Page 1

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 31, NO. 3, JUNE 2016

921

Real-Time Overhead Transmission-Line Monitoring for Dynamic Rating Working Group on Monitoring & Rating of Subcommittee 15.11 on Overhead Lines

Abstract—This paper discusses the wide range of real-time line monitoring devices which can be used to determine the dynamic thermal rating of an overhead transmission line with the power system operating normally or during a system contingency. The most common types of real-time monitors are described including those that measure the line clearance, conductor temperature, and weather data in the line right of way. The strengths and weaknesses of the various monitoring methods are evaluated, concluding that some are more effective during system normal and others during system contingency conditions. Index Terms—Dynamic line rating, effective perpendicular (EP) wind speed, line section, power system operator, real-time line monitor, static thermal rating.

I. INTRODUCTION

P

OWER transmission utilities now have the capability to remotely monitor certain mechanical and thermal characteristics of their overhead transmission lines in real time. While electrical parameters, such as line currents and bus voltages, have routinely been measured and communicated to system operators, real-time line clearances, conductor temperatures, and weather data, such as wind and solar heating in line corridors, have not. In recent years, relatively inexpensive, reliable, and accurate instruments have become commercially available to measure weather (e.g. ultrasonic anemometers), transmission-line sag tension (e.g. precision, temperature-compensated load cells), and conductor temperature. Also, relatively low-cost communication methods (e.g. spread-spectrum radio, etc.) are now available to allow remote instrument data to be transmitted in real time to the utility system operations centers. The process of real-time line and weather data monitoring along overhead lines, and the calculation of dynamic line ratings (DLRs) based on it, is an excellent example of a practical “smart grid” application but an unusually demanding one. It is demanding because maintaining adequate electrical clearances and avoiding premature conductor system aging are essential to public safety; and the calculated dynamic line ratings (DLRs) provided to the operator must be determined with unusual care with high instrument reliability.

Manuscript received October 30, 2013; revised November 23, 2014; accepted December 15, 2014. Date of publication December 18, 2014; date of current version May 20, 2016. Paper no. TPWRD-01234-2013. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2014.2383915

II. STATIC AND DYNAMIC THERMAL LINE RATINGS Power flow on overhead transmission lines is limited for a variety of reasons but the thermal limitations involved keep the conductor temperature below the line's maximum-allowable conductor temperature (MACT), therefore maintaining acceptable electrical clearances along the line and avoiding excessive aging of the conductor system. The system operator normally must act to keep the power flow less than the line rating under all system conditions. Static line ratings (SLRs) equal the maximum line current for which the line conductor temperature is less than the MACT under suitably conservative weather assumptions which are either fixed or vary seasonally. SLRs do not vary with actual weather conditions or time of day though some utilities adjust SLRs as a function of peak daily air temperature for a region. DLRs are also equal to the line current for which the conductor temperature is less than the MACT, but since the weather conditions vary in time, DLRs are only valid for a limited time into the future called the thermal rating period (e,g., the next hour). At the beginning of the DLR rating period, the weather conditions for the rating period must be predicted based on recent real-time data and/or statistical analysis of historical line corridor weather data. The DLR is updated at the end of each data time interval (e.g., 10 min) when a new prediction is made for the next DLR rating period. Since EP wind speed can change rapidly in a manner which is at least partly random, the DLR value may change abruptly at times. Under most conditions, the DLR may be expected to be higher than the SLR of a monitored line. This potential increase in the utilization of existing lines is one of the justifications for real-time line monitoring and compensates the user for the volatility and limited predictability of DLR. III. POWER FLOW ON OVERHEAD TRANSMISSION LINES Power-flow limits for overhead transmission line circuits are not always thermal. For long ac lines, because of concerns with regard to system stability and excessive voltage drop, power flow may be limited to the line's surge impedance loading which may be no more than 30% of the line's thermal capacity. Even with short lines, whose maximum power flow is equal to the line's thermal limit, the power flow with the system operating normally may need to be kept well below the line's thermal limit to avoid overloading the line after a contingency event. In order to realize the primary advantage of DLRs (higher line ratings), the monitoring system and rating analysis must work under both system normal conditions and during contingencies.

