Active Damping of LLCL‐Filter Resonance Based on LC‐Trap Voltage or Current Feedback

The principle and design of the LLCL filter are introduced in Chapter 3. In addition, the design and modeling of passive damped grid‐connected inverters are investigated in Chapters 5 and 6. In the previous chapters, we also show that the LLCL filter is emerging into grid‐connected converters owing to its smaller size compared to the conventional LCL filter but is still burdened by resonance complications when used with a grid converter. Hence, an LLCL‐filtered converter must be passively or actively damped. Active damping is presently more efficient and can easily be realized by feeding back a state variable. For the LCL filter, the variable is usually the middle capacitor current. However, with computational delays, a simple proportional damper cannot be used with the capacitor current. Instead, a high‐pass damper must be used, which may cause undesired noise complications depending on the operating conditions. In this chapter, the same capacitor current damper has been investigated for the LLCL filter, and its limitations are clarified. Cases with and without delays were also considered. The investigation has also been extended to consider LC‐trap voltage feedback, which is based on the formulated transfer functions and experimental results, permitting a simple proportional damper to be used. Lesser noise complications and faster dynamics can be achieved. Circuit equivalences for the presented dampers have also been derived, from which quick damping insights can be easily drawn. Finally, the expected performance of the damper is verified in the experiment of the grid converter.