Capacitive Emulation for LCL-Filtered Grid-Connected Converters

This paper proposes an effective capacitive emulation (CE) technique to reduce the grid current distortion when the converter currents are controlled (converter current feedback (CCF)) in grid-connected converters with an LCL filter. Although the CCF scheme is preferable to the grid current feedback (GCF) scheme in terms of stability and inherent current limitation of the power semiconductors, the former presents a problem of additional grid-current harmonic distortion due to the LCL capacitor, which generates a distorted current by line voltage differentiation. To solve this problem, the distorting capacitive current is reproduced on the converter output, so that both cancel out each other on the grid. That is, the converter emulates a negative capacitance while delivering the active and reactive power at the fundamental frequency. This is achieved by adding an estimation of the distorting capacitive current to the converter current reference. Moreover, an effective CE technique requires a current control capable of tracking all harmonics added to the current reference and compensating for any control delay. To compensate this delay, the paper proposes a buffer-based method to advance and filter the current reference. The effectiveness of the CE method has been tested on a 10-kVA transformerless inverter with CCF and a simple proportional-integral (PI) control with grid-voltage feedforward cancellation. Experimental results prove that the proposed solution reduces the line current THD (total harmonic distortion) compared with the GCF strategy with proportional-resonant (PR) control.