Photo courtesy of State Grid Corp.
Typical 1000 kV AC tangent tower.
Typical V-string used at ± 800 kV.
composite insulators used on 1000 kV AC and ± 800 kV DC lines.
for maximum operational reliability, creepage distance and length of the composite insulators used on ± 800 kV DC lines was increased even further.
According to the withstand voltage curves from Fig. 1a, the required creepage distance for 1000 kV AC composite insulators used in medium to heavy pollution areas at altitudes of less than 1000 m should be from 27,970 to 32,000 mm (corresponding to an insulator length of 7.6 ~ 8.7 m). These insulators would have alternating shed geometry and a ratio of creepage to arcing distance of 3.68. Moreover, as a precaution to ensure such creepage, insulator length would be increased by 20%. Because pollution performance is even more critical at DC, V-type composite insulators were selected for China’s first new ± 800 kV DC lines. According to the withstand voltage curves in Fig. 1b, the creepage distance of these insulators had to be at least 28,520 mm (i.e. an insulator length of 9.2 m) in areas of light pollution and below 1000 m altitude. In this case, the geometry would be one big and two small sheds, with a ratio of creepage to arcing distance of 3.1.
2. Voltage Distribution & Choice of Corona Rings Usually, voltage distribution along a UHV DC insulator is highly non-uniform and greatly affected by ionization due to corona. The proper design of grading a. AC 1000 kV porcelain and composite insulators
INMR® Q3 2010
A three-dimensional finite element methodology was used to calculate the electric field on an I-string at 1000 kV AC and a V-string at ± 800 kV DC. The effect of the tower and conductor were also considered in the calculation, yielding a general design scheme as shown in Figure 2. For UHV composite insulator strings, one big and one small ring have been placed on the conductor side and a single ring on the tower side. Such a configuration was found to offer improved electric field distribution along the insulator surface, especially at the end-fittings on the conductor side as well as the interface of the sheath with the core rod. 3. Requirements for Core Rods and Rod-Sheath Interface
b. DC ± 800 kV porcelain and composite insulators
In order to improve electrical performance at the interface of the core rod and sheath, desired test
Fig. 1: Pollution flashover characteristics of porcelain and composite insulators used on UHV lines.
For medium to heavy pollution areas below 1000 m altitude, the creepage distance required would increase to a minimum of 31,310 mm (i.e. an insulator length of 10.1 m). Moreover,
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rings in such applications is therefore extremely important.
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