Impact of Surface Topologies on the electrical properties of high voltage power transmission lines has a huge impact as there is an increased demand for efficient transmission of power from the generating end to the end users. The high-voltage transmission lines are the most important component in the power transmission infrastructure. Hence, there is an increasing need for the optimization of high-voltage power transmission lines and the exploration of factors influencing their performance.
There are numerous factors affecting the efficiency of the transmission lines. One among them is the surface topology of the transmission line. The surface topology of the transmission lines has a major impact on losses such as corona discharge, conductivity, and skin effect that in turn results in an increased transmission loss. This sets the stage for understanding the surface topologies for high-voltage power transmission lines and optimizing them.
This review document aims to explore the existing literature on the impact of different surface topologies on the electrical properties of high-voltage power transmission lines. According to the literature, the phenomenon that is highly dependent on the surface topology is the corona effect. Laboratory test reports and several conclusions drawn by the researchers in their published documents are reviewed to understand the Impact of Surface Topologies on the Electrical Properties of High Voltage Power Transmission Lines.
2. The effect of surface morphologies on the corona effect
Corona can be defined as a non-linear phenomenon involved in the initial phase of electrical discharges, resulting in the flow of electric energy from a conductor to the ionized medium. [1] The corona effect in transmission lines is a phenomenon characterized by the ionization of air surrounding high-voltage conductors, leading to the creation of a visible corona discharge. As the electric potential on the transmission line increases, the air surrounding the conductor experiences stress, causing the air molecules to undergo partial breakdown. This partial ionization results in the release of charged particles, generating a luminous glow or halo around the conductor. The corona effect is associated with energy losses, audible noise, and radio interference, posing challenges to the efficiency and reliability of high-voltage power transmission systems.
2.1 The impact of surface conditions on corona discharge
The corona effect depends upon the shape, material, and conditions of the conductors. The rough and irregular surface i.e., unevenness of the surface, decreases the value of breakdown voltage. This decrease in breakdown voltage due to concentrated electric field at rough spots gives rise to more corona effect. [2] The roughness of the conductor is usually caused due to the deposition of dirt, dust, and scratches. Raindrops, snow, fog, and condensation accumulated on the conductor surface are also sources of surface irregularities that can increase corona. [2]
Each conductor possesses a corona inception gradient, which serves as an indicative measure defining its corona performance. Corona inception refers to the onset of positive streamer discharges in proximity to the conductor. This occurs when the voltage gradient on the conductor’s surface reaches a critical value. [3]
From equation (1), m is the surface factor of the conductor. For an ideal conductor, m is equal to 1. But even minute imperfections on the conductor surface tend to diminish this value. Experimental investigations have revealed that practically stranded conductors, which exhibit surface irregularities like scratches and nicks, may exhibit a surface roughness factor ranging between 0.6 and 0.8. In extreme scenarios, the roughness factor of certain conductors may even be as low as 0.2. [3]
The surface roughness factor stands out as a crucial initial factor in assessing the corona inception of a transmission line. Initially, it is commonly assumed that new, stranded conductors would possess a surface roughness factor of 0.8. However, this assumption lacks precision and requires adjustment. Keeping this in consideration, tests were conducted on various conductors to establish a more scientifically grounded approach for estimating the value of the surface roughness factor.
2.1.1 Study of Surface roughness effects on the corona discharge intensity of long-term operating conductors
A study was conducted by a group of scientists into the impact of surface roughness on corona discharge, focusing on a 30-year-old operating conductor from the inaugural 500-kV transmission line in China. Numerous substances were discovered adhering to the conductor’s surface. As a result of corrosion and deposits, the surfaces of the operated conductor were considerably rougher compared to those of new conductors.
The surface of the operating conductor was found to have significantly greater coarseness and unevenness compared to the new one. To quantify the degree of surface roughness in this study, roughness parameters and average roughness were utilized. For old conductor and the new conductor, the average roughness values were measured at 7.19 µm and 0.80 µm, respectively. These rough surface conditions have the potential to distort the electric field of long-term operating conductors.
