Microgrids can be engineered to connect to utility systems with switchgear and protection systems that are meant to ensure safe and smooth operation through all modes – i.e., fully grid-connected, islanded, and during transition, while an islanded system is preparing to re-connect to the grid. Some – but not all – microgrids are fully synchronized to the alternating current (AC) on the utility feeder line, matching the microgrid’s power to the voltage, frequency, and phase angle of the utility’s po werMore
Distributed energy resources (DER) have served Minnesota residents and communities since the earliest days of electricity-powered machines. Indeed, before the Rural Electrification Act of 1936 and electrification efforts in the 1930s and ’40s, such companies as Delco-Light sold gasoline-burning “electric plants” for homes and farms that hadn’t yet been connected to utility systems. Many farms in Minnesota used “wind chargers” combined with lead-acid batteries and internal-combustion generators to provide power for lighting, pumping, welding, and other farm applications. The LeJay Manufacturing Company in Minneapolis distributed a popular farm equipment catalog in the 1930s and ’40s that featured parts and instructions for wind chargers and related electrical equipment.3
Such isolated power systems might be considered “quasi-microgrids”: not true microgrids in the strictest sense of the word. In most of the industrialized world, a microgrid is assumed to have a connection to a utility grid, except during times when it’s intentionally islanded – disconnected to operate in isolation, for instance during a storm outage.4 (See Appendix B: Microgrid Definitions.)
Minnesota also has ample experience with two other forms of quasi-microgrids: cogeneration (or “CHP” for combined heat and power) plants, such as those at numerous paper mills and other industrial sites; and district energy systems, like those in downtown St. Paul and Duluth. These systems, however, are more limited in their functions than today’s sophisticated islandable microgrids. In the case of district energy plants, they generally provide thermal energy and not electricity service, and most CHP plants serve only dedicated loads, independent from utility service. Nevertheless, these systems provide familiar analogues for the Minnesota microgrids concept – and in some cases they might also represent the building blocks for bona fide microgrids.