Basic Criteria in Designing LCL Filters for Grid Connected Converters

The presence of distributed resources (DR) and the subject of distributed generation (DG) are rapidly growing in modern distribution networks. Many of these resources use renewable energies, and many of them provide electrical energy in the form of dc voltage source. Consequently, the connection of such resources to the utility grid needs a dc/ac conversion stage. Voltage source converters (VSC) are widely used for this application as they have a mature technology. These converters are usually connected to the utility grid through a simple L filter to reduce the current harmonics injected to the grid. However, higher order filters, like LCL filters, can provide better harmonic attenuation and reduce the filter size at the same time. This paper is concerned with the subject of grid connected converters via LCL filter topology. The advantages of using LCL filter as compared to simple L filter are discussed. Mathematical expressions and plots are given to show the performance of these filters. The design criteria are stated. Different design issues like the total size of magnetic components, ripple current and filter resonant frequency are addressed. Design guidelines are given for optimal selection of filter components