Wide area monitoring technology is coming into its own in ensuring the reliability of highly interconnected power systems. The next step is for the incorporation of control and protection functions to become a commercial reality.
Bob Fesmire, Petra Reinhardt and Albert Leirbukt, ABB
In 2003 and 2004, major blackouts in Europe and North America demonstrated that even the world’s most robust power systems were still vulnerable to cascading disturbances that could spread across a broad geographic area. The power industry has since wrestled with the question of how to prevent such events, and much attention has been paid to the need for greater investment in the grid and, in the regulatory arena, for the establishment of reliability standards.
Typically, somewhere in the discussion the need for more accurate and timely information in the control room comes up, but usually this takes a back seat to the bigger (i.e. more costly and contentious) issue of transmission expansion. However, the need for enhanced system monitoring and control was evident the moment the lights went out, if not before. The blackout in northeast USA in particular was exacerbated by the inability of a given grid operator to see system conditions beyond its own control area.
Figure 1. A supplement to existing SCADA/EMS systems, WAMS use a combination of technologies to provide a highly accurate and dynamic view of the grid
Today, there is broad consensus that highly interconnected power systems require a new level of monitoring and control, as evidenced by the final reports from UCTE and NERC on the above mentioned outages. Grid operators need to be able to assess system conditions over a very large area in a real-time environment. This is what wide area monitoring systems (WAMS) now offer. A supplement to existing SCADA/EMS systems, WAMS use a combination of technologies to provide a highly accurate and dynamic view of the grid. The capabilities of these systems are also being extended to include control operations, and soon will likely incorporate automated wide area protection schemes as well. The collective term for these distinct functional areas (monitoring, control and protection) has become known as Wide Area Applications.
WAMS rely primarily on phasor measurement units (PMUs), which represent a quantum advancement in performance over traditional RTUs. PMUs gather current and voltage magnitude and phasor measurements i.e. magnitude and phase angle for every cycle of the network fundamental frequency. Using time synchronization from a GPS satellite and a high-speed communications network, WAMS coordinate this flow of data from disparate points on the grid via a data collection system in the control center. The information can be displayed to operators and fed into a wide array of applications. In the area of monitoring, these include:
- Power Oscillation Monitoring, in which an algorithm uses voltage and current phasors or frequency to detect power swings, identify frequency and damping of specific oscillation modes, and trigger notifications.
- Phase Angle Monitoring, in which the voltage phase angle separation between selected nodes in the transmission grid is monitored, and operator notifications or system protection scheme actions can be triggered.
- Frequency Stability Monitoring, which detects imbalances between generation and load, estimates their impact, and provides operators with suggested remedies.
- Voltage Stability Monitoring, which monitors active power margin to support operator actions to maintain voltage.
- Line Thermal Monitoring, which uses phasor measurements to estimate actual average line temperature, enabling dynamic line rating and increased available transmission capacity for transfer interfaces that are limited by line temperature criteria.
The result is a highly accurate depiction of real-time grid conditions across a large area, which in turn increases operator awareness, helps to enhance security of supply and reach informed planning and operational decisions.
High profile project
Although WAMS have been in use for several years, the technology is still regarded as “new”. This is changing, however, as more and more transmission system operators gain experience with WAMS.
Perhaps the most newsworthy example to date of the benefits of wide area monitoring occurred in 2004 when the two separated regions of UCTE were resynchronized with the help of PMUs deployed in the two previously asynchronous regions. Etrans, the independent grid coordinator in Switzerland, had already installed PMUs on its critical north-south transmission corridor in 2003 to assess actual loads, the impact on system security after a sudden loss of segments of the sometimes heavily loaded corridor, and the effect of wide area oscillations on the European grid generally. Croatia’s HEP had also installed WAMS, which would help monitor the resynchronization process as well. In the lead-up to the reunification of the grid, a PMU in Greece was connected to the Etrans control center in Laufenberg. With the support of real-time data provided by these systems, higher observability was achieved and the reunification process was completed without incident in less than 90 minutes on October 10, 2004.
