An improved droop controller for grid-connected voltage source inverter in microgrid

This paper addresses the low-frequency relative stability problem of the grid-connected voltage source inverter (VSI) based distributed generation (DG) unit in microgrid, in which the conventional active power-frequency (P-ω) and reactive power-voltage (Q-E) droop controllers are commonly used to share the loads. In the sense of the characteristic equation and eigenvalues, the relative stability and the transient response are remarkably affected by the change of the droop gains and/or stable operating points, resulting in the poor damping and eventually instability. An improved P-ω and Q-E droop scheme based on the static droop characteristics combined with a derivative term is presented to preserve the system stability and obtain an improved damping performance. Subsequently, the dynamic performance of the system can be adjusted without affecting the static droop gains to damp the oscillatory modes of the power sharing controllers. Finally, the desired transient and steady-state responses can be obtained.