0885-8977 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.


922

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 31, NO. 3, JUNE 2016

Fig. 1. Line section consisting of four spans having different conductor temperatures.

With the system operating normally in the absence of major equipment outages, most lines, cables, and substation terminal equipment operate at a fraction of the high power flows that they must safely handle during a contingency event. Therefore, “everyday” transmission-line conductor temperatures are typically only 5 C to 15 C above air temperature. Real-time monitoring instruments must provide accurate data on which to determine DLRs when the power flow is low though with certain radial connections to a wind farm, for example, higher DLRs can allow increased power transfer. During a system contingency, where one or more major system components experiences an unscheduled outage, the power flow on the monitored line may increase rapidly, even approaching the line's thermal rating. Real-time monitors must operate accurately under such high-current–high conductor temperature conditions to provide the operator with an accurate estimate of the DLR when electrical clearances are close to mandated minimums and conductor temperatures are near the MACT of the line. During a contingency event, DLRs based on real-time monitor data can help the system operator avoid unnecessary load shedding. The two conditions—system normal and contingency— present quite different physical situations with regard to real-time monitors and, as we shall see, certain monitoring systems are better at helping the system operator under system normal conditions and others are better under contingencies. IV. TENSION EQUALIZATION AND TEMPERATURE VARIATION Overhead transmission lines consist of line sections (typically 1 to 10 mi long) which are terminated at each end by “strain” or dead-end” structures. Within each line section, the conductor is supported by suspension insulators (see Fig. 1) that allow a high degree of tension equalization between spans, even when span lengths are unequal or when the conductor temperature varies along the line section (due to variations in wind speed and direction). If the tension in each span is the same, the sag in each span, (m), is proportional to the square of the span length, (m) (1) where (N/m) is conductor weight per unit length, and (N) is the horizontal component of conductor tension. There are limits beyond which tension equalization is no longer correct. As discussed in [1] and [2], tension equalization between suspension spans works particularly well where: changes in elevation along the line section are small, suspension spans are of approximately equal length, I string insulators are long, and conductor temperatures do not exceed 50 C

Fig. 2. Temperature variation between suspension spans in a sag-section due to variation in wind speed and relative direction.

to 70 C. The assumption of tension equalization works less well for lines when: terrain is uneven; suspensions spans are of unequal length; insulators are either short I strings or post insulators; and conductor temperatures exceed 70 C. In the latter case, the line sag behavior should be modeled including the balance of forces at each support. Bare stranded overhead conductors have been shown to be poor axial heat conductors [3]. At high current, the natural variation in wind speed and direction along a line section can produce large variations in conductor temperature between spans as shown in Fig. 2. Although Fig. 2 shows conductor temperature variation due to random differences in wind speed in direction along the line, sheltering by trees and terrain can also produce large temperature differences at high current levels . In this situation, a single weather station or temperature monitor placed in the line section, is unlikely to yield a good estimate of the average conductor temperature or the equalized tension, though several monitor locations could do so. A single sag or tension monitor, however, can produce a more accurate estimate of sag clearance along the line section and, if the relationship between tension and the average conductor temperature is known, a DLR which better corresponds to minimum clearances. The relationship between tension and average conductor temperature for a line section, is called the “State Change Equation” (SCE) and its proper derivation requires both conventional mathematics and field experimentation. An accurate SCE usually begins with a mathematical sag-tension model but needs to be verified or adjusted for non-ideal support point behavior (dead-ends may move) and/or, with ACSR conductors, the non-linear thermal elongation rate at high temperature may be incorrect. The use of the SCE is discussed at greater length in Section VII of this paper. With sag or tension monitors in place, the SCE could be determined with great accuracy if the line current were raised repeatedly to levels close to the line static rating. Unfortunately, for most lines, high temperature excursions are rare and it is usually impossible to obtain permission to raise the line current just for this purpose.


