Emergency Power Supply System for Critical Infrastructures: Design and Large Scale Hardware Demonstration

Seamless recovery and sustained power to critical infrastructures (CIs), after grid failure, is a crucial need arising in disaster scenarios that are increasingly becoming more frequent. Accreditation standards recommend CIs to have emergency power supply system (EPSS) in order to form a local microgrid network with backup resources (generation units/renewable resources) in case of sudden power blackouts of main grid supply. The standards also recommend enhancements such as seamless transition capability during on-grid to off-grid jump and vice versa, efficient and optimal operation while maintaining the standard regulation and power quality limits during power contingencies, and operation with sustenance of reliable power for long periods of time. This article is proposing a comprehensive design of the EPSS for uninterrupted operation of CIs by employing novel techniques, such as $\mathrm {1)}$ mode-dependent droop controlled grid-forming inverters for seamless transition capability; $\mathrm {2)}$ fast-acting optimal net-load management engine for efficient and optimal operations maintaining regulation and power quality limits; $\mathrm {3)}$ optimization-based horizon of viability engine for longer, sustained and viable operation in the aftermath of grid failure as recommended in standards. Two stage validations are conducted using defined test cases while considering various types of contingencies on a 3- ${\phi }$ , 60 Hz, 480 V, 450 kVA test system based on a realistic electrical network of a commercial scale medical center. The comprehensive controller-hardware-in-loop and power-hardware-in-the-loop based experiments corroborate the efficacy and robustness of the proposed strategy for EPSS employed in the CI. The mode-dependent droop controller enables seamless transition of the critical infrastructure between the off-grid and the on-grid modes. Inverters operate in the gird-forming mode when the CI is off-grid and also when the CI is on-grid while keeping frequency and voltage regulated between ±0.5 Hz and $\pm 10\%$ of the nominal respectively. The fast-acting net-load management engine maintains the balance in total available generation and the power demand by determining the optimal decisions within 100 ms during various contingencies while regulating the voltage, frequency, and total harmonic distortion (THD) of the system under the strict limit recommended by the grid codes. The horizon of viability optimization engine showcases advantages over conventional scheduling engines with higher utilization of critical loads and maintaining higher energy at the end of an 1-hour experiment.