P 17 External Insulation Functional Specification for the Upgrade of a 400 kV AC Transmission Line to 500 kV DC Dr WL Vosloo, R Stephen, P Naidoo Dr I Gutman , Dr D Muftic and Prof N Ijumba Matimba - Midas 400 kV AC Line Zimbabwe Botswana Mozambique Namibia Pretoria Swaziland Bloemfontein Lesotho Matimba Durban Midas Cape Town Mosselbay Port Elizabeth • Mostly 520B type towers • Diamond quad Zebra conductors • Triple “V” glass cap and pin suspension insulator and twin horizontal strain insulator strings Insulator String Assemblies Tower type Orientation Connecting length Creepage distance Specific creepage distance (Um = 420 kV) Disk type used Suspension 520 B and E “v” 3382 * 6992 16.7 mm/kV 19 x U190BS Suspension 518 H “V” 3315 * 7480 17.8 mm/kV 17 x U300BS Strain 518 C and D horizontal twin 3510 7920 18.9 mm/kV 18 x U300BS * Ignoring the centenary shape of the insulator string. Type Diameter Connecting length Creepage distance Minimum failing load Metal fitting size Fitting type Weight U190BS 279 178 368 190 kN 24 Ball and Socket 7.5 kg U300BS 320 195 440 300 kN 28 Ball and Socket 12.4 kg Environmental Conditions • • • • • • • • Light pollution levels (ESDD < 0.06 mg/cm and NSDD < 0.1 mg/cm ) 400 to 500 mm rain pa with about 8 dry months Less than 1 fog days pa Lightning ground flash density of about 1 to 8 Tower footing resistance of about 20 Altitude of about 1280 m above sea level 1 to 10 veldt fires pa Bird activity present 2 2 Line performance (line faults per annum) REASON 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Bird Cane Fire Matimba – Lightning Midas Other Pollution Veld Fire 4 0 3 0 0 0 2 0 1 3 0 1 0 0 2 1 0 0 1 0 6 0 0 2 1 0 7 0 0 0 2 0 8 0 0 3 1 0 1 2 0 0 8 0 4 3 0 9 0 0 0 1 0 6 Fires, Lightning and Birds 1 0 0 0 0 1 1 0 3 0 0 0 Grand Total 21 0 35 10 0 22 SCD - Existing AC Line Experience Kf x SCDDC 0.75 x Kf x SCDDC Glass Silicone Rubber = 2.0 39 29 x 1.3 = 1.9 47 35 1.2 x 1.6 = 1.9 59 44 1.0 x 1.9 = 1.9 72 54 SCDAC SCDAC x √3 / √2 = SCDDC Pollution catch factor DC to AC withstand factor 16 19.6 2.0 x 1.0 20 24.5 1.5 25 30.6 31 37.9 Total factor (Kf) VERY HEAVY CONTAMINATION AREA China (SR) 35 44 LIGHT CONTAMINATION AREA SCD - Pollution, Russia and IEC Guides NSDD mg/cm² New IEC 60815 Very heavy Heavy Medium Russian Guide The maximum expected ESDD of 0.06 mg/cm2 corresponds approximately to a surface conductivity of 9 µS. This, according to the Russian Guide in turn corresponds to a SCDAC of about 18 mm/kV indicating a medium pollution environment. Light Very light ESDD mg/cm² SCDAC SCDAC x √3 / √2 = SCDDC Pollution catch factor DC to AC withstand factor 17.4 21.3 2.0 x 1.0 20 24.5 1.5 x 1.3 Kf x SCDDC 0.75 x Kf x SCDDC Glass Silicone Rubber = 2.0 42.6 32 = 1.9 47 35 Total factor (Kf) SCD - Climate, Pollution, Lab and IST • • • • • SCDAC SCDAC x √3 / √2 = SCDDC Pollution catch factor 17.2 20.9 2.0 Mean time between flashover = 10 years ESDD = 0.06 and NSDD = 0.1 mg/cm2 Number of critical wetting events per year = 10 Altitude = 1300 m 100 km of line exposed to the same pollution event DC to AC withstand factor x 1.0 Total factor (Kf) = 2.0 Kf x SCDDC 0.75 x Kf x SCDDC Glass Silicone Rubber 41.8 31 Specific Creepage Distance (SCD) Input data SCDDC (mm/kV) Existing AC Line Experience 29 Climatic and Pollution measurements and using IEC 35 Climatic and Pollution measurements and using the Russian Guide 32 Climatic and Pollution measurements and laboratory test data and using the IST 31 SCDDC of between 29 and 35 mm/kV would be a sufficient requirement for the Matimba – Midas area Other Considerations • Open aerodynamic profile with a shed spacing to shed projection ratio of approximately one • Use silicone rubber insulating material • Maximum connecting length of 4137 mm • Use of corona rings on both ends of the insulator • V-string configuration Questions for Further Investigation 1. What should we specify for dc insulators, for example specific creepage, profile, material etc? 2. What is the effect of polarity, for example can we use the same specifications for both, and how do they differ from ac? 3. What are the ageing modes of the various materials (rod, shed and sheath)? 4. What is the effect of dc on pollution catch and overall insulator performance? 5. What is the relative performance of the various dc insulator designs available in the market? 6. Can we use existing site severity assessment techniques and apply them for dc insulator selection, and if necessary how should we compensate? Conclusions From an external insulation viewpoint the upgrading of the 400 kV AC Matimba – Midas Transmission line to operate at 500 kV DC appears possible. The following basic functional specifications for the external insulation are recommended: • Specific creepage distance between 29 to 35 mm/kV • Open aerodynamic profile with a shed spacing to shed projection ratio of approximately one • Use silicone rubber insulating material • Maximum connecting length of 4137 mm • Use of corona rings on both ends of the insulator • V-string configuration The above is based on the current available information and a number of questions for further research have been identified. A combination of laboratory and field testing is proposed for this research. Thank You Specific creepage distance between 29 to 35 mm/kV Open aerodynamic profile with a shed spacing to shed projection ratio of approximately one Use silicone rubber insulating material Maximum connecting length of 4137 mm Use of corona rings on both ends of the insulator V-string configuration Thank You