Seamless Transition of Critical Infrastructures Using Droop-Controlled Grid-Forming Inverters

Seamless recovery of power to critical infrastructures, after grid failure, is a crucial need arising in scenarios that are increasingly becoming more frequent. In this article, we propose a seamless transition strategy using a single and unified mode-dependent droop-controlled grid-forming inverters. The control strategy achieves the following objectives: First, regulates the output active and reactive power by the droop-controlled inverters to a desired value while operating in on-grid mode; second, seamless transition and recovery of power injections into the load after grid failure by inverters that operates in grid-forming mode all the time; and third, requires only a single bit of information on the grid/network status for the mode transition. A framework for assessing the stability of the system and to guide the choice of parameters for controllers is developed using control-oriented modeling. A controller hardware-in-the-loop-based real-time simulation study on a test system based on the realistic electrical network of a commercial-scale medical center is conducted for initial prototyping of the control strategy. A hardware experiment is conducted with two $\mathbf {3}$-$\boldsymbol{\phi }$, $\mathbf {480}$-V, $\mathbf {125}$-kVA grid-forming inverters, a $\mathbf {3}$-$\boldsymbol{\phi }$, $\mathbf {480}$-V, $\mathbf {270}$-kVA grid simulator, a physical grid switch, and a physical load bank. The experimental data establish the effectiveness of the always grid-forming operation and control of inverters in meeting power delivery objectives when on-grid and off-grid under various kinds of loads and scenarios while minimizing transients during transitions. Furthermore, performance comparison with the existing strategies showcases the advantage of the proposed strategy.