Due to the accumulation of various substances on the conductor surfaces, corrosion and deposits caused the surfaces of the operated conductors to become significantly rougher compared to new ones. Consequently, corona discharges on the operating conductor were more intense than on the new one due to the extremely rough surfaces. This resulted in the corona inception voltage of the operating conductor used in this investigation being 13.6% lower than that of the new conductor. Additionally, the apparent noise (AN) and radio noise (RN) of the operating conductor were higher compared to the new one. [4]
2.2 Experiment
In order to explore the effect of surface morphologies on the corona effect, an experiment is conducted with high voltage conductor operated in the presence of fine particulate matter.
2.2.1 Experiment setup
The experimental arrangement involved employing a cylindrical corona cage featuring an iron conductor at its center to generate a non-uniform electric field distribution within the chamber. The conductor had a diameter of 5 mm, with a corona inception voltage of approximately 47 kV and a corona inception electric field of around 38.6 kV/cm. [5]
The DC source boasted a maximum output voltage of 80 kV and a maximum output current of 15 mA. To maintain a charge-free medium inside the chamber, the positive-polarity DC voltage was set at approximately 50 kV. This ensured that the non-uniform electric-field distribution remained unaltered by any free space charge.
2.2.2 Experiment result
Under the influence of the applied DC voltage, small particle aggregations were observed locally. Specifically, the surface morphologies of the conductor exhibited several parallel chains of particles as a whole. The voltage level and duration of testing played a significant role in the formation of these morphologies. Furthermore, the emergence of these morphologies altered the surface roughness. Increased applied voltage and testing time led to heightened surface roughness, consequently elevating the total ground level electric field and ion flow density. This phenomenon implied that heightened surface roughness intensified corona discharge.
3. Impact of surface roughness on electrical conductivity
Till now, this review document was fully referring to the impact of surface roughness on the corona effect of high voltage transmission lines. When a current is applied, a smooth surface with limited irregularities can offer lower resistance, hence better conductivity, whereas a rough surface may cause increased scattering of charge carriers and create additional resistance to the flow of electricity. [6]
But, when it comes to the electrical conductivity of the transmission lines, not enough evidence can be found in the literature that proves the reduction in the conductivity of the high voltage transmission line with the increase in surface resistance. However, there are numerous studies about the impact of surface resistance on the thin conductors and surface resistance impact on contact resistance of electrical conductors.
Researchers have studied the electrical conductance of ultra-thin copper films featuring triangular surface roughness through ab initio calculations. Varying the roughness amplitude from 1 to 4 atomic layers, they observed a notable decrease in conductivity even with the presence of minute roughness structures, attributed to electron scattering at the Fermi surface. Furthermore, they also derived a concise analytical formula linking conductivity to roughness amplitude and uncovered contrasting effects of aluminum (Al) and tantalum (Ta) surface coverages on the electrical conductivity of these thin copper films. [7]
Smoother surfaces typically provide better contact and, as a result, lower electrical resistance. This is because a smoother surface increases the actual contact area between conducting surfaces, allowing for more efficient current transfer. A rough surface, in contrast, decreases the actual contact area, which can lead to increased contact resistance due to less area for the current to pass through and more localized heating. [6]
3.1 Impact of surface roughness on the electrical conductance of thin film
The resistance of a conductor reduces with the reduction in its cross-section. First-principles calculations show that atomic-scale surface roughness dramatically affects the electrical conductivity of thin films. Atomic clusters, 1–3 atoms high, deposited on the flat Cu(001) surface of an 11 monolayer thick film lead to a 30−40% reduction of its conductance. This is attributed to the destruction of isotropic Fermi surface sheets. We provide a simple parametrized formula, correlating the size of the surface-added structures to the film conductance, and also demonstrate that Ta and Al surface monolayers on rough Cu surfaces cause a conductance decrease and increase, respectively. [7]
Apart from the effects of surface roughness on the transmission lines and the thin film conductors, electronic circuits operating at high frequencies are also affected by surface roughness. A scientific literature published by the National Library of Medicine suggests that when a circuit is operated in the frequency range of 100 GHz and the RMS surface roughness was less than 50 nm, the effect of surface roughness on the transmission line impedance and reflection coefficient was limited and negligible. [8] Also, the maximum operating frequency of the transmission lines was also affected by the Ra value.
