Mark MacAlpine

Anti-icing performance of RTV coatings on porcelain insulators by controlling the leakage current

Considerable work has been done on preventing ice formation on insulators and the modification of the surface characteristics by increasing the contact angle and decreasing the adherent force have been tried with some degree of success. Heating by electric current has however proved to be an effective and practical method for de-icing transmission lines, but difficult to apply to insulators. A room temperature vulcanized (RTV) silicone rubber coating containing sufficient carbon black to render it partially conducting but not enough carbon to lose its surface hydrophobicity, has been investigated to determine if the heat generated would suffice to inhibit ice growth on the insulators. The heat exchange progress was analyzed and the leakage current through it to prevent ice forming was estimated and coatings developed accordingly. The anti-icing performances of these RTV silicone rubber coatings with different leakage current magnitudes were compared in a climate chamber. The results showed that the surface heating effect, together with the hydrophobicity, could significantly reduce the formation of ice on insulators.

Corona-generated space charge effects on electric field distribution for an indoor corona cage and a monopolar test line

The effects of corona-generated space charge on the resultant electric field near a conductor were determined both for a simulated test line and a corona cage, since both are used in predicting the corona behavior of EHV and UHV transmission lines. Programs were developed for the calculation of the potential, the electric field and the space charge distributions for a simulated test line and a corona cage. Algorithms based on the charge simulation and the finite-element methods were used to solve Poissons equation; and the method of characteristics to solve the current continuity equation. The resultant electric field on the ground plane of the two configurations was measured with a rotating field-mill, and the results fitted well with the calculated data. It was found the electric field at the surface of the conductor increased linearly, for both experimental configurations, before the voltage reached the corona inception value; but was reduced once corona-generated space charge was produced. The space charge and density distribution in the corona cage differed greatly from that around the test line: the electric field at the surface of the conductor in the test line decreased faster with voltage than that in the corona cage, the relative difference increasing as Vapp/Von was raised. The difference reached an asymptote of about 16% when the applied voltage was about 25% above the corona inception level.

Influence of composite insulator shed design on contamination flashover performance at high altitudes

The dc arcing performance of composite insulators with contaminated surfaces has been investigated at high altitudes because of the program of building ±800 kV dc transmission lines in China, because much of their intended routes will be at relatively high altitudes. As the length of the arcs on the composite insulator depends on the repeat distance and the arcs are initiated on the core rod between the sheds, the effect of the repeat distance and the shed dimensions on the dc flashover voltages was studied for composite insulators under contaminated conditions. The experimental results confirmed that the shed design parameters do have a great influence on the flashover voltages. It was found that, with the same insulation distance, the flashover voltage had a maximum at a particular value of the radial extent of the shed and of the between-repeat distance, and that the influence of these two factors on the flashover voltage was independent of each other. It was also found that, for the alternate large and small shed design, the ratio of the radial extent of the major shed to the minor shed should not be too great.

Influence of Pulse Rise Time on the Inactivation of Staphylococcus Aureus by Pulsed Electric Fields

In this paper, the effect of rise time (200 ns and 2 μs, respectively) of rectangular pulse on the inactivation of Staphylococcus aureus has been studied. Apple juice with three different conductivities (1.5 mS/cm, 2.0 mS/cm, and 2.5 mS/cm) was inactivated 75 μs at an electric field strength of 25 kV/cm, 30 kV/cm, and 35 kV/cm, respectively. The experiment results showed that the pulse with shorter rise time had higher effect of inactivation and lower solution temperature increases after treatment, and the difference between te inactivation effects was more than 0.5 logs in general. Fast Fourier transform analysis showed that the pulse with shorter rise time had higher amplitude of low frequencies and more high-frequency components, which resulted in a higher transmembrane potential of Staphylococcus aureus cells and higher effect on intracellular material.

