Xiaobo Meng

Negative corona inception voltages in rod-plane gaps at various air pressures and humidities

The variation of negative dc corona inception voltages with air pressure and humidity, using a point/plane electrode system, was studied using a perspex chamber in which the pressure and humidity could be varied. A method of calculation was developed to determine the corona inception voltages over a range of air pressures and humidities. The effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages for the same range of pressures and humidities. A comparison of the negative and positive corona inception mechanisms was also made. Both the negative and positive corona inception voltages decreased with a decrease in the air pressure and an increase in the humidity. The greater values of the effective ionization intensity found in the critical zone for the negative corona accounted for their lower inception voltages. The mechanisms of the affect of the photo-absorption coefficient and Townsends second coefficient on the negative and positive corona were also considered.

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.

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.

Study of dynamic response of overhead transmission lines to different wind speeds

The conductors swinging out of sync caused by wind is a common phenomenon on overhead transmission lines, especially in the coastal zone the wind is usually very strong, and conductors non-synchronous swinging is also very severe. Many examples of power system failure were caused by conductor swinging out of sync in recent years. The study of conductor swinging out of sync becomes a hot topic in China. This paper focus on the study of dynamic response of transmission lines to different wind speeds. In this paper, a 3 degree of freedom (DOF) dynamic model of multi-span line suitable for analyzing wire swinging out of sync is presented. The time responses of the wire swinging out of sync are acquired, finally find out the relationship of the minimum interphase distance and wind speed. providing a theoretical supporting for how to effectively avoid air breakdown under different wind speeds.

Influence of pressure and humidity on the amplitude — Phase distribution of AC corona pulse

Corona problem is one of the important problems in the design and operation of the transmission lines. The corona discharge of high voltage transmission lines, results in power loss, radio interference and audible noise, which become more serious at the high altitudes. Besides, it has a great influence on the safe and robust operation of high voltage apparatus. A study carried out in the chamber made of perspex, was that AC corona characteristics varied with air pressure and humidity through the rod plane electrodes. The experiments showed that the inception voltage of AC corona was reduced with air pressure decreasing or humidity increasing. Meanwhile, the amplitude - phase distribution of AC corona pulse was displayed under different air pressure and humidity. Under low humidity (RH=25% or 55%), the streamer pulses only appear in the rise edge of positive half cycle, and the number of streamer pulses is few. The negative streamer pulses occur around the peak value of the negative half cycle. The streamer pulses appear in the rise and fall edge of positive half cycle at the same time with high humidity (RH=85%), and the number of streamer pulses increases obviously. When the humidity is higher (RH=95%), the streamer pulses appear only in the fall edge of positive half cycle. Furthermore, the negative streamer pulses around the peak of the negative half cycle also disappear.

Numerical model of negative DC corona inception voltage of stranded conductors

Energy loss, radio interference, and audible noise interference caused by corona discharge bring many special problems to the design and operation of the transmission lines. The smooth conductor is adopted in the traditional calculation model. The electric field intensity of stranded conductor which is used in practice is larger than that of smooth conductor. Therefore, a computation model is adopted for evaluating the corona inception voltage of stranded conductors. The electric field intensity is calculated by the charge simulation method in the computation model. Then the negative corona inception voltage is calculated according to the negative corona criterion. The results from numerical computation are compared with that from test in the others papers. Good agreement is obtained between the computation and test results, which demonstrate the validity of the numerical simulation program. The numerical simulation program may therefore be applied to study characteristics of negative DC corona inception voltage of stranded conductors. Surface roughness coefficient is in inverse proportion to the conductor radius, which is in proportion to conductor strand number.

Influence of air pressure on positive DC corona inception voltage of stranded conductors in the corona cage

The transmission lines pass by the high altitude region with low air pressure, which brings many special problems to the design and operation of the transmission lines. The corona cage is widely used to study the corona performance of conductors for its good economy and maneuverability. A computation model is adopted for evaluating the corona inception voltages of conductors in the corona cage, which is established according to the gas discharge theory. The charge simulation method is introduced to the computation model to calculate the electric field intensity. Then the positive corona inception voltage are calculated according to the positive corona criterion. The influence of air pressure on the corona inception voltage of conductors is analysed systematically. Corona inception voltage decreases as air pressure is lowered. The corona inception voltage may decrease by 9% per 0.01MPa bases on the standard atmospheric pressure, which is mainly due to the enlargement of ionization zone as a result of the increase of effective ionization coefficient.

Characteristics of streamer propagation along the insulation surface: Influence of air pressure and humidity

Most of the UHV transmission lines passed the high altitude and high humidity area. It was necessary to study the influence of air pressure and humidity on characteristics of flashover along the insulation surfaces. In the three-electrode arrangement, the characteristics of streamer propagation in air alone and along the silicone rubber surface were measured at Shenzhen, Kunming, and Yangbajing, respectively. The data of the streamer ‘stability’ propagation field and velocity were measured at the different air pressure and humidity. Empirical equations were presented expressing the effects of the air pressure and humidity on the streamer ‘stability’ propagation field and velocity. Furthermore, the variation of the streamer ‘stability’ propagation field and velocity with air pressure and humidity was analysed. The test results showed that the streamer stability propagation field was in proportion to air pressure or humidity. Under the same electric field, the streamer propagation velocity was in inverse proportion to air pressure or humidity. Therefore, it was easier for streamer propagation in the lower air pressure or humidity. Furthermore, the effect of the humidity and air pressure on stability fields of streamer propagation along silicone rubber surface was larger than that in air alone.

Characteristics of streamer propagation along insulation surface: quantitative influence of permittivity and surface properties

An investigation into the quantitative influence of permittivity and surface properties on characteristics of streamer propagation along insulation surface was presented. In the three-electrode arrangement, the variation of the streamer stable propagation electric field and streamer propagation velocity along the dielectric materials was measured through three photomultipliers. The measured dielectric materials were consisted of two different dielectric materials with the thickness of 5 mm piled by the side of each other. The influence of the permittivity (the capacitive effect) and the surface properties on the characteristic of streamer propagation along the surface of dielectric materials were obtained by varying the ratio of two material volumes. The streamer propagation velocity and electric field for streamer stable propagation along two combinations of nylon and PTFE, silicone rubber and PTFE were measured, respectively. The conclusion was that the surface properties of dielectric materials had large effect on streamer propagation. Furthermore, surface properties of silicone rubber were most unfavorable for streamer propagation in the three dielectric materials.

AC corona pulses amplitude and frequency related to pressure and humidity

Compared with DC corona, the physical process of AC corona is more complicated. Through the rod plane electrodes, AC corona characteristics varied with air pressure and humidity were researched in the chamber made of Perspex. The relationships between average amplitude of AC corona pulse and air pressure or humidity, between the frequency of AC corona pulse and air pressure or humidity were studied. The average amplitude of AC corona pulse was inversely proportional to humidity. The amplitude decreased in positive half-cycle and increased in negative half-cycle with the fall of air pressure. Whats more, the repetition frequency of AC corona pulses increased with the rise of the humidity. The frequency had little change with the fall of air pressure in the positive half-cycle, while rising in negative half-cycle obviously.