Research On Lightning Protection Against Simultaneity Tripout Of Double-circuit Transmission Lines

With the rapidly growth of construction-land price, double-circuit transmission lines on the same tower are widely used to increase the transmission capacity in the same width of the transmission corridor at present. Compared with regular single-circuit transmission lines, the double-circuit transmission lines face more serious lightning-protection problems, in which simultaneous trip-out caused by lightning stroke is the worst. How to avoid the simultaneous trip-out accidents in double-circuit transmission lines becomes the key issue that seriously affects the stability of the power grid and need to be solved as soon as possible.In this paper, electromagnetic transient program (EMTP) is used to recur the accidents which happened in220kV double-circuit lines in practice, and the simultaneous trip-out mechanism in double-circuit transmission lines on the same tower is revealed. The study shows that:although the power frequency voltage is not the primary cause of lightning flashover, it has a significant impact on the flashover sequence of the conductors; grounding resistance has a great impact on the lightning withstand level, it is suggested that the grounding resistance of the double-circuit transmission lines on the same tower is not larger than10Ω in heavy lightning area.This paper also analyses the application effect of the unbalanced insulation on double-circuit transmission lines.The results show that by means of ensuring enough difference between insulation levels for the two circuits, it can make sure that when double circuit transmission lines on the same tower is lightning-stricken the phase-to-phase flashover only happens at the circuit with the lower insulation level, therefore the serious fault of simultaneous trip-out of two circuits can be avoided at the cost of trip-out of one circuit. Calculation results show that when the degree of insulation unbalancedness for double220kV or110kV circuit transmission lines on the same tower reaches to30%, or in other word, the difference in insulator pieces of a string between two220kV circuits or two110kV circuits is4or3pieces respectively, the unbalanced insulation can restrict the circuit, where phase-to-phase flashover happens. Besides, the effect of such a measure is reliable and suitable for double circuit transmission lines with various conductor arrangements of phase sequence to avoid simultaneous trip-out of both circuits for double circuit transmission lines on the same tower due to lightning stroke.