IEEE Power Perspectives
Microgrids: Consulting-specifying engineers must be ready
Microgrids appear poised for a wide adoption. As a result, consulting-specifying engineers should be aware of how microgrids affect substation designs in commercial and industrial applications.
Microgrids are typically grid-tied entities that balance and even orchestrate multiple-generation sources (often including distributed generation) and multiple loads. One major advantage is that microgrids can “island” themselves from the grid, when the latter is experiencing faults, voltage volatility, or other power quality-related disturbances.
When designing a substation for a commercial and/or industrial facility that intends to become part of a microgrid – or wants to preserve that option in the future – certain specifications and considerations must be addressed early on in the process. This is particularly true if the microgrid owner/operator seeks to put excess power back onto the grid.
In fact, early consideration of a microgrid can be cost-effective for an organization. Working the needed equipment into the substation design prior to adopting a microgrid can be a cost-effective way to keep an organization's options open. Retrofitting after the fact is much more expensive and less optimal in practice.
The consulting-specifying engineer should utilize the IEEE 1547 "Standard for Interconnecting Distributed Resources with Electric Power Systems." The IEEE 1547 series defines interface points, which are points to be monitored and metered. In addition, the standards provide protection and control strategies for substations handling a bi-directional flow of power. These standards are currently being further developed in recognition of the widespread interest in microgrid capabilities. Utilities are beginning to design the grid in a way that recognizes all loads beyond merely the interconnection with the microgrid, and they will be supported by a microgrid’s generation sources.
The IEEE 1547 series of standards will provide the consulting-specifying engineer with the knowledge they need to address substation design in fields such as equipment, monitoring, wiring, and metering. Just as an electrical grid needs a supervisory control and data acquisition (SCADA) system and a control room, a microgrid requires a control center of sorts, which could be just a PC on a desk. But generation sources need to be matched to loads, regardless of a microgrid’s scale. Perhaps loads are increased or reduced to match current generation output, but the two sides must be matched.
The consulting-specifying engineer needs two controlling technologies in the substation in the form of remote terminal units (RTUs). First is for the utility, and the second is for the microgrid owner/operator and the “control center.” This provides the utility with data on the microgrid’s status. If the microgrid islands itself, the utility’s grid must adjust for the drop-off in load.
In fact, if the facility plans to oversize its distributed generation capacity to inject excess power back onto the grid through net metering, then substation design must accommodate the bi-directional flow of power. The objective is to address the utility’s need for situational awareness, meaning the visibility it needs to monitor and manage how that bi-directional flow affects its grid. The consulting-specifying engineer must design for a greater degree of instrumentation in a substation that will handle a bi-directional flow of power. The design depends on scale. If the bi-directional flow never exceeds 1 to 2 MW, situational awareness is achieved in a specific manner. If the scale of bi-directional power is in the 100 to 200 MW range, however, a different set of design considerations apply.
The consulting specifying engineer must also understand the local utility’s requirements for situational awareness. For example, what data points does the utility require for its own monitoring, control, and protection schemes? If this is not assessed and addressed early on in the design process, unanticipated costs are likely to crop up later. The consulting-specifying engineer may even play the role of a mediator between client and utility in this context.
Utility requirements for the substation may be specific, extending down to the level of vendor and device. For situational awareness, a utility may require specific RTUs and specific protocols to integrate with its supervisory control and data acquisition (SCADA) and energy management system. At transmission-level voltages, the utility may require specific protective relay equipment. Utility line crews may need to access the substation in question to confirm that no power will flow while they’re working. Thus, utility-side needs will influence the physical design of the consulting engineer client’s substation, even the access controls.
Given the cost-effectiveness of addressing these issues up-front in the substation design for the client, and the critical nature of utility concerns that must be met, the consulting-specifying engineer is well-advised to begin these conversations at the earliest opportunity.
Sam Sciacca is an active senior member in the IEEE and the International Electrotechnical Commission (IEC) in the area of utility automation.