Performance Investigation of LCL Filters in Grid Connected Converters

The use of dispersed or distributed resources (DR) is rapidly increasing in modern distribution networks because of their potential advantages. As many types of these resources generate electrical energy in the form of DC voltage source, they need a DC/AC converter for energy transfer to the grid. Static converters are usually connected to the utility grid through a simple L filter to reduce the injected current harmonics. However, 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 using LCL filter topology. Basic expressions related to LCL filters are derived and compared to that of L filter. Design criteria with respect to filter size, ripple current and resonance frequency are addressed. The control strategy is briefly described and it is shown that how the system can be stabilized by proper state plus output feedback