Stability-Oriented Design for LCL-LC-Trap Filters in Grid-Connected Applications Considering Certain Variation of Grid Impedance

In the distributed power generation systems, to suppress the switching harmonics, LCL filter combined with LC trap is attracting wide attention. However, in the digital control system, due to the folding effect of extra resonance poles, such LCL -LC- Trap filter behaves quite different from LCL filter. If the design and control of LCL -LC- Trap filter do not seriously consider the folding effect, the inverter can even be unstable. It is quite necessary to formulate a robust design and control dedicated for LCL -LC- Trap filter. First, this study analyzes the resonance of LCL -LC- Trap filter and shows the difference from LCL filter. To suppress the extra peak and phase sag caused by the folding effect, a passive resistor is then added. Unlike the typical passive damping, power loss of the passive resistor for suppressing the folding effect is minimal. By solving the folding effect, LCL -LC-Trap filter is treated as LCL filter within the Nyquist frequency. Then, a stability-oriented design based on resonance-frequency selection is provided, with which the inverter with LCL -LC- Trap filter can achieve the optimal phase margin in the default case while being robust for filter parameters variation. At last, results have been provided to verify the analysis and design.