Classify insulators; string insulator configurations

Overhead line insulators support the conductors and isolate them electrically from the towers.

Classification

• Pin type: fixed on a pin on the cross-arm. Used up to 33 kV.
• Suspension / disc: discs hung in a string, used above 33 kV.
• Strain: a disc string in the horizontal plane at dead-ends, sharp angles and river crossings.
• Shackle: used on LV distribution lines and service drops.
• Post: used in substations and compact lines.
• Stay insulator: fitted in stay wires to keep the lower part dead.

Advantages of string insulators for HV lines

1. Each disc is rated about 11 kV, so any voltage is handled by adding discs.
2. Cheaper than pin insulators above 33 kV.
3. A failed disc is replaced alone.
4. The flexible string reduces mechanical stress.
5. The conductor below the cross-arm is partly shielded from lightning.
6. The voltage can be upgraded later by adding discs.

Configurations of string insulators

1. I-string (single suspension): one vertical string. It is simple and cheap but swings freely, so it needs a longer cross-arm.
2. Double I-string: two parallel strings for heavy conductors or important crossings (higher mechanical strength and reliability).
3. V-string: two strings in a V shape. The conductor cannot swing, so the tower window and right-of-way are smaller. Used at 400 kV and above.
4. Tension (strain) string: horizontal, single or double, at angle and dead-end towers.
5. Y-string or inverted V, and long-rod / composite strings: used for compact EHV lines.
6. Jumper (pilot) string: holds the jumper loop at tension towers to keep clearance.