WORKING GROUP ON MONITORING & RATING OF SUBCOMMITTEE 15.11 ON OVERHEAD LINES: REAL-TIME OVERHEAD TRANSMISSION-LINE MONITORING 923

V. THERMAL RATING OF OVERHEAD LINES Static (i.e. “book”) thermal ratings (current limits) for overhead transmission lines are typically based on “suitably conservative” weather conditions which consist of a low-speed perpendicular wind (e.g., 0.6 m/s or 2 ft/s), a seasonally high air temperature (e.g. 40 C) and full solar heating (e.g., 1000 or 110 ) [1]. While the weather conditions assumed in rating calculations vary between utility systems, within each system, they are generally applied to all sizes and types of bare overhead transmission conductors in all lines. This is done regardless of the transmission line's physical location or its MACT. In regions with well-defined seasons, including most of the US, static ratings may be calculated for summer and winter which usually differ in the assumed maximum seasonal air temperature. Some utilities take advantage of variations in daily air temperature by adjusting seasonal static line ratings for daily peak air temperatures though [4] points out that making such adjustments more frequently than once a day can lead to rating errors. The availability of real-time weather and sag-tension data leads to the possibility of calculating variable (dynamic) transmission line ratings that reflect “actual” rather than suitably conservative weather conditions. When the real-time data includes line current, the conductor temperature can be “tracked” as it varies with weather and current. If the weather conditions can be predicted for the next hour or more, then the dynamic line rating can also calculated. Dynamic line ratings predicted from real-time weather data can relieve constraints on power transfer due to the normal power flow on the line itself or to contingency flows after the loss of other usually higher voltage lines. The volatility of dynamic line ratings and their limited predictability must be weighed against the higher thermal ratings obtained at low cost, with minimum outage time, and with little or no regulatory delay. Dynamic line ratings may also serve as a temporary solution when applied while awaiting a more permanent fix, such as re-conductoring or rebuilding an existing line. The process of “tracking” conductor temperature and calculating both static and dynamic thermal line ratings is defined in IEEE Standard 738 [5], originally published in 1986 and updated again in 2012. Members of IEEE Subcommittee 15.11 have been the primary contributors to this guide which provides a numerical algorithm by which the bare conductor temperature can be calculated for any combination of weather conditions and line current. Also, a maximum current (i.e. thermal rating) can be calculated given the MACT of a line and either assumed or predicted weather conditions. The IEEE-738 numerical algorithm works equally well with constant or time-varying line current and weather conditions. IEEE 738 describes the following “heat balance” equation for a bare conductor (refer to IEEE 738 for a complete definition of terms) (2) where is the convection heat loss (W/m), is the radiation heat loss (W/m), is the heat capacity of the conductor

, is the average temperature of the conductor strands , is time (in seconds), is the heat gain from solar radiation (W/m), is the rms current (in amperes), and is the temperature-dependent resistance of the conductor . The convection heat loss per unit length depends on the conductor temperature rise above air temperature as well as the average wind speed and direction relative to the conductor. This is the largest heat loss term, even with still air. The convection term can be calculated on the basis of wind speed and direction or on the basis of an effective perpendicular (EP) wind speed which gives the same convection heat loss. Static line ratings are conventionally calculated for an EP wind speed (e.g. 2 ft/s) rather than a wind speed and direction. The radiation heat loss per unit length is normally between 25% and 35% of the convection loss even at high conductor temperatures. Radiation is affected by the “darkness” of the conductor surface, called emissivity (between 0 and 1). The “heat-storage” term in (2) is zero when the solar heating, convective heat loss, and radiative heat loss terms, as well as the electrical current are constant in time. If the weather conditions or line current vary with time, the conductor temperature also varies with a thermal time constant that ranges from less than 5 min for smallest conductors to as much as 20 min for the largest bare-stranded overhead conductors. The solar heat input per unit length is not dependent on the conductor temperature or weather parameters. It is a function of the conductor's location and orientation on earth, the sun's position in the sky, and the “darkness” of the conductor's surface called absorptivity (between 0 and 1). It is typically 10% to 30% of the joule heating term . This heat balance equation is the key to turning real-time weather data a line thermal rating (amps or megavolt amperes) that the power system operator can use. Equation (2) can also be used to calculate the convection heat loss per unit length if the other terms can be determined. Since line corridor wind speed is the most important line rating variable, in other words, the heat balance equation allows an overhead line to be used as a “hot wire anemometer” if the line current, solar radiation, and ambient and average conductor temperatures are known. This observation is critical to understanding the best application of certain types of real-time monitors. VI. CALCULATING EP WIND SPEED WITH ANEMOMETERS The heat balance (2) and the rest of IEEE 738 allow calculation of the conductor temperature and the conductor rating when the air temperature, solar heat intensity, and the wind speed and direction are known. Therefore, weather monitoring instruments, placed along the line corridor, can be used as the basis for DLR calculations. Some of the critical issues affecting dynamic rating accuracy include: • If the weather monitoring station is outside the transmission line right-of-way (ROW), the measured wind speed and direction may not equal that which the conductor experiences within the ROW. • If the wind anemometer is within the ROW, and is at the same elevation as the line conductors, it may not be properly located at a “sheltered” span location and there may


