Researchers at the Bucharest University of Economic Studies compared grid-forming (GFM) and grid-following (GFL) inverters in a modeled residential solar-plus-storage system. Their analysis found that GFM technology performed better in simulations of grid disturbances and offered stronger financial results under the study’s assumptions.

The team modeled a 5 kW solar array paired with a 10 kWh lithium iron phosphate battery. Both inverter types were tested under the same conditions using electromagnetic transient simulations in MATLAB/Simulink, including a 1.2 kW increase in load and a 20% voltage drop lasting 200 milliseconds.

With GFM, the maximum rate of change of frequency fell from 18.99 Hz/s to 6.34 Hz/s compared with GFL. Maximum frequency deviation declined from 0.62 Hz to 0.18 Hz, and frequency settling time shortened from 7.5 seconds to 3 seconds. The simulations also showed that maximum voltage deviation decreased from 0.20 to 0.08 per unit, while voltage recovery time dropped from five seconds to two.

The researchers said GFL systems rely on the grid for voltage and frequency references, while GFM inverters can establish those references themselves. This allows them to support the grid and operate in islanded mode, potentially maintaining supply during outages.

The financial assessment assumed a 15-year project life and a 6% discount rate. Initial investment was set at €7,000 for GFL and €8,500 for GFM. The model included operating costs, savings from self-consumption, revenue from exports and energy arbitrage, as well as estimates for ancillary services, avoided outage costs, battery degradation and reduced renewable-energy curtailment.

Under those assumptions, GFM produced annual net cash flow of €1,142, compared with €615 for GFL. Its net present value was €2,591 and its internal rate of return was about 10.4%; the corresponding GFL figures were a negative €1,027 and about 3.7%. Although the conventional levelized cost of energy was higher for GFM—€0.206/kWh versus €0.174/kWh—the adjusted figure fell to €0.118/kWh when flexibility and resilience benefits were counted. The researchers estimated that the additional €1,500 investment would be recovered in about 2.85 years.

The study’s sensitivity analyses found GFM remained economically viable across the electricity-price, ancillary-service revenue and battery-cost scenarios considered. The researchers cautioned that profitability depends on access to flexibility markets and on how grid services are valued, and said the assumptions need further testing with seasonal operating data.

The findings were published in the Journal of Energy Storage in a study titled “Can grid-forming inverters pay off? A techno-economic analysis of photovoltaic and battery systems in low-inertia power systems.”

Sources: pv magazine.