Research On Harmonic Suppression Strategies Of LCL Based Grid-connected Inverters In Weak Grid

Distributed power generation can flexibly utilize clean energy resources,its energy production and consumption are nearly completed,with low pollution and high utilization,which is the representative of new directions of energy development.As a bridge for new energy access to the grid,grid-connected inverters play a vital role in the energy conversion of new energy generation.When new energy is connected to the AC grid through the converter,the harmonic distortion enriched in the grid will interact with the inverter,affecting the quality of its output power.In addition,the weak grid environment with grid impedance will also have a negligible impact on the robustness of the grid-connected converter,causing harmonic resonance.In this paper,the LCL filtered grid-connected inverter is studied for its resonant damping control strategy and harmonic suppression method.The impedance adaptation of the active damping and harmonic suppression strategies in weak grids is discussed.By improving the feedforward and multi-resonance control strategies,the robustness of inverters to the grid impedance is improved.Aiming at the suppression problem of the natural resonance peak of LCL filter,the equivalent model of three-phase grid-connected converter is established in the coordinate system.Firstly,the resonance suppression mechanism of capacitive current feedback damping control is explained based on the control model.Considering the delay of digital control,the influence of delay on active damping effect and system stability is derived,which provides a detailed basis for system parameter selection and damping link design.The simulation proves the effectiveness of the damping control and the system resonance caused by improper selection of the damping coefficient when considering the delay.In order to ensure the current quality of the inverter into the grid under harmonic disturbance of the power grid,the transfer function of the SRF-PLL phase-locked loop is derived from the perspective of reducing the given error.Accurate phase locking under harmonic perturbations is achieved by increasing the feedforward cross-decoupling structure and reducing the bandwidth.Then the harmonic suppression principle of MR control and three different feedforward modes is studied.The limitations of proportional feedforward and first-order feedforward for higher harmonic suppression and the effect of delay on full feedforward effect are pointed out.For the stability problem under weak grid,the adaptability of damping controlconsidering delay to grid impedance is discussed.It is pointed out that the active damping control of the unit control delay has good weak network stability when the natural resonant frequency is designed to be less than six-six sampling frequency.Then,the theoretical analysis of the problem of poor robustness of weak network for MR control and feedforward control is carried out.The phase compensation and the additional pre-bandpass filter are used respectively to improve the adaptability of the weak grid for harmonic control.Finally,experiments were carried out on a 3kW three-phase voltage type grid-connected inverter experimental prototype.The grid impedance is simulated by means of series reactors and the harmonics of the grid are simulated by means of programmable AC power injection disturbances.The above method is used to perform harmonic and resonance suppression under different conditions,which is validated by theoretical analysis.