924

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 31, NO. 3, JUNE 2016

Fig. 4. Traditional propeller-type and ultrasonic anemometers (on left) and conductor model directional anemometer (on right).

The resolution of such improved data is often on the order of 0.5 m/s. Fig. 3. Forecast wind speed and direction from NOAA national weather service [6].

not be enough devices to assure good “coverage” of rating conditions long the line. • Whether within the ROW or without, the anemometer may not be capable of measuring speed and direction accurately below 1 m/s. • If average wind speeds are reported, the random variations of wind speed and direction may not be averaged properly. There are numerous instruments which can be used to collect weather data along an overhead line route. Some of the most widely applied devices are described in the following. A. “Airport” Weather Data Wind anemometers produce a direct measurement of wind speed and direction. The older cup-type anemometers, used at many NOAA airport stations, are rugged and dependable but typically stall at wind speeds below 1 to 2 m/s. Also, airport wind measurements are usually made only for a few minutes each hour, yielding poor quality data for nearby transmission line rights-of-way. The instruments are in open areas, free from obstructions and sheltering, over smooth grassy terrain, at a measurement height of 10 m, and are thus not representative of power line wind exposure. Finally, such measurements are usually not available in real-time. B. Internet Weather Data [6], [7] Weather data and hour-ahead forecasts can be obtained for many locations around the world over the Internet, as shown in Fig. 3. These are generally based on high-altitude measurements, filtered by a digital terrain model to 5-km resolution at ground level. Prior to adoption of these data sources into new Smart Grid initiatives, it is essential to establish the long-term accuracy and applicability to line rating calculations. Recent improvements to the map resolutions, which for example previously cut off wind speeds of 5 knots (8.4 ft/s), suggest that the forecasts may be helpful to the overhead line thermal rating process in the future.

C. Substation Wind Measurements Validation of low wind speed forecasts, such as those given in Fig. 3, must rely on comparisons with high accuracy ultrasonic anemometers. These are the instruments that can produce high quality wind speed and direction measurements down to speeds of 0.1 m/s (0.3 f/s) in long-term outdoor exposure. If placed in a transmission line substation, they produce much better estimates of wind speed in the transmission line right-of-way than airport measurements, especially for the line sections closest to the substation. However, sheltering by trees and terrain in the ROW can only be estimated, and field measurements have shown that wind speeds and directions along the line may be quite different from those measured in the substation. Placement in the substation simplifies maintenance, reduces vandalism, and may provide means for secure communication of real-time data through existing infrastructure. The calculated rating may be reduced by a percentage to account for the uncertainty in distant spans. D. In-ROW Weather Measurements High accuracy ultrasonic anemometers (Shown in Fig. 4) can be placed within the line's right-of-way, combined with air temperature and solar sensors in a weather station but the anemometers must be placed at the average conductor height for line rating purposes. Both wind speed and direction can be quite variable within a typical 5 or 10 minute averaging interval. When measuring wind with an anemometer, one must take care to determine a correct effective value before using it to calculate a rating. One cannot simply average wind direction. For example, if the wind were at from parallel for a minute and then turned to for the next minute, the straight average would be 0 (parallel). The effective direction, though, is not parallel (it's 20 from parallel). One other issue with the use of time-averaged data concerns the calculation of forced convection with the heat balance equations of IEEE 738 since these equations do not consider the effects of conductor stranding nor the impact of large scale turbulence at low wind speeds [8] Measurement of a simulated conductor temperature, the actual line conductor temperature, or line sag or tension, all avoid


