A 230-Kv Spar Arm H-Frame Transmission Line C. C. DODGE

FELLOW AIEE

DURING the past 16 years, the Virginia Electric and Power Company has had in service over 600

circuit miles of 115- and 138-kv wood pole H-f rame trans­mission lines with spar-arm construction. Experience with this construction has been so satisfactory that the new 96-mile 230-kv transmission line between the com­pany's Chesterfield and Possum Point power stations is being built with spar construction for both crossarms and braces. Each end of the line will be tied into the existing 115-kv system through 230/115-kv autotrans-formers and a tap connection will be made at the Lake­side substation north of Richmond.

A comparison of impulse flashovers of the air gap between conductor and grounding downlead with the flashover of the insulators plus the wood under blowout, led to the decision to use a 19-foot phase spacing with 12 standard 10- by 534-inch disks in suspension strings. Flashovers at 4 microseconds with a li/£- by 40-micro-second positive full wave have been used as a basis for comparison. The 4-microsecond flashover of the 12 in­sulator flashover is co-ordinated with the air-gap flashover. 1,510 kv, which corresponds to the air-gap flashover of the conductor to grounding downlead with an insulator string swing of 28.7 degrees. If the line is installed on level terrain, it requires a wind pressure of 6.25 pounds per square foot on bare conductor to produce this angle of swing.

The angle of insulator string swing for a given wind load will vary considerably in different spans over rough terrain, depending on the amount of vertical loading at each structure. A lower limit on the vertical loading has been set by specifying that a wind pressure of 4 pounds per square foot must not result in an angle of insulator swing greater than the 28.7 degrees at which the insula­tor flashover is co-ordinated with the air-gap flashover.

Fig. 1 shows the type of bracing used on tangent structures. Hardware items have been selected to permit assembly of the structure with a minimum of field drilling of holes and a minimum of labor. Conductor is 795,000-circular-mil 45/7-stranding ASCR (aluminum cable, steel reinforced) with a maximum design tension of 8,800 pounds under National Electric Code (5th edition) heavy loading. Two ^-inch 7-strand high-strength steel overhead static wires are used.

Structures with 70-foot poles on level ground result in a normal span of 700 feet, or 7.5 structures per mile, with a minimum ground clearance of 29 feet. The actual average number of structures is 8.3 per mile because of extra clearances required at crossings and a rather large proportion of angles in the line.

Two full-size structures were built and loaded trans­versely to the point of failure in order to check the hardware fittings and the effect of uplift. Additional

uplift tests were made on pole stubs set under different soil conditions and with crushed rock backfill. One structure was tested with vertical loads equivalent to i/2-inch of ice on a 1,000-foot average span. A transverse force of 14,725 pounds caused a cross-brace bolt on the upper end of the brace to shear. The second structure was tested with vertical loads equivalent to bare con­ductor on a 1,000-foot average span. A transverse force of 15,525 pounds caused the upper end of a cross-brace to split. Both of these transverse forces represent over 414 times maximum design loading for 1,000-foot aver­age spans. The uplift tests demonstrated the value of crushed rock for backfilling pole holes. This method has been used on average spans over 700 feet.

Fig. 1. AH spar construction tangent structure.

Construction started February 1, 1956, and 51 miles of line were completed in four months. Maximum use is made of mechanized equipment. Pole holes are dug mechanically and structures are erected with a crawler crane after assembly on the ground. A mobile aerial tower is used for applying armor rods and clipping in conductor. Mobile radio units on automotive equipment and portable pack units are of invaluable assistance in providing necessary communication for construction activities.

Digest of paper 57-154, recommended by the AIEE Committee on Transmission and Distribution and approved by the AIEE Technical Operations Department for pres­entation at the AIEE Winter General Meetings, New York, N. Y., Jan. 21-25, 1957. Scheduled for publication in AIEE Power Apparatus and Systems, 1957.

C C. Dodge is with the Stone & Webster Engineering Corporation, Boston, Mass.

APRIL 1957 Dodge— Transmission Line 307