Insulator String Length Research Articles

Study on the minimum polarity distance of suspension tower for UHVDV transmission line in ultrahigh altitude area

The design experience of ± 800 kV UHVDC transmission line in 3500 m and above is less. Based on the basic design conditions of ± 800 kV DC transmission line project, the minimum polarity distance of the suspension tower in an ultrahigh altitude area is investigated and the effects of the electromagnetic environment, insulator string length, V-string angle string and air gap on polarity distance are analyzed. Finally, the minimum polarity distance of the suspension towers under different altitudes, V-string angle string and icing area conditions are calculated, which provides technical support for the tower design of UHVDC transmission lines in ultrahigh altitude areas.

Calculation Method for Dynamic Stiffness of Multispan Overhead Transmission Lines

The dynamic motion of a span is coupled to the other spans in transmission lines. From the continuity conditions, force equilibrium conditions, and dynamical equations of every span conductor and insulator string, a close-form expression for the dynamic stiffness of a coupled two spans in harmonic motion is presented. Unlike some existing theories of single span conductor, the effect between the conductors and insulator strings is considered here. By means of example calculations, the validity of the dynamic stiffness of two-span is demonstrated by the consistency of the results determined by the ABAQUS. Meanwhile, the dynamic stiffness and natural frequency are discussed under variation of the span lengths ratio, Irvine parameter, and insulator string length. Moreover, the modal shape function corresponding natural frequency is derived, and the contribution of localization mode is studied. The results show that the contribution is either sensitive to or independent of transmission line parameters in certain parameter ranges. Finally, generalizing the work of two-span, the dynamic stiffness of arbitrary span number is obtained.

Reduction of the number of faults caused by lightning for transmission line

In this paper, author proposed an algorithm to determine method which is used to reduce the fault number caused by lightning on a transmission line. The algorithm is developed based on three methods including increase in the insulator string length, surge arrester installation, and surge arrester installation in combination with increase in the insulator string length. The output of algorithm is the proposed method which makes the number of faults caused by lightning satisfy the requirement and its investment is the cheapest in them. Based on this algorithm, the reasonable method (the number of tower positions needing to improve) to reduce the number of lightning caused faults of the transmission line can determine if we know the ratio of the required number lightning caused faults to the current value. This algorithm is applied to a transmission line to propose an improving method so that the fault number satisfies the requirement. This algorithm is implemented to a 220kV transmission line. The calculation results show the reasonable method and it also indicates in detail which towers should install surge arrester, which towers should increase the insulator string length.

Free vibration analysis of transmission lines based on the dynamic stiffness method.

An improved mathematical model used to study the coupling characteristic of the multi-span transmission lines is developed. Based on the solution method for single-span cable, an expression for the dynamic stiffness of two-span transmission lines with an arbitrary inclination angle is formulated. The continuity of displacements and forces at a suspension point is used to derive the dynamic stiffness. Interactions between insulator strings and adjacent spans are accommodated. Considering the infinite dynamic stiffness corresponds to the natural frequencies of the transmission lines, the finite-element method (FEM) is employed to assess the validity of the dynamic stiffness. In the numerical investigation, attention is focused on the effect of the inclination angles, Irvine parameter, insulator string length and damping parameter. In addition, the modal function corresponding to the natural frequencies is derived. Then, the results of comprehensive parametric studies are presented and discussed. Special attention is paid to the effect of the Irvine parameter and damping parameter on the in-plane modal shapes. Finally, according to the theoretical model of two-span transmission lines, the generalized dynamic stiffness of transmission lines with an arbitrary number of spans and inclination angles is derived. The method can be used as the basis of the vibration analysis on a wide variety of multi-span transmission lines.

Free vibration of transmission lines with multiple insulator strings using refined models

A theoretical dynamic model of two-span and three-span transmission lines with multiple insulator strings is presented. Unlike existing single-span models, the model has a considerable potential that the effect of the motion along the axial direction of transmission lines and insulator string swing on adjacent spans is considered. The equations of in-plane and out-plane motion of multi-span are set up by using a combination of moment equilibrium of insulator string swing and free vibration equation of each span, in which the longitudinal component of inertia is considered due to a motion of the support. Accurate and approximate solutions for the equations governing multi-span vibrations under self-weight are achieved with the continuity conditions of the displacement and force. Based on the theoretical formulas, a rich variety of symmetrical and asymmetrical modal features of adjacent spans interaction occurring in various parameters of transmission lines are discussed. The results show that the natural frequencies of corresponding in-plane and out-plane modes are heavily dependent on span number, the ratio of span lengths, Irvine's parameter λ and insulator string length. Furthermore, the finite element results based on the ABAQUS are reported, which establish the validity of the theory.

