Coupled inductance design for grid-connected photovoltaic inverters

The coupled inductor with larger inductance is beneficial to improve the inverter output current quality but instead of causing additional power loss due to the increased series parasitic resistance. Conversely, once the inductance is turned down, the part of the filter power loss caused by the growing ripple current becomes gathering. To clarify the ratio of the ripple and fundamental current on the coupled inductor power loss, a generalised algebraic formula based on the LC filter model is presented to predict the ripple current magnitude at specific fundamental current in each switching cycle. The coupled inductor power loss contributed by the ripple current and the fundamental current can be, respectively, predicted under various inductances and thus in favour of choosing appropriate inductor parameter for power efficiency optimisation. Although this approach is derived based on the LC filter, it is also available for the LCL filter by using the model reduction technique. A guideline of a unity inductance split factor for the LCL filter is proven with maximum fundamental current gain and is adopted for choosing the grid-side and inverter-side inductances of the LCL filter in this study. Experimental results from various ratings of the grid-tied single phase inverter validate the feasibility of the proposed strategy with satisfactory power efficiency.