Photos taken during the 2012 Hurricane Sandy disaster almost literally turned the spotlight onto microgrids.
Images posted on social media and in the news during the storm showed swaths of Manhattan plunged into darkness as power outages cut off electricity to large parts of America’s biggest city.
Just as striking, however, were blossoms of light visible against the otherwise black skyline.
Many of these lighted outposts had separated from the grid and were now generating electricity on their own. These microgrids were islands of light in a sea of darkness. Facilities such as hospitals were able to provide critical services both during and after the storm because of microgrids.
Now, Ameren Corp. has completed a $5 million microgrid at its Technology Applications Center adjacent to the University of Illinois campus in Champaign, Ill. The facility is one of the only utility-scale microgrids in the United States that serves live customer loads on an actual utility distribution feeder.
If the grid-connected electric distribution line fails or is knocked out by a storm, the Ameren microgrid is intended to seamlessly transition to island mode and provide 180 residences and 12 commercial buildings with power from dedicated wind, solar, and natural gas resources, backed up by a bank of lithium-ion batteries.
The microgrid, which has already been show to work as intended, will now be tested in order to learn how it can improve electric reliability, says Ron Pate, Ameren Illinois senior vice president of Operations and Technical Services. It also will be tested to receive and respond to market price signals, potentially adding value to still wider microgrid deployment.
An Array of DERs
The U.S. Energy Department defines a microgrid as “a group of interconnected loads and distributed energy resources (DER) with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid [and can] connect and disconnect from the grid to enable it to operate in both grid connected or island mode.”
The Ameren microgrid can produce up to 1,475 kilowatts. Leased generation assets at the site include:
- Northern Power Systems Wind Turbine – 100 kilowatts
- Yingli Solar Array – 125 kilowatts
- Caterpillar Natural Gas Generator – 1,000 kilowatts
- S&C Electric Co. Battery Storage – 250 kilowatts
Pate says the battery storage equipment is a critical element of the Ameren facility. The Ameren microgrid is among the first in the United States to adopt S&C’s PureWave SMS-250 Storage Management System, which provides 500 kilowatt-hours of lithium-ion battery energy storage. The SMS-250 stores energy from both the utility and the facility’s generation sources, provides the voltage signal for the microgrid, enables renewable integration and smoothing, and controls microgrid frequency and power quality.
So, what happens when the grid goes down? The battery storage, designed to provide electricity for around 60 seconds, allows renewable and natural gas generating assets to ramp up and come online with no service interruption for customers. In that way, battery storage is the “key to the seamless transition to the microgrid,” Pate says.
Sitting Out Front
Heretofore, most microgrids have operated behind the meter to support a specific building—like one of those New York City hospitals that had electric power in the aftermath of Hurricane Sandy—and a specific load that generally is well balanced, says Tamer Rousan, supervising engineer for distribution and automation at Ameren Illinois. Because the Ameren microgrid sits in front of the meter and is connected to the grid, it is fully exposed to system imbalances and issues related to harmonics and frequency regulation.
For example, Rousan says that the microgrid’s natural gas generator set is designed to trip offline if imbalances exceed 10 percent. System monitoring and control equipment, therefore, are especially important to manage the grid-facing system.
The Ameren microgrid uses two levels of control to address these operational challenges. First, each generating asset—solar, wind, natural gas, and storage—has a controller interface to manage its output and frequency. Also important, but a bit more complex, Rousan says, is communication between control devices and each of the distributed resources that provides tighter frequency and voltage control. The controllers share information, then make decisions regarding frequency and voltage adjustments across the assets.
Rousan says that if the microgrid’s solar array is putting out, say, 125 volts and the wind turbine is generating 120 V while the nominal output is set at 122 V, the controller trims solar output and adds wind to balance the microgrid’s overall output.
“Normal operation is different than island mode,” Rousan says, because the microgrid must be able to adapt instantaneously to operational changes as they occur on the much larger Ameren system.
A third layer of operational control will enable the distributed generating resources to respond to market price signals. These might indicate, for example, that it makes good economic sense for the microgrid to activate the battery storage or switch on the natural gas-fired gensets. The controls will also be able to forecast solar and wind conditions as well as load changes.
The microgrid was completed at the end of 2016 and was tested during early 2017. As part of that testing, Ameren installed a 200 kilowatt load downstream of the switch that feeds electricity to the 192 customers on the microgrid. In turn, each distributed generating resource was tested to benchmark how it performed both in island mode and when connected to the grid.
The tests confirmed that the utility can proactively and seamlessly transition the circuit to island mode and shift the load onto the microgrid. “We are able to operate the two systems in parallel, bring up the generating assets and disconnect from the utility,” Rousan says.
The Ameren microgrid operates at utility-scale voltages of between 4-kilovolts and 34.5-kilovolts, with multiple levels of control.
In addition to the microgrid, Ameren Illinois will begin installing 83,000 smart meters at customer locations in Champaign County. These two-way devices will provide customers with energy use data and access to software intended to help them reduce energy consumption and save money.
More broadly, the utility will explore possible opportunities to use microgrid technology as a service for customers willing to pay a premium for enhanced reliability. What’s more, Pate says, microgrids might also help the utility delay—or avoid altogether—major capital expenditures for circuit upgrades in some locations.
Contributing Editor David Wagman has been covering energy issues for three decades, focusing on all forms of electric power generation, regulation, and business models. He is particularly interested in the ongoing electrification of advanced economies and the effects that distributed generating resources could have on efforts to decarbonize national grids. Wagman, who is based in Colorado, is currently editorial director for IEEE Engineering 360, a search engine and information resource for the engineering, industrial, and technical communities.