DC flashover performance of ice‐covered insulators under complex ambient conditions

The electrical characteristics of ice-covered insulators in coexisting conditions of high altitude, icing and pollution are a basis for external insulation selection of transmission lines in icing regions. Ice accretion on insulators has been studied for decades, while few studies focus on the flashover performance of ice-covered insulators polluted by two types of contamination, that is the solid layer method and icing water conductivity method, simultaneously. On the basis of the investigations on DC flashover performance of ice-covered insulators in the artificial climate chamber, the influence of different parameters, such as insulator string length, icing water conductivity, salt deposit density as well as atmospheric pressure on DC icing flashover voltage of insulator strings is studied. According to the experimental results, the relationships between the flashover voltage and each of such factors are determined and analysed. Moreover, the correction formula for DC flashover voltage of ice-covered insulators in complex ambient conditions is proposed.

Study of DC flashover performance of ice-covered insulators at high altitude

This paper examines the direct current (DC) icing flashover performance of typical DC porcelain, glass, and composite insulators in a complex environment of high altitude, icing, and pollution. The effects of the insulator string length, ice thickness, pollution level, freezing water conductivity, air pressure, and arrangement of insulator strings on the DC icing flashover performance are analyzed, and the correction formula for the DC icing flashover voltage is obtained. The DC flashover process of ice-covered insulators at high altitude is classified into dynamic changes of surface arcs, air-gap arcs, and residual resistance of the ice layer, as observed through a high-speed camera, and a DC flashover circuit model explaining the flashover performance for ice-covered insulators at high altitude is established.

Electrical and material characterization of field-aged 400 kV silicone rubber composite insulators

This paper presents the electrical and material characteristics of field-aged silicone rubber composite insulators, which have been deployed for 15 years on a 400 kV transmission line in a coastal region of the UK. There were no indications of reduced performance in service. Observations indicate non-uniform aging of the insulators on the different surfaces of the insulator sheds and core, along the insulator string length and in the different compass orientations. A uniquely large number of contact angle measurements, made on each of the insulators' different surfaces, confirm the visual aspect of nonuniform aging. Electrical investigations of the insulators have been performed in terms of leakage current analysis, ac flashover / withstand and switching-surge impulse flashover. A correlation is seen between the ac leakage current and the hydrophobicity measurements. Energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR) analysis of the changes of the materials' surface chemistry is presented. The role of solar radiation and organic growth appears critical to the observed non-uniform aging and must be considered if the longer term operation of the insulator is to be forecast. It is proposed that a reduction of the difference between positive and negative wet-flashover voltages may be a good indicator of early insulator aging.

Comparison of DC Pollution Flashover Performances of Various Types of Porcelain, Glass, and Composite Insulators

Based on the artificial pollution tests, the flashover performance of various types of DC porcelain and glass suspension insulators as well as composite long-rod insulators were analyzed and compared. The test results show that there is a nearly linear relation between the pollution flashover voltage and the disc-type insulator string length. The flashover voltage gradients of the insulators are affected by their materials and shed shapes. The antipollution performances of glass insulators are superior to those of porcelain insulators with the same profile. The flashover voltage gradients of composite insulators are higher than those of porcelain or glass ones. The exponent characterizing the influence of salt deposit density on the pollution flashover voltage is dependent on the profile and the material of insulators, and the values of the composite insulators' exponents are smaller than those of porcelain or glass insulators, namely, the influence of the pollution on the composite insulators is relatively less. The effectiveness of leakage distances of porcelain or glass insulators is less than 0.9 while that of composite insulators is higher than 0.9.

