The use of interface-filter for reducing current harmonics has been commonly used for inverters connected to the power grid. However, the power grid in a typical distribution system is non-ideal, presenting itself as a voltage source with significant impedance. Thus, an inverter using an interface-filter may interact with other inverters connecting to the grid via the non-ideal grid. In this paper, the irreversible instability phenomenon of the three-phase voltage source inverter connected to a three-phase voltage source with significant impedance is studied. We show that resonant oscillation occurs in the distributed power grid, even if the voltage source inverter with interface-filter is well designed. Specifically, the stable operating region in selected parameter space is studied. A small-signal loop analysis is performed to predict this stability problem and to locate the boundary of the instability using an impedance approach. The phenomenon and design-oriented stability boundaries are verified by cycle-by-cycle simulations.