Writing a Handbook for Large-Scale PV

With the increased popularity of solar energy among industrial, commercial, and residential users comes increased pressure on utility companies to significantly expand the amount of solar energy they distribute. At the same time, utilities must ensure the same high quality power on which customers rely.

To understand the impacts and mitigate the effects of large amounts of solar photovoltaic (PV) on the distribution grid, the National Renewable Energy Laboratory (NREL) is leading a series of studies related to the interconnection of high penetrations of PV into distribution systems throughout southern California.

The project aims to create a framework for understanding what utilities need to do to integrate solar PV systems at high levels, how these high penetrations of solar impact the distribution system, and what installation requirements should be imposed on the interconnection agreements required by utilities. One of the key project deliverables is a handbook of best practices for any state or utility looking at interconnecting large PV distribution systems.

“We hope to make distribution system operators more comfortable,” says Barry Mather, NREL project manager. “In California, there are very rough rules of thumb for planning and implementing large-scale PV. This used to be okay because PV systems weren’t very large, so distribution planners didn’t worry about them too much. But if everyone has solar on their roofs, it becomes a big deal.”

Field testing is an essential component of the project. By installing PV on commercial rooftop systems owned and operated by Southern California Edison (SCE) – the utility partner whose distribution system is being used as the field testing mechanism – impacts of high penetration PV on any point in the distribution are assessed.

The project will evaluate several systems as part of SCE’s plan to install 500 megawatts of distributed solar in its service territory during a 5-year period. This represents the highest level of distributed PV in any utility service territory in the United States. The installation’s biggest challenge will be voltage regulation.

“When you start putting PV onto those distribution systems that feed power to houses and industrial and commercial customers, it reverses the power flow with large consequences on voltage levels,” says Mather. “We’re not proposing a redesign of these feeders, but a way to coordinate between the existing systems to ensure good voltage and high-quality power at all times.”

One of the most surprising findings to date is that voltage regulation is being done solely through switch capacitor banks throughout the distribution feeder. “We’re going to shy away from a centralized control approach. The voltage regulation function needs to be able to act in an independent manner, otherwise it becomes an overly complex control problem,” Mather explains. Understanding the benefits of this control will provide more value to distribution system operators in the long run. “Utilities love those sorts of experiences – they can learn from them without having to go through them directly.”

In addition to NREL and SCE, partners on this project include:

  • Quanta Technology, helping develop models of the distribution system
  • Clean Power Research, providing resource data for the different PV system locations
  • Electrical Distribution Design, developing the necessary distribution modeling tools for evaluating high-penetration PV scenarios
  • SATCON, developing and manufacturing the advanced PV inverters.

Mather concludes, “We have all the players in place to address the problem from a total systems integration approach. It allows us to do much more than just analyze the problem. At the end of the day, we’re going to fix it.”

Learn more about the NREL project or read the Southern California Edison High-Penetration Photovoltaic Project – Year 1 report.



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