By an analysis of the inertia time constants and system eigenvalues of inverter-interfaced distributed sources，the filter，the equivalent line of an inverter-interfaced microgrid，it is proposed that microgrids of different voltage levels can analyze the small signal stability using different state-space equations.For an inverter-interfaced microgrid of400V bus，steady-state equations of the electrical grid part should be adopted，while for a microgrid of 10 kV bus or higher voltage levels，differential equations should be used.Eigenvalue analysis and simulations have demonstrated the correctness of the conclusions.