Battery energy storage system for frequency support in microgrids and with enhanced control features for uninterruptible supply of local loads

•A battery energy storage system to support the frequency in autonomous microgrids.•Original frequency controller to better damp the frequency oscillations.•The frequency controller covers the main two control levels, namely primary and secondary.•Enhanced control functions to ensure uninterruptible power supply to local sensitive loads.•Simulations and experimental results validate the proposed control solution.

This paper proposes a battery energy storage system (BESS) to support the frequency control process within microgrids (MG) with high penetration of renewable energy sources (RES). The solution includes features that enhance the system’s stability and security of supply. The BESS can operate connected to MG or islanded and the transition between the two states is seamlessly coordinated by an original method. The BESS active power response is governed by an improved frequency controller on two layers, namely primary and secondary. It responds to frequency deviations by combining a conventional droop control method with a virtual inertia function to improve the system’s stability. The proposed BESS may also compensate the power of the local loads, so that the MG frequency transients can be reduced and, depending on the remaining inverter capacity, voltage support in the point of common coupling with the MG may be provided. If the MG power quality degrades in terms of the voltage and frequency, the BESS and the local load are disconnected from the MG and continue operating islanded. The BESS is reconnected to the MG after a smoothly resynchronization of the local voltage with the MG, without disturbing the local loads supply. Simulation and experimental results assesses the proposed control solutions.