Development of anti-icing coatings applied to insulators in China

Icing of transmission lines due to extreme weather conditions has caused many failures in power networks worldwide [1]. As China has much of its land 2,000 m above sea level, intense snow and ice storms at this altitude have caused significant losses to the economy, and the harsh conditions have made life very difficult in these regions [2]. The winter of 2008/2009 was particularly harsh with freezing rain, which continued for more than three weeks in the central provinces, causing 218 ice flashovers. In particular, Hunan and Jiangxi Provinces experienced the severest storms resulting in damage to 37% of the 500-kV power transmission towers, collapsing many towers. Considerable work has been done in China on understanding the reasons for, and preventing, the icing flashover of insulators [3]-[10], but in this article the various anti-icing and de-icing methods for insulators currently being investigated in China are discussed with particular reference to the application of semiconductive silicone rubber coatings on insulators.

Extreme bending of composite insulators on 750 kV compact transmission lines

In designing and constructing the first 750 kV double-circuit compact transmission line, much pioneering work is necessitated in the choice of tower, insulator strings and phase-to-phase spacers. In this paper the mechanical characteristics of three possible phase-to-phase composite spacers for such compact transmission lines are considered in respect of very large bending of the spacer. A large deflection theory is introduced to calculate the lateral deflections and longitudinal compressions caused by applied axial loads and these are compared with the results of tests on three spacers of different lengths and diameters. Agreement between calculation and test results was good and the necessity for the larger diameter demonstrated. No permanent damage due to the extreme bending was found and two of the spacer samples were found to be suitable for use in the compact transmission lines with a good safety margin in respect of such bending.

Simulations of the controlling effect of interphase spacers on conductor galloping

Conductor galloping on overhead transmission lines can be a major problem and the interphase spacers is a countermeasure which has been found to be effective. There is, however, little theoretical basis for their use and for defining their design parameters. In this paper, a galloping simulation model is used in which the aerodynamic force is replaced by a sinusoidal excitation with a frequency equal to the natural vertical galloping frequency of the line. This is used to analyze the ability of interphase spacers to control the galloping amplitude. A full-scale free oscillation test on a triple-span transmission line was conducted and good agreement was obtained with the simulation calculation. Then by simulation, the changes of galloping amplitude and conductor tension were determined in terms of the initial excitation amplitude, both with and without interphase spacers. The dynamic spacer tension during galloping was also computed. The results clearly indicated the effectiveness of interphase spacers to control galloping, and show that the methods used will be useful and applicable to the design of interphase spacers to control galloping and to determine where they can best be deployed.

Simulation of positive DC corona streamer pulse characteristics at various air pressures and humidities

The simulation of positive dc corona streamer pulse characteristics at various air pressures and humidities for a rod-plane electrode system has been investigated. A flux-corrected transport (FCT) algorithm, based on a non-uniform mesh, and a successive over-relaxation (SOR) algorithm were applied to solve the continuity equations and Poisson equation in a fluid model to produce positive streamer pulse waveforms. The values of the various coefficients for humid air, considered as a gas mixture of air and water vapor and necessary for the calculations, were estimated by linear interpolation from the values for the separate ‘gases’. The simulation results were in general agreement with the experimental ones of our previous study. The positive streamer average amplitude decreased as the pressure was reduced or the humidity was raised. The phenomenon was explained by the decreasing of the average electric field and the conductivity in the discharge channel.

The effect of Applied-voltage on photon-number of corona discharge on transmission lines defect

Defects of high-voltage transmission lines are often accompanied with corona discharge; and corona discharge would produce ultraviolet. Based on this principle, the UV imager can be used in detecting the defect. Electric field strength is the main factor affecting the corona discharge, which is determined by power-supply and experimental model. In this paper, UV measurement of different defects was studied, and the effect of Applied-voltage on corona discharge of defects was acquired. A quadratic relationship exists between the UV strength produced by corona discharge and applied voltage. The quadratic coefficient A is between 0.31~0.37, which is increase with the length increasing at the experimental conditions in this paper.