Some commercial and industrial customers have determined that self-generation or energy efficiency improvements at their facility could make their operation more economic and more environmentally friendly. Many of these businesses have chosen cogeneration. The greater availability and the wider choice of suitable technologies have meant that cogeneration has become an attractive and practical proposition for a wide range of commercial and industrial customers.
Cogeneration also represents the backbone of many microgrids. To the broader grid, a microgrid can be viewed as a distributed energy resource (DER) that supplies the power needs of the commercial facility on whose property it is located. The DER operates in synchronous mode with the distribution system that also serves the load of the facility.
The microgrid’s control system is constantly monitoring the interconnection with the distribution system. When an imbalance occurs on the distribution system - voltage or frequency variations beyond acceptable limits- the control system should be intelligent enough to disconnect the microgrid from the distribution system in a safe and smooth manner thus maintaining undisturbed power to the facility.
Today’s microgrids are modern, small-scale versions of the centralized electric system. Microgrids generally achieve specific local goals, such as reliability, carbon emission reduction, diversification of energy sources, and cost reduction, established by the community being served. Microgrids also provide an excellent catalyst or environment to support localized transactive energy opportunities, especially those that can support functionality both within the microgrid and for the larger distribution system to which the microgrid is connected.
Today, we exist within a hybrid system that is transitioning from load following to supply following. In most cases, the central coordination is able to bend far enough that the grid does not break; but it can only bend so far. We have to find innovative ways to make it bend further as renewables and intermittent resources increase daily. These intermittent resources challenge the grid’s flexibility on an increasing basis.
The Department of Energy’s Smart Grid R&D Program considers microgrids to be a key building block for a smart grid and has established microgrid R&D as a key area of focus. The National Rural Electric Cooperative Association is also looking at the opportunities offered by microgrids. In its 2014 paper, NRECA states its concept that the control systems at different levels should have the same architecture, so that standard methods for integration can be developed.
What is needed in the agile grid, they argue, is to define a standard for the grid and microgrid interface and this interface should be fractal, so that the grid to which the microgrid interfaces could be the broad grid, another microgrid, or a federation of microgrids.
This concept is consistent with the GridWise Architecture Council’s view that transactive energy systems (TES) can exist at many levels within the overall grid infrastructure or behind a single customer’s meter. This conceptual design means that a TES could be comprised of components that are separate transactive energy systems or that any device could potentially participate as an element of a TES.
This fractal nature and applicability of control in markets has already been applied in some pilots and will be a way to achieve the multi-objective control and optimization challenge. The need to integrate markets and control is one reason why transactive energy embraces both the economics and engineering of the power system.
Microgrids make this achievable by optimizing both of these conditions locally. Centralized control by humans is likely not sustainable for the whole grid going forward and more automated control with human supervision will be needed. Transactive Energy may be the most developed framework thus far to lead the transformation from stable central generation to distributed generation variability.
Mark Knight is executive consultant of CGI Utilities and Robert Burke is a principal analyst at ISO New England. Both were early supporters of the GridWise Architecture Council.