NY UTILITY Examines Integrating Dynamic Line Ratings

By Jennifer Mayadas-Dering,

Liana Hopkins and Arnie Schuff, NYPA

Difficulties today surrounding financing, planning and gaining approval for new facilities are challenging utilities to re-evaluate the normal and post-contingency loading of existing transmission lines. In response, dynamic rating and real-time monitoring of transmission lines are becoming important tools in attempting to maintain system reliability while optimizing power flows. Dynamic ratings can be considered a low-cost alternative for increased transmission capacity.

Wind generator facilities are popping up faster than the transmission system can keep up. In New York, 1,200 MW of wind generation is installed, and an additional 8,000 MW is planned to be installed. Transmission owners cannot always construct new circuits fast enough to meet the growing demands. As a result, transmission capacity is testing design limits, and ultimately, curtailment of renewable resources is the only remaining option.

Transmission circuit capacity is generally imposed by the “static rating.” Static or “book” ratings are calculated using worst-case weather and pre-loading conditions. They apply for an extended period. (Transmission operators may exceed the static rating by 10 to 20 percent for brief periods during system emergencies based on system operations guidelines.)

Dynamic ratings are typically 5 to 15 percent and could go higher than 15 percent above conventional static ratings. Application of dynamic ratings can benefit transmission utilities and system operation in several ways–in particular by increasing power flow through the existing transmission corridors with minimal investments. With the recent increase of wind generation integration into the power system, it has been found that overhead conductor thermal capacity increases during maximum wind farm output. During high wind, conductor thermal capacity likely could be increased for periods through the use of dynamic ratings, allowing more efficient use of wind generation.

Dynamic vs. Static Ratings

When applying static ratings, no attempt is made to determine the equipment temperatures. System operators can be reasonably certain that the equipment temperatures remain below their maximum allowable values so long as the current is kept below the static rating and the environmental conditions assumed in the rating calculation are conservative (e.g., high ambient temperatures and low wind conditions).

Dynamic ratings are calculated using similar techniques to the methods used for static rating calculations. The difference, however, is that dynamic ratings are based on real-time measurements or near-term predictions of weather conditions and circuit loading as well as possibly real-time measurement of component parameters such as temperature, rather than on worst-case assumptions. Unlike static ratings, dynamic ratings are valid for a limited time.

For overhead conductors, dynamic ratings can be calculated using three methodologies:

1. Weather-based dynamic line ratings. The dynamic line rating is calculated using a heat-balance equation based on air temperature, solar heating and wind speed and direction. The weather-based model is accurate if the stations are positioned appropriately to measure the weather seen by the line conductors. This is also the simplest methodology because no instruments need to be mounted on the line itself. The measurement of wind speed and direction by an anemometer is not dependent on the line’s electrical loading, so the method works well under pre- and post-contingency loading. One of the challenges is often communicating weather information from remote sections of the line and incorporating it into the supervisory control and data acquisition (SCADA) system. Weather stations are also only measurements of the ambient conditions at a single location and do not account for changes along the line section.
2. Conductor temperature-based dynamic line ratings. The real-time conductor temperature is converted to an equivalent wind speed perpendicular to the line. Then the wind speed is used in combination with other data to calculate the dynamic line rating. The advantage of this methodology is that the user has a direct measurement of conductor temperature. The disadvantage is that the instruments measuring conductor temperature must be mounted on energized conductors, the measured temperature must be communicated to a station some distance away and the measured temperature may not be the best estimate of the average conductor temperature along the line. Sensors are being developed to communicate with one another and will enable temperature measurement from multiple points along a line.
3. Sag/tension-based dynamic line ratings. Conductor sag or clearance at maximum allowable conductor temperature is the main limiting parameter in the design of a transmission line. Sag (or tension) -monitoring equipment respond to weather conditions along an entire line section being monitored rather than to weather conditions at a single point along the line. Therefore, ratings based on a single sag (or tension) monitor are equivalent to several weather stations along a line section. Most lines consist of multiple line sections, so one sag (or tension) monitor does not indicate the rating of the whole line. Similar to the temperature monitors, the real-time sag (or tension) is converted to an equivalent wind speed. Then, the line rating is calculated using the heat-balance equation.

As part of its ongoing research in this area, the Electric Power Research Institute (EPRI) has developed monitors, Sagometer and Backscatter Conductor temperature sensors, as well as rating calculation methodologies, the Dynamic Thermal Circuit Rating (DTCR) software, as well as other products. The combination of all these products and field test results is referred to as EPRI’s DTCR Technology. DTCR was developed for optimizing the power transfer capacity of existing transmission assets at relatively low cost. This technology has matured to the point where transmission owners can engage in field demonstrations and applications.

Operating With Dynamic Ratings

When transmission line flows are limited by their thermal capacity rather than voltage or stability limits, dynamic thermal ratings offer many advantages and challenges compared with the traditional static thermal ratings.

Because dynamic thermal ratings normally are higher than static ratings, the advantages of their use are:

  • Better situational awareness in the control rooms, allowing operations staff to react to real—time, post-contingency load flows that otherwise might exceed static ratings,
  • Potential for cost savings during generation dispatch calculations because dynamic ratings are higher than static ratings,
  • Increased reliability during system emergencies by combining real-time and probabilistic rating methods to give the system operator better guidance during N-1, N-2 and potential blackout events,
  • Better response to Federal Energy Regulatory Commission (FERC) demands for improved reliability and operator response to reduce system disruptions. In addition, refining the approach to static ratings in a way that is scientifically justifiable and can satisfy FERC requirements for rating assignments,
  • Real-time monitoring of the thermal state and rating of a line or circuit that maximizes power throughput safely and reliably,
  • Enhanced decision making in high load periods–both planned and because of emergencies,
  • Deferment of capital expenditures for new assets or significant upgrades,
  • Enhanced performance and reliability of aging assets, and
  • Rating calculations and studies are invisible and can be done without public hearings or any other permitting issues.

