The amount of power that an overhead transmission line can transfer is affected by the conductor’s ability to radiate thermal energy. This is determined by the conductor’s surface “darkness,” called its emissivity. An old, weathered conductor cools itself by thermal radiation much better than a new, shiny conductor. You can see this effect when working on a car exhaust system. With a shiny stainless steel muffler and tailpipe, you’ll not be able to detect its hot temperature by putting your hand near it as well as if the metal had a darker rusted surface. Similarly, you can see the emissivity effect after taking a stainless steel brownie pan out of the oven. When putting your hand near its side, you will not be able to sense its hot temperature nearly as well as if the pan had a darker steel or cast iron surface. (Of course if you touch it, all doubt will be removed that it’s truly hot.)
Determining a transmission line’s emissivity improves the accuracy of the thermal rating, regardless of whether the rating is a static rating or is determined in real-time using sag, line temperature, weather or any other technique. Accurate emissivity improves the accuracy of both normal and emergency ratings. Since it is has been difficult to measure conductor emissivity, guesses of conservative values are often used by transmission owners. Measuring emissivity allows eliminating this unnecessary conservatism and allows harnessing more of the conductor’s true ampacity (i.e., current-carying capacity).
The emissivity of a heated surface is the ratio of the radiant energy emitted by that surface to the radiant energy emitted by an ideal emitter. Emissivity is like emission efficiency, with 0.0 representing a surface that emits no radiation and 1.0 representing a perfect emitter (the so-called ‘blackbody”). The following figure shows that the rating of a Drake conductor increases with increasing emissivity.
If the conductor’s emissivity were assumed to be 0.5 – a very common assumption – while the actual emissivity were 0.8 – a realistic number for an older line – the rating would be boosted by 5%. Not a bad increase considering the minimal effort and expense. Consider how valuable it could be if all your transmission lines increased their power transfer capability by 5%!
Older lines have the most to gain by determining the actual emissivity. This is useful since older lines are more apt to run up against their thermal limits. As a line is energized for years exposed to the environment, its surface darkens and its emissivity increases due to oxidation and contamination. That’s a good thing for cooling and line rating purposes; older lines can easily have an emissivity above 0.8. House and Taylor2 did some work, which is often cited for estimating conductor emissivity as a function of years in service and operating voltage. The data, however, has a large spread of emissivity values, so it is hard to make definitive use of the numbers.
EPRI’s Emissivity Test Instrument (ETI)
EPRI, in collaboration with Pike Electric, has developed the Emissivity Test Instrument to accurately measure the emissivity of conductor samples specifically for the purpose of line rating. A short conductor sample is prepared and placed in the ETI chamber. Then, the requested parameters are entered into the ETI operational software, and the system automatically performs the test.
How it works
An internal heater continuously heats the conductor
sample, which is equivalent to heating the conductor by current. The radiated heat loss from a conductor depends on the conductor temperature, the ambient temperature, the conductor’s diameter, and the emissivity according to the following equation.
And where:
qr = radiated heat loss, in watts
D = diameter of the conductor, in inches
= emissivity of the conductor, unitless
Tc = temperature of the conductor, in degrees C
Ta = ambient temperature, in degrees C
Solution Examples
Emissivity is determined by measuring the temperatures and the radiated heat loss. In regular air, the radiated heat loss is difficult to figure out, because a hot conductor simultaneously undergoes quite a bit of complex convective heat loss at the same time (because hot air rises). The ETI eliminates convective heat loss by testing the conductor sample in a vacuum chamber. Once steady-state is achieved, the power supplied to the internal heater must simply equal the radiated heat loss out of the conductor. The heater power and temperatures are measured in this steady state condition, and the emissivity is then calculated by the previous equation.
Emissivity can be thought of as a surface “darkness,” but that is a simplification. Emissivity is really the “darkness” at the specific wavelengths that are being radiated. However, human eyes respond to various visible wavelengths, so the visual appearance can be misleading.
