High voltage suspension insulator is normally used with grading rings to reduce electrical stress concentration on and within the insulator and to reduce radio and/or television interference.
Grading discs and/or rings are applied to polymer insulators for higher operating voltages to reduce electrical stress concentration on and within the insulator and to reduce radio and/or television interference. General recommendations of grading ring application are as follows.
| System voltage | 138kv and below | 161kv | 230kv | 345kv | 500kv | |
| Grading rings (discs) application | Top (tower side) |
None | None | None | ø4´´ disc | ø8´´ Ring* |
| Bottom (line side) |
None | ø4´´ disc | ø8´´ Ring* | Ø11´´ ring | Ø15´´ ring | |
During installed the grading rings, we should be adjustments with grading rings
| System voltage | 161kv | 230kv | 345kv | 500kv | |
| Adjustment of tabular values for 25k, 30k, 50k and 80k SML insulators | Arcing distance, inch(mm) | -0.8(-20) | -2.0(-50) | -2.8(-70) | -5.5(-140) |
| Low freq. F/O dry, kV | -5 | -15 | -25 | -50 | |
| Low freq. F/O wet, kV | -5 | -15 | -20 | -45 | |
| Critical impulse F/O pos., kV | -10 | -30 | -45 | -85 | |
| Critical impulse F/O neg., kV | -15 | -30 | -45 | -90 | |
| Net weight, lb(kg) | +1(+0.45) | +3.1(+1.4) | +5.4(+2.5) | +14.3(+6.5) | |
Selection suspension insulator should consider some factors
Mechanical rating strength
To select Shed Profile on the basis of leakage distance. If medium contamination area, See the shed profile is SS
To select Number of Sheds on the basis of section length
To select configuration of End Fitting, socket & ball, clevis & tongue etc.
To select Grading Ring application on the basis of system voltage
Shed profiles for composite suspension insulator
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