Housings on the DC voltage dividers at Longquan, Jiangling and Yidu were all silicone rubber type with basically the same section size. The grading rings on the equipment at Jiangling and Longquan stations were also the same although the voltage divider at Yidu had a different single ring structure. In 2009/2010, staff at the State Grid decided to apply RTV coatings to the composite housings of all three and there have been no further reports of problems since then.
Fault at Tianshengqiao Converter Station
In May 2005, again during a period of heavy rain, a flashover occurred on the porcelain housing of the voltage divider on Pole I at Tianshengqiao and white traces remained visible afterwards on the high voltage portion of the equipment. To prevent a recurrence, operating staff at the station decided to add booster sheds to the porcelain and also applied an RTV silicone coating (see Fig. 2). There have been no further flashover incidents with this apparatus.
Different External Insulation for Main ±500 kV DC Voltage Dividers
The housings of the voltage dividers used in the Ge-Nan and Tian-Guang DC projects were made from porcelain with relatively low specific creepage distance. However, once coated with RTV, creepage distance was seen as equivalent to that of a compositehoused voltage divider. Moreover, at the Tianshengqiao and Nanqiao Converter Stations, the creepage distance of porcelain-housed voltage dividers, once coated with RTV and with added booster sheds, exceeded that of composite-housed units.
In addition, the design of the DC field busbar voltage dividers used on the Yi-Hua and De-Bao systems has been improved by reducing the size of their grading rings at the high voltage end. This permits more efficient use of the composite housing’s creepage. The external insulation on these different types of ±500 kV DC voltage dividers is described in Table 3 and shown in Fig. 3.
Failures Due to Insufficient Creepage
After analyzing past insulation faults affecting ±500 kV DC voltage dividers in China, the causes of these various problems could be grouped into the following categories:
a. Failures Due to Insufficient Creepage
The minimum specific creepage of equipment used on early ±500 kV DC projects in China (e.g. Ge-Nan and Tian-Guang) was only about 40 to 41 mm/kV. However, this value proved insufficient and indeed almost all equipment used in the DC field, including voltage dividers, experienced varying degrees of discharge phenomena – some even when operating at lower voltages. External insulation failures caused by lack of sufficient creepage were found best resolved by adding booster sheds or applying RTV coatings to increase external insulation strength.
b. Failures Due to Decline in Hydrophobicity
Flashovers of external insulation on ±500 kV DC voltage dividers at Jiangling, Longquan and Yidu Converter Stations under persistent rain and fog were traced mainly to decline in the hydrophobicity of their composite housings and resulting diminished performance under conditions of high pollution and wetting.
While water dripping from the sheds of a saturated insulator due to gravity takes some deposited surface salts with it (effectively cleaning the surface), a decrease in hydrophobicity means that a continuous film of conductive moisture can more easily form on the surface of polluted insulators. The result is a serious net reduction in dielectric strength. Indeed, research has demonstrated that hydrophobicity has such a significant influence that the pollution flashover voltage of composite insulators under good hydrophobicity is 2.4 times higher per unit length than when this property has been lost. For example, under pollution conditions of ESDD = 0.10 mg/cm2 and NSDD = 0.60 mg/cm2, pollution withstand of composite insulators whose surface hydrophobicity is lost is approximately 47 mm/kV.