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Knowing Your Grid Forming Inverter

In remote locations where the traditional utility power grid is inaccessible, standalone microgrids, powered by renewable energies like solar and wind turbines, are playing a crucial role in generating electricity for the local community. In addition to generating power to meet the electricity needs of islands and remote communities across the world, microgrids also see application within conventional power grid locations by helping utilities in times of outages by delivering reliable power to neighborhoods, hospitals and mission-critical buildings.

Year after year, power outages are becoming a common phenomenon during storms and hurricanes. We all remember the havoc caused by superstorm Sandy of 2012. Hopefully, with the emergence of microgrids, the devastation of power outages can be controlled effectively.
Wall Street Journal, market research firm Greentech Media’s chief executive Scott Clavenna told writer Michael Fitzgerald that “the need for resiliency has led to the development of microgrids.” Consequently, the U.S. Department of Energy (DoE) has begun funding the design and development of microgrids, while several state governments have also begun rolling out microgrid programs for emergency use, says the WSJ report. The state of Connecticut, for instance, this year will be unveiling a number of microgrids for emergency use.
Because renewable energy sources generate DC or variable AC voltages, they require an inverter to interface with and help regulate or “form” the microgrid. Accordingly, these inverters are called grid forming inverters (GFI). Unlike grid-tie inverters (GTI) which typically interface to a utility grid with well-regulated voltage and frequency, GFIs are tied to microgrids where they are required to assist in the regulation of both voltage and frequency.

Since several different renewable energy sources typically feed a single microgrid, GFIs must be capable of operating in parallel. In essence, these grid forming inverters must be designed as parallel-able voltage sources with very good load sharing capability, while maintaining a stable AC output voltage and frequency with varying loads.
Frequency regulation is typically achieved by controlling real power (kW) while voltage regulation results from reactive power (kVAR) control.  Furthermore, droop control methodologies can be employed to allow GFIs to operate in parallel without the need for communication or synchronization.