Adaptive Decentralized Droop Controller to Preserve Power Sharing Stability of Paralleled Inverters in Distributed Generation Microgrids

TLDR

The paper tackles the low‑frequency relative stability issue that arises when inverter‑based distributed generation units are paralleled in microgrids. It proposes an adaptive decentralized droop controller to preserve power‑sharing stability among these paralleled inverters. The controller combines static droop characteristics with an adaptive transient droop function whose gains are scheduled through small‑signal analysis along each unit’s loading trajectory, using filtered active and reactive powers as indices to ensure smooth, stable power injection. The resulting two‑degree‑of‑freedom controller allows independent tuning of dynamic performance without altering static droop gain, actively damps power oscillations across operating conditions, and delivers stable, robust operation of the paralleled inverter system.

Abstract

This paper addresses the low-frequency relative stability problem in paralleled inverter-based distributed generation (DG) units in microgrids. In the sense of the small-signal dynamics of a microgrid, it can be shown that as the demanded power of each inverter changes, the low-frequency modes of the power sharing dynamics drift to new locations and the relative stability is remarkably affected, and eventually, instability can be yielded. To preserve the power sharing stability, an adaptive decentralized droop controller of paralleled inverter-based DG units is presented in this paper. The proposed power sharing strategy is based on the static droop characteristics combined with an adaptive transient droop function. Unlike conventional droop controllers, which yield 1-DOF tunable controller, the proposed droop controller yields 2-DOF tunable controller. Subsequently, the dynamic performance of the power sharing mechanism can be adjusted, without affecting the static droop gain, to damp the oscillatory modes of the power sharing controller. To account for the power modes immigration at different loading conditions, the transient droop gains are adaptively scheduled via small-signal analysis of the power sharing mechanism along the loading trajectory of each DG unit to yield the desired transient and steady-state response. The gain adaptation scheme utilizes the filtered active and reactive powers as indices; therefore, a stable and smooth power injection performance can be obtained at different loading conditions. The adaptive nature of the proposed controller ensures active damping of power oscillations at different operating conditions, and yields a stable and robust performance of the paralleled inverter system.