Limitations of the ruling span method for overhead line conductors at high operating temperatures

This report summarises the work by the Task Force to review the accuracy of the ruling span method for conductors operated at high temperatures. The basics of the ruling span approximation method have been examined. The traditional ruling span approach can be used with little or no error for a typical overhead line crossing a rolling terrain to predict sags in suspension spans for conductor operating temperatures in the range of 50/spl deg/C to 70/spl deg/C. Sensitivity studies were performed using conductors "Lapwing" and "Tern" in order to quantify such ruling span assumptions as the effect of the longitudinal swing of suspension and line post insulators on conductor sags at high temperatures, and the effect of the suspension insulator string length on the equalization of conductor tensions in adjacent spans. Significant errors in estimating the sag at conductor temperature above 100/spl deg/C may occur if the tension differences are not taken into consideration in line sections consisting of a series of spans of nonequal lengths. It was confirmed that the ruling span method is the most practical way to string conductors in multi-span line sections.

Natural insulator contamination test results on various shed shapes in heavy industrial contamination areas

In Shanghai, China, industrial contamination is very severe and periodic insulator washing is performed not only in substations but also on transmission lines. In order to review the present insulator contamination design and maintenance practices, a series of periodical contamination measurements under de-energized conditions were made. Also insulators that had been energized at two sites for one year were removed, and withstand voltage tests were conducted in an artificial fog chamber. Unexpectedly high ESDD (Equivalent Salt Deposit Density) values >1 mg/cm/sup 2/ and NSDD (Nonsoluble Deposit Density) values over >5 mg/cm/sup 2/ were measured. Aerodynamic type insulators showed a smaller accumulation of contamination than deep-rib type insulators. However, no significant difference can be seen between aerodynamic type and deep-rib type insulators in the withstand voltage per unit insulator string length. CaSO/sub 4/1/2H/sub 2/O (Bassanite) is identified as the most dominant soluble contaminant. >

Withstand voltage characteristics of insulator string covered with snow or ice

The authors describe the results of an investigation into the withstand voltage characteristics of an insulator string covered with snow or ice and the application of these results to the insulation design and maintenance of a transmission line in a heavy snow area. It was found that the characteristics of withstand voltage were affected by the specific gravity of snow, the conductivity of water melted from snow, the height of snow, and the ratio of snow-covered length. The lightning impulse flashover voltage of insulator string covered with snow is much lower than that of nonsnow-covered insulator, indicating that the efficiency of lightning protection greatly decreases under snowing conditions. When transmission lines pass through a heavy snow area, an extension of the transmission system fault is feared in 54 and 275 kV transmission lines because the existing insulator string length does not have sufficient dielectric strength for temporary AC overvoltage and switching surge. >

Simultaneous pole and neutral faults on an HVDC line with a dedicated metallic return

The dedicated metallic return conductor (DMR) of an HVDC transmission line may flashover as a consequence of a pole insulation failure or backflash over simultaneously with a pole during a lightning-induced event. The transient voltage surges induced on the DMR of the Quebec-New England HVDC intertie by pole faults are analyzed, and the sensitivity of simultaneous pole and DMR backflashover probability to variations in line insulating and footing impedance, is evaluated. It is found that for any reasonable DMR insulator string length, any lighting backflashover of a pole will be accompanied by a DMR flashover. Because coincident flashovers from lighting cannot be practically eliminated, the only practical solution for pole plus DMR faults on the New England portion of the Quebec-New England HVDC intertie is to install a high-speed ground switch at Sandy Pond, MA. The DMR insulator string length was increased to four units on the Sandy Pond-Comerford (NH) section, and to five units on the Comerford-Quebec order section in order to minimize the frequency for which this ground switch must operate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Switching-Surge Flashover of EHV-UHV Towers

Design curves are presented for switching-surge flashover of tower window gaps 20 to 39 feet (6.1 to 11.9 meters), bundle conductor-to-ground gaps spaced 20 to 50 feet (6.1 to 15.2 meters), and bundle conductor-to-tower leg gaps spaced 14 to 29 feet (4.3 to 8.8 meters). The critical flashover voltage range is from 1100 to 2700 kV. Together with the above, the following variables were investigated: time to crest, insulator string length, rain, window-shape, tower width, conductor entrance angle, and conductor configuration. In addition, work was accomplished on standard deviation, time to flashover, and flashover patterns.