A new solution for grid flexibility

Credit: Dreamstime
Credit: Dreamstime
Real-time information provided by dynamic line rating technology can play a vital role in increasing the power-carrying capacity of existing overhead line assets and reducing congestion writes Sandy Aivaliotis

Oncor is a regulated electric transmission and distribution service provider operating approximately 1500 circuits serving 10 million customers across the US state of Texas.

This grid is subjected to significant variability in demand. Oil and gas developments have prompted rapid load growth in specific areas, while wind and coal have created changes in generation. In addition to these larger trends, weather and load create daily fluctuations in pricing, further increasing the unpredictability of the load on any particular line.

Utilities are unable to distribute enough power to meet the demand, leading to congestion. A few lines suffer from congestion over sustained periods of time, with financial implications ranging up to hundreds of millions of dollars.

Over the three-year period monitored, approximately 200 lines would experience sporadic to chronic congestion under contingency conditions. Since ERCOT (the Electric Reliability Council of Texas) dispatches the system to avoid contingency overloads, these constraints account for approximately $172 million in annual congestion costs, even though during actual operation the lines are seldom loaded to their limits. If Oncor could utilize more of the latent capacity that exists in every line, customer costs could be reduced not only through access to lower cost generation, but also a reduction in transmission charges.

There is an obvious requirement for a flexible solution capable of meeting the uncertainty of transmission grid need. Yet, since grid topologies must adjust constantly to reflect load growth and changes in the nature of generation and transmission grid enhancements, the demand on individual transmission lines can be difficult to anticipate. It is possible that loads can appear and disappear within the planning and construction duration of a traditional upgrade. Smart grid technologies such as Dynamic Line Rating (DLR) are required to meet the fast-changing line capacity requirements of today’s grids.

During the cost-sharing Smart Grid Demonstration Project (SGDP) with the US Department of Energy, over 24 months Oncor operated 13 transmission lines by integrating DLR directly into its Electric Management System (EMS) and streaming the DLR information to the Independent System Operator’s (ISO) Security Constrained Economic Dispatch (SCED). This innovative combination of new and existing technologies to fully automate and stream DLR allowed the ISO and Oncor to utilize the actual real-time transmission line capacity to enhance asset optimization and optimize generation dispatch, leading to reduced congestion, increased reliability and heightened wide-area system awareness (WASA). By completely automating the process, Oncor achieved this breakthrough without adding to the workload of its grid operators in the system control centre.

Assumption-based operation

Overhead conductors exhibit a certain level of ‘sag’ when strung between their supporting towers. This is determined by both the physical characteristics of the conductor (related to materials and design) and the conductor temperature – as a conductor carries more current it heats up and sags.

Transmission lines must be operated so that the overhead line conductor maintains a minimum clearance from the ground or other objects allowed on the right-of-way to ensure safe, reliable operation. This safe clearance is determined by specific parameters that characterize the position of the conductor.

These parameters include:

  • the conductor properties;
  • the line loading (in Amps); and
  • ambient conditions along the transmission line including the ambient temperature, net solar impact on the conductor and effective wind speed blowing across the conductor.

The current flowing in the line causes it to heat up due to resistive heating. The sun can further heat the conductor while the ambient temperature can either heat or cool it. By far the most dominant variable comes from wind which, when blowing over the conductor, provides cooling.

The net heat balance between these influences effectively makes the conductor a spatial thermometer. Each conductor has a time constant associated with its mass – meaning that the conductor temperature does not change instantaneously in response to a changing net heat balance; instead there is a lag in response. The conductor sag, tension and temperature are a uniquely coupled system.

The majority of utilities have established what is referred to as a Static Line Rating (SLR) which represents the level of current loading the conductor can carry while maintaining the desired operating safety and clearance criteria.

