Energy storage systems impact on the short-term frequency stability of distributed autonomous microgrids, an analysis using aggregate models

This study analyses the integration impact of battery energy storage systems (BESSs) on the short-term frequency control in autonomous microgrids (MGs). Short-term frequency stability relates with the primary or speed control level, as defined in the regulations of the classical grids. The focus is on autonomous MGs that dynamically behave similarly to the classical power systems. This is the systems case with classical distributed generators (DGs), but which can also contain renewable energy sources (RESs) in a certain penetration level. During MG islanded operation, the local generators take over most of the frequency control process, by means of their automatic generation control, which include inertia response and primary control. However, RES-based DGs are rarely able to provide grid frequency support, as they lack controllability and usually the power conversion chain does not have the possibility of storing and releasing energy when required by the system. Therefore the need of boosting the MG power reserves by adding energy storage systems is often a requirement. The study highlights the improvement in the MG short-term frequency stability brought by an original BESS control structure enhanced with both inertial response and an adaptive droop characteristic during battery state-of-charge limitations. The conducted analysis is accomplished by adopting aggregated models for the involved control mechanisms. The developed model is analysed in frequency domain, whereas an experimental test bench including a real-time digital simulator with BESS controller in a hardware-in-the-loop structure is used for assessing the system performances.