Identification of resonances in parallel connected grid inverters with LC- and LCL-filters

Modern high power grid inverters use usually LC-or LCL-filter at grid interface. Simpler L-filter does not prove sufficient harmonic attenuation without resulting in to very large and bulky filter components. Higher order filters realize better attenuation with smaller components. However, these filters introduce additional resonances to the system, which may cause instabilities without proper care. Connecting more than one inverter with individual LC- or LCL-filters in parallel will result in shifted resonances. The identification of the resonances in parallel connected LC- and LCL-filter system is essential for proper operation of the grid-connected inverter. For instance the efficiency of active damping is highly depended on the accurate knowledge of the filter resonances. In this paper the transfer functions for parallel-connected filter systems are derived and resonances analyzed. A method for resonance identification is presented.