Phasor measurement technology garnered the attention of electric utilities following a significant disruption on the Western U.S. power grid in August 1996. At the time, Southern California Edison (SCE) was conducting preliminary research on using synchrophasor measurements for electric utility application.

The utility also was collaborating with the Western Electricity Coordinating Council (WECC), U.S. Department of Energy (DOE), California Energy Commission and Electric Power Research Institute to advance the technology for preventing wide-area catastrophic outage events and enabling quicker restoration of systems after major disturbances.

What Phasors Do

By 1996, growing evidence indicated phasor technology could offer several benefits:

• Provide in-depth insight into post-event analysis of disturbances by capturing system dynamics
• Allow instantaneous assessment of system performance and stability (situational awareness)
• Enable data visualization and real-time monitoring of power system electrical quantities
• Determine available transmission capacity in real time.

In response, SCE began deploying phasor measurement units (PMUs) at its 500-kV and 230-kV substations to validate these claims. By 2007, the utility had 18 PMUs and two phasor data concentrators installed, and was monitoring and analyzing system disturbances on its system as well as the WECC interconnection.

Synchrophasor technology measures system states on the transmission grid by computing the magnitude (rms) and phase angle of system voltage and currents. This provides electric utilities the ability to determine power flow limits based on the phase angle difference between the 60-cycle voltages at the ends of transmission lines. Being able to measure these voltages in real time enabled SCE to pioneer technologies that quickly alert grid operators of growing phase angle separations (an indicator of stress in the transmission system), so the utility can take the appropriate actions for avoiding large-scale blackouts.

Data to Information

SCE realized the need for and began developing software that enabled engineers, planners and grid operators to synthesize the data captured by the PMUs on its system.

In 2000, the utility unveiled Power System Outlook (PSO), which provides offline viewing with analytical capabilities for monitoring and analyzing data regarding grid conditions. It also enables the assessment of dynamic behavior of the system by evaluating inter-area oscillation frequencies and damping ratios.

PSO made it possible for SCE engineers to analyze a variety of phasor measurements: voltage, current, power, reactive power, frequency and frequency deviation, rate of frequency change at PMU locations, phase angle differences from selectable bus references and percent of deviation for voltage. PSO also proved capable of feeding data measurements to a voltage phasor display.

The Northeast blackout of August 2003, which affected 50 million utility customers in eight states and Canada, validated the need for more advanced synchrophasor applications that could at least monitor events on the grid as they occur. As utilities in the Eastern United States began aggressive phasor development and implementation programs, SCE became an active member of the North American Synchrophasor Initiative (NASPI). The utility also developed a system operations tool called SCE Synchrophasor Measurement Analysis in Real Time (SCE SMART).

SCE SMART built on the analytical functions of the PSO software by adding enhanced real-time tools for event recording, playback capability and continuous data archiving. This provides operators and engineers with synchronized data on system stress and stability at an unprecedented 30 scans/second. SCE's grid control center then stores streaming, event and compressed data files and uses the software to monitor voltage, frequency, power imports and path flows on the system.

Intermittent Resources

Using the PSO and SCE SMART software, SCE has been investigating the impacts of integrating greater amounts of intermittent renewable energy into its grid. The software is playing a prominent role in SCE's Tehachapi Renewable Transmission project, which is expected to produce 4,500 MW of wind generation.

A primary feature of phasor data is its model validation capabilities, and SCE is using its phasor data to enhance the modeling and simulation work using a real-time digital simulator (RTDS) at the utility's Advanced Technology laboratories. The study examines the impact of the intermittent nature of renewable resources on the grid, and phasor data greatly improves and validates the accuracy of the models.

Research projects such as these empower SCE engineers to investigate other phasor applications such as intelligent control of bulk power components, remedial action schemes and eventually closed-loop control.

A First in Voltage Control

The utility's static VAR compensation (SVC) project at its Rector Substation is the first application of its kind to use synchronized phasor measurements in a closed-loop dynamic control scheme. The objective of the project is to integrate the synchrophasor voltage data from the PMU installed at Big Creek and use it in the SVC controller to manage the complex system generation and maintain voltage stability on the system at Rector. Thus far, SCE has been successful at avoiding overvoltage conditions at the Big Creek Generation Station, where a PMU monitors and streams data back to the utility's grid control center for use by system operators.

Presently, SCE is working to develop a voltage and VAR control system at one of its bulk power substations using phasor measurement technology. The objective of this program is to coordinate multiple reactive power (VAR) devices so they do not counteract each other's functionality. This program will automate voltage control function at bulk power substations while optimizing VAR resources.

Adaptive Protection System

While the use of phasor data for protection is still in its early stages, SCE is participating in a DOE grant to examine the potential use of the data in this capacity. SCE is one of two utilities working on a Virginia Tech demonstration project that uses phasor data to monitor system stress and to develop, test and deploy an adaptive protection system for commercial application. The project will integrate highly redundant bulk power protection with information derived from phasor data, reconfiguring the system to require either two-out-of-three voting or single-relay tripping according to the amount of system stress.

This process is based on the fact that conservative relay settings can result in over-tripping. Under normal (unstressed) conditions, an unnecessary outage on a line does not necessarily impact system reliability, whereas during stressed conditions, an unnecessary line outage can adversely impact system reliability, resulting in cascading outages and, eventually, a widespread blackout. To be confident there is indeed a fault on the line, the control process will use phasors to inform the control system whether or not the electrical system is stressed. When the system is stressed and a fault occurs, at least two out of three relays on the line will have to detect the fault to allow the trip. If the system is not stressed, any relay detecting a fault will initiate the trip.