
Pablo Ruiz, NewGrid and The Brattle Group
For long-term transmission planning, the topology of the existing transmission grid is typically assumed to be fixed, and not a decision variable. But this may be about to change in the U.S. with FERC Order 1920, which requires consideration of transmission switching among other technologies. Using transmission switching as a decision variable can provide significant value by reflecting that the grid topology can be optimized to reliably re-route flow around expected bottlenecks, thereby increasing transfer capacities. In this blog we provide thoughts for the appropriate use of topology optimization and transmission switching in grid planning.
Definitions
First, it is helpful to define some terms.
Transmission switching is the act of reconfiguring the network by opening or closing switching devices, usually circuit breakers. While Order 1920 adds to this definition “in pre-determined circumstances based on prior analyses well in advance of the operational time horizon,” in section 1246, page 884, this addition is overly limiting. Analysis ahead of the operational time horizon is indeed preferred for any operational decision, but frequently it is not possible. Unanticipated events, such as forced and emergency outages, can place the system in scenarios that are different from those analyzed in advance. Today, transmission switching is already used to address transmission limitations in such unexpected situations in addition to scenarios that allow for advance studies.
Topology optimization is “the act of determining the optimal use of the transmission system, which may involve many different transmission facilities,” as indicated in Order 1920. Topology optimization as a technology is not tied to a timeframe; rather, the timeframe is determined by the process to which the technology is applied. For example, in long-term transmission planning processes, the topology optimization goal is to optimize the use of the transmission system under the scenarios considered in the planning analyses, whereas in operations its objective is to optimize the transmission system use under the realized system conditions.
Transmission Planning Context
An argument for the current practice of keeping transmission topology fixed in planning is the fact that planning processes understate operational transmission needs, including because there is:
- Limited modeling of transmission outages (beyond contingency constraints)
- Limited modeling of extreme events that require very large transmission support
- Understated growth scenarios for various resources that require transmission, including renewable generation expansion, and, now, load growth
Not reflecting transmission switching options in planning helps (to some extent) preserves operational degrees of freedom to address the transmission needs that would materialize in operations but are not captured in planning studies. In general, however, it would be best for planning processes to more appropriately reflect both operational transmission needs and operational decisions to manage those needs. The industry needs to continue moving in that direction to provide more robust transmission planning.
Thoughts on Transmission Switching as a Solution Used in Planning
Even in the current transmission planning context, there are transmission switching applications that would be appropriate and would support more efficient and desirable transmission outcomes.
- Corrective transmission switching as part of Corrective Action Plans (CAP) for transmission needs that involve multiple outages, as allowed by NERC TPL001-4. In these cases, transmission switching should be prioritized over other allowable CAP actions that would be costly and prone to failures, such as use of must-run units, and it must be prioritized over load-shedding actions. (Transmission switching solutions may be available to replace load shedding in CAPs, see slides 23-24 of this presentation for a few examples.)
- Persistent transmission switching solutions that are beneficial to meet a transmission need reflected in some of the planning power flow cases, and that can be implemented in all of the seasonal cases while meeting all reliability criteria without adversely impacting other system needs. Such solutions should be considered as the new baseline topology i.e., the associated breakers would become normally open. As an example application of a persistent transmission switching solution, the most expensive constraint in the Southwest Power Pool (SPP) in 2019 was permanently resolved with a transmission reconfiguration, see the SPP State of the Market 2019, pages 194-195, 199, and 214.
- Temporary transmission switching as a fast-track planning solution. While temporary switching is usually thought of as an operational solution, it has planning applications. To the extent that there are reconfigurations available to meet different planning scenarios, they could be used to quickly buy time and enable larger and more cost-effective transmission expansion projects in the future (compared to smaller piecemeal solutions). In cases when a large transmission expansion project cannot be cost-justified in the near-term planning horizon, temporary switching may allow reliability and/or economic issues driving the need for the project to be mitigated in the interim.
- Indicative transmission switching to inform transmission plans and understand the value of transmission flexibility. This needs to be considered to justify different substation designs and arrangements, such as to purposely connect different branches contiguous to each other or in different busbars, or to enable various reconfiguration options in operations. It can also be used to identify, quantify the value of, and justify transmission lines that explicitly enable more flexible reconfiguration options that can address a broader range of challenging scenarios. A transmission solution informed by topology optimization is likely to look different from a solution that is not, offering increased flexibility and significantly better performance during operations. As part of topology optimization studies, we routinely find examples of substations that were clearly not designed with reconfigurations in mind and limit the opportunities to mitigate congestion in operations.
- Complementary transmission switching to increase the benefit-to-cost ratio of transmission expansion projects. Normally, constraints on the underlying transmission system limit the utilization of high-capacity transmission lines, especially new ones. In planning studies, this results in understating the full benefits of larger projects. Transmission switching can be used on the underlying system to reliably re-direct flow to the high-capacity projects, improving their utilization and benefits.
In all cases of transmission switching use in planning, operational review is needed, as there could be system conditions not reflected in the planning cases that could preclude the use and require a modification of the planned reconfiguration.
Finding Transmission Switching Solutions
The question remains of how to practically identify appropriate transmission switching solutions for the planning applications listed above. Traditionally, in operations this task has been performed relying on the knowledge of the system of expert staff. This is already challenging in operations, limiting the use of switching options unless assisted by topology optimization software. In planning applications, it would be a monumental task without topology optimization technology, given the complexity of meshed transmission networks and the many scenarios that should be considered in addition to the thousands of contingencies associated to each scenario.
The effective use of transmission switching in planning must thus be informed by topology optimization technology to identify the candidate reconfigurations to be analyzed.
Pablo A. Ruiz
CEO and CTO, NewGrid
Senior Consultant, The Brattle Group
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