A Control Method for Balancing the SoC of Distributed Batteriesin Islanded Converter-Interfaced Microgrids

In a low-voltage islanded microgrid powered by renewable energy sources, the energy storage systems (ESSs) are considerednecessary, in order to maintain the power balance. Since a microgrid can be composed of several distributed ESSs (DESSs), acoordinated control of their state-of-charge (SoC) should be implemented, ensuring the prolonged lifespan. This paper proposes anew decentralized control method for balancing the SoC of DESSs in islanded microgrids, without physical communication. EachDESS injects a current distortion at 175 Hz, when its SoC changes by 10%. This distortion is recognized by every DESS, througha phase-locked loop (PLL). In order to distinguish the origin of the distortion, each DESS injects a distortion of different timeduration. This intermediate frequency has been selected in order to avoid the concurrence with the usual harmonics. The DESSstake advantage of this information and inject a current proportional to the SoC. Implementing this strategy, a comparable numberof charging/discharging cycles for each DESS are achieved. Furthermore, an active filter operation, implemented in the𝑑𝑞rotatingframe for each individual harmonic, is integrated in the control of the distributed generation units, supplying nonlinear loads withhigh-quality voltage. The effectiveness of this method is verified by detailed simulation results.