Modeling and stability of autonomous dc microgrids with converter-controlled energy storage systems

The dc nature of most of the distributed loads and renewable energy sources (RESs), as well as other advantages of small dc electric networks such as higher efficiency, no need for frequency and reactive power regulation, have recently led dc microgrids to take a considerable attention in modern distribution systems. The challenging issue of controlling and analyzing an autonomous dc microgrid with different RES units operating at their maximum power point (MPP), through the sharing regulation of energy storage systems (ESS) is presented in this paper. In particular, droop-based dc voltage regulation is implemented exclusively on the converter interface of each ESS involved. The well-known cascaded mode control technique is adopted with the fast inner-loop current controllers to be included in the analysis of the complete dc microgrid system. Also, in order to concentrate on the system dynamic performance, as it is achieved by the ESS charging or discharging, an adequately detailed model of the Li-ion battery is used; in contrary, the different RESs are inserted as simple current sources, with varying current values determined by the supervisory MPP tracking mechanism, and are considered as unpredictable piece-wise constant inputs. As proven in the paper and verified by extensive simulations, this scheme can guarantee stability and convergence to steady state equilibrium under several varying resistive loads and RES operating conditions.