The number of single-phase faults in the transmission networks in wintertime increases substantially during cold atmosphere precipitation and after ice accretions followed by a rise in air temperature above 0/spl deg/C. >

Flashover problems caused by ice build up on insulators

Composite insulators have been used in power system for more than 20 years in China. Compared to porcelain and glass insulators, the pollution flashover performance of composite insulators is better, whereas the icing flashover performance is worse. Shed configuration is a key factor that influences flashover voltage of composite insulators covered ice or snow in icing regions. The DC flashover tests of artificially ice-covered 110 kV composite insulators with different shed configuration were carried out in the multi-function artificial climate chamber. The test results revealed that there are huge differences between the icing flashover voltages of the composite insulators with different shed configuration. The test results indicate that icing flashover performance of the improved composite insulators for icing regions is not always superior to that of the standard composite insulators.

Effect of shed configuration on DC flashover performance of ice-covered 110 kV composite insulators

This study, based on AC and DC flashover tests of porcelain, glass and composite insulators under the conditions of low air pressure and pollution, discusses the differences between AC and DC flashover performance and discharge process of ice-covered insulators. Results of the study indicate that the influence of air pressure on ice-covered insulator string's AC flashover voltage is greater than that on DC flashover voltage, whereas the influence of pollution on both AC and DC flashover voltage is almost the same. Good agreement is achieved between experimental results and the equivalent electrical circuit model proposed previously. Moreover, n (characteristic index of the influence of air pressure on ice-covered insulators' flashover voltage) differs slightly when calculated with the icing-water-conductivity method and solid-layer method, respectively, and the duration from the occurrence of local arc to flashover is longer under the condition of AC than DC. Although arc floats are more seriously under the condition of DC.

Comparison between AC and DC flashover performance and discharge process of ice-covered insulators under the conditions of low air pressure and pollution

A laboratory experiment is performed in a cold chamber to produce microdischarges between frozen hydrometeors. A charged icicle is placed close to an uncharged one connected to a transient analyser. The electric current signature obtained during each event is then studied and characterized in function of both temperature and net charge. The existence of a transition temperature of about −15°C is detected, whereas net charge clearly governs the microdischarge characteristics. Furthermore, the radiated electric field is calculated and the spectral analysis, by means of fast Fourier transform, shows that this elementary phenomenon radiates within the VHF range. Comparisons are made with previous similar work by Chauzy and Kably (1989) on microdischarges between water drops.

Microdischarges between ice particles

Ice morphology was established from field failure observations on electrical insulators, considering partial and full bridging conditions. Small scale laboratory simulations showed an important effect of droplet ejection on the probability of flashover. During the experiments, factors including applied water conductivity, icicle length, combinations of air gap and icicle length and duration of melting period were investigated using typical in-service stress levels. According to the experimental results, applied water conductivity and icicle length affected the residual ice layer resistance. Flashover voltage decreased by 33% as the conductivity increased from 150 μS/cm to 750 μS/cm and reduced by 10% as icicle length declined from 6 to 4 cm for constant 2-cm air gap. Fully bridged icicles required extra energy for ice melting, leading to higher flashover voltage compared to partially bridged icicles of the same morphology. A single 3-cm air gap had 14% higher flashover voltage than a pair of 1.5 cm air gaps.

Effects of electric field distribution and water drop ejection on flashover of icicles in plane-to-plane gaps

In order to gain a better understanding of the DC discharge performance of a string of ice-covered insulators with icicles, discharges were performed with three shapes of artificial icicles under DC voltages in a cold chamber. The DC flashover voltage is shown to increase with a decrease in temperature because of the simultaneous increase of the resistance of the icicle. This increase in resistance occurs as the thickness of the water film on the icicle surface decreases exponentially with the decrease in temperature. The melted weight W/sub m/ of the icicles was greater for negative polarity than for positive polarity when melting occurred by corona discharge. These differences in W/sub m/ may confirm reports that negative flashover voltage for ice-accreted insulators is lower than positive flashover voltage. There was a hot region on the icicle surface of the rod type near the boundary between the electrode in the ice and the ice. >

K. Hokari

Papers

DC discharge performance at the tip of an icicle