The SLR is established at a designated set of ambient weather conditions defining the ambient temperature, effective wind speed and level of solar radiation. A very common set of parameters for this is 40à‚°C, 0.61 m/s perpendicular wind and full solar radiation. Some utilities use an Adjusted Ambient Rating (AAR) that adjusts the rating to account for the ambient temperature along the line.

In effect, the majority of overhead transmission lines are operated according to a set of assumptions that project their operating temperature and therefore sag, rather than measuring it. This lack of real-time information results in lines operating at loads well below their actual safe limit – or, in other words, there is a significant level of unused spare capacity that could help to address grid congestion issues if it can be accessed.

Across the utility sector, some may use “dynamic” to refer to any line ratings which change throughout the day, such as when ratings are adjusted for ambient temperature, seasonal or day versus night. In the context of this SGDP project, we are discussing Dynamic Line Ratings (DLR) where the condition of the conductor is directly monitored in real time (i.e., continuously). Further, because we have integrated the DLR rating automatically into system telemetry for real-time operations, we refer to the ratings as integrated Dynamic Line Rating (iDLR).

Real-time DLR technology, such as Nexans’ CAT-1 system, utilizes sensors that monitor the key conductor parameters and calculates those that cannot be directly measured, providing a calibrated basis to project the impact of additional current on the conductor’s average temperature. The goal is to maximize the line rating for existing ambient conditions.

DLR is intended to adjust the SLR to the rating appropriate for the real-time ambient weather conditions. Ambient temperature and solar radiation are relatively stable over distance, but they have a very modest impact on the ratings compared to the wind blowing across the conductor. However, wind exhibits considerable spatial variability and can vary almost metre by metre along the span.

Fortunately, DLR technology can capture that spatial variability and determine the average effective wind speed on the entire line section. The conductor’s time constant matches well with sampling rates and the cycle time for state estimator analysis that uses the DLR data to minimize wide variations in ratings due to transient parameters.

Once a line rating is determined, a protocol is required to bring this information to the operating environment so that the real-time rating can be applied to operate the grid area in which the line is located.

DLR Systems

The key parameter to determining the DLR is the effective wind speed along the stringing section.

For a Drake conductor (a common transmission line conductor) using typical summer climate assumptions, a decrease in effective wind speed from 0.2 m/s to 0.1 m/s results in a rating reduction of 14 per cent. Since the wind speeds of interest are very low, in the range 0-2.0 m/s, there are no meteorological services that monitor these levels along the line.

Even a distributed network of anemometers would require too extensive a deployment of instrumentation by a utility to be viable. The more practical solution is to obtain the effective wind speed by deriving the value from the conductor temperature.

Deploying the DLR system requires a selection of monitoring locations in order to develop the rating for each line section, differentiated from the next by conductor size, stringing and loading design criteria, line orientation to prevailing winds and line loading.

If the line has long tangent sections, it may be necessary to put more than one sensor device in the line section. The SGDP project validated that a tension or conductor position monitor can easily characterize the performance of 8 km of line.

From a capacity standpoint, the data shows that the DLR exceeds the SLR rating 99 per cent of the time, and exceeds it by more than 110 per cent between 93 per cent and 97 per cent of the time.

The results of the SGDP project illustrate that real-time monitoring and application of DLR is essential to optimal application of the methodology. This is because the dynamic ratings for each line vary in real time and over the long term. It is not sufficient to collect data for a certain length of time on a few lines to characterize the dynamic rating potential of a transmission line or a system of lines.

There is a significant level of unused spare
There is a significant level of unused spare capacity that could help to address grid congestion issues if it can be accessed.
Credit: Oncor

Applying the technology

The importance of DLR is to capture real-time information and then calculate the maximum current that can be carried by the entire line while maintaining safe operating criteria – setting a maximum operating temperature that maintains the required ground clearance.

Any DLR system should be evaluated with respect to cybersecurity concerns relating to data integrity and system security.

There are two strategic ways to apply the DLR information: system operations query system and autonomous streaming telemetry to state estimator.

