When electric transmission lines are constrained in such a way that there is inadequate capacity on existing lines to efficiently deliver the power to meet demand in certain areas, or to transport energy from high-generation areas to high-consumption regions, the cost of the energy rises; more expensive sources of power are used to meet the demand or the system operates less reliably.

These economic impacts are known as congestion, and they can amount to a substantial financial burden on the operator.

There are several solutions to the transmission constraint problem, including construction of new generation, construction of new transmission facilities, rebuilding and/or reconductoring of existing transmission assets – and dynamic line rating (DLR). All of these options except DLR are capital intensive, may have long lead times and often experience strong public and regulatory opposition.

"DLR means assessing the capacity of a given line in real time rather than according to a fixed figure"

DLR means assessing the capacity of a given line in real time rather than according to a fixed figure (‘static rating’) which is inevitably set to correspond to the low end of the range of circumstances over which the line operates, and its potential value has been recognised by the US Department of Energy which has provided co-funding under its Smart Grid Demonstration programme (SGDP) with the aim of demonstrating that DLR technology is capable of resolving many transmission capacity constraint problems with a system that is reliable, safe and cost competitive. The technology employed for the trial was supplied by Nexans.

The project, co-funded with Oncor Electric Delivery Company, a US transmission and distribution utility in central Texas, has developed and deployed an extensive and advanced DLR installation to carry out a 3 year field trial of the technology to provide real-time information on conductor temperature – enabling system operators to make fully informed decisions on how hard they can drive their overhead power lines (Figures 1 and 2).

The SGDP DLR deployment is the first application of DLR technology to feed transmission line real-time dynamic ratings directly into the system operations State Estimator and load dispatch program, which optimises the matching of generation with load demand on a security, reliability and economy basis.

Results of DLR demonstration

The project’s final report has recently been published.

"Real-time information…can make an important contribution to increasing the power carrying capacity of existing overhead line assets and reducing congestion"

It shows that the real-time information provided by the Nexans’ technology (Figures 3 and 5) can make an important contribution to increasing the power carrying capacity of existing overhead line assets and reducing congestion.

For the core component of the SGDP, Oncor installed the DLR technology on eight 138 kV to 345 kV transmission circuits located in central Texas, where it enabled power capacity to be increased by up to 14%.

Tip Goodwin, DLR project manager at Oncor says, "Technologies like DLR give transmission owners like Oncor significantly increased visibility and flexibility to operate more reliably and efficiently. That’s important not only for our residential customers, but also for the more than 400 communities we serve that are looking to grow existing businesses and attract new businesses. While electricity infrastructure may not receive the headlines that tax incentives do, economic and reliable electricity is at the top of the list of priorities for businesses."

The final report notes that "The SGDP Project has been a complete success, having demonstrated that Dynamic Line Ratings are a practical and efficient tool to increase the capacity of a transmission line, which will enable transmission providers and system operators to mitigate congestion, increase system reliability and redeploy capital to its most efficient uses through a least regrets strategy."

DLR was being evaluated as a potential key component of Oncor’s 5-year capital investment programme that will average $1 billion per year. The programme’s strategy is to invest in technologies and equipment to improve reliability and efficiency of the company’s infrastructure.

“Our investment programme is not about just adding more infrastructure. We want to be able to use our existing assets more efficiently and effectively because that’s a more economical use of our investment dollars. DLR clearly demonstrated that we could improve the efficiency of our existing assets in an economical manner," said Jim Greer, Oncor chief operating officer. "Already we are looking at other areas of our grid where we can install this capability for future investments." In fact Oncor has now deployed additional DLR systems in the Odessa-Midland region of Texas in a commercially funded follow-on project.

How integrated dynamic line rating works

"iDLR collects transmission line parameters at remote locations on the lines and calculates the real-time line rating"

The integrated dynamic line rating (iDLR) collects transmission line parameters at remote locations on the lines calculates the real-time line rating based on the equivalent conductor temperature, ambient temperature and influence of wind and solar radiation on the stringing section, transmits the data to the transmission energy management system, validates its integrity and passes it on to Oncor and ERCOT (Electric Reliability Council of Texas) respective system operations. The iDLR system is automatic and transparent to ERCOT system operations, that is, it operates in parallel with all other system status telemetry collected through Supervisory Control and Data Acquisition (SCADA) employed across the company.

The Nexans DLR technology employs an algorithm that transforms real-time sensor data into a conductor temperature and calculates the maximum current capacity – the Dynamic Line Rating – which maintains the overhead line sag within safe clearance limits. The DLR is updated every 5-10 minutes, providing operators with much clearer visibility than traditional Static Line Ratings, which use pre-determined weather assumptions, or Ambient-Adjusted Rating, which takes into account the ambient air temperature.

Deployment of DLR equipment

Deployment of the DLR equipment was completed in the following stages.

  • Thirty-seven loadcell CAT-1 devices were installed on eight line segments.
  • Eighteen were installed on 345 kV steel tower lines.
  • Nineteen were installed on 138 kV wooden H-frame lines.
  • Eight CATMasters were installed in substations to route the data into the Oncor SCADA system.
  • An IntelliCAT server was installed at the transmission grid management.
  • The EMS system was upgraded to manage DLR data incorporation into the ERCOT telemetry.
  • Five sagometers were installed for the Sag Verification Study.
  • Two non-contact real-time monitoring and optimisation of transmission lines systems (RT-TLMS) were installed, one for 138 kV monitoring and one for dual-circuit 345 kV.

Basis of calculation

The maximum operating temperature is set by the utility and is typically 75 °C, 90 °C or 125 °C. Knowing the maximum sag allowed, the maximum electrical operating load can be determined, based on the specific conductor and ambient climatic conditions at the time of electrical loading using either IEEE 738 or CIGR. TB 207 calculation.

Traditionally, the climatic conditions for minimum clearance were assumed to be a worst case scenario of minimum wind speed, high ambient temperature and full solar radiation, eg, 2 ft/s wind, full sun, and ambient temperature of 104 °F (40 °C). Other ambient conditions can be used for this calculation, but the given values are predominant in the United States. They represent a typical maximum ambient temperature for the service area, a typical minimum effective wind speed and full solar exposure.


The Oncor report highlights the following conclusions and breakthroughs:

  • Increased line capacity. DLR provided up to 14% additional capacity above the ambient temperature-adjusted ratings. The incremental capacity was available from 83.5 to 90.5% of the time.
  • Reduced congestion. The project found that 5% additional capacity could relieve congestion by up to 60% on the target lines with DLR installed, while 10% additional capacity would eliminate practically 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.
  • Market integration. The iDLR system at Oncor feeds real time conductor ratings to ERCOT, the market operator, which then incorporates the additional capacity into its Security Constrained Economic Dispatch process. With zero operator intervention, DLR capacity is used to increase market efficiency.
  • Transmission planning. By providing additional capacity on transmission lines where a full upgrade cannot yet be justified, DLR can be utilised in the planning process to enable a ‘least regrets’ capital strategy.
  • Best practices. The project managers have developed a guide to assist other transmission owners considering DLR technology for their own systems.

Staff report from the May 2014 issue of ModernPowerSystems