4. Impact of surface roughness on insulators and support structures
Insulators are essential components of high-voltage transmission lines. They are used to electrically isolate transmission lines from each other and the ground. Proper insulation is vital for preventing current leakage and ensuring the efficient transmission of electrical power. The surface roughness of insulators has a significant effect on the performance of the insulator. It affects the leakage current, flashover voltage, and resistance to environmental stressors.
Surface roughness can create micro-scale irregularities that promote the accumulation of contaminants, such as dust, salt, or pollution deposits. These contaminants can form conductive paths across the insulator surface, increasing leakage current and compromising insulation performance. Rough surfaces are more prone to contamination buildup, leading to higher leakage currents compared to smooth surfaces. [9]
The voltage at which insulators experience a breakdown and allow current to flow across their surface, is influenced by surface roughness. Rough surfaces exhibit lower flashover voltages compared to smooth surfaces due to enhanced electric field concentration at surface irregularities. The accumulated charges on the insulator surface become a key factor to incur surface flashover. The charge accumulation process is closely related to the surface condition. [10]
5. Wind swing flashover of transmission lines
Wind swing flashover or windage flashover occurs when strong winds and rain cause conductors to swing or move erratically. These movements can lead to flashovers (electrical discharges) between conductors or between conductors and the ground.
This phenomenon is not directly related to the surface parameters of the transmission lines. However the surface roughness will change the wind speed to indirectly affect the wind swing angle. [11] Transmission lines, with their elongated structure, are particularly susceptible to wind-induced swing, especially in areas with high surface roughness. When wind swings cause the conductors to oscillate, they may encounter nearby objects or vegetation, leading to flashover events due to contamination or arcing.
7. Eddy current impact of transmission losses, affected by surface topologies
Eddy currents are circulating currents induced in a conductor subjected to changing magnetic flex.
Surface roughness typically increases the magnitude of eddy currents induced in a conductor. This is because rough surfaces provide more paths for the magnetic flux to penetrate the conductor, leading to a larger area over which the eddy currents can circulate. As a result, higher surface roughness tends to enhance the interaction between the changing magnetic field and the conductor material, thereby increasing the amplitude of the eddy currents. [12]
Eddy current effects could change the current distribution in the coil conductors and alter the impedance of the coil, which in turn affects the characteristics of the discharge current. [13]
8. Impact of surface topology on skin effect
The skin effect is the restriction of the flow of alternating current to the surface of a conductor. This restriction is caused by the alternating magnetic field that the current itself generates within the conductor. The higher the frequency of the alternation, the thinner the layer of the conductor into which the current will be driven by this magnetic field. [14]
Since the skin effect has the least effect at lower frequencies, so does not have a huge impact on losses occurring in the transmission lines.
9. Conclusion
This review document delves into the intricate relationship between surface roughness and its impacts on the electrical properties and performance of high-voltage power transmission lines. I have studied several research articles to explore the impact of surface topologies of high voltage transmission lines on various phenomena such as corona discharge, electrical conductivity, and wind swing flashover.
The corona effect, a significant concern in high voltage transmission systems, is influenced by surface roughness, with rough surfaces leading to increased corona discharge intensity due to concentrated electric fields at irregularities. Studies demonstrate that surface roughness affects the corona inception gradient, thereby influencing the onset of corona discharges. Experimental investigations, including long-term studies on operating conductors, emphasize the correlation between surface roughness and corona discharge intensity, with rougher surfaces exhibiting higher corona inception voltages and increased noise levels.
Furthermore, surface roughness has implications for electrical conductivity, particularly in thin conductors, where even minute roughness structures can lead to decreased conductivity due to electron scattering. While limited evidence directly links surface resistance to the conductivity of high-voltage transmission lines, studies on ultra-thin copper films provide insights into the impact of surface roughness on electrical conductance.
Moreover, the influence of surface roughness extends to wind swing flashover events, albeit indirectly. Surface roughness alters wind speed patterns, thereby affecting wind swing angles, which can lead to oscillations of transmission line conductors and potential flashover events, particularly in areas with high surface roughness.
Hence it is evident that surface roughness is a critical factor to consider in the design, maintenance, and optimization of high-voltage power transmission lines.
10. References
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