US power grid stable but faces challenges
- May 11, 2020 7:57 am GMT
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OAK RIDGE, Tenn. - The continental United States has an electrical power system that "is reasonably stable and robust" compared with the grids of many other nations.
"A large countrywide blackout is very unlikely," said Yilu Liu, the Governor's Chair professor for Power Electronics at the University of Tennessee and Oak Ridge National Laboratory since 2009. She recently gave a talk to the Friends of ORNL via a Zoom videoconference.
She noted that the grids in the Bahamas and Puerto Rico are less stable than the United States.
In answers to questions, she indicated that operation of the U.S. electric grid is not seriously threatened by the coronavirus pandemic, lightning or cyber attacks. But, she acknowledged the grid may be vulnerable to a large-scale blackout (such as the Quebec blackout of 1989) as a result of electromagnetic pulses from a rare solar storm that can destroy electronic controls and transformers.
Liu indicated that challenges to grid stability include modernization to reduce the nation's carbon footprint. Coal-fired power plants are being retired and renewable energy sources, mainly solar and wind technologies, as well as batteries and other energy storage technologies, are being added to the power system. The modernized grid also has to accommodate new demands such as the need to regularly charge the batteries of an increasing number of electric vehicles.
Renewable energy sources, Liu said, are often located far from population centers, and the electricity they supply may not always match electrical demand. She is working on helping solve these problems as deputy director of CURENT, the acronym for the Center for Ultra-Wide-Area Resilient Electric Energy Transmission Networks, an engineering research center jointly sponsored by the National Science Foundation and the Department of Energy.
A native of China with M.S. and Ph.D. degrees in electrical engineering from Ohio State University, Liu led the effort as a Virginia Tech faculty member to start Grid Eye to monitor and understand the flow of electrical energy through the nation's power grid.
Grid Eye, which is hosted at UT and ORNL, is also known as the Frequency Monitoring Network (FNET). This low-cost sensing system consists of frequency disturbance recorders (FDRs) plugged into 120-volt outlets. The compact devices, which have GPS antennas and connect to the internet, transmit real-time data on the grid's electrical frequency and voltage angle (indicator of the amount of power transmitted).
Hundreds of Grid Eye FDRs monitor the stability of the wide-area grid just as an electrocardiogram measures and records the electrical activity of the heart to determine its health.
Frequency - the number of waves that pass a fixed point every second - is measured in the hertz unit, named after a German physicist.
"You are trying to keep the power system frequency at 60 Hz but it is dancing around 60 Hz all the time," said Liu, a member of the National Academy of Engineering.
"Whenever there is a major disturbance, which the U.S. gets one to three times a year, that dance becomes more violent." Grid Eye daily detects and records all the fluctuations in grid frequencies not only in the United States but in many other countries. Larger fluctuations, Liu noted, can indicate a generator drop from the grid, a major loss of electrical demand, an earthquake or tripping of renewable energy sources from the grid, temporarily cutting off the flow of electricity.
"Grid Eye is the only system in the U.S. that gives a complete view of what's happening across the grid in North America," Liu said. "It's important to be able to see all areas of the grid, not just one region. What may happen in Florida will affect systems as far away as the Dakotas." Liu and her collaborators Eric Zhan and Wenxuan Yao, both researchers in ORNL's Power and Energy Systems Group, received an R & D 100 award in 2018 for inventing a next-generation FNET sensor. The UT-ORNL app called the Mobile Universal Grid Analyzer enables utility operators to monitor the grid in the field using smart phones and tablets. Now, operators have the situational awareness to catch early signs of abnormal trends so they can act more quickly to restore stability to the power system.
With its improved resolution, Liu said, Grideye constantly monitors the power system like a racecar driver who never blinks while making sharp turns at high speed.
Grid Eye, Liu added, detects oscillations - potentially harmful fluctuations resulting from generator trips, load changes or faults - and provides real-time alerts, enabling utilities to dampen the oscillations in time to prevent damage and stability loss.
"We get alerts each day," she said.
But during the Super Bowl each year, the many alerts received do not usually indicate a generator trip or load shedding.
"The alerts represent the activities of millions of people during commercials," Liu said. "People get up and go to the restroom or the refrigerator.
That causes wild swings in the frequency." Liu said the recorded frequency signatures in the FNET database can be used for criminal investigations of audio or video recordings to ascertain if any tampering (cuts or inserts) has occurred.
The power grid frequency is one of the signals that can be filtered out of the recording and compared with reference frequencies in the FNET database to determine when and where the recording was made and whether it is intact.