Practical difficulties with the dynamic ratings implementation include the following:

  • SCADA/energy management system (EMS) flexibility and capability to incorporate dynamic ratings,
  • Instrumentation reliability (instruments mounted on overhead line structures are vulnerable to extreme weather conditions, high electric and magnetic fields and vandalism),
  • Availability and reliability of communications links to SCADA (in many power systems, the number and capability of communication links connecting remote substations to SCADA may be a limitation in implementing dynamic ratings),
  • Rating variability, one of the greatest challenges associated with dynamic ratings to system operators. One method of limiting rating variability is through a combination of averaging and limiting the range of dynamic rating values, and
  • Engineering acceptance. Asset management and operations personnel typically question the accuracy of the dynamic ratings.

Demonstration Project With EPRI DTCR technology

The New York Power Authority (NYPA) is engaging with EPRI in a demonstration project that will evaluate the instrumentation and dynamic thermal ratings for overhead transmission lines. An area of possible application is emerging where higher ratings are needed because of wind generation penetration, and when turbines are operating, one expects higher dynamic ratings because of increased wind speed.

The project will use EPRI’s DTCR software, which uses real-time or historical weather and electrical load data to calculate dynamic ratings for overhead lines in real time based on actual load and weather conditions that generally are accessed through the utility’s SCADA/EMS system. The studies will occur in areas of high wind infiltration.

Required real-time data will be provided using different methodologies. Temperature monitors (backscatter sensors), video sagometers and tension-monitoring equipment (CAT1) may be installed on one of NYPA’s 230-kV lines with a lot of wind capacity connected to it. Real-time measured data (temperature, sag, tension, weather conditions) will need to be communicated to a central computer. EPRI will work with NYPA on defining the best data-logging and communication configuration. Dynamic ratings will be computed for the different methodologies and compared. EPRI will work with NYPA operators to develop an appropriate technique for integrating and displaying the DTCR output into NYPA’s energy control center.

Regardless of the application objective, the dynamic rating information must be communicated to operations and integrated into normal operating procedures.

In the changing world of power generation and transmission, and with the fast integration of variable generation on the transmission system, the do-nothing option for transmission owners is no longer available. Utilities must accept, incorporate and drive new technologies such as dynamic rating in system operations.

Jennifer Mayadas-Dering has worked within energy services, nuclear licensing, project management and engineering at the NYPA since 1994. She is the manager of the operations planning group within the power supply business group.

Liana Hopkins joined the New York Power Authority in 1986. She is presently a senior system planning engineer with the operations planning group.

Arnold J. Schuff began his career with the Long Island Lighting Co. Since 1986, Schuff has held several positions within the NYPA system planning department. He is the manager of the transmission planning group.

Dynamic Rating and the Smart Grid

Dynamic rating is considered a smart grid advanced application because it involves the monitoring of real-time system data that can be used in various applications:

  • Real-time monitors yield a continual flow of data to system operations–line sag, tension or both, wind speed, conductor temperature, etc.–traditionally not available to operators,
  • Monitored data can be processed to spot trends (increasing now, higher than yesterday at the same time, etc.),
  • Real-time monitored data may be turned into useful operator predictive advice (e.g., time till critical temperature, percent load reduction needed, and planner information. For example, conductor strength reduced by X percent after most recent emergency, clearance limit exceeded 2 percent of time during August, etc.).

A Conversation With ERCOT on Dynamic Rating

By Kathleen Davis, senior editor

KD: The Electric Reliability Council of Texas (ERCOT) started dynamic rating for your transmission lines in 2005. Since then, what has been the greatest benefit?

JD: ERCOT is able to maximize the use of the transmission system to transfer more efficient generation to the load. For example, ERCOT ran an analysis of the zonal transfer capabilities during a typical winter day in 2007. The results are shown in the chart. They clearly indicate that transfer capability is increased with the utilization of dynamic ratings instead of using the static (worst-case) ratings of the transmission lines.

Click here to enlarge image

KD: Have you discovered any drawbacks?

JD: No.

KD: What was the impetus for this change? Why not continue to simply use a general worst-case scenario estimate?

JD: The reason for the change was to reduce congestion cost by utilizing the available transmission capacity during cooler weather conditions. The worst-case scenario that drives the static ratings of lines is based upon ambient temperatures in the range of 98.5 F and 104 F.

Transmission lines have higher transfer capabilities at cooler temperatures. A dynamic line rating system allows the operator to maximize the line capabilities at the cooler temperatures and to adjust the ratings dynamically based upon real-time temperatures.

KD: Would you recommend dynamic rating to be implemented across the board for transmission lines?

JD: Yes.

Answering for ERCOT: John Dumas, manager, operations planning, Electric Reliability Council of Texas (ERCOT)

John Dumas has worked 22 years in the electric utility industry, four of which have been with ERCOT. The operations planning group is responsible for load forecasting, wind forecasting, ancillary services and short-term planning (voltage stability analysis and special protection scheme and remedial action plan analysis).

On the Net: http.://ercot.com

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The Clarion Energy Content Team is made up of editors from various publications, including POWERGRID International, Power Engineering, Renewable Energy World, Hydro Review, Smart Energy International, and Power Engineering International. Contact the content lead for this publication at Jennifer.Runyon@ClarionEvents.com.

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