WORKING GROUP ON MONITORING & RATING OF SUBCOMMITTEE 15.11 ON OVERHEAD LINES: REAL-TIME OVERHEAD TRANSMISSION-LINE MONITORING 925

Fig. 5. Conductor temperature monitors with radio links.

these possible inaccuracies. One such monitor is shown in Fig. 4 to the right of the anemometers. The conductor model device is placed in the ROW at the height of the line conductor and oriented in the direction of the span. One half of the simulated conductor is internally heated at a known rate and the other half is heated only by the sun. The monitor reports the effective perpendicular wind speed at the point of measurement and, since the surface of the tube is machined to simulate the line conductor surface stranding, it avoids any inaccuracy from ignoring surface stranding effects on wind cooling. The accuracy of dynamic line ratings calculated based on such weather instruments does not depend on the line current but presents two limitations: the instruments provide no direct measure of conductor sag position at high current; and the rating reflects only local wind effects in the span within which they are placed. VII. CALCULATING EP WIND SPEED WITH TEMPERATURE MONITORS In conjunction with air temperature and solar intensity instruments, but in place of using an anemometer, the conductor temperature can be measured at multiple points along the line using conductor temperature monitors such as those shown in Fig. 5. Temperature monitors will affect the conductor temperature due to their mass and disruption of wind flow over the conductor so they must be carefully calibrated in a laboratory prior to installation. In order to determine the line's thermal rating near the monitor, the measured temperature is converted to an effective perpendicular wind speed using (2). A typical result is shown in Fig. 6. Depending on weather changes over distance, sheltering, and placement within the span, the measured spot temperature may not be representative of conditions beyond the span in which it is placed. The accuracy of dynamic line ratings calculated based on conductor temperature monitors and air temperature and solar intensity instruments does depend on the line current. Under system normal conditions, the typical low line current ( , 0.5 A/kcmil) is likely to produce a very inaccurate estimate of the local effective wind speed at the monitor location. Direct measurement of conductor temperature reduces the uncertainty associated with calculation based on anemometer measurements but does not provide a direct measurement of

Fig. 6. Relation between conductor temperature and perpendicular effective wind speed for drake ACSR carrying 450 A.

Fig. 7. Video sag and load cell tension monitors.

conductor sag position at high temperature so some monitors include position sensors. VIII. CALCULATING EP WIND SPEED WITH SAG-TENSION MONITORS If sag or tension monitors (see Fig. 7) are used for line monitoring, thermal rating calculations require a double conversion from sag/tension to conductor temperature and then from conductor temperature to perpendicular wind speed. If the tension variation between spans in the line section is small, then the advantage of sag-tension monitors [5] is that the calculated effective perpendicular wind speed represents the average convection effect over an entire line section rather than just the single span in which it is mounted. If the tension variation from span to span in the line section is significant, then the use of a sag-tension monitor does not reflect the average wind speed along the line section but only local convection. The accuracy of DLRs based on conductor sag-tension monitors and air temperature and solar intensity instruments depends on the line current and the line design. Under system normal conditions, the low line current is likely to produce a very inaccurate estimate of the average effective wind speed in the line section within which it is mounted. On the other hand, under contingency conditions, the high current is likely to produce a more accurate line rating. As shown in Fig. 8, the determination of line section average wind speed requires the conversion of tension to average conductor temperature followed by the conversion of conductor


926

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 31, NO. 3, JUNE 2016

NERC and other auditing agencies feel that the use of DLR is justifiable and the process of calculation and monitor maintenance well-documented. X. REAL-TIME TL DATA CUSTOMERS

