Source: U.S. Energy Information Administration
The term smart grid covers a range of devices and systems that leverage recent advances in digital technology and communications to improve the efficiency, performance, and reliability of the existing electric power system infrastructure. Although the smart grid is most frequently discussed in terms of advanced electric meters and other distribution system technologies, it also includes important enhancements to the transmission system. In particular, phasor measurement units, or PMUs, are a new "smart" technology being deployed throughout North America (see map, click to enlarge), monitoring what happens on the transmission grid.
This map shows a snapshot of PMU deployment as of March 2012. The North American SynchroPhasor Initiative (NASPI) reports there are about 500 networked PMUs installed. NASPI expects that approximately 1,000 PMUs will be in place and networked by the end of 2014, a timeline associated with the Department of Energy's Smart Grid Investment Grant (SGIG) program.
Electric power systems are enormous integrated machines. In the United States and Canada, there are four large electric systems, called interconnections, that underpin the provision of electricity service to all consumers. For example, the Eastern Interconnection covers an area from the Atlantic Coast nearly as far west as the Rockies in the United States as well as much of Canada. For these integrated systems to operate reliably, system operators must continuously match electricity generation to electricity demand, within tight tolerances, as demand changes throughout the day. Further, the operation of each component of the electric power system—every generator, transformer, and transmission circuit—must be closely synchronized.
A mismatch between supply and demand or a breakdown in synchronization can put stress on the grid. If these problems are not rapidly identified and corrected, the result can be deterioration in power quality or power outages. PMU data can be used to monitor and mitigate these problems.
Comparing data between points on an electric system is a good way to reveal stress on the system and home in on the source of the problem. PMUs monitor the characteristics of electricity flowing through a particular location, for instance, at the point where a generator connects to the bulk power system, or at a substation. The ability to compare time-synchronized data on the same timescale, among widely separated locations, is a relatively new achievement, based on two major improvements:
• Speed. PMUs make measurements at short time intervals—typically 30 times per second—significantly faster than the conventional supervisory control and data acquisition (SCADA) technology, which makes measurements only every few seconds. (For comparison, electricity alternates at a frequency of 60 times per second on the system.) The more-frequent measurements from the PMUs can reveal system dynamics that would not be apparent with the older SCADA systems (see chart below).
• Synchronization. All PMUs across an interconnection are kept in precise time synchronization using GPS, leading to the term "synchrophasor data" (in this context, the term "phasor" comes from the mathematical representation of the measurement). This synchronization provides the capability to easily compare system data among geographically dispersed units, creating wide-area visibility across large power systems, which was not previously possible using older technology.
PMUs can accumulate large amounts of data. Telecommunications technology—via fiber optic, cable, or satellite—plays an important role in compiling synchrophasor data (example: a company forwarding data from 60 PMUs might require two dedicated T1 lines.). Developing the necessary communications networks is currently a factor limiting many real-time applications of synchrophasor data. PMU capability is comparatively inexpensive—often an optional function on standard equipment—but the costs of adding the necessary networking infrastructure are usually larger, as seen in the data collected as part of the SGIG program.
Developing methods for accumulating, analyzing, and distributing vast amounts of data is a challenge throughout the range of smart grid technologies. While there are a number of software applications in use and under development that take phasor data and turn it into actionable information, the industry is still working to understand what high-speed phasor data reveal about the grid's behavior. That insight is needed to make software applications fully usable.
In time, applications for PMUs may include integrating intermittent renewable generators and automating controls for transmission system and demand response equipment, as well as developing increasingly-efficient use of electric power system infrastructure. On a real-time basis, power system operators and reliability coordinators might use synchrophasor data to enhance situational awareness, preventing transmission grid failures, isolating problems, and speeding outage restoration when blackouts do occur. One application may be automated responses to system disruptions, which can execute faster than manual action.
Offline, synchrophasor data are already being used to improve electric system models used in real-time operations and planning, as well as for disturbance analysis and forensic investigation. For example, the investigation into the causes of the 2003 Northeast Blackout required many person-years of labor, but the investigation of the 2011 blackout in the Southwest is proceeding much more quickly, in large part due to the availability of PMU data. Official recommendations made after the 2003 blackout called out the need for time-synchronized data and its use in wide-area situational awareness.
For more information on PMUs and the use of synchrophasor data, see the recent report from the North American Electric Reliability Corporation (2010).