Insulated Cross arm and Worldwide Experience of Compact Lines
PRINCIPLE OF INSULATED CROSS ARM
- Freedom of Movement provided to the conductor determines the clearance required for the line.
- Clearance should be provided for both inward and outward swing of the conductor.
- For Suspension Tower, Freedom of Movement is very high and hence more clearance will be required.
- In Insulated cross arm, Insulators are directly fixed to the tower.
- This eliminates the use of metallic cross arm.
- With the freedom of movement restricted, the conductors will be unable to swing, which reduces the clearance level and making the line compact.
- Post works under Compression and Brace works under Tension.
- With advancement in composite insulator technology, the use of insulated cross arm in high voltage transmission is making an impact.
Countries conducted studies on compact line design
IBER-DUERO, S.A. carried out a study to optimize line design of a short length of 2.34km of a 132kV transmission line. Composite insulators were considered to form a rotary cross-arm where a rigid column insulator was placed horizontally working in tension and long rod composite insulator used as a tie-bar working in tension. Both were joined to form an apex from which conductor clamp is suspended. The other ends of both insulators were fixed to the tower by universal joints to allow rotation of the cross-arm about an inclined axis in case of broken wire condition.
ENEL conducted studies to determine limits of line compaction. Rotating insulator cross-arms were considered.
EDF carried out studies to develop methods for design of compact high voltage lines at 90kV. An experimental line of 1200m was built to study the feasibility of various compaction techniques. Composite insulators were adopted for insulated cross-arms to form both rigid and pivoted types.
Studies extensive tests were carried out to develop polymeric inter-phase spacers for 66kV transmission lines to find effective means for dealing with conductor galloping under ice loading. Inherent flexibility of polymeric composite insulators helps in attenuating vibrations of conductors during galloping.