Super-high bandwidth secondary control of AC microgrids

This paper proposes a control structure for ac microgrids that enables increase of the secondary voltage and frequency control bandwidth by more than two orders of magnitude compared to state-of-the-are methods. This is enabled by a finite control set model predictive control (FCS-MPC) strategy that is deployed at the primary level. It allows the voltage regulation performance of voltage source converters (VSCs) at the upper bandwidth limit defined by the physical parameters of the system. Multiple FCS-MPC regulated converters can then be interfaced in parallel using a conventional droop strategy, where resistive virtual impedance is used to program the control laws. As there is almost no filtering necessary in the primary control stage, nor there exists a low-pass filtering behavior of conventional nested linear loops, the secondary control can be programmed with much higher bandwidth than in the case of state-of-the-art methods. In particular, case study from a dual-inverter system showed around 400 times faster voltage and frequency restoration compared to recent references in the field.