Study On External Insulation Characteristics And Configuration Of Composite Insulating Material Tower

Epoxy resin composite insulation material has the advantages of stable performance, strong environmental adaptability and excellent insulation performance to increase the insulation distance, which can improve the level of resistance to lightning, and reduce the width of transmission line corridor. The special insulation structure of the composite insulation tower and the characteristics of the tower with a large number of metal conductors with floating potential make the lightning protection and anti-contamination characteristic become new problems. The insulation characteristics and configuration of composite insulation tower are the key technologies to be solved. At present, the study of the grounding method of the lightning shielding wire of the composite insulating material tower is mainly based on the experiment, which needs further theoretical proof. On the other hand, the experiment and analysis of the influence of the tower insulation configuration on the external insulating material of tower under wet condition need to be further researched. In this paper, the numerical simulation and field test are combined to explore and deeply study the external insulation characteristics and insulation configuration of the composite insulating material tower, and provide the basis for the extension and application of composite insulating material tower. The contents of this paper are as follows:(1) Selection of grounding mode of lightning shielding wireThis paper designs two kinds of grounding connection method of lightning shielding wire for HOkV double-circuit lattice composite insulating material tower. The lightning shielding wire is grounded in two ways:inside of double-circuit line or the inside of the tower head. Electrostatic field simulation is carried out for the arrangement of the tower head to be applied to the lightning impulse voltage test. The results show that when the height of bottom of the tower head is not less than 8m, the length of the test conductor is not less than 18m and the radius of the downlead wire is not less than 0.01m, the electric field intensity of conductor and ground wire is very close to that in practical operating conditions of 110kV double-circuit lattice composite insulation tower. The surface electric field strength of the conductor and the downlead wire under the two kinds of grounding modes are compared. Lightning impulse voltage test is carried out on the tower head. When downlead wire is grounded from the inside of the tower head,50%lightning impulse breakdown voltage is 1578kV. The discharge path passes through the gap between the latticed composite rods. The discharge path is bent under the influence of the composite rod and the floating potential metal parts. At high wind speeds and other meteorological conditions, the arc is likely to burn the surface of the composite rod. When downlead wire is grounded from the inside of double-circuit lines, there is no significant relationship between the phase-to-ground 50% lightning impulse breakdown voltage and the distance from the grounding downlead wire to the tower. The minimum value of U50% is 1438kV. Although the value is slightly smaller than the former method, composite insulating material tower greatly improves lightning withstand level, and increases 50% lightning impulse breakdown voltage of phase-to-ground-gap, compared with the traditional steel tower. When the distance is not less than 0.45m, the discharge path is not affected by the composite insulating material tower. When the distance is less than 0.35m, the discharge path will be bifurcated or bent due to the influence of the metal parts having floating potential on the tower. Therefore, the downlead wire of lightning shielding wire of double-circuit lattice composite insulating material tower should be outside of the tower and inside of double-circuit line. Distance of downlead wire and tower should not be less than 0.45m.(2) Investigation on contamination discharge characteristics of composite insulating material towerArtificial contamination endurance and flashover test are carried out when the composite insulating material tower head is jointed with glass insulator string and composite insulator respectively. When jointed with glass insulator string and under maximum operating phase voltage, serious intermittent discharge occurs on the surface of contamination layer of insulators and metal parts around the tower head. The surface of the contamination layer among some metal bolts is broken by short arcs. The peak value of leakage current reaches 23mA. When jointed with composite insulator and under the same voltage, intermittent corona discharges are present only on metal fittings at the ends of the insulator. Leakage current peak is less than 0.6mA.When jointed with glass insulator string, the enhancement of the local electric field around the metal bolt cannot eliminate corona discharge on the composite insulation tower in the artificial pollution test. It is due to the existence of these floating potential metal parts leads to corona discharge on the tower head. Flashover arc only bridges insulator umbrella skirts in the flashover test. The voltage when bridging arc appears and the tower head jointed with composite insulator is higher than that of glass insulator string. The voltage when bridging arc appears increases from 109kV to 145kV, because the grading rings installed at both ends of the composite insulator effectively improve the local electric field distribution.(3) Pollution discharge mechanism of composite materialThe phenomenon of discharge in composite insulation material towers pollution test is reproduced on small size composite insulation piece. The test and electric field simulation results show that, the existence of the floating potential metal conductor distorts surrounding electric field strength, which prompts the formation of the drying band. The metal parts heated by the discharge arc continuously distribute heat outwardly. This behavior maintains the presence of the drying band around the metal parts. The electric field simulation results show that the electric field decreases rapidly at the wet and dry junctions as the length of the drying zone increases. Therefore, when the drying bands of the layer develop to a certain length, the discharge activity cannot be maintained due to the reduction of the distortion electric field. The corona discharge caused by floating potential metal body appear to be intermittent, from weak to strong, and then weakened, finally disappeared. The presence of the floating potential metal body is a necessary condition for corona discharge of the composite insulating material tower head. First of all, discharge arc is generated on the glass insulator, then the leakage current in the contamination layer near the metal part is increased. So that the electric field intensity that has some degree is further increased, resulting in drying zone and corona discharge.(4) Distortion characteristics of the electric field on the surface of the contamination layer by the floating potential metal conductorThe quasi-static electric field method is used to calculate and analyze the effect of the floating potential metal conductor with different shapes, different radius of curvature and different relative positions on the surface potential and electric field distribution of the wet contamination layer on the small size composite insulation piece. The electric field of the surface of the contaminated specimen is distorted by the presence of the floating potential metal conductor. The difference of the electric field intensity distribution characteristics between the metal conductor and the bulk and strip suspension potentials is obvious. The effect of floating potential metal conductors of different shapes on the distribution of electric field intensity is significantly different. The electric field strength of the different contact areas of the floating potential metal conductor and the contamination layer is also obviously different.(5) Simulation analysis of insulation configuration of composite insulating material tower The quasi-electrostatic field method is used to simulate the voltage bearing ratio of different parts of the tower jointed with different types of insulators under wet and contaminated conditions. The configuration composite insulating material coordinating with the composite insulator is defective. Composite insulators bear almost all the voltage. It is harmful to the safe and stable operation of transmission lines in wet and contaminated environment.With the increase of insulator number, the voltage undertook by the insulator is increased rapidly, and the maximum value of the surface electric field intensity of the composite insulation material tower head is decreased rapidly. In the principle of the maximum values of surface electric field intensity on the contamination layer of both composite insulating material tower and the top insulator do not exceed the initial discharge field strength, the suitable number of insulator in glass insulator string is three or four. The effect of voltage equalizing measures of the floating potential conductor on the whole or local electric field distribution of the composite insulation material tower is not significant.