Fig. 8. Conversion of horizontal line tension to average temperature, followed for stringing section. by effective cross-wind speed,

temperature to wind speed. As with the use of a conductor temperature monitor, accuracy improves as the conductor temperature increases but the use of a sag-tension monitor also requires an accurate state Change equation relating the conductor temperature to sag-tension. IX. ADVANTAGES/DISADVANTAGES OF DLRS In most applications, the percentage increase in line rating obtained by real-time monitoring is less than that obtained with conventional methods of uprating. The potential increase in line rating with DLR, depends in part in how conservative or aggressive the SLR weather assumptions are. If SLRs are calculated with suitably conservative wind speeds (e.g. 0.6 m/s), then DLR methods are more likely to produce large line rating increases. If SLRs were based on aggressive wind speed assumptions (e.g. 1.2 m/s), then DLR may offer a smaller potential rating increase. The typical increase in DLR due to the use of real-time monitors and DLR calculation methods is in the range of 5% to 20%, though instances outside that range have occurred. Larger increases in SLR can usually be obtained by structure modification or reconductoring, but, if the modest rating increase is sufficient, DLR can normally be implemented quickly without public hearings or the need for regulatory approval and the cost of monitors and communications may be much lower. Also, most real-time monitoring systems are portable and could be useful in future applications if no longer needed where initially installed. A key question regarding the advantages of real-time monitoring of overhead lines is: (1) Who pays for the monitoring systems and for their implementation and (2) Who stands to gain from the increase in circuit capacity that these methods offer? Financial incentives for transmission asset owners are often centered on capital investment not on circuit capacity, thus a far more expensive line uprating method such as reconductoring may generate more income to the owner than the installation of any DLR method. Finally, there is the acceptance of DLR data by regulatory agencies. Utilities will not employ such methods until FERC/

The system operations group can utilize real-time line monitor to: • modify and adjust the power flow on the transmission system to avoid congestion and to relieve potential line overloads; • develop short-term forecasts line rating capacity; • use higher dynamic thermal ratings to relieve congestion and utilize the most economical generation sources; • improve operator decisions during system emergencies where real-time knowledge of the line thermal state (sagtension or temperature) can help avoid excessive or unneeded remedial actions. Of course, system operators needs to have reliable protocols in place (agreed to by their independent system operator (ISO) or regional transmission organization (RTO) if they exist). But several other groups within the power transmission system organization are potential customers for field data, either real-time or statistical, on overhead transmission lines, they include: • maintenance and operations planning; • engineering design; • system planning; • asset management. If maintenance and operation planning have sufficient confidence in DLR forecasts based on real-time monitoring, then these forecasts could be used to schedule maintenance outages. Engineering & standards personnel may be able to utilize statistical analyses of field data to improve line design, line uprating, and line analysis processes. These people usually have longer time frames and a need for considerable accuracy, which may require longer data collection periods and wider combinations of data from multiple monitoring sites. Real-time transmission line data can be stored, organized, and sorted to identify trends and data parameter relationships not always obvious in a more deterministic environment. Predictor models can be either developed or refined using the real-time transmission line data as either averages or statistically based predictor models. XI. PREDICTION, VOLATILITY, DISPLAY, INCREASE IN RATING

AND

USEFUL

The two primary customers for real-time information— system operations and maintenance—require predictions of line ratings since neither community is capable of responding to problems instantaneously. For example, at 8 AM, the system operator may want to know if the peak load on critical circuits will exceed the thermal capacity during the afternoon load peak at 2 PM, 6 hours into the future. Knowing that there is a problem “now” may not be nearly as useful as knowing the likelihood of a problem occurring in the near future. In addition, certain aspects of dynamic ratings present challenges in any installation:


WORKING GROUP ON MONITORING & RATING OF SUBCOMMITTEE 15.11 ON OVERHEAD LINES: REAL-TIME OVERHEAD TRANSMISSION-LINE MONITORING 927