The difference between the two methods is how system operations accesses the DLR in order to apply them.

System operations query system: In this application, the DLR data is made available to System Operations on a control room screen.

Oncor uses an ambient temperature-adjusted rating based on the ambient temperature from a weather station assigned to each line.

This format calls for the operator to recognize a need for additional ratings, recognize that the line has DLR available and then decide to allow the system to operate at that level.

This places considerable dependence on the operator to take additional operational decisions.

The SGDP project demonstrated the benefits of using DLR in an autonomous protocol that streams the data automatically to the system state estimator so that the dynamic rating can be applied continuously in real-time.

This technique provides several advantages for grid operations. The transmission grid is operating closer to its true functional capability, is continuously monitored and managed by the most recent and relevant data, and relieves the system operator of additional workload.

Within the protocol, the system has built-in quality and integrity checks that are executed before passing the data to the state estimator. These validate that the ratings returned by the DLR system are within an acceptable and expected range. If the validity checks fail, the system reverts to the line rating that is normally applied.

One of the concerns that many operators have in making the query-based decision when applying DLR is that they are unsure of what the rating may do in the near term.

If they are increasing the line capacity based on DLR ratings, they may have concerns about how long the rating will remain elevated and how it could drop in the short term. Furthermore, each different control room shift has operators with different experience with line performance and their decisions may be different case to case, shift to shift.

Continuous monitoring

iDLR ratings have the advantage that the operator is not burdened with making additional decisions. The real-time nature of iDLR also adds a component of Wide Area System Awareness (WASA) to the line operation by continuously monitoring the line’s state.

The cycle time for monitoring the line and reporting to the state estimator can be adjusted and synchronized to the state estimator’s cycle of managing the grid, so that the system becomes self-resolving. If the wind should die off, the conductor heats up and iDLR levels reduce, the system automatically adjusts the operation to maintain system and line reliability.

The iDLR streaming protocol has proven to be a more reliable and consistent application of DLR than the operator query based system. The continuous WASA aspects of iDLR provide system reliability as well as increased capacity when available. The reduced operator work load, reduced personnel training requirements and consistent availability make the iDLR stream of ratings a valuable tool in the operating environment.

Overhead conductors exhibit a certain level of 'sag' when strung between their supporting towers.
Overhead conductors exhibit a certain level of ‘sag’ when strung between their supporting towers.
Credit: Oncor

Project results

The final report published on the Oncor SGDP Project highlighted several conclusions and breakthroughs.

Oncor found that using DLR increased capacity by 14 per cent above the ambient temperature-adjusted ratings. The incremental capacity was available from 83.5 per cent to 90.5 per cent of the time.

In addition, the project found that 5 per cent additional capacity could relieve congestion by up to 60 per cent on the target lines with DLR installed, while 10 per cent additional capacity would practically eliminate all congestion on the target lines. Congestion on the Oncor transmission lines in 2011 and 2012 cost more than $148 million and $197 million respectively. Increasing capacity is useful on overhead lines where a full upgrade cannot yet be justified.

By providing additional capacity, DLR can be utilized in the planning process to enable a least-regrets capital strategy, minimizing any potential stranded investment.

The integrated Dynamic Line Rating (iDLR) system feeds real-time conductor ratings to ERCOT, the market operator, who then incorporates the additional capacity into its Security Constrained Economic Dispatch process. With zero operator intervention, DLR capacity is used to increase market efficiency.

As evidence of the extensibility of DLR technology, in June 2013 Oncor deployed additional DLR systems in the Odessa-Midland region of Texas in a commercially funded follow-on project.

For other transmission owners considering using DLR, the project authors have developed a guide.

While this project has focused on a US application, the principles apply worldwide. And we are already seeing significant interest in Europe.

Sandy Aivaliotis is senior vice-president, Operations, Technology and Business Development at Nexans’ The Valley Group

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