Land acquisition problems coupled with the high cost of developing new rights-of-way led the Electricity Generating Authority of Thailand (EGAT) to the decision to upgrade an existing 230-kV double-circuit line with a 500-kV double-circuit compact line. The 230-kV line's rights-of-way would continue to be used for the new 500-kV compact line. Some 80 km (50 miles) long, the line is routed through agricultural surroundings as well as industrial, commercial and residential city developments in the greater Bangkok area of Thailand.

The majority of the EGAT's existing 500-kV double-circuit lines were constructed with a right-of-way width of 60 m (197 ft). However, the design criteria developed by EGAT for the new compact line required a right-of-way of 40 m (131 ft).

Based on the system security, planning and operating condition N-1, the 500-kV compact line was designed on the assumption that maintenance would be conducted with the circuit de-energized. However, EGAT realized this would be a huge constraint. When system integrity, reliability and operating revenues are at a premium, a transmission circuit outage is unacceptable. Therefore, at the design stage, EGAT examined methods and maintenance techniques for working with live lines.

Line Compaction

Designed and constructed within the right-of-way of an existing 230-kV line, the 500-kV compact line had smaller available clearances than EGAT's conventional 500-kV lines. To optimize the design of the compact line, EGAT considered several factors:

• The higher-voltage line had an environmental impact, namely on the visual aspect, electric and magnetic fields, audible noise and radio interference.

• The right-of-way of the existing 230-kV line was one of the main constraints of the design of the 500-kV circuits.

• The line route in the Bangkok area traversed a flat terrain with very soft soil. These soil characteristics required the use of long pile tower foundations, a factor that increased the construction cost, affecting optimization of the design parameters.

• The design was based on the use of Quad 1272-kcmil ACSR/GA conductors with a bundle spacing of 457 mm (18 inches).

To accommodate the smaller-than-normal right-of-way width for the new 500-kV compact line, EGAT selected a line-compaction technique that calls for optimum design of the tower windows to bring the conductors closer together.

EGAT studied four compact tower configurations, each with V-insulator strings and a conductor clearance of 3.5 m (11.5 ft) to tower and 11 m (36 ft) to ground. EGAT found that a portal steel structure, with all phases positioned inside the tower window, was also more compact than the alternatives. A second portal tower would be lower in height; therefore, the visual impact would be reduced. However, the most cost-effective compact configuration was the lattice-steel tower.

Electrical Design Studies

Switching overvoltage was one aspect of the tower design that had to be considered. Without using any overvoltage control, a switching overvoltage magnitude of 1650 kV_PEAK (3.7 p.u. [1 p.u. = 450 kV_PEAK]) could be produced at the remote-end substation during reclose operations. That level of switching overvoltage would result in the need for larger air-gap clearances in the geometry of the compact tower design.

A surge arrester with a minimum-rated voltage of 420 kV was selected for installation at the line terminal to limit the switching overvoltage — reducing it to the residual voltage of the surge arrester — to less than 930 kV_PEAK (2.1 p.u.), the value used to design the air-gap clearances.

Electrical tower clearance also had to be considered. The electrical strength of the air gaps had to be coordinated so that line energization would be successful (i.e., a flashover risk of less than 6.2×10^-7). The clearance distance, or gap factor, was calculated following CIGRÉ Guide No. 72, which allows a 3.25-m (10.7-ft) clearance to tower and an 11-m minimum ground clearance. The minimum conductor blowout clearance at the edge of the right-of-way was determined under a wind pressure of 50 kg/sq m (10.2 lb/sq ft).

Design Optimization

The majority of EGAT's existing transmission line designs were ineffective and not economical. To optimize the design of the new 500-kV compact line, a reliability-based design concept was adopted. The cost-effective 500-kV compact line design is based on the criteria shown in the adjacent table.

The design constraints for the new line had safety clearances that were inadequate for conventional live-line maintenance; therefore, it was necessary to develop innovative training techniques to enable application of live-line maintenance.

Live-Line Techniques

EGAT studied live-line work and maintenance procedures to select the most effective techniques, paying special attention to safety and reliability of operation. The most essential conditions for live-line work are to minimize the risk of injury to those working the energized lines and to prevent flashovers between components at different potential.

EGAT reviewed four basic live-line techniques: the de-energized technique, insulated or hot glove technique, hot stick technique and bare-hand technique.

EGAT has long-term experience using the bare-hand technique for the live-line maintenance of conventional 500-kV lines. However, with the safe working distances being decreased on the compact line, the bare-hand technique had to be reconsidered.

This resulted in the bare-hand technique with insulated working platforms being selected for use on 500-kV compact lines. In practice, the safe working distances are 20% less than those prevalent on conventional 500-kV lines, therefore, switching impulse withstand voltage tests were performed.