The Research On Grid Converter Control Strategy Under Weak Grid

With the popularity of renewable energy, the distributed generation system becomes a hot topic, and grid converter is to achieve energy exchange between the grid and the distributed generation system. The grid is designed as an ideal voltage source in the conventional converter system. But in the distributed generation system, ideal grid model will no longer apply. Due to the transmission line impedance, transformer leakage inductance and so on, the actual grid exhibits weak grid characteristics. The grid converter and the grid formed an interactive system, the interaction between the grid converter and the grid could have an impact on the stability of the grid system.This paper studies the control strategy of the grid converter under weak grid conditions, a detailed analysis of the stability of the grid system with the interaction between the grid converter and the grid is proposed, and the paper also select the PLL method which is applied to the grid converter under weak grid conditions.First, impedance based stability criterion has been described as a theory-based to analyze interactive system stability. The grid converter output impedance is modeled with L and LCL filter respectively Since the LCL filter would introduces additional resonance peak to the grid converter and then make the grid system unstable, we use the passive damping method where the damping resistor in series with the capacitor branch to suppress resonance peaks, the passive damping method increases the damping to make the system stable. The quantitative analysis in the frequency domain of the current harmonic resonance between the grid converter and the weak grid is proposed. Theoretical analysis shows that in the vicinity of the frequency of the grid impedance equals to the converter impedance is easy to produce a certain degree of harmonic resonance, compared to the converter with L filter, the converter with LCL filter is easier to produce harmonic resonance under the weak grid. The simulation in the time domain verifies the theoretical analysis.Next, this paper selected the phase locked loop based on second-order generalized integrator approach to lock phase, the paper validates the phase locked loop is effective and correct under different grid conditions through simulation, indicating that the method is suitable for the three-phase grid converter under weak grid conditions.Finally, we set up a three-phase grid converter experimental platform to verify the stability analysis for the grid converter system under weak grid conditions, and grid urrent waveforms and harmonic analysis results are given with different inters unaer lifferent grid conditions to verify the correctness of theoretical analysis and simulation esults.