1) real-time ratings depend on wind speed and direction which can change rapidly; this yields line ratings that can increase or decrease quickly; 2) high-speed, high-altitude winds can be predicted with some degree of reliability but low speed, low altitude winds within transmission-line corridors cannot typically be predicted more than 1 to 4 h in advance; 3) system operations personnel are accustomed to seasonally constant circuit ratings, incorporated into operations software and displays; the integration of real-time data while avoiding distraction can be challenging. These aspects of overhead line DLRs are discussed in more detail as follows. A. Prediction of Real-Time Line Ratings While DLRs are difficult to predict more than 1 to 4 hours in advance, a minimum dynamic line rating may be estimated based on predictions of air temperature and solar heating (which are predictable) combined with a statistically derived minimum wind speed. B. Volatility of Ratings Volatility of dynamic line ratings can be reduced by a variety of methods but the two main methods involve either capping the dynamic rating at some percentage above the static (giving up those brief high rating excursions) or by averaging over time. The danger in averaging over time involves missing short periods of calm during which clearance problems might develop. C. Effective Operator Displays [9] There have been many attempts to develop effective but simple displays that operators may find helpful. Clearly, any visual display should be confined to those time periods when the actual or post-contingency calculated line current approaches the rating. Displays involving speedometers, traffic lights and bar charts have been suggested. When interfacing with a computer program such as an automatic load shedding scheme, there is no display issue but the specific data to be provided may have to be unique to the software. XII. CONCLUSIONS Real-time monitors for overhead transmission lines can provide a basis for dynamic line ratings. While static line ratings are calculated using suitably conservative weather conditions, which are usually applied to all lines in the system, dynamic

line ratings are calculated using actual weather conditions in a particular line corridor. Real-time communication of data from accurate weather instruments, suitably placed in the line corridor, can provide the basis for rating calculations that are independent of the line power flow. At moderate to high power flows, conductor temperature and sag-tension monitoring devices are potentially more accurate and the sag-tension devices provide DLR based on a direct measure of conductor sag clearances during system contingencies. Dynamic line ratings present challenges regarding volatility, predictability, and operator display integration. However, if the relatively modest levels of rating increase (5% to 20%) over static ratings is sufficient, DLR offers utilities a rapidly deployable, low cost method of increasing line ratings, which can be installed without the need for public hearings and without the need to take key lines out of service. ACKNOWLEDGMENT The Working Group members are Dale Douglass (Chair), William Chisholm, Glenn Davidson, Ian Grant, Keith Lindsey, Mark Lancaster, Dan Lawry, Tom McCarthy, Carlos Nascimento, Mohammad Pasha, Jerry Reding, Tapani Seppa, Janos Toth, and Peter Waltz. Otto Lynch and Bruce Freimark served as primary subcommittee 15.11 reviewers. REFERENCES [1] CIGRE Technical Brochure 324, “Sag-tension calculation methods for overhead lines,” Study Committee B2, Jun. 2007. [2] “Limitations of the ruling span method for overhead line conductors at high operating temperatures,” IEEE Trans. Power Del., vol. 14, no. 2, pp. 549–560, Apr. 1999. [3] B. Clairmont, D. Douglass, J. Iglesias, and Z. Peter, CIGRE Paper B2-108, “Radial and longitudinal temperature gradients in bare stranded conductors with high current densities,” Aug. 2012. [4] CIGRE Technical Brochure 299, “Guide for selection of weather parameters for bare overhead conductor ratings,” Working Group 22.12, Aug. 2006. [5] Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors, IEEE 738-2012, Jun. 2012, Power Energy Soc. [6] National Weather Service. (2014 Oct. 15), Graphical forecasts. [Online]. Available: http://preview.weather.gov/graphical [7] S. F. Manuel, V. De Pondeca, G. Manikin, G. DiMego, and S. Benjamin et al., “The real-time mesoscale analysis at NOAA's national centers for environmental prediction: Current status and development,” Weather Forecast., vol. 26, pp. 593–612, Apr. 2011. [8] Working Group 22.12, “Thermal behavior of overhead conductors,” CIGRE Tech. Brochure 207, Aug. 2002. [9] Working Group B2.36, “Guide for application of direct real-time monitoring systems,” CIGRE Tech. Brochure 498, Jun. 2012.

Real-Time Overhead Transmission-Line Monitoring for Dynamic Rating  

This paper discusses the wide range of real-time line monitoring devices which can be used to determine the dynamic thermal rating of an ove...

Real-Time Overhead Transmission-Line Monitoring for Dynamic Rating  

This paper discusses the wide range of real-time line monitoring devices which can be used to determine the dynamic thermal rating of an ove...

Advertisement