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A 500-kV double-circuit compact transmission line solves Bangkok's right-of-way problem.
Rapid development in the Greater Bangkok area is leading to a substantial increase in the demand for energy. This expansion is particularly challenging for the Electricity Generating Authority of Thailand (EGAT). As land acquisition for additional substations and rights of way (R/W) for new transmission circuits is increasingly difficult and expensive, EGAT decided to upgrade the power-transfer capability of the existing 230-kV double-circuit lines by constructing new 500-kV double-circuit compact lines.
Bangkok's Existing 230-kV System
The existing transmission system for the Greater Bangkok area and EGAT's planned reinforcement requires upgrading four sections of the existing 230-kV transmission lines-a total length of 80 km (50 miles)-to 500 kV (Fig. 1).
The existing 230-kV lines, which have a 40-m (131-ft) R/W, are constructed on routes that pass through agricultural, industrial, commercial and residential areas. Most of EGAT's 500-kV double-circuit lines with normal design criteria were constructed in 60-m (197-ft) R/W.
In considering line-compaction techniques to erect the proposed 500-kV double-circuit line within the existing R/W, EGAT referred to designs from Bonneville Power Administration (BPA). The BPA compact design produced the desired objectives and improved electrical performance with lower costs. As a result of consultation with BPA, EGAT relaxed some design standards to make live-line work easier on the 500-kV compact line.
Corona and Field Effects Analysis
Reduction of the corona and field effect (electric and magnetic field [EMF]) levels at the edge of the RIW were two of the most important considerations. Table 1 shows EGAT's permissible limits for the parameters of the R/W for the new 500-kV compactline design.
These field effects were evaluated for the four compact tower configurations studied using a circuit-phase current of 4000 A; 4 x 1272 kcmil ACSR bundled conductors (457 mm [0.73 in] spacing); and a minimum ground clearance of 11 m (36 ft).
Alternative Tower Designs
EGAT initially studied the four different compact tower configurations, all with V-insulator strings and a preliminary conductor clearance of 3.5 in (11.5 ft) to tower and 11 in (36 ft) to ground (Fig. 2).
The conventional steel-lattice tower (Fig. 2a) was the base case. The single steel-pole tower in Fig. 2b offered compaction; however, the portal steel tower (Fig. 2c) offered greater...