Phase-to-Phase Voltage (V) 334 substations. Its power system load exhibits an interesting cycle of a daytime peak, followed by a solar and wind power ramp-down, followed by an evening system load ramp-up that reaches a nighttime peak that is higher than the daytime total. This cycle is important to the challenges of integrating wind and solar into the power system (see Gevorgian and O'Neill 2016). Some hypothetical scenarios for the Puerto Rico Electric Power Authority (PREPA) net load at different installed PV capacities appear in Figure 9 (note the 2 1.6 1.2 0.8 0.4 0 -0.4 -0.8 -1.2 -1.6 -2 ×104 7.35 7.4 7.45 7.5 7.55 7.6 Time (s) (a) 7.65 7.7 7.75 7.8 7.35 7.4 7.45 7.5 7.55 7.65 7.7 7.75 7.8 BESS Current (A) 4 3.2 2.4 1.6 0.8 0 -0.8 -1.6 -2.4 -3.2 -4 similarities to the California " duck " curve). High levels of PV generation will cause sharp changes in the real-time ramping needs, so more resources with a faster ramping capacity will be required to meet these steep evening and morning net load ramps. At very high PV penetration levels, the " belly of the duck " may actually go negative, necessitating either curtailment or energy storage capacity to shift it. As described earlier, energy storage can be a potential resource for flexibility and the provision of reliability services within such PV resource portfolios. 7.6 Time (s) (b) Figure 5. The soft black start of a PV system using a BESS (zoomed in to inverter voltage ramping period). (a) The 13.2-kV bus voltages (line to line) (V). (b) The BESS currents (A). 1,300 BESS PV Load Diesel Wind Power (kW) 800 300 -200 -700 -1,200 7 a.m. PV-BESS 10 a.m. 1 p.m. Wind-BESS 4 p.m. 7 p.m. 10 p.m. 1 a.m. Figure 6. An example 24-h operation of a 100% renewable islanded grid at NREL's Flatirons facility. 82 I E E E E l e c t r i f i cati o n M agaz ine / MARCH 2021 4 a.m. 7 a.m.