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