Transmission systems around the world are being squeezed between two great forces. On one side, increasing demand, energy trading and economic pressures are pushing transmission owners and grid operators to maximize the use of transmission assets. On the other, reliability concerns—especially since 2003’s spate of major disruptions—are forcing these same players to be more vigilant about just how far they push the grid’s infrastructure.
At the most basic level, this situation presents a choice between two courses of action: build more lines, or do more with existing ones. Clearly, there are places today where the only viable solution is to add transmission capacity. However, given the cost, time and siting issues associated with such projects, it’s not surprising that utilities now are focusing more than ever on getting more out of their existing systems.
Recent advances in measurement, communications and analytic technologies have produced a range of new options. In particular, wide area measurement systems (WAMS) have come to the fore as a means to address not just immediate reliability concerns but also operations issues and long-term system planning as well.
Phasor Measurement – the Key to WAMS
Critical nodes in today’s transmission grids are usually monitored using static or quasi-dynamic data based on RMS measurements. Traditional SCADA/EMS systems, however, can provide only a limited picture of dynamic grid conditions. Indeed, these systems can take a minute to deliver a snapshot of a system whose characteristics are changing at the speed of light.
In contrast to conventional control systems, where RTUs are used for acquisition of RMS values of currents and voltages, a Wide Area Monitoring System acquires current, voltage and frequency phasor measurements collected by Phasor Measurement Units (PMUs), from selected locations in the power system. The measured quantities include both magnitudes and phase angles, and are time-synchronized via Global Positioning System (GPS) receivers down to a time resolution of one microsecond.
The availability of highly accurate synchronized phasor measurements has enabled a new level of monitoring capability. By comparing local phasor measurements, operators can observe not only the steady-state, but also the dynamic state of critical nodes in their transmission and sub-transmission networks. This improvement facilitates better, faster analyses of grid conditions, which in turn provide operators with more time and more options to preserve system stability. It also represents a quantum leap in the quality of data on which transmission planning and everyday operational decisions are based.
Defining Power System Phenomena
Utility needs and problems are often formulated in very loose terms, such as "intelligent load shedding", "protecting the system against major disturbances", and "counteracting cascaded line tripping". In order to address these needs, they have to be broken down to physical phenomena that can be mitigated by a management scheme making use of information from WAMS. Some examples include:
• Transient angle instability
• Small signal angle instability
• Frequency instability
• Short-term voltage instability
• Long-term voltage instability
• Cascading outages
To address each of these phenomena, a reliable (based on redundancy and robustness) system has to be designed with respect to input variables, decision criteria and output actions. Parallel systems counteracting different phenomena, and different layers of safety nets, can also be designed and coordinated. PMUs allow for direct and fast angle measurement, instead of indirect power measurement, and enable the development of more accurate control algorithms for emergency control or protective actions.
Architecture Options
The impact of WAMS on protection schemes and short-term reliability is obvious. So, how can utilities take advantage of these advances? The answer, to be sure, will depend on the unique needs of a given organization.
Enhancements to SCADA/EMS
At one end of the spectrum, enhancements to the existing EMS/SCADA can be made. However, the possibilities of extending the SCADA/EMS system with new functions tend to be limited. Therefore it might be relevant to provide new SCADA/EMS functions as "stand alone" solutions (e.g., load shedding) more or less independent of the ordinary SCADA/EMS system.
"Flat Architecture" with System Protection Terminals
Protection devices or terminals are traditionally used in protecting equipment (lines, transformers, etc.). Modern protection devices have sufficient computing and communications capabilities that they are capable of performing beyond the traditional functions. When connected together via communications links, these devices can process intelligent algorithms based on data collected locally or by other devices via data sharing.
In a "flat architecture," de-centralized, specially developed interconnected system protection terminals are installed in substations, where actions are to be made or measurements taken. Actions are preferably local (i.e., transfer trips should be avoided) to increase security. Relevant power system variable data is transferred through the communication system that ties the terminals together. Different schemes—against voltage instability and frequency instability, for example—can be implemented in the same hardware.
Multilayered Architecture
While the above two designs attempt to extend the "reach" of existing control domains (protection terminal being one domain, and EMS being the other), there is no guarantee that the end solution will address all the operator’s needs. A multilayered architecture provides a more comprehensive alternative.
