The complexity of operating large electric systems in the United States and Canada is increasing as power systems expand to include more independent and small distributed generation. Power systems have become so interdependent that the events in one area can cascade and have significant impacts on other areas that appear, at first glance, to be relatively remote from the source of the initial event. This was recently demonstrated by the Northeast U.S. blackout of Aug. 14, 2003. New tools such as Synchronized Phasor Measurement Systems (SPMS), which use advances in communications, computers and Global Positioning System (GPS) technologies, are needed for monitoring and managing system stresses and dynamic system security of large power systems.

“The information captured by these types of systems will not only allow system operators to make better informed real-time decisions, it will enable energy providers to maintain or even enhance reliability of service in this more complex environment,” said Jim Kelly, vice president of Engineering and Technical Services at Southern California Edison. “Electricity is not like other commodities; it is fundamental to almost every facet of modern life and prosperity. As such, the adoption of practical technologies that safeguard reliability is not just desirable, it's our obligation as stewards of the system.”

Synchronized Phasor Measurement Technology

Southern California Edison Co. (SCE; Rosemead, California, U.S.), which is part of the Western Electric Coordinating Council in the United States, has been working on the SPMS for the last eight years. SCE has installed a network of Phasor Measurement Units (PMUs), Phasor Data Concentrators (PDCs), a high-speed/high-reliability fiber-optic communications system and suitable data storage servers to collect the phasor data. SCE also exchanges data from a PDC installed by Bonneville Power Administration (BPA; Portland, Oregon, U.S.) at its Dittmer Station in real time. This data interchange is required to view the complete North-South WECC system picture. SCE has also developed a program called “Power System Outlook” (PSO), to view and analyze the data collected by the PDCs. The data from this phasor measurement system have been collected and analyzed over several years for different types of disturbances and has provided useful insight into the operation of large power systems.

When large and interconnected power systems are pushed to their limits for power transfer economics, management of stress is essential. One of the simplest stress measurements is the phase-angle separation between distant generation units. Large power systems, as in the WECC, operate at large phase-angle separations, but this can only be done when continuous voltage support is available. This obviously has significant implications for system design, placement of voltage support devices and protocols. The SPMS system is a useful tool to track these system parameters.

Also, systems are generally designed for maximum loading conditions and for outage of an element (N-1 conditions) at these loading levels. Because these conditions typically occur only for a short duration, adequate operating margins are typically available. Some disturbances, however, occur not because of the loss of one element, but due to multiple contingencies occurring over a time period. Often, the system/line loadings and margins are not adjusted when line outages occur, especially if they are outside the affected operator's control area. SPMS is a tool that can help in monitoring wide-area systems and managing stress by tracking the phase-angle separation, voltage phasors and line loadings. The system also can track inter-area system oscillations, which can warn of dynamic instability.

The SPMS is sometimes referred to more generically as a Wide Area Measurement System (WAMS). Use of SPMS or WAMS can enable operators to monitor and often control critical system operating indices, which are essential for secure operation of a large power system, including static phase-angle limits (system stress); critical intermediate voltage support when operating at large phase-angle separation; dynamic/transient phasor movements indicating dynamic/transient swings among different areas; modal inter-area oscillation frequencies and their modal damping; and path loadings, line flows and key tie line status.

The PMUs in use at SCE record three-phase voltages, currents and frequencies at a rate of 12 samples per cycle and convert the voltage and current phase quantities to positive sequence phasor data. Using the GPS system, the PMUs time-tag the phasors recorded at the different PMU locations with a high degree of accuracy. These phasor data are transmitted to a central location to the PDCs every two cycles (30 samples per second) and tabulated based on the time stamp. The basic records in the disturbance file consist of the time-tagged voltage, current phasors and frequency deviations. The megawatts and megavars also are calculated from this phasor data.

Figure 1 shows the PMUs installed at SCE showing transmission of data to PDCs. The data from the various PMUs and the BPA PDC are transmitted to the SCE PDCs located at the SCE Grid Control Center and also transferred and stored on IT servers. SCE staff can access the data via the network. Files also are time-compressed in various time lengths for viewing a longer time span as necessary. The files are stored as event files or as 3-minute stream files. The event files are created by the PDC whenever there is a disturbance in the system and the frequency, rate of change of frequency, voltage or voltage deviation set limits are exceeded. The stream files are continuously downloaded and recorded. The stream files also are used to create time-compressed files. The event file has 1 minute of pre-trigger and 2 minutes of post-trigger data. The data can be viewed using the PSO program or the Phasorfile viewer developed by BPA.

Phase-Angle Separation

The phase-angle separation between two locations can be considered as a direct indicator of the stress on that part of the system. The static phase-angle is the static stress from the normal operating condition of the system. The dynamic phase-angle change caused by a system event is the direct dynamic stress caused by that event on that part of the system. The dynamic stress can increase the static phase angle between two monitored locations, or it can reduce the static phase angle between these locations. When the dynamic change in the phase angle causes the phase angle separation to increase on an already stressed system, then it is adding stress and can lead the system into separation. SCE is monitoring the static and dynamic stress by monitoring the phase angle separation between Grand Coulee Generating Station in the BPA area and Devers and Vincent Substations in the SCE area. It has been observed that at the time of two system events, this static phase angle had exceeded 90 degrees and the loss of some transmission lines or load in the northwest increased the stress further, causing a system disturbance on Aug. 10, 1996, and a system event on Aug. 4, 2000.

Figure 2 shows a plot of voltage phasors on 500-kV substations along the ac Pacific Intertie lines just before the Aug. 4, 2000, system event. This plot has been created by merging the two disturbance files created for this event by the BPA and SCE. Voltage phasors' magnitude and their relative phase angles can be seen in this figure. The reference has been selected to be Grand Coulee in the BPA area. The file has been created by merging the BPA and the SCE event files.

