Generic Transmission Constraints in ERCOT: What Are They, and How Do They Impact Shift Factors?

Constraints in the flow of electricity are fundamental to understanding both key market dynamics and the physical nature of the power grid.

Market participants such as generators and retail-serving entities must understand the significant impact of constraints on wholesale price formation. Power traders studying energy flows at the nodal level and making predictions must understand the shift factors of those nodes on each constraint. All stakeholders should consider the role of existing constraints in the evolution of the resource mix as certain technologies exit and new ones enter, especially technologies like wind, solar, and batteries. 

This is an introduction to constraints generally, with a specific focus on one type of constraint in ERCOT called generic transmission constraints (GTCs) and their corresponding shift factors.

What Is a Transmission Constraint?

Transmission lines carry electricity at high voltages over long distances from one geographical region to another. Power is constantly flowing, but the equipment used to transmit the power has thermal limits on the amount of power it can safely carry. Constraints occur when the amount of power flowing over a line or through a piece of equipment approaches those physical limits and must be managed so as to not exceed them. 

In the ERCOT region, GTCs are one particular flavor of transmission constraint that shapes market fundamentals such as shift factors.

While the naming convention for these GTCs are unique to ERCOT, this constraint type is not. GTCs are constraints that act like internal interfaces, or interties, between geographic regions within ERCOT. 

Other ISOs have similar types of internal interfaces, called by other names. 

What Exactly Is a GTC in ERCOT? 

Simply put, a GTC is a constraint, which ERCOT creates, consisting of two or more transmission elements. In contrast with a typical constraint relating to an individual line or piece of equipment, GTCs are a specialized constraint that allow ERCOT to manage the flow of power over a group of lines or other elements instead of just one. 

The total flow of power over the group of lines or elements is subject to a limit defined by ERCOT, called the generic transmission limit. 

Why Does ERCOT Create This Special Flavor of Constraint? 

GTCs allow the grid operator to manage the flow of power between geographic areas to mitigate issues that individual constraints don’t necessarily address. These can include certain non-thermal operating limits such as stability and voltage. For example, one reason ERCOT may create a GTC is to manage the flow of power from an area where there is a large amount of wind generation to another area where the load centers are located on the other side of the grid. 

Managing the resulting impacts to stability and voltage boils down to ensuring that the grid continues to operate reliably across the entire ERCOT footprint. 

Consider WESTEX GTC, one example of the 18 GTCs ERCOT currently employs. Texas is the largest wind producer in the nation, operating 25% of the total wind generation in the US. The vast majority of the 36 GW of installed wind capacity is located in West Texas behind the WESTEX GTC. This wind capacity is on the opposite side of the state from major cities such as Houston, Austin, and Dallas, with a limited set of transmission lines that can carry the power from one end of the state to the other. When there are large export power flows out of a region, instability can result in the area. See Figure 1 below for a visualization of the WESTEX GTC from ERCOT’s June 2020 West Texas Export Stability Assessment. The North/South line shows the approximate bifurcation of the grid for this GTC (abundant generation to the West, load centers to the East). 

The WESTEX GTC is made up of the following 16 transmission lines, from ERCOT’s Generic Transmission Constraint Definitions updated on November 28, 2022.

The representation of the WESTEX GTC by the purple line in Figure 1 is just that – a representation – and not the physical location of the GTC itself. You can search transmission elements across Yes Energy’s suite of products using the transmission system mapping layer. 

One way to think of GTCs (and internal interfaces in general) is that they are similar to the border of a country. A border represents a division between geographical areas on a map, but the border itself is not always physical. The roads that cut through a border are like the transmission lines carrying power from one region to another. In this analogy, the GTC is like the nonphysical representation of the border. 

Recently, we’ve heard questions concerning unique behavior in the shift factors for GTCs. Some GTCs are associated with typical shift factors our clients are generally familiar with, ranging in decimal form from -1 to 1, while other GTCs appear to have shift factors of just 1 (or -1). 

Consider this visualization of WESTEX GTC in Yes Energy’s Constraint Profile module:

The squares represent price nodes, and the blue and red gradients represent the shift factors of those nodes on the WESTEX GTC ranging from -1 to 1.

The shift factors associated with WESTEX behave similarly to typical single-element thermal constraints. Contrast these shift factors with those associated with another ERCOT GTC, called NE_LOB:

NE_LOB represents another stability limit associated with wind farms, this time in South Texas. The GTC consists of four transmission equipment elements connecting along the North Edinburg – Lobo 345 kV line. Notice that the shift factors associated with this GTC look different, both on the map and numerically.

Each of the price nodes behind the NE_LOB GTC has a shift factor of -1 on the constraint, as opposed to floating values ranging from -1 to 1. 

The Types of GTCs

There are two kinds of GTCs: “closed loop” and “open loop.” A closed loop GTC represents a constraint that encircles a region of the system in a closed fashion, which means the resources inside of the region have a 100% shift factor on the flow of the GTC. An open loop GTC means that the GTC cuts across the plane instead of encircling a closed region, much like any other typical single line constraint. 

NE_LOB is an example of a closed loop GTC. The resources inside of the closed loop region have a shift factor of -1, because each MW of output from those resources are fully transferred over the GTC. There is no alternative way for the power to flow out of the region. 

By contrast, WESTEX is an open loop GTC, which simply means that the GTC cuts across the plane instead of encircling a closed region, much like any other typical single line constraint. Therefore, generator price nodes will have varying shift factors on either side of the GTC, because the flow of power from any one resource can’t be fully transferred over that binding constraint.

What Does a Shift Factor Represent?

Remember, when the flow of power over a transmission line or element is beginning to approach the limits that the line or element can carry, a constraint occurs. When the constraint is binding, the flow of power from a nearby generator’s output becomes limited as well. Only so much of that generator’s output can flow over the constrained line. 

Shift factors tell us what proportion of the generator’s output can flow over the constraint, or in this case, the GTC specifically. A simple equation represents this:

Shift Factor_{Generator/Price Node}  X MW_{Generator/Price Node} = MW flow over constraint

For example, consider a hypothetical shift factor = 0.75:

0.75 X 1 MW = 0.75 MW flow over constraint

Every electrical bus, or generator node, has a shift factor for every constraint on the system. When selecting a specific constraint, such as the WESTEX or NE_LOB GTCs in the images above, Yes Energy’s Constraint Profile module displays the shift factors of every node for that constraint.

Monitoring GTCs in Real Time

Utilizing sensor inputs from Live Power, we can start to look at the correlation between flow on actual transmission elements and the binding of a GTC. In the chart below, the shaded area displays an aggregated view of several elements comprising the WESTEX GTC. As line loading increases, we can watch the GTC bind in the real-time market.

Live Power sensor data is a nonpublic source of transmission flow data and generator outputs available through the Yes Energy services. 

Conclusion

In summary, constraints represent limits on the transmission of power across individual transmission elements on the grid. These limits constrain the flow of pockets of generation over the grid, thereby shaping key market fundamentals like shift factors. 

GTCs are a special flavor of constraint representing a group of transmission elements, with its own defined group limit. While the naming convention of GTC is unique to ERCOT, conceptually GTCs are similar to the interfaces that exist within all ISO regions representing the internal flow of power from one constrained area to another. 

The distinction between a “closed loop” and “open loop” GTC is crucial for understanding the unique behavior of shift factors associated with GTCs. 

Yes Energy provides key data concerning power grid constraints like GTCs and the market knowledge necessary to interpret the data and understand the impact on market fundamentals.

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