Response of Transmission Line Conductors Under Downburst Wind

Electricity is transmitted by Transmission Lines (TLs) from the source of production to the distribution system and then to the end consumers. Failure of a TL can lead to significant economic losses and to negative social consequences resulting from the interruption of power. High Intensity Winds (HIW), in the form of downbursts and tornadoes, are believed to be responsible for more than 80% of the weather-related failure of TLs around the world. The studies reported in this thesis are part of an ongoing extensive research program at Western University focusing on the response of TLs under HIW. Previous investigations conducted to study the behavior and to assess the failure of TLs under downburst wind indicated the importance of accounting for the forces transmitted from the conductors to the towers. The current thesis focuses on the response of TL conductors subjected to downburst wind while considering various terrain exposures. The thesis is written using the Integrated Article format and includes various complementary studies. First, an effective numerical technique to analyze transmission line (TL) conductors subjected to HIW events is developed. This is followed by a derivation of a simplified closed form solution to estimate the forces transmitted from the conductors to the towers due to downburst winds. Then, an expression for the conductor aerodynamic damping, which is a main parameter affecting the conductors’ dynamic behavior, corresponding to downburst wind, is derived and validated. Afterwards, dynamic behaviour of TL conductors under downburst and synoptic winds corresponding to open terrain exposure is investigated. In order to account for other terrain exposures, a new roughness model adequate for Large Eddy Simulation (LES) of moderate-rough to rough terrain exposures typically encountered by TLs is developed and validated. Then this model is used in conducting LES of downbursts for various terrain exposures in order to: (i) characterize the downburst turbulence, (ii) investigate the dynamic behavior of TL conductors under downburst wind corresponding to different exposures. The research accomplished in this thesis, in terms of development of efficient structural analysis tools and characterization of the wind field, provides an advancement in knowledge about the behavior of transmission lines in general and conductors in particular during downburst events

NDM-551: TOWARDS A ROBUST WIND TUNNEL BASED EVALUATION OF EXTREME WIND LOADS

Proper modeling of the boundary layer flow is essential for wind load evaluation under extreme events such as hurricanes. This layer is formed due to the interaction of wind with natural or man-made obstacles over the surface of the earth. Such interactions generate drag forces proportional to the roughness of the ground which shape the characteristics of the boundary layer, including mean velocity and turbulence intensity profiles, as well as the spectral contents and correlations. In the current study, a robust technique for evaluating extreme wind loads using wind tunnels is proposed and validated. The technique is based on automatic identification of effective ground roughness at a site of interest using aerial Google images and reproducing it through automated roughness blocks mounted on the wind tunnel floor, dynamic turntable to recreate the wind direction effects, and high resolution pressure and load measurements. In addition to enhancing the efficiency, these automations limit the subjectivity involved in this type studies

NDM-536: EFFECT OF WIND SPEED AND TERRAIN EXPOSURE ON THE WIND PRESSURES FOR ELEVATED STEEL CONICAL TANKS

Steel liquid storage tanks in the form of truncated cones are commonly used as containment vessels for water supply or storing chemicals. A number of failures have been recorded in the past few decades for steel liquid tanks and silos under wind loading. A steel conical tank vessel will have a relatively small thickness making it susceptible to buckling under wind loads especially when they are not fully-filled. In this study, a wind tunnel pressure test is performed on an elevated conical tank in order to estimate the external wind pressures when immersed into a boundary layer. The tested tank configuration represents combined conical tanks where the cone is capped with a cylinder. In addition, the effect of terrain exposure and wind speed on the pressure values and wind forces is assessed. The mean and rms pressure coefficients are presented for different test cases in addition to the mean and rms total drag forces that are obtained by integrating the pressure coefficient over the tank model’s surface. It is found that the total mean and rms drag forces are highly-dependent on Reynolds number which is a function of wind speed and they have a maximum value at mid-height for the lower cylinder, at top for the conical part, and at bottom for the upper cylindrical part