Monitoring Voltage Support

Monitoring voltage support at various intermediate substations on the ac tie lines and ensuring adequate reactive margin is important when operating at large phase-angle separation. In Fig. 2, the voltage support at Keeler, Malin and Big Eddy Substations is the key for the system to operate at such large phase-angle separation. The phasors at these substations should be close to the circumference; that is that the voltage should be about 525 kV. This voltage support and adequate reactive margin should be available for static system loading and also during dynamic or transient loading.

Figure 3 shows the voltage phasors of the WECC North-South system for a different event that occurred on June 14, 2004. These are phasors just before a significant event when several transmission lines and generating units tripped in the Arizona and California area, resulting in a loss of more than 4700 MW in the Southwest United States. Fortunately, the system was operating at a much lower stress level — initially at a phase angle of about 50 degrees between Grand Coulee in the Northwest and Devers/Vincent in the Southwest — and the system was able to ride through and survived this disturbance.

Figure 4 shows the phasor displays for the June 14, 2004, event with the phasors displaced to their peak. The plot shows the phasor separation between Grand Coulee in the north and Vincent/Devers Substations in the south, as well as the intermediate voltage supports at Big Eddy, Malin, Keeler and some other substations.

The magnitude of the phasors at Malin and Keeler reduces considerably during the peak of the transient. The Malin voltage, in fact, reduced to about 0.81 per unit, indicating exhausted reactive margin. Any further additional stress could have caused separation at Malin. Figure 5 shows the phase-angle plots for this event. The dynamic phase angle changed from about 55 degrees to about 145 degrees at its peak. Because the system was operating at low stress, the modal oscillations damped rapidly.

Monitoring Inter-area System Oscillation

A system will show high dynamic stability when the static stress is low but may show poor dynamic stability when the static stress is high. Low inter-area modal oscillation damping could be an early indicator of a weakening system. As the static phase angle or the system loading increases, the system may start showing the signs of instability. This is generally seen in the reduced damping of the system and increasing time durations for the oscillations of the system. The modal oscillations are generally present in the system, although at a small level. The oscillation magnitude can increase as the system is stressed. The oscillation frequencies are dependent on the system inertia and the system transmission strength; however, the damping may depend more on the system stress or the phase-angle separation. Monitoring these oscillations and their damping can alert if the system starts weakening or is likely to become dynamically unstable. The loss of a major transmission path can result in increasing the stress, necessitating an assessment of whether the system should be able to withstand this increased stress as it is or whether appropriate remedial measures are required. At times, the system itself may be stressed to such levels that it starts showing low damping and an increase in oscillations. The SPMS can monitor the ac system oscillation modes, damping and oscillating power to identify signs of instability and to avoid system breakup by taking appropriate remedial actions. Figure 6 shows the voltage magnitude plots from this SPMS system for the Aug. 4, 2000, system event. The system had low damping and the inter-area system oscillations continued for about 60 seconds. The oscillations damped when a capacitor bank provided var support at Keeler Substation.

Figure 7 shows the voltage phase-angle plot for this event from different PMU locations. The reference is Colstrip, which leads Grand Coulee by about 22 degrees. The phase-angle difference between Grand Coulee and Devers is initially about 90 degrees (static stress) before the event, which increases by about 18 degrees to about 108 degrees. The combined effect of the static and dynamic stress is that the system oscillated for about 60 seconds, showing low damping of this mode.

The Power System Outlook program (Fig. 8) can calculate the modal frequencies and their damping at any PMU location. The program shows a table of the first four modes and their damping and time constant.

Monitoring df/dt for Identifying Generation Loss

The PSO program can also display the change of frequency with respect to time (df/dt) at different PMU locations. By monitoring df/dt, it is possible to identify the area/location and the severity of the system disturbances, such as generation and load drops. The df/dt plot shown in Fig. 9 shows the df/dt recorded at Grand Coulee from loss of about 2300 MW of generation on July 15, 2002, at about 15:04 PDT.

The SPMS has a high potential for monitoring large power systems. Considerable development and progress has been made at Southern California Edison Company in using the SPMS technology for monitoring the system stability and system stresses, modes of dynamic oscillations and their damping.

SCE is continuing to work with this technology to provide real-time voltage phasor angle and magnitude displays to grid operators and the California ISO so that the system stress, dynamic system oscillations and associated damping can be monitored continuously for improved system reliability.

Editor's Note

This paper represents the views of its author and does not necessarily represent the views of SCE.

Bharat Bhargava is a consulting engineer in the T&D Technology Integration area of SCE, where he has worked for the last 26 years. He is actively involved in phasor measurements, transient analysis of power systems, SSR studies, system dynamics studies, power quality, railway electrification, capacitor switching and insulation coordination studies. Bhargava graduated from Delhi University in 1961, subsequently earning a master's degree from Rensselear Polytechnic Institute in 1976. Bhargava is a senior member of PAS, IAS Communication and Vehicular Societies of IEEE and a member of CIGRÉ.
Bharat.Bhargava@sce.com

George Rodriguez is currently manager of Edison's T/S Engineering Technology Integration group, which conducts transmission and distribution R&D and advanced engineering projects for Edison's T/D Business Unit. Rodriguez has over 25 years of utility program and project management experience in developing and commercializing energy-related technologies. He has managed research activities in advanced energy storage technologies, including super-conducting magnetic energy storage and advanced battery storage systems. Rodriguez has a master's in electrical/electronic engineering (with a power option) from the California State Polytechnic University in Pomona. He is a professional engineer and a 25-year member of the IEEE.