Real-Time Ampacity and Ground Clearance Software
- Written by Kevin M. Klein, Paul L. Springer and W. Z. Black
The temperatures of transmission lines have traditionally been estimated by means of ampacity tables that are calculated from expected average weather conditions and values that have been considered valid for a whole season. But in reality, a conductor's position and temperature vary greatly with weather conditions over much shorter time intervals. One solution is to base real-time estimates on instrument readings, but an even better method uses software to calculate the mechanical and thermal status of the conductor.
The ambitious goals of the smart grid can only be met if T&D hardware elements, particularly overhead conductors, are managed and operated intelligently. To gain full advantage of the smart grid, more sophisticated and innovative software must be adopted to optimize the overall efficiency of the power delivery system. Fortunately, software exists that is proven to accurately predict the real-time rating of overhead conductors.
In the past, the temperatures of overhead conductors were estimated by employing ampacity tables that were calculated on the basis of seasonal-averaged weather conditions. Conductor ratings remained unrealistically static for a single season. In reality, the conductor’s position and temperature vary greatly with weather conditions, particularly with wind velocity, wind direction and air temperature.
Implementing real-time ratings, instead of static temperature and ground clearance data, will improve on that outdated technology, allowing operators to make decisions that maximize the safety, reliability and revenue generating capabilities of the power delivery system. When real-time conductor data are available, operators can apply corrective action when emergencies occur, thereby preventing the possibility of system collapse. Furthermore, accurate up-to-date data on the precise temperature and ground clearance of overhead conductors will avoid unnecessary transmission constraints when they may be incorrectly predicted by static ampacity estimates. Real-time data from the field will also improve reliability and power quality—a necessity in an increasingly digital age.
To provide accurate conductor temperature and sag, a number of instrument-based methods have been proposed. Typically, these methods require that the circuit be taken out of service while instrumentation is attached to the line. Cost and maintenance issues become paramount when relying on these types of experimental means to measure the ground clearance, conductor temperature or tension. An alternative method based on using software to calculate the real-time mechanical and thermal status of the conductor exists. It has unique advantages over the instrumentation-based option.
The software-based option is constructed on a platform of combined ampacity and sag/tension programs, which avoids cost and maintenance issues that must be considered when applying an instrument-based conductor rating method. When using a software rating approach, no instrumentation needs to be attached to the conductor and the only hardware necessary to support the program is a weather station that records wind speed, wind direction and ambient temperature. Cost for this equipment is minimal and weather stations are known to operate reliably over many years of service.
The real-time rating program combines mature ampacity and sag/tension programs that have been used extensively throughout the power industry for over 25 years. The combined software provides conductor sag, tension and temperature in real time as the conductor responds to minute-by-minute changes in both weather conditions and line current. The program is able to report conductor temperature and ground clearance to a SCADA system so that control decisions can be made intelligently using accurate, up-to-date knowledge of the conductor's status.
In tests funded by Georgia Tech's National Electric Energy Testing Research and Applications Center (NEETRAC), and conducted at the request of utilities and manufacturers assembled by Southwire, a team at Oak Ridge National Laboratory in Tennessee evaluated the accuracy of the real-time rating software. During a four-month period in 2007, computer outputs for temperature, sag and tension were compared with test measurements from a full-scale, four-span outdoor test facility.
The test facility consisted of three steel supporting structures in which the conductor formed a closed loop in the shape of the letter "U". The supporting structures were 183 meters apart and the conductor was looped at the end structure so that no splices were utilized. The test conductor was a specially designed Bluejay ACSR conductor that included a temperature-sensing fiber optic element threaded between the steel and aluminum strands.
The test site included a laser range-finder to record real-time conductor position, load cells to provide a measure of the conductor tension, and a single weather station. Conductor centerline and surface temperatures were measured using the fiber optic temperature sensing system located near the conductor center and thermocouples attached to the conductor's outer surface. The optical fiber was capable of measuring temperatures along the axis of the conductor on two meter intervals, while the thermocouples were positioned every 45 meters. Nearly 40 million data measurements were statistically compared with the computer output. The integrated rating program provided results with a 95 percent confidence interval for conductor temperatures within ±10°C, sags within ±0.3m and tensions within 1800 Newtons (400 pounds) for a mean conductor temperature of 75°C. These accuracy values were maintained for a wide array of weather conditions, realistic conductor currents and conductor temperatures up to 175°C.
The integrated ampacity-sag/tension program proved to be robust for all conditions that were experienced during the overhead comparative tests. At no time did the software experience any computational errors or fail to operate satisfactorily. The fiber optic system and the thermocouples operated fairly reliably and they suffered only a single outage that resulted from a lightning strike. In contrast, the laser range finder did not prove to be as robust. It failed to record sag values during periods of high humidity as well as times when the sun interfered with its ability to acquire the target that was attached to the conductor.
Both hardware and software approaches to real-time rating of conductors contain uncertainties. However, the experiences gained during the four-month test program demonstrated that a computer program, in conjunction with a weather station, can successfully be used to predict the real-time thermal and mechanical behavior of an energized overhead conductor within reasonable errors. Implementation of a software-based ratings system promotes an intelligent smart grid concept by providing both conductor temperature and ground clearance data in real-time.
Thus, a software-based rating method is an economic alternative to hardware-based schemes because a computer program eliminates the need to install and maintain equipment that must be attached to the conductor. A software-based rating method avoids the need to de-energize the conductor during installation periods, and also reduces concerns about the reliability issues that must be considered when depending on a hardware-based rating system.