By Sonia Aggarwal, Eric Gimon, and the Experts of America’s Power Plan

Ask a distribution grid engineer in Germany or Hawaii how work is going these days, and you’re in for an earful. And policymakers would be smart to listen—while discussions of voltage regulation or “transient stability” can sound overly technical, the truth is that voltage stability in the distribution network is essential to taking advantage of distributed energy resources (DERs).

Understanding the importance of voltage stability

Distribution engineers work on the medium and low voltage grids, as opposed to transmission engineers, who focus on the high-voltage system.  Distribution engineers aim to maintain reliability by planning for peak demand and ensuring that power line voltage stays within a safe range (usually +/- 5–10%) as it reaches into homes and businesses.  Controlling voltage is important for efficiency, safety, and reliability throughout the grid system.  Historically, fluctuations in demand from individual homes and businesses have not been closely monitored, since they are mostly uncorrelated, and overall demand at a service transformer—which can be estimated and measured much more easily than individual customer demand—is what has traditionally been used to estimate distribution equipment lifetimes and local grid stability.  In general, that means there is much less data available about what is happening on the distribution system itself—especially behind the transformer.  As a result, reliability in the medium- and low-voltage grids has mostly been maintained by sizing equipment a little bit bigger than necessary and accepting some inefficiencies.  While this approach has been acceptable in the past, many distribution planners and engineers are now lamenting the lack of “visibility” into operation and control of the distribution system.

Unlike transmission engineers, who run complex power flow models to plan needed infrastructure and operate the grid, distribution engineers often plan based on engineering guidelines and rules of thumb about how many wires are needed and how big they should be, what size transformer is needed, and which other components are needed to maintain reliability.  Instead of relying on data-intensive computer models and real-time monitoring to keep track of what upgrades are needed, distribution planners and engineers run static calculations (validated by measurements of “power quality” from time to time) to understand how the distribution grid is performing.  This can make it a bit tricky to plan for more dynamic distributed generation and demand response programs, which affect the distribution grid by causing rapid fluctuations in electrical current and voltage.

Every time you turn on the microwave or the washing machine, the power you’re drawing from the grid spikes and the voltage on the line near your home decreases.  When your popcorn is done or your clothes are clean, the power draw drops and voltage spikes.  Distribution engineers are quite used to these kinds of fluctuations in voltage on the power lines, although large local fluctuations can still wreak havoc on sensitive electrical equipment.  Because you and all your neighbors aren’t taking orders from the Mayor about precisely when to eat popcorn, these fluctuations in demand all happen at different times and are “seen” in aggregate by the transmission grid as relatively smooth demand.  Transmission engineers and grid operators can then dispatch centralized power plants and adjust large power transformers (sometimes automatically) to meet that relatively smooth demand.  This is how distribution and transmission engineers have traditionally worked together to maintain grid reliability.

Technology is changing voltage stability on the distribution grid

With high shares of distributed energy resources, new technologies begin to erase traditional boundaries and demand more technical sophistication.  A new operational element will be added to the distribution system—correlated fluctuation.  Consider these two possible scenarios:

• The majority of homes in one particular neighborhood have solar on their roofs. They’re producing a great deal of power at noon, so much that some of the energy is feeding up into the grid to power surrounding neighborhoods.  Suddenly, a large cloud comes overhead and—at once—the power supply into a given line drops.  Minutes later, as the cloud passes, the power supply once again comes online, causing a spike in voltage.
• A real-time pricing scheme interacts with automated home controls to manage an oversupply of energy on the grid. The automated systems all respond at once when the pricing signal goes out, turning on electric hot water heaters in homes and businesses across the territory.  This results in a sudden surge of demand.

Both scenarios highlight bulk grid requirements that are impacting the distribution system more and more—additional generation close to demand in the first example, and a way to absorb an oversupply of generation in the second example.  What’s more, both scenarios result in voltage instabilities that can overwhelm distribution equipment and circuits that were built for fewer fluctuations.  As a result, the distribution system finds itself needing to act more and more like the transmission system, using improved situational awareness and more dynamic and distributed control.

This is the kind of thing that keeps Hawaiian and German distribution engineers up at night.  But even in Hawaii (where distributed solar makes up 24% of peak load in HECO’s territory) and in Germany (where it makes up 35% of peak load), these correlated fluctuations have not yet caused any major reliability issues.  Of course, Germany’s Distribution System Operators already rely on power electronics to help manage distributed resources.  These percentages are a relief for the vast majority of the world, where penetrations are currently only at a fraction of one percent.  Still, these are important scenarios to consider when planning for distributed energy solutions to contribute to a well-functioning, least-cost power system.

New technologies can help anticipate and manage emerging voltage stability issues

Several technological solutions are emerging now to manage the distribution-level effects of transmission-like needs.  For example, smart inverters and smart appliances could be pre-loaded with settings that randomize when they come on and go off, enabled to adapt to location-specific schedules, or programmed to automatically modify production when they sense abnormal conditions.  More physical sensors in the distribution grid can also help manage variability, both by feeding better information into planning processes as well as by allowing more optimized ongoing operations.  Likewise, better data management can make a big difference—predicting demand profiles, and related infrastructure needs, will benefit from knowledge of important characteristics (location, size, controls) of distributed energy resources, in conjunction with better integration of weather data.

Smart policy can provide fertile ground for these technological solutions—and others like them—to be deployed early, before distribution grid stability or power quality ever become an issue.  Regulators and policymakers can take several important steps now:

• For regions that anticipate a great deal of distributed resources, allow cost recovery for sensors and other equipment that provide situational awareness and visibility into the distribution grid.
• Support the use of cost-effective, innovative equipment that can correct for dynamic fluctuations—such as power electronics-based in-line voltage regulators—as alternatives to traditional distribution grid capacity expansion aimed at accommodating peaks.
• Begin including medium- and low-voltage power quality on the list of utility performance metrics.
• Ensure utilities take advantage of third-party assets, such as internet-connected energy management or reporting systems, and develop new information and control layers to manage the distribution grid.
• Give distribution system operators the capability to manage pricing schemes that can prevent the markets from creating critical conditions on the grid.

Preparing for a dynamic power grid that takes advantage of distributed energy resources is very doable, but it pays to take the proactive steps listed here while penetration levels are low.  Awareness and early action will mean we manage this transition smoothly and cheaply.

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Thank you to Maria Tome, Jeff Lo, Gerhard Walker, and Aram Shumavon for their input on this piece. The authors are responsible for its final content.