NDM-529: NUMERICAL EVALUATION OF WIND LOADS ON A TALL BUILDING LOCATED IN A CITY CENTRE

Estimation of wind-induced loads and responses is an essential step in tall building design process. Wind load for super tall buildings is commonly evaluated using boundary layer wind tunnel (BLWT) tests. However, the recent development in computational power and techniques is encouraging designers to explore numerical wind load evaluations using a Computational Fluid Dynamics (CFD) approaches. CFD can provide a faster estimation for building loads and responses with lower cost and satisfactory accuracy for preliminary design stages. The current study investigates the accuracy of evaluating wind pressure and building responses of a typical tall building (CAARC building). Two configurations are investigated, which are (1) standalone building and (2) located in a city center. Large Eddy Simulation (LES) numerical model is utilized adopting a newly developed synthesizing turbulence generator named Consistent Discrete Random Flow Generator (CDRFG). The adopted inflow technique is believed to provide good representation of wind statistics (i.e. velocity and turbulence profiles, spectra and coherency). Pressure distributions and building responses from the current study match with those obtained from boundary layer wind tunnel tests. The average difference between the pressure values between the current model and the BLWT is 4%. While the difference in building responses resulted from the LES model to those from BLWT is 6%. It was found that utilizing CDRFG in LES models provides an accurate estimation for building aerodynamic performance in an efficient computational time owing to its capability of supporting parallel processing

NDM-514: LARGE EDDY SIMULATION OF WIND INDUCED LOADS ON A LOW RISE BUILDING WITH COMPLEX ROOF GEOMETRY

Wind induced damage on low-rise buildings with complex roof geometry is common in coastal areas of USA, such as Florida and Louisiana. Available design codes provide information about the design of regular roof geometries (e.g. hip/gable roofs), but refer to wind tunnel modelling for complex roof geometries. Due to time and financial constraints physical modelling may not always be possible to carry out. Computational modelling through Large Eddy Simulation (LES) has been used successfully for several wind engineering applications. This paper presents comparisons between LES and previously obtained wind tunnel data of mean and peak pressure coefficients on a low rise building with complex roof geometry. Two different cases, namely: isolated building and the effect of neighbouring buildings have been considered for the most critical wind direction of 135 degrees. Results show that the mean pressure coefficients on the low rise building roof for the case with adjacent buildings were somewhat lower in magnitude (less suction) than the isolated case. In general, excellent matching was obtained within a factor of 1.1 between wind tunnel and LES for all roof locations except at the roof ridge, where the latter predicted somewhat lower mean and peak pressure coefficient values than wind tunnel data. The velocity streamlines obtained from LES provide an excellent overview of the airflow around the buildings. This study shows the efficacy of LES for assessing wind loads on building roofs with complex geometry, since existing codes do not provide any quantitative assessment methods for such problems

NDM-538: WIND TUNNEL TESTING OF A MULTIPLE SPAN AEROELASTIC TRANSMISSION LINE SUBJECTED TO DOWNBURST WIN

A 1:50 scale aeroelastic wind tunnel test for a multi-span transmission line system is conducted at the WindEEE dome under downburst wind. WindEEE is a novel three-dimensional wind testing facility capable of simulating downbursts and tornadoes. This test simulates a transmission line consisting of v-shaped guyed towers holding three conductor bundles. Details about the model design, the wind field and the test setup are provided. A downburst loading case that is critical for the line design and causes unbalanced tension load on the conductors is investigated in the current study. Resulting line responses obtained from the test are compared with a previously developed finite element model by the research group at the University of Western Ontario. The comparison shows a good agreement which validates the finite element models. Results obtained from this test will be very useful to understand the behavior of the lines under downburst wind

NDM-534: SENSITIVITY OF WIND INDUCED DYNAMIC RESPONSE OF A TRANSMISSION LINE TO VARIATIONS IN WIND SPEED

This paper studies the dynamic behavior of a multi-span transmission line system under synoptic wind considering various speeds to determine the range of wind speeds in which the system experiences resonance. A finite element numerical model was developed for the purpose of this study. This model is employed to assess the dynamic behavior of a self-supported lattice tower line under various wind speeds. Dynamic Amplification Factor (DAF), defined as the ratio between the peak total response to the peak quasi-static response, is evaluated. It is found that conductors’ responses exhibit large DAF compared to the towers especially at low wind speeds (v ≤ 25 m/s). This results from the low natural frequency of the conductors (0.19 Hz) which is close to the wind load frequency while the natural frequency of the tower is equal to 2.36 Hz. In addition, the conductors’ aerodynamic damping decreases with the decrease of wind speed which leads to higher dynamic effect while the tower’s aerodynamic damping plays a minor role. The results of the dynamic analysis conducted in this study are also used to compare the gust response factors (GFT), defined as the ratio between peak total response to the mean response, to those obtained from the ASCE code (GFT-ASCE). It has been noticed that the gust response factors obtained from the ASCE code lead to conservative peak responses for both towers and conductors of the chosen line