Dynamic line rating: increasing grid capacity and renewable energy penetration

This summer the Federal Energy Regulatory Commission issued Order No. 2023 Docket No. RM22-14-000, Improvements to Generator Interconnection Procedures and Agreements, adopting reforms to address interconnection queue backlogs, improve certainty, and implement new transmission technologies. The reforms are classified into three categories: Reforms to Implement a First-Ready, First-Served Cluster Study Process, Reforms to Increase the Speed of Interconnection Queue Processing, and the one that is of greatest interest for this article, Reforms to Incorporate Technological Advancements into the Interconnection Process.

The path towards the energy transition requires changes in current interconnection processes

The growth of new resources seeking to interconnect to the transmission system and the differing characteristics of those resources have created new challenges for the generator interconnection process, resulting in interconnection queue backlogs and uncertainty regarding the cost and timing of interconnecting to the transmission system, increasing costs for consumers. Backlogs in the generator interconnection process, in turn, can create reliability issues as needed new generating facilities are unable to come online in an efficient and timely manner. In light of this, the approved reforms will ensure that interconnection customers are able to interconnect to the transmission system in a reliable, efficient, transparent, and timely manner [1].

The reform related to the incorporation of technological advancements into the interconnection process arises under the premise that the deployment of these transmission technologies may reduce interconnection costs by providing lower-cost network upgrades to interconnect new generating facilities. The Commission proposed to require transmission providers to consider the following technologies within the interconnection studies upon request of the developers: advanced power flow control, transmission switching, dynamic line ratings, static synchronous compensators, and static VAR compensators.

While all the technologies proposed by the Commission refer to devices, dynamic line rating corresponds to a transmission technique, resulting in a series of comments for and against by the specialized entities that were part of the debate. Why was this proposal so debated? What does it consist of?

Real-time thermal ratings enable efficient transmission grid management

Historically utilities have operated transmission systems conservatively in order to provide high reliability through moderate transmission line loading and redundancy. The system is planned to guarantee the highest possible security and quality of supply, which involves using conservative worst-case assumptions at the planning stage [2]. 

The loadability of long transmission lines is often limited by surge impedance loading or stability constraints, voltage profile, and energy losses, but for short lines, the maximum load capacity is dictated by their thermal rating, which depends on the maximum allowable conductor temperature [3].

IEEE Std. 738-2012 describes a numerical method, by which the core and surface temperatures of a bare-stranded overhead conductor are related to the electrical current and weather conditions [4]. The IEEE method is based on the steady-state heat balance equation considering the balance between heat dissipated and absorbed, see Equation 1.

qC+qR=qS+qJ (1)

Where:

qC: Convective cooling

qR: Radiative cooling

qS: Solar heating

qJ: Joule effect heating

Convective cooling depends mainly on wind speed (WS) and wind direction (Wd), whilst radiative cooling is influenced by the temperature at which the conductor operates (TS) and ambient temperature (Ta). Direct and diffuse solar radiation (QB, QD), are relevant for solar heating calculation. Joule effect heating is a result of the electric current (I), this is precisely the variable to be solved when weather conditions are analyzed.

Ampacities proposed by manufacturers of bare stranded overhead conductors are calculated with very conservative weather conditions: QB = 1000 W/m2, Ta = 25°C and WS = 0.61 m/s, this last consideration arose from a study realized in the 1930s, where it was determined that this wind speed was not registered more than 5% of the time during the summer season. Consequently, the possibility of overheating a conductor is almost negligible.

In practice, the current capacity obtained by applying the IEEE Standard with very conservative environmental conditions defines the capacity of the transmission lines limited by thermal constraints, giving rise to static line rating.

Nevertheless, weather conditions are not static; therefore, the thermal rating of a transmission line is not either. When favorable weather conditions occur transmission lines operate with a low utilization level. Recognize that certain weather conditions can impact conductor temperature and cause a change in the transmission line capacity reveals that the possibility of using variable line ratings could increase the utilization of overhead lines, bringing with it relief of congested transmission connections and higher renewable energy integration in the grid, as well as better grid management in case of contingencies.

Dynamic line rating is a technique that can dynamically increase the current carrying capacity of electric transmission lines. In a dynamic line rating framework, ampacity is considered a dynamic variable giving a conservative estimate of the critical value at which the line may be operated. In the current power system scenario, where the rise of power from variable renewable sources puts stress on the existing infrastructure, making necessary transmission upgrades, dynamic line rating can represent a solution for accommodating higher renewable production whilst minimizing or postponing network reinforcements [2].

A dynamic line rating system implementation does not increase line capacity by itself; rather, it reveals the real-time line capacity through monitoring systems. Monitoring devices can have different functions, some of them are used for measuring conductor temperature, weather conditions, or conductor mechanical tension.

Success operational implementations in the US

Even though the Commission recognizes this technique as one of the most promising Grid Enhancing Technologies, it was not included in the final rule under the premise the technique is more beneficial during grid operations, and it might not be appropriate for transmission planning. 

There are several operational applications where this technique has demonstrated its functionality. The first application where an energy market operates with real-time line ratings is managed by ERCOT, in Texas. Since 2013, ERCOT has received real-time ratings from Oncor Electric Delivery Company, which developed and deployed an extensive and advanced dynamic line rating installation to demonstrate that this technology is capable of solving many transmission capacity constraint problems with a system that is reliable, safe, and very cost competitive. This system feeds and loads a dispatch program, which optimizes the matching of generation with load demand on a security, reliability, and economic basis, [5] and [6].

PJM implemented a dynamic line rating system in October 2022 as part of a PPL Electric Utilities project for three transmission lines in northeastern Pennsylvania. The project expects to expand capacity and promote market efficiency on three historically congested lines (the two-circuit Susquehanna-Harwood path and the Juniata-Cumberland line). PPL Electric Utilities estimates that this project can save customers $23 million annually in congestion costs.

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References

[1] FEDERAL ENERGY REGULATORY COMMISSION, Order No. 2023, Docket No. RM22-14-000.

[2] Michiorri A, Nguyen H, Alessandrini S, Bjornar J, Dierer S, Ferrero E, Nygaard B, Pinson P, Thomaidis N and Uski S. Forecasting for Dynamic Line Rating.

[3] Davis M. A new thermal rating approach: The real time thermal rating system for strategic overhead conductor transmission lines. Part I – General description and justification of the real time thermal rating system. 

[4] Institute of Electrical and Electronics Engineers. IEEE Standard for calculating the current-temperature of bare overhead conductors. IEEE Std.738-2012.

[5] Oncor Electric Delivery Company. Oncor Electric Delivery Smart Grid Program. DE- OE0000320. 2013. 

[6] Electric Reliability Council of Texas. ERCOT Nodal Protocols. Section3: Management Activities for the ERCOT System. 2017.