Primary Frequency Control in Islanded Microgrids Using Solid-State Transformers as Virtual Synchronous Machines

The high penetration of power electronics interfaced distributed generators (DGs) in microgrids reduces their inertia; this can lead to large frequency excursions during changes in generation or demand. A promising method to increase the inertia of such microgrids is to operate power electronic converters as virtual synchronous machines (VSM) so that they emulate the inertia of synchronous machines. Solid-state transformers (SSTs) have become more popular recently as a way of controlling the active power flow in power systems. Most of such SSTs have one intermediate DC bus capacitor that can be used as an inertia source in the power system. As a result, SSTs can be used as a VSM in islanded operation of microgrids to increase the inertia. In this paper, a new controller for a typical SST is proposed to increase the inertia of a power system when it is operated as an islanded microgrid. The dynamic behavior of the islanded microgrid with an SST with proposed controller is studied. In order to perform such a study, the modified IEEE 37 bus system is used as an islanded microgrid. The system is modeled and studied in MATLAB/SIMULINK. Simulation results confirm that SSTs can be used effectively to increase power systems inertia and improve the frequency control of islanded microgrids.