There are up to three layers in this architecture. The bottom layer is made up of PMUs or PMUs with additional protection functionality. The next layer up consists of several Local Protection Centers (LPCs), each of which interfaces directly with a number of PMUs. The LPCs act as mass storage for PMU readings that are accessible from the top layer, the System Protection Center (SPC). With this approach, the local protection center forms a system protection scheme while the interconnected coordinated system forms a more comprehensive defense for the network as a whole.
More Than Just Protection
WAMS is most frequently associated with protection, but in truth it is more fundamental than that. WAMS is an enabling technology, and while its contribution to preserving grid integrity in an emergency is clear, it represents an equally important advance for the analytic and planning activities that will preserve grid integrity over the long term.
Phasor measurements can be utilized offline, for example, as input for calibration and verification of the dynamic models of various transmission system components. More accurate models mean better planning, and that ultimately results in a more efficient, more resilient grid.
WAMS can have a significant impact on day-to-day operations as well. Having a more precise understanding of the conditions at a specific intertie, for example, would allow an operator to push that connection closer to its operating limits without sacrificing reliability. This is precisely what ETrans, the Swiss grid operator, sought to do with its first WAMS installation.
The company wanted to more closely monitor large power transfers across its North-South corridor. They placed four PMUs at strategic locations at both ends of the lines, and found that the difference in voltage phase angles across the system proved to be a very good indication of system stress caused by power export to the south. There is, in fact, an almost linear relationship between the transferred power and the voltage phase angle difference. Additionally, line losses and impedance can be computed very accurately, which in turn provide the means to derive average line temperature. Armed with this information, ETrans can more confidently push the limits of the transmission assets by, for example, increasing the load on a given line when ambient temperatures and wind allow.
One Size Doesn’t Necessarily Fit All
The potential to improve power system performance and reliability using smart controls instead of more high voltage equipment appears to be great. The advantages of phasor measurement, and of WAMS in general, extend well beyond the more publicized short-term reliability applications. However, implementing these technologies should be undertaken with a focus on the specific needs of the given utility. Some organizations may wish to introduce a complete system in order to address a large number of issues at once, while others may want to proceed more slowly with small installations in parallel with present systems. WAMS is still new in terms of commercial application, and while such systems are likely to become common, for now their benefits will be best realized one step at a time.
Bios...
Lawrence Broski works in sales in ABB's Network Management business unit in Edmonton, Alberta.
Dr. Daniel Karlsson is an Application Senior Specialist at ABB working in Sweden.
Sethuraman Ganesan is a Senior Application Engineer at ABB, loacated in Allentown, Pennsylvania.
At the most basic level, this situation presents a choice between two courses of action: build more lines, or do more with existing ones. Clearly, there are places today where the only viable solution is to add transmission capacity. However, given the cost, time and siting issues associated with such projects, it’s not surprising that utilities now are focusing more than ever on getting more out of their existing systems.
Recent advances in measurement, communications and analytic technologies have produced a range of new options. In particular, wide area measurement systems (WAMS) have come to the fore as a means to address not just immediate reliability concerns but also operations issues and long-term system planning as well.
Phasor Measurement – the Key to WAMS
Critical nodes in today’s transmission grids are usually monitored using static or quasi-dynamic data based on RMS measurements. Traditional SCADA/EMS systems, however, can provide only a limited picture of dynamic grid conditions. Indeed, these systems can take a minute to deliver a snapshot of a system whose characteristics are changing at the speed of light.
In contrast to conventional control systems, where RTUs are used for acquisition of RMS values of currents and voltages, a Wide Area Monitoring System acquires current, voltage and frequency phasor measurements collected by Phasor Measurement Units (PMUs), from selected locations in the power system. The measured quantities include both magnitudes and phase angles, and are time-synchronized via Global Positioning System (GPS) receivers down to a time resolution of one microsecond.
The availability of highly accurate synchronized phasor measurements has enabled a new level of monitoring capability. By comparing local phasor measurements, operators can observe not only the steady-state, but also the dynamic state of critical nodes in their transmission and sub-transmission networks. This improvement facilitates better, faster analyses of grid conditions, which in turn provide operators with more time and more options to preserve system stability. It also represents a quantum leap in the quality of data on which transmission planning and everyday operational decisions are based.
Defining Power System Phenomena
Utility needs and problems are often formulated in very loose terms, such as "intelligent load shedding", "protecting the system against major disturbances", and "counteracting cascaded line tripping". In order to address these needs, they have to be broken down to physical phenomena that can be mitigated by a management scheme making use of information from WAMS. Some examples include:
• Transient angle instability
• Small signal angle instability
• Frequency instability
• Short-term voltage instability
• Long-term voltage instability
• Cascading outages
To address each of these phenomena, a reliable (based on redundancy and robustness) system has to be designed with respect to input variables, decision criteria and output actions. Parallel systems counteracting different phenomena, and different layers of safety nets, can also be designed and coordinated. PMUs allow for direct and fast angle measurement, instead of indirect power measurement, and enable the development of more accurate control algorithms for emergency control or protective actions.
