CAR: A visualization of power

Stephen Lee,
Electric Power Research Institute, Palo Alto, CA, USA

Transmission bottlenecks can be extremely limiting in competitive power markets, but a new software system can help participants visualise market activity. The software has been likened to air traffic control systems, and is set to improve market reliability and efficiency.

The power market can be viewed as a community of electricity participants involved in power transactions affecting one another. These participant generators, transmission owners, grid operators, traders, load-serving entities, and ultimately the customers ” will be able to plan their activities more efficiently if they know how transmission limitations will affect them. A newly developed software technology for power trading, Community Activity Room (CAR), uses the metaphor of a multi-sided room to show the ranges of operation within which market activities can freely take place.

CAR graphics define the limits of the power market, locating congested bottlenecks and suggesting the combinations of net import and net export from various control areas that will avoid congestion. The CAR concept brings transmission planning and operation to the next generation of probabilistic power system reliability assessment and promotes integration of reliability and market efficiency.

CAR technology

With the new restructured environment where complex transmission bottlenecks are limiting the efficiency and reliability of emerging power markets, the concept of the CAR was developed by the author during a planning study of the interregional transmission transfer capacities of the Eastern Interconnection of North America, in response to the need to provide maximum information from a limited number of computer cases, and where complex results had to be visualized.

The CAR technology takes a large set of high dimensional transmission constraints, based on a list of potential transmission outages, and reduces them to a visual, three-dimensional set of equations, akin to walls of a room. Only potentially binding transmission bottlenecks are graphically painted in colour, showing increasing degrees of congestion as market activities increase.

Figure 1. An example of a 3-D CAR for a system with four interconnected regions
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The current state of the power market is represented by a floating point of light (bulb) inside the CAR. Operators monitor the location of the bulb and are warned when it nears a wall. The CAR will warn the operators how far it is from the walls, and which wall is closest to it. When the market moves the bulb beyond the walls, the CAR will show the shortest way to get back inside. This may be achieved by market mechanisms or reliability procedures, or both.

There are two types of walls ” hard and soft. Hard walls represent the boundary of operation beyond which a transmission facility will be continuously overloaded, or instability will occur. Soft walls represent the potential boundary of unsatisfactory operation if a particular transmission outage contingency happens. They represent a buffer zone wherein if a particular outage happens, there is no immediate overload and the operator will then move the operating point closer to the centre of the room. When an outage happens, a new set of walls will be computed immediately.

An example of a two-dimensional visualization is shown in Figure 2. The horizontal axis represents net export out of the NW region, and the vertical axis represents net export out of the NE region. The SW net export is zero for all points in Figure 2. The system shown consists of four regions connected to one another only. Therefore, the SE region’s net export can be determined from the net exports of the NW, NE and SW regions by SE export.

Figure 2. CAR painted as potential overload levels shown in colour bands of 200 MW steps, with SW export = 1029 MW
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The white zone in Figure 2 shows the operating space within which no constraint is violated, i.e., no potential overloads due to contingencies would occur. The blue zone shows where up to 200 MW of potential overloads may happen, if the system operates in that zone. The yellow zone shows the state space where between 200 to 400 MW of potential overloads may occur, and so on, with 200 MW being the step size of each colour band.

The larger blue dot in the blue zone of the chart marks the current operating point of the system, or the light-bulb, with a positive NW export and a negative NW export. Because the bulb lies outside of the white zone, the CAR tells the operator the shortest distance and direction to move back inside. The small green dot marks the location.This information will be valuable in an emergency situation of heavy congestion for the most efficient way to relieve congestion.

Under normal conditions, the large blue dot is inside the white zone. In that case, a black line segment will be shown marking the location of the point on the closest wall (constraint) as projected onto this plane.

Proven technology

The foundation technology behind the CAR is proven. The mathematical basis of the CAR is sound. The technology is a creative application of transmission transfer capacity studies to the operating arena. The equations are based on linearization around a full AC power flow model. It is as accurate as the data used in computing the walls. For example, for online applications, the availability status of transmission lines and generators can be modeled by the power flow equations from which the walls would be derived.

Personal computers can be used to continuously derive the equations for the walls, based on a large set of possible operating conditions. These walls can be saved for ready retrieval when new operating conditions are matched against the database. This approach to online reliability assessment will change a number of energy control centre applications. Pattern recognition would be applied to quickly assess the current operating point and match it with previously studied cases. At the same time, the computer will derive a new set of equations for the walls that are most accurate for the current operating point, thus adding to the knowledge base.


The CAR technology is ingenious in the way many ideas and applications fit together as neatly as a hand in a glove. In the first place, this technology reduces the complex high dimensional transmission constraints into lower dimensions with easily understandable implications for the market. Secondly, the addition of colour-coded probabilities of outages converts today’s deterministic transmission reliability criteria into probabilistic criteria of the future. When that is put into practice, it could engender new market products for risk management or insurance against blackouts. EPRI has a plan to use the CAR technology to develop an online probabilistic reliability monitor.

The CAR has linked transmission planning and operation into a unified framework, whereby statistics on congestion and bottlenecks can be communicated to all stakeholders interested in expanding the size of the CAR, within which the power market will achieve greater efficiency.

Making continuous use of computer power to increase the knowledge base of power market operation, the CAR will take traditional energy management systems from a single-point analysis of the current operating point to the next generation where the full range of state space around the current operating point will be presented to the operators. This can be compared to an air traffic radar system that shows all planes on the same screen instead of a warning system which only gives the air traffic controller a yes or no answer about whether an aircraft is within a fixed radius from another one.

The CAR has the potential to display market prices in the same way as congestion is displayed. The CAR also has the potential to serve as a real-time toll collection system for transmission usage because the loading of transmission lines of interest can be monitored and analytically attributed to portfolios of market activities. This system, which can complement any power market design, could provide a much needed way to increase the financial incentives for investing in new transmission lines by assuring an attractive but fair return to investors. The tolls would vary, depending on the degree of congestion.

Commercial feasibility

The CAR prototype was developed in less than three months, a timeframe that gives us a high degree of confidence that this promising technology can be brought to market quickly. Commercialization depends only on the availability of data and the supporting computer and communication infrastructure, both of which are available in many power systems.

Megawatt transfer limits and voltage drop constraints can now be handled by the system; and future work will address voltage stability and dynamic stability in the same framework. Current technology is limited to separate assessments of voltage stability and dynamic stability for a single operating point. The CAR provides a framework for displaying all three types of constraints at the same time.


In North America, wholesale power transactions are electronically tagged from the starting point to the ending point. This system is the North American Electric Reliability Council (NERC) E-tags. EPRI and NERC provide to authorized Reliability Coordinators an online web-based display of power market transactions in bubble diagrams, called the TagNet display.

These data enable the current location of the floating light bulb to be calculated. As a monitoring device of the transmission grid’s state of health, the online system accumulates statistics on wholesale power market schedules, congestion indices and limiting bottlenecks. This is useful for transmission planning.

The CAR provides a visual display of electricity market activity in action in a manner analogous to air traffic control systems. It has large commercial potential for transmission pricing and grid stability, as well as risk management and insurance products. More importantly, it offers tremendous potential to launch the electric power industry into the next era of highly efficient power markets worldwide.

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