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.

Table 1. EGAT's permissible limits for corona and EMF.

Parameter	Maximum in R/W	Edge of R/W
Electric field	15 kV/m	2 kV/m
Magnetic field	-	15 mT
Audible noise	-	55 dB (A)
Radio interference	-	40 dB

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 R/W 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 compact-line design.

These field effects were evaluated for the four compact tower configurations studied using a circuit-phase current of 4000 A; 4 × 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 m (11.5 ft) to tower and 11 m (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 compaction, as all phase conductors are inside the tower window. Although the second portal tower (Fig. 2d) is lower in height and, therefore, its visual impact at a distance is reduced, EGAT did not pursue this configuration because of the inability to maintain one circuit with the second circuit energized.

To obtain an indication of the acceptable corona and field effects levels along the existing corridors, EGAT included towers from three existing conventional steel-lattice towers supporting 230-kV double-circuit lines in its Bangkok analysis:

• 230-kV line with twin 1272 kcmil ACSR 42/7 conductor, ground clearance of 7.5 m (25 ft)

• 230-kV line with single 1272 kcmil ACSR 42/7 conductor, ground clearance of 7.