Frequency Control of Microgrids in Autonomous Mode by a Novel Control Scheme Based on Droop Characteristics

Abstract

Microgrids as a new evolving paradigm operate in two distinct modes of operation: the utility-grid-connected mode and the autonomous (islanded) mode. In transition from the utility-grid-connected mode to the autonomous mode, the frequency of a microgrid is seriously affected due to the slow response of the renewable resources, especially when the microgrid is totally composed of inverter-based micro-sources without any synchronous generators. Therefore, to operate the microgrid securely, a kind of storage system, like a sufficiently charged battery, is needed. In this article, a novel control system based on an adaptive droop characteristic is proposed to control the frequency of the microgrid upon isolating from the utility grid, simultaneously transferring the state of charge of the storage system to a safe condition. The proposed droop speed control helps to balance the supply/demand in the autonomous mode by charging/discharging the batteries and increasing/decreasing the power outputs of the micro-sources in such a way as to keep the state of charge of the storage device in a stand-by condition for further connection/disconnection to the grid after the balance is set up. The proposed method operates locally, and no extra communication link is needed. Sophisticated models are used to simulate the performance of the proposed method.

Keywords:

• microgrid
• droop control
• state of charge
• frequency control
• power system security