Architecture Options
The impact of WAMS on protection schemes and short-term reliability is obvious. So, how can utilities take advantage of these advances? The answer, to be sure, will depend on the unique needs of a given organization.
Enhancements to SCADA/EMS
At one end of the spectrum, enhancements to the existing EMS/SCADA can be made. However, the possibilities of extending the SCADA/EMS system with new functions tend to be limited. Therefore it might be relevant to provide new SCADA/EMS functions as "stand alone" solutions (e.g., load shedding) more or less independent of the ordinary SCADA/EMS system.
"Flat Architecture" with System Protection Terminals
Protection devices or terminals are traditionally used in protecting equipment (lines, transformers, etc.). Modern protection devices have sufficient computing and communications capabilities that they are capable of performing beyond the traditional functions. When connected together via communications links, these devices can process intelligent algorithms based on data collected locally or by other devices via data sharing.
In a "flat architecture," de-centralized, specially developed interconnected system protection terminals are installed in substations, where actions are to be made or measurements taken. Actions are preferably local (i.e., transfer trips should be avoided) to increase security. Relevant power system variable data is transferred through the communication system that ties the terminals together. Different schemes—against voltage instability and frequency instability, for example—can be implemented in the same hardware.
Multilayered Architecture
While the above two designs attempt to extend the "reach" of existing control domains (protection terminal being one domain, and EMS being the other), there is no guarantee that the end solution will address all the operator’s needs. A multilayered architecture provides a more comprehensive alternative.
There are up to three layers in this architecture. The bottom layer is made up of PMUs or PMUs with additional protection functionality. The next layer up consists of several Local Protection Centers (LPCs), each of which interfaces directly with a number of PMUs. The LPCs act as mass storage for PMU readings that are accessible from the top layer, the System Protection Center (SPC). With this approach, the local protection center forms a system protection scheme while the interconnected coordinated system forms a more comprehensive defense for the network as a whole.
More Than Just Protection
WAMS is most frequently associated with protection, but in truth it is more fundamental than that. WAMS is an enabling technology, and while its contribution to preserving grid integrity in an emergency is clear, it represents an equally important advance for the analytic and planning activities that will preserve grid integrity over the long term.
Phasor measurements can be utilized offline, for example, as input for calibration and verification of the dynamic models of various transmission system components. More accurate models mean better planning, and that ultimately results in a more efficient, more resilient grid.
WAMS can have a significant impact on day-to-day operations as well. Having a more precise understanding of the conditions at a specific intertie, for example, would allow an operator to push that connection closer to its operating limits without sacrificing reliability. This is precisely what ETrans, the Swiss grid operator, sought to do with its first WAMS installation.
The company wanted to more closely monitor large power transfers across its North-South corridor. They placed four PMUs at strategic locations at both ends of the lines, and found that the difference in voltage phase angles across the system proved to be a very good indication of system stress caused by power export to the south. There is, in fact, an almost linear relationship between the transferred power and the voltage phase angle difference. Additionally, line losses and impedance can be computed very accurately, which in turn provide the means to derive average line temperature. Armed with this information, ETrans can more confidently push the limits of the transmission assets by, for example, increasing the load on a given line when ambient temperatures and wind allow.
One Size Doesn’t Necessarily Fit All
The potential to improve power system performance and reliability using smart controls instead of more high voltage equipment appears to be great. The advantages of phasor measurement, and of WAMS in general, extend well beyond the more publicized short-term reliability applications. However, implementing these technologies should be undertaken with a focus on the specific needs of the given utility. Some organizations may wish to introduce a complete system in order to address a large number of issues at once, while others may want to proceed more slowly with small installations in parallel with present systems. WAMS is still new in terms of commercial application, and while such systems are likely to become common, for now their benefits will be best realized one step at a time.
Bios...
Lawrence Broski works in sales in ABB's Network Management business unit in Edmonton, Alberta.
Dr. Daniel Karlsson is an Application Senior Specialist at ABB working in Sweden.
Sethuraman Ganesan is a Senior Application Engineer at ABB, loacated in Allentown, Pennsylvania.
