Study On The Influence Of The AC Field On Conductor Icing And Corona Onset Characteristics

China has a large land area and most places about70%are at high altitudes whilethe micro climate characteristics are various, due to the development strategy of “powertransmission from west to East, mutual supply between South and North, nationwideinterconnection”, the transmission lines have to go across some extremely complexenvironments such as icy, snowy or rainy climates and so on during the transmissionprocess. And roughness brought by external surroundings will directly affect the safetyoperation of the power grid. Super-cooled water droplets may cause the micro-dischargeunder the affection of winds when they are close to the transmission line surface, andthe ice on conductor surface will change the original shapes or even causes the partialcorona effect, which may lead to the conductor’s corona onset voltage decline andcorona loss. Electromagnetic pollution, circuit aging problems caused by thisphenomenon should be paid more attention. At present, the domestic and foreignscholars have carried out some researches on transmission line icing, but most of themignored the practical significance of energized icing, so there are some differencesbetween the theoretical conclusions and engineering practices. And in addition, theydidn’t study the influence law of ice on corona onset voltage and corona dischargethoroughly. Therefore in this paper, researches were carried out on the impact ofdifferent ice coating types on the corona onset characteristics of transmission line whichhas important academic significance and engineering value.In this paper, the artificial climate chamber was used to simulate three kinds ofcommon icing types, namely glaze, rime and mixed-phase icing, while seven different(in diameters, types and splits) kinds of conductors were iced in this chamber underdistinct field strength. Optical radiation mechanism based on the cathode surface,combined with the UV imaging technology and the photon number fitting were firsttime used to analyze the corona onset voltage of conductors before and after the icecoating. Results show that: the shapes of conductors’ surfaces are very different whenthey were iced in different fields. Corona onset voltage values of bare wire are largerthan those of iced conductors, even minimal icing will decline the corona onset voltagevalues to about40%~60%of bare wires’ values. As the field strength increases from0to20kV/cm, the inception voltage of glaze and hard rime icing surface firstlydecreased and then increased, while the soft rime surface showed a fluctuation trend which dropped, rose, dropped again and rose again, the difference was due to thedistinct ice shapes which formed in diverse field strengths. Corona onset voltage of theglaze and mixed-phase icing surface declined continuously along with the increase ofthe ice extent, while the rime surface first decreased and then increased, the change rateof them slowed down gradually with the ice amount saturated. The conductivity withdifferent concentrations had little effect on the ice morphology, and it would not affectthe corona inception voltage of rime and mixed-phase icing, higher conductivityincreased the discharge area of glaze surface and led to corona inception voltage dropeventually.This paper presents for the first time the ice roughness coefficient W tocharacterize how the icing affects the field strength and corona onset voltage. To studythe influence of ice coating on the wire surface field, this paper established the fielddistribution model according to the different icing shapes, maximum field strength oficed surface was calculated by using the finite element method, the concept of icingroughness coefficient W was put forward to characterize the how the icing affect thefield distribution and corona onset voltage. Results show that: different AC field leads todistinct ice morphology and roughness, so the electric field distortion on the wiresurface by icicle is different. Compared to the bare wire, ice will raise the electric fieldstrength on conductor surface greatly, this is because the ice surface is so rough that theicicle tips will distort the field. Increase of ice extent will distort the electric field ofglaze surface more seriously, while the increase speeds of partial field strength slowsdown. Rime’s dry growth characteristics will produce greater ice thickness, thus mayweaken the distortion effect of ice-tree, so the maximum field strength of rime surfacewill be reduced with the increasing icing time. During the same icing time, distortioneffect on electric field always shows a trend namely a thin wire> coarse wire> bundleconductor. The ice roughness coefficient W and corona onset voltage Ucare fitted byMatlab, the empirical formula obtained can quickly calculate the glaze and rime icingconductor’s corona onset voltage.To study the corona discharge caused by super-cooled water droplets close to thewire surface, the test was established for the first time in Hunan province Xuefengmountain, combined with the corona cage and Q-V method to analyze the coronadischarge quantity and discharge power during the energized icing, results show that:the force tendency of water droplets in the field after charged was decided by the ionconcentration, the force extent will increase as the background field strength rises. At the beginning of icing, corona discharge on the wire surface is the mainly caused by themicro discharge, then during the icing process is mainly caused by the water dropletsand rough surface, and the corona discharge was mostly caused by the icicle tips afterthe icing stops. Micro discharge and corona discharge on the conductor surface are allrandom which approximate the normal distribution. The positive and negative coronadischarge will reach the peak level with the increase in icing time, and then the coronadischarge will be saturated or even slightly reduced later. The total discharge quantitiesof positive and negative are almost the same, only a small part of the discharge cycle isnot consistent. Discharge capacity and power of glaze are smaller than the rime under5~10kV/cm, but in15~20kV/cm the situation is opposite. Different conductivity hasno effect on the rime discharge capacity, while the discharge quantity and dischargepower of glaze increased as the conductivity rose, the glaze discharge amplitude in20kV/cm influenced less than10kV/cm by conductivity.