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Microgrid are gaining popularity due to several advantages like potential for fuel savings and resiliency in case of grid catastrophic failures. In a microgrid, many energy sources like wind and solar farms are connected to the grid through inverters with different power ratings and LCL filter parameters. The inverters incorporated in these systems might have a different frequency response and stability ranges than those inverters with identical LCL filter values. This paper establishes the model and analyzes the stability of a system with multiple paralleled- and grid-connected inverters with different LCL filter paramenters using the grid-side currents as feedback signals. The analysis results showed that a method similar to the interactive and common current analysis technique used on inverters with identical LCL filters can be implemented on a system with different LCL filers to calculate the maximum values of the inverters’ current controller gains without having to derive the complicated equations of the MIMO system. 

Distributed generation is gaining greater penetration levels in distribution grids due to government incentives for integrating distributed energy resources (DERs) and DER cost reductions. The frequency response of a grid-connected single inverter changes as other inverters are connected in parallel due to the couplings among grid inductance and/or inverter output filters. The selection of the inverter- or grid-side currents as feedback control signals is then not trivial because each one has tradeoffs. This paper analyses the system stability for multiple parallel- and grid-connected inverters using the inverter- or gridside currents as feedback signals. Modeling of both feedback signals is performed using the current separation technique. Moreover, the stability range for different conditions including active damping is analyzed through the root locus technique. The grid-side current has a wider range of stability, but the inverterside current allows for higher values of the proportional gain near the critical frequency and no extra sensors are needed since measurement of the inverter current is needed for protection in high-power applications.