APG, Austria’s largest producer and distributor of electric power, operates in a highly meshed system with interties to all of the surrounding countries. Congestion is a problem, with three 220 kV lines moving power southward from generators in the northeast of the country. The reliability issue came into sharp focus in 2003 when the trip of a neighbouring 380 kV line nearly caused the complete shutdown of the 220 kV system. APG decided to deploy WAMS as an immediate measure to monitor system conditions while three phase-shifting transformers were installed to help prevent further problems. The transformer installations are slated for completion this year. Once in place, APG intends to deploy even more PMUs to assess the effectiveness of the phase-shifting transformers and then optimize their use into the future.
WAM systems are not limited to Europe, however. EGAT, the state-owned grid operator in Thailand, last year installed WAMS on the country’s long north-south transmission corridor. Operations on this line were limited due to power oscillations. This presented concerns not only about reliability but also about the transmission system’s ability to support the push towards privatization of the generation sector. Now the Thai operators can monitor damping, frequency and amplitude of oscillations and take corrective action before minor fluctuations become major disturbances. In this way, the WAMS serves as a warning system.
Several other projects are underway addressing user-specific requirements and partially involving R&D. After gathering experience with wide area measurement technology since 2001, Norway’s grid operator, Statnett, initiated a collaborative R&D effort with ABB and Sintef Energy Research Institute in 2005 to develop and deploy an integrated wide area solution for the Norwegian transmission grid. The capabilities envisioned in the project call for a system that can detect power oscillations, offer protection against voltage collapse, provide high speed communication of dynamic information, and manage related applications in the SCADA/EMS system.
The system is designed to monitor oscillations and voltage instabilities based on PMU and RTU/IED information. The outputs from the wide area applications can later also be utilized to control systems for FACTS and substation automation over common protocols. Most importantly, the solution enables seamless integration of wide area applications’ outputs with Statnett’s existing SCADA/EMS system, the ABB Network Manager solution.
Figure 2. APG’s WAM system setup
Statnett’s system is now in the phase for testing and evaluation of the post-processing algorithms, with prototype testing scheduled for later this year. The solution will be used for monitoring purposes initially, but the evolution from a monitoring system to one that incorporates actions based on data from the monitoring functions is clear.
In what may be the most ambitious WAMS implementation to date, America’s Tennessee Valley Authority is leading an initiative to outfit the entire US Eastern Interconnect with PMUs that will be linked to TVA’s control center. The project got underway in 2002 when the power agency installed the first five PMUs on its own system. Since then, more have been added within TVA and in other locations on the Eastern Interconnect so that now there are over 30 units in place. The project is growing, with more PMU installations as well as the development of applications that will make use of the volumes of phasor measurement data the system is providing. Line thermal monitoring is at the top of the list. Congestion is a major problem from both a reliability and market perspective in the eastern US. TVA hopes to leverage the line thermal monitoring function to gain a more accurate picture of actual transfer capacity at any given time, perhaps facilitating the use of dynamic line ratings.
While some obstacles remain before WAM technology realizes its full potential, clearly these systems have already made their mark. Reliability, market efficiency and even the optimization of transmission owners’ assets are all enhanced by the use of WAMS. Now, as wide area monitoring extends into wide area control, grid operators are able to make decisions with the benefit of a view of the transmission system that was unattainable a decade ago. As the technology continues to advance and as it is adopted by an expanding community of users, it won’t be long before an integrated system providing wide area monitoring, control and protection functions becomes a commercial reality.
Figure 3. On-line display of frequency, damping and magnitude of oscillation
Perhaps most intriguing of all, though, is what might be done with the treasure trove of data that these systems provide. How will power systems models be improved? Will the additional information be used and perhaps exchanged to enhance the quality of power system operation in the light of increasing complexities in multi-national and highly meshed grids? What impact might WAMS have on wholesale power markets? As with similar advances in other areas of utility IT, often the questions are not even asked before the data to answer them is already available.