbalanced environment,proportional complex number integral control strategy during grid frequency drift and adaptive control strategy including impedance identification of grid impedance.The main idea is that,Through more advanced control strategy,grid-connected inverters can be more flexible and multifunctional to cope with grid-connected situation in microgrid environment.First of all,the current quality in PV grid-connected inverters is deteriorated due to interference of grid voltage and couplings between the phases in dq frame,especially under the unbalanced grid conditions.Therefore,a positive and negative sequence feed-forward technique was proposed for a alternative solution.The double synchronous reference frame is utilized to eliminate the negative-sequence component and extract the positive-sequence component for decoupling control.In order to avoid dead-time effect,compound control of repetitive controller with PI controller is used as a current harmonic compensator.Then,the performance of conventional controller of single-phase gridconnected inverters possibly deteriorate during grid frequency drift.In this paper,an improved adaptive proportional complex integral(PCI)controller with frequency locking loop(FLL)was proposed,the FLL was utilized to adjust the resonant frequency of PCI automatically,and the zero current steady error was achieved by PCI controller.Further,In the single phase grid-connected inverter with an inductivecapacitive-inductive(LCL)filter,the performance of predictive controller maybe deteriorate during the variation of the plant model because of the grid impedance change.In this part,a robust predictive current controller based on internal model observer(IMO)is proposed.the ratio of sampling frequency to resonant frequency is adjusted to ensure that the LCL resonant frequency is above the critical frequency.So,active damping is not required.At the same time,the digital delay was compensated by deadbeat predictive current controller.Furthermore,an internal model disturbance observer is designed to conquer the influence of model mismatch and suppress measurement error,then the stability of the IMO was ensured through constructing Lyapunov function.Finally,theory analysis,simulation,and experimental results are presented to illustrate the effectiveness of the proposed strategy above. |
