As more distributed energy resources (DER) — primarily solar — are connected to electric utility power distribution systems, the equipment that makes it all work safely and reliably is under increasing stress. Compounding this, much of this equipment — conductors, transformers, relays, breakers, etc. — is reaching or even beyond its intended end of life.
As solar generation is installed in distributed locations, traditional planning and distribution power system management assumptions are being challenged. Additionally, increased load in traditionally off-peak times from electric vehicle (EV) charging is adding complexity. Bidirectional power flows on aging infrastructure are causing thermal overload of the primary conductors farthest from the source station, limiting the hosting capabilities of the lines.
For many decades, electric utility distribution systems could be designed and managed with a great deal of predictability. Historically, most distribution systems have been configured in a radial pattern with circuits originating at distribution substations. Distribution planners were able to assume that power would flow predictably one way over the lines and that voltage would decrease as it moved farther away from the substation. In this model, transformers and other equipment would cool overnight as load could be expected to decrease.
Now, as new DER sources are connected to the distribution system, those assumptions are no longer as valid. Voltage will be higher near a source of solar generation, and this power is likely to fluctuate multiple times each day as cloud cover and other factors come into play. Further, EV charging shifts demand from the more traditional load curves.
As the penetration level of DER rises, it causes a couple of issues that potentially lead to grid and distribution system instability. The first is that voltage levels are increasing beyond the limits set by safety codes and standards. When power reaches the meter at homes or businesses, voltage levels must be consistent to be sure all mechanical systems, appliances, motors, and devices are not damaged. Voltage regulation is now increasingly required throughout the distribution power system.
The second issue is that sudden upsurges or losses of solar power can affect frequency of the alternating current, creating power quality issues. These sudden fluctuations can knock power out of phase.
Distribution planners increasingly must assume that power supply and load can appear anywhere, requiring protective devices and voltage settings to be adaptive to fluctuations that can sometimes move up to 100 times a day on a single circuit. Phase imbalances can appear and disappear in minutes as sources of DER come online.
Supply chain challenges
Of course, many electric utilities undertaking system upgrades are facing the same supply chain and labor challenges that are slowing projects in many other sectors. For example, a 10-mile distribution system upgrade now underway for one of our clients will take at least eight months to build, but the schedule is further delayed as cable material lead times have increased from eight weeks to roughly 22 weeks, with equipment and components up to six months out. With many thousands of miles of circuits needing upgrades, this pace will certainly become problematic as many distribution system sections already are facing stress.
The obvious solution will be a staged approach, with projects addressing the most urgent needs first and including upgrades that will enable system planners to anticipate and manage sectors that are likely to experience the most DER connections and load growth.
As electric utilities grapple with the surge in needed upgrades for both transmission grids and distribution systems, some adjustments in filings for regulatory approvals for large capital programs will be needed.
Thinking long term
The need for a broader, holistic planning approach is gaining momentum as more electric utilities begin to realize the full scope of today’s challenges. Still, these long-term planning needs are sometimes taking a back seat to other priorities.
For one Midwest electric utility currently undergoing a large capital program to harden existing distribution assets against severe weather impacts, the budget does not allow capacity additions that could truly prepare this power distribution system for the future. Though the program will reduce duration and number of customers impacted by outages, the system will be ill-prepared for DER connections and the inevitable increase in EV charging demand. To alleviate issues like these, electric utilities will need to work closely across departments and functions, communicating clearly with their regulators about the relatedness of various in-flight programs.
Adam Rehfeld is business line manager for distribution grid modernization at Burns & McDonnell. With more than 15 years of experience, Adam specializes in leading teams to transform increasingly complex power grid challenges into safe, reliable and cost-effective solutions.