Performance Comparison of Single and Dual Loop Control Scheme for Grid Connected Inverter with LCL Filter

Normally, utility-connected inverters are equipped with LCL filters to reduce the output current harmonics. But the system that is equipped with an LCL filter has a stability problem and requires a more complex control scheme. This paper presents a detailed performance comparison of single loop and dual loop control structure for grid-connected inverters with an LCL filter. The dual loop control structure effect on the resonance of the LCL is also discussed. The outer loop regulates the current and ensures the tracking of reference, while the inner loop improves stability and dampens the resonance. In this paper, grid current is taken for the outer loop feedback variable, and different parameters ${\mathrm {I}}_{\mathrm {c}}$ and ${\mathrm {I}}_{\mathrm {L}}$ are used as inner loop feedback. Stability analysis and harmonic impedance offered to the grid voltage by inner feedback variables ${\mathrm {I}}_{\mathrm {c}}$ and ${\mathrm {I}}_{\mathrm {L}}$ are discussed in detail. The effect of resistive damping on stability is also discussed in this paper. Generally, it is very difficult to measure the grid impedance exactly, so the performance of the control schemes is also evaluated over a range of grid side inductances. Simulation results using Simulink show the effectiveness of the proposed strategy.