Searching for Real-Time Measurements of Electric Grid Health

Searching for Real-Time Measurements of Electric Grid Health

September 29, 2014

Today's electric grid faces an array of threats and opportunities—threats from aging equipment, catastrophic weather and other factors, and opportunities from clean, customer-sourced of power such as solar and wind generation, and demand response (DR). A key to managing the new grid is to be able to measure its health in real-time—faster than second to second. Measuring a key grid parameter, "synchrophasors," offers a solution to the real-time measurement problem.

A new project, funded by the Advanced Research Projects Agency-Energy (ARPA-E), aims to give electricity grid operators a futuristic, microsecond-to-microsecond measurement of the state of electric distribution lines using "microsynchrophasors." Scientists at the University of California, Lawrence Berkeley National Laboratory (Berkeley Lab), the California Institute for Energy and Environment (CIEE), and Power Standards Lab of Alameda, California, are performing the research.

The project's goal," says Emma Steward, an engineer in the Environmental Energy Technologies Division (EETD) of Berkeley Lab, "is to learn how to use phase angles [phasors] on distribution systems to better understand what's happening on these systems."

Transmission and distribution lines carry our power from sources to consumers. Transmission lines carry power over long distances at high voltages. The distribution transformers and lines step down this power, sending it to homes, businesses and industrial facilities. Synchrophasors are already beginning to be deployed on transmission lines. The groundbreaking ARPA-E research seeks to bring this real-time measurement capability to the electrical distribution system.

From One-way to Multi-Directional Power Flow

Just 20 years ago, most of the power flowing through the electricity grid was traveling from mainly utility-owned power plants to customers. Those plants were usually coal- or natural gas-fired, or nuclear. Almost all of the customers, except some large industrial facilities with onsite power generation, were consumers, not producers of power.

Things changed in one generation. Today, renewable power sources provide a growing percentage of power on the grid, helping reduce climate by lowering greenhouse gas emissions. These sources are intermittent—the power flows when the sun shines or the wind blows.

Another change from the past is that customers have options to interact with the grid. They can reduce their power consumption during periods of high power prices through demand response (DR) programs. DR is becoming automated (AutoDR) and widely adopted thanks to R&D at Berkeley Lab's Demand Response Research Center (DRRC).

Industrial and commercial power consumers are creating microgrids—producing power for their own facilities distributed through small grids that can be disconnected from the larger electric grid, and reconnected to the grid when the consumer needs additional power, or has an excess to sell.

The electric power industry is more complicated today, but it offers more opportunities for clean energy and customer engagement.

These changes require electric system operators (known as independent system operators, ISOs) to have a more accurate read of the state and the health of the power grid in real-time. Instabilities in power, caused by fluctuations in supply and demand from the intermittency of renewables, sudden changes in demand response availability, demand changes caused by weather, catastrophe, or equipment failure, and the general aging of the grid infrastructure mean that there are more threats to delivering power reliably and continuously. The electric power industry needed a better way to know what's happening on the grid in real time, to respond more quickly to these threats.

Synchrophasors: A revolution in grid measurement

The electric power industry found a solution in the form of synchrophasors.

Power is traditionally transmitted in the form of an alternating current. The current looks like a sinusoidal wave, with peaks and troughs. However, the waveform produced by each plant will not peak and bottom out at the same time as any other plant—the power that plants produce is not perfectly synchronized. Phase angle is a measure of the difference between two sinusoidal waves—it measures how far the peaks and troughs are from one another.

Power engineers know how to measure a quantity called a phasor at any point on an electrical grid with an instrument called a phasor measurement unit (PMU). Phasors are a measure of the phase angle, and the magnitude of voltage at a certain point on the grid. Measuring a large number of phasors at exactly the same time, a measurement known as a synchrophasor, tells grid managers something about the health of the grid.

Synchrophasors can tell them, for example, whether the demand for power matches supply, whether the power is flowing in the correct directions from supply to demand, and if there are any fluctuations in power that might cause grid instability. The precise, nearly simultaneous measurements are made possible by GPS systems, which can provide timing down to the microsecond time scale (one-one millionth of a second).

Today, synchrophasor measurements on the transmission lines of the electric power grid are becoming more common. Thanks in part to research and demonstration projects managed by the Consortium for Electricity Reliability Solutions (CERTS), based at Berkeley Lab, ISOs throughout the U.S. are expanding their use of synchrophasors.

Measuring conditions on distribution lines, however, provides a distinct challenge, one that the ARPA-E project is addressing.

Distribution Systems Need More Accurate Measurements

Distribution systems are the portions of the grid where electric power is stepped down from a transmission line's high voltages to voltages appropriate to household, commercial and industrial customers by transformers at substations, and transmitted to homes, businesses and industrial facilities.

"We think that measurements of 'microsynchrophasors' will provide a better visualization of what's going on, and it will allow ISOs to detect problems on the distribution system and plan their responses before problems run out of control," says Stewart."

Because the power flow through transmission lines is smaller, the equipment must be able to measure phase angles that are at least an order of magnitude smaller than on transmission lines. The signal will also be noisier because of interference from the devices connected to the grid by consumers and from utility equipment at transformer stations. One of the research partners, Power Standards Lab, has developed a microsynchrophasor measurement unit (µPMU) capable of making the accurate measurements required on the distribution grid. Synchrophasors are typically measured 24 times per cycle. The prototype µPMU can take 512 measurements per cycle.

Multiple Program Elements

The research partners are each leading a different research activity. UC Berkeley team members are developing a network capable of recording and processing data and communicating with µPMUs installed in the field. They are also studying how the data can support diagnostic of problems on distribution systems. PSL developed the µPMU device and is evaluating its performance. The California Institute for Energy and the Environment is studying how µPMU can be used in controlling applications on the grid such as managing microgrid connection to the larger grid. Berkeley Lab leads field-testing of µPMUs on several utility grids.

With ARPA-E's funding, Berkeley Lab is testing µPMUs in four electric utilities' service territories. When the pilot installations are complete, there will be one at each partner utility. About 10 µPMUs will be split over one or two distribution feeders at each site. In addition, seven µPMUs have been or will be installed on one of Berkeley Lab's distribution feeders (Figure 2). Data from the devices are collected wirelessly and sent via a 4G network to a database on the UC Berkeley campus. The testing will last two years. Stewart and her colleagues are also modeling distribution circuits, and will use data from the field tests to validate their model.

"One application of this research will be to understand what happens when a microgrid synchronizes and desynchronizes from the electric grid," says Stewart. The measurements can help coordinate resources between microgrids and the electric grid.

The research will also evaluate whether the systems could improve the management of demand response. "A goal of the study is characterizing the loads during demand resources," Stewart notes. "When customers turn loads on and off at the same time, it could affect the state of the grid.

Microsynchrophasor measurements are ideal for detecting and locating faults that might lead to instabilities on the grid resulting from sudden imbalances in supply and demand." They also have the potential to help system operators better manage the use of intermittent renewable sources of power, and to help match supply to demand through demand response programs and grid storage.

"Utilities are excited about this research," says Stewart. "They are always looking for more information about the state of the grid."

Allan Chen