That is, a surface that appears light can be actually dark for cooling purposes. For example, white paint has about the same emissivity as black paint because these look about the same in the infrared spectrum where heat is being radiated. Emissivity also varies with wavelength, and a hot conductor radiates a spectrum of wavelengths (about 3 to 30 um.)
The surface material and oxide influences the radiated spectrum shape. You can see the complexity increasing. Infrared techniques only “see” a portion of the emissive wavelengths, so they make an approximation for the whole radiated spectrum. Total emissivity is the value of interest for conductor thermal rating, because it is the effective emissivity over all emission wavelengths.
The emissivity of a surface also varies with the angle of radiation, because a surface does not radiate equally in all directions. Some emissivity measurement devices require a smooth perpendicular surface and are therefore unable to perform measurements on round conductor. The complex surface due to conductor stranding also complicates attempts to determine emissivity. Emissivity can also vary around the periphery of conductors, possibly due to the bottom being more darkened than the top. Infrared cameras measure over a narrow angular view, so the measured emissivity is only applicable for energy coming directly out of the surface (perpendicular) rather than for all angles (hemispherical).
For real surfaces, the perpendicular emissivity will typically be greater than the hemispherical emissivity. For conductor cooling and rating, hemispherical emissivity, or the effective emissivity over all emission angles, is the parameter of interest. Finally, it should be recognized that emissivity can also vary with surface temperature.
The ETI determines the emissivity applicable to line rating, i.e., the total hemispherical emissivity at the applicable conductor rating temperature. The ETI measures sample temperature and watts, which is a very direct approach to determining emissivity, making minimal assumptions. The complex effects of emissivity variation with temperature, wavelength, position around the conductor, and the effects of conductor stranding are automatically accounted for using this method.
Measuring Emissivity with the ETI
To measure a conductor sample’s emissivity, the sample is first cut to 15 inches and the inner strands are removed and replaced with a cylindrical cartridge heater. Since the test is done in a vacuum, it would be difficult for heat to travel from the heater to the sample. Therefore, the void between the heater and sample is filled with metal powder to create good conduction.
Stainless wire bands are applied to keep the sample from expanding. The sample ends are capped with a mirror finish material to reduce heat loss and to allow compensating for what little heat does exit the end. Also, a thermocouple is inserted under the outer layer.
The conductor sample is placed inside of the cylindrical vacuum vessel, which has a high absorptivity interior. Insulating hangers hold the sample in the cylinder. A vacuum pump removes air from the vacuum vessel.
The ETI software prompts the user for parameters such as the test name, diameter, and desired rating temperature. The conductor emissivity may vary with temperature, so it is desirable to measure the emissivity of the conductor at the temperature at which the conductor will be rated. Using this user-specified target temperature, the ETI software automatically adjusts the heater voltage until the desired conductor temperature is reached.
Once the conductor reaches steady-state, the measurements are recorded, small adjustments are made for heat losses through the wiring, banding, and end caps, and the sample emissivity is calculated very directly by equation 1. Because of the long thermal time constant of a conductor in a vacuum, this process may take 3-6 hours to complete. However, the process is automated. The ETI produces a pdf report listing the emissivity applicable for thermally rating the conductor.
It is anticipated that whenever a line is retensioned or reconductored, samples could be retained and sent to EPRI for testing. Testing these samples will give the line owner emissivity information about other similar lines. EPRI is accumulating an emissivity knowledge base that will improve emissivity assumptions and possibly enable the future development of a live-line emissivity measurement technique.
In a time when it is difficult to build or rebuild overhead lines, line owners are looking for ways to increase the capacity of the existing lines. Some options include real-time rating using actual rather than worst-case weather conditions, surveying the line and removing any sag buffer built-in at installation, and retensioning the conductor. Measuring conductor emissivity is an option that can be combined with these other methods.
