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With the fast development of China's power grid construction, many transmission lines' corridors are inevitable across mountain with luxuriant vegetation. The occurrence of forest fires under transmission lines may lead to power interruptions, seriously affecting the stability of the grid operation. The characteristics of fire-induced flashovers are analyzed by statistical data of 220-500-kV lines trips in recent years. The line trip out rate caused by forest fires varied according to seasons and climate. The fire-induced flashover is much more serious than lighting trips. AC and DC voltage discharge tests of transmission line gap under different fire conditions are carried out by using wood crib fire. Test results indicate that: the gap's insulation strength under fire conditions is reduced to about 20% of that under pure air; the values of breakdown voltage have been decreased about 30% after alkali metal added; the gap breakdown under DC voltage has obvious polarity effect, and the flame shape and leakage current waveform are different under positive DC voltage and negative DC voltage. The analysis of gap breakdown mechanism under fire conditions shows that temperature, flame conductivity and ashes are main factors. Their combined effect significantly reduces the gap's insulation strength. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

An experimental scale-down model of ultra high voltage direct current converter transformer is designed and manufactured to study the wave process in the winding under impulse voltage. Nine outgoing lines are configured on different winding coils of the scale-down model. The voltage waveforms of different outgoing lines are obtained by the lightning impulse voltage. The simulation model of scale-down transformer is established to calculate the distributed parameter matrices through the finite-element method. The equivalent circuit of the transformer winding is proposed to calculate the voltage waveforms and the potential gradient distribution. The comparative analysis of test and simulation results shows that the oscillating voltage occurs in the winding, and the simulation results can reflect the wave process characteristics. [ABSTRACT FROM AUTHOR]

Based on the similarity theory, a reduced-scale model is proposed to study the voltage distribution characteristics in ultrahighvoltage direct current converter transformer windings under the impulse voltage. Through comparing the distribution characteristics of electric field intensity and magnetic flux intensity of the scale model with that of the original model, the effectiveness of the reduced-scale method is verified. Furthermore, the actual production design is also considered in the model. The matrices of the inductance, capacitance, and resistance parameters for the reduced-scale model are obtained by finite element method. Then, the multi-conductor transmission line model is set up to study the propagation process of impulse voltage along the windings of the converter transformer. The voltage distribution curve is obtained under a standard lighting impulse voltage and the turn-to-turn fault is taken into consideration as well. The simulation results are the basis for the following experimental study. [ABSTRACT FROM AUTHOR]