Inverter-based generators today are available as grid following (GFL) devices or grid forming (GFM) devices (see Figure 2 below by NREL). Batteries provide a deterministic energy source to which the GFM and GFL Battery Energy Storage Systems (BESS) rely. GFL inverters are current sources and GFM are voltage sources. Historically, inverters were introduced to the Bulk Power System (BPS) in the grid following mode, mostly as PV and Wind generation plants. In this GFL mode, the inverter has full control over its current wave form (amplitude and angle). The inverter can achieve the requested set-points faster as it controls the current directly. GFL BESS inverters provide grid services that compliment other renewable energy sources (PV & Wind) such as primary reserve, frequency control/regulation and ramp rate control of PV plants.
Figure 2
Reference [2] Research Roadmap on Grid-Forming Inverters; Lin, Yashen, Joseph H. Eto, Brian B. Johnson, Jack D. Flicker, Robert H. Lasseter, Hugo N. Villegas Pico, Gab-Su Seo, Brian J. Pierre, and Abraham Ellis. 2020. Research Roadmap on Grid-Forming Inverters. Golden, CO: National Renewable Energy Laboratory. NREL/TP-5D00-73476. https://www.nrel.gov/docs/fy21osti/73476.pdf.
GFM inverters have risen from the need for microgrids, black starting of critical grid infrastructure, inertia, Rate of Change of Frequency (RoCoF) limitation, synchronous fault current support and to improve the stability of low short circuit ratio (SCR) grid interconnections just to name a few. Figure 3 shows that defined inertia as additional functionality on top of droops can provide GFM with inertia where needed. In GFM mode, the inverter can control the active power and reactive power to the given set-points; but, not by controlling the current rather by directly adjusting the output voltage (like synchronous generators).
For BESS designs today, there are many factors that need to be evaluated when selecting and quantifying both inverters and battery products to meet a projects requirement. These include Vdc voltage range compatibility for active and reactive power, interconnection requirements, system use cases, and safety considerations among others.
The BESS portion of a power plant today is a customized solution designed specifically for each project. The batteries are arranged in racks in a series connection to achieve the designed DC voltage. The battery racks are paralleled to achieve the designed circuit amperage. Integrators offer containerized solutions based on standard 20’ and 40’ containers. Large battery systems are also integrated in a site building often near the substation. Each design is specific in the needs for access, maintenance, fire detection/suppression, lighting, HVAC, etc. There is a trend to offer integrated solutions in standard building blocks to minimize costly customizations. The control of BESS power plants, whether GFM or GFL, often requires a plant-level controller to coordinate the required inverter functionality to achieve the desired plant response at the point of interconnection (POI).