Emissivity Tests of PSE&G Conductor
For the past five years PSE&G has been re-vitalizing its 138 kV overhead electric transmission system by re-conductoring, re-insulating and replacing hardware components. These lines were predominantly designed and constructed between 1925 and 1947.
One facet of this re-vitalization project involves the random selection and evaluation of hardware components being replaced, such as conductor, insulators, line splices, connectors, and removed structural members. The components are sent to independent laboratories for inspection, dissection and testing using applicable industry standards in order to determine their present condition and, where possible, their approximate remaining life. These evaluations provide a number of benefits, including a better understanding of line aging that can be applied to other lines of similar vintage and revealing possible design weaknesses. The results may also drive changes in line inspection practices and work priorities. Just as important is information that can help to validate past engineering judgment or assumptions, such as emissivity in this particular case.
This was the motivation for sending both new and in-service Condor ACSR conductors to EPRI for emissivity testing using the new ETI. PSE&G wanted to test in-service aged conductor and compare the measured emissivity values against established criteria used in rating bare overhead conductors. With little effort there may be opportunity to fine-tune line ratings if it is found that actual conductor emissivity values are different than established utility rating criteria.
PSE&G is a Pennsylvania-Jersey-Maryland (PJM) Interconnection member company and adheres to the PJM rating methodology for rating overhead transmission lines. The PJM rating methodology goes back to the early 1970’s at which time a Task Force was developed to formulate a rating system for adoption by member companies.
At that time, the Task Force made arrangements for emissivity tests to be made on four 230 kV ACSR conductor samples; one sample from new stock and three samples with different periods of in-service time (10 months, 8 years and 36 years). These samples were taken from a non-industrial area in the vicinity of Washington, D.C. The values obtained from careful NASA testing compared favorably with other emissivity studies available at the time. As a result, the Task Force selected an emissivity of 0.7 for the PJM area. PJM currently follows IEEE Standard 738 for its rating methodology.
EPRI’s ETI measured the emissivity for the Condor conductor sample to be 0.63 [+/- 0.02] as compared to the prescribed value of 0.7. The conductor sample was estimated to be in-service approximately 15 years in an industrial area in New Jersey. This emissivity would result in a slightly lower rating if based on this test sample alone. Additional in-service samples need to be tested before any consideration is given to changing the present emissivity value. In support of EPRI’s development efforts with the ETI, PSE&G expects to send additional samples of different conductor sizes, types, and various years of operation to EPRI for testing.
With the availability of the ETI and present EPRI research, opportunity exists to address a number of open questions relative to emissivity such as, how long does it take for a conductor to reach its final emissivity value? How does emissivity change with conductor type (T2, for example), size, and service environment? Having an instrument that can readily evaluate emissivity provides utilities with a means to determine and fine-tune one of their own rating parameters, and perhaps increase their line ratings with minimal cost.
About the Authors
Ray Ferraro joined Public Service Electric & Gas Company in 1973 and is currently a Technology Development & Transfer Consultant in the Emergent Technology and Transfer Department. His previous position was Principal Engineer in the Electric Transmission Engineering Department where he served for 14 years. Mr. Ferraro received his Masters Degree in Electrical Engineering in 1982 from Fairleigh Dickinson University. Contact: raymond.ferraro@pseg.com
Bernie Clairmont joined EPRI in 1986 as a Research Engineer, and is presently a Sr. Project Manager at the EPRI high voltage laboratory in Lenox, Massachusetts. Previously he was a faculty member of the physics department at North Adams State College and a research consultant. At EPRI’s Lenox facility, Bernie has led investigations of corona and field effects of transmission lines, magnetic field management, and thermal ratings. Contact: bclairmo@epri.com
Dan Lawry has a BSEE degree from Clarkson University. He worked for Power Technologies, Inc. since 1993 in the area of thermal uprating of overhead lines and other outdoor power equipment. He now works for Pike Electric in supporting the ThermalRate line rating system. Contact: dlawry@pike.com
