Striving Toward 100% Renewable Microgrids: NREL

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Studying How to Create 100% Renewable Microgrids

If you’re interested in following the latest research into microgrids — especially 100 percent renewable microgrids — keep an eye on the National Renewable Energy Laboratory in Golden, Col., which has a research microgrid.

Among the microgrids under study are microgrids on islands, microgrids on military bases, and microgrids that are part of a utility grid, says Shanti Pless, senior research engineer for NREL.

NREL’s research microgrid, part of its Energy Systems Integration Facility, is a microgrid with PV simulators and grid simulators, he says. Research partners can plug in and test new energy technologies on real and simulated power systems before they connect them to the grid.

This allows NREL and its partners to simulate and test what will happen when certain components — solar inverters, for example — are hooked up to the grid.

Partners, such as utilities, can connect equipment and then begin testing in a few days.

NREL is studying how much renewable energy can be integrated into a microgrid, says Pless. That’s an important topic because a number of utilities, especially on islands like Hawaii, have available to them more renewable energy than they can integrate.

NREL is demonstrating that, in places like Hawaii, it is indeed possible to have microgrids with higher-than-previously-allowed levels of renewable energy, says Pless.

Hawaii has a new law (SB 5445) requiring 100 percent renewables by 2045.

“Renewable microgrids are possible with the right amount of storage and management of generation and loads in a grid. We’re demonstrating that can be done,” he says.

Using smart inverters is key to integrating more renewables, Pless says.

The lab conducted “inverter load rejection over-voltage” (LRO) tests, along with SolarCity, examining ways to quantify over-voltage events — a major barrier to bringing more distributed energy onto the grid.

LRO events take place when a local feeder or breaker opens and the power output from a distributed energy resource exceeds the local load. The NREL testing helped address challenges created by PV inverters, says NREL.

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Energy Storage
Energy Storage in Smart Microgrids
All power systems also require the capability to balance, in real time, differences between load demand and generator output. This balancing capability—which keeps system frequency at the level where it’s designed to operate—is critical to ensure the stability and reliability of the overall power system, for both centralized power grids as well as microgrids. In a centralized grid, fossil-fuel-fired generating plants typically will ramp up or down to ensure demand and generation output remains in balance.
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The NREL Tests were designed to determine the length and magnitude of transient over-voltage events created by several commercial PV inverters during load-rejection conditions.

As a result of this work, HECO filed with the Hawaii Public Utilities Commission a plan to boost its penetration limits for rooftop solar — from 120 percent of minimum daytime load to 250 percent of minimum daytime load.

“If those increases are implemented, they will represent the highest threshold for solar penetration on distribution circuits in United States,” says NREL.

A whole new set of inverters, called smart inverters, can protect against over-voltage problems. “Smart inverters  can enable more PV systems to be integrated onto the grid,” Pless says.

“The industry is striving for 100 percent renewable microgrids at the island scale,” says Pless. “The hardware and technology is coming along. It’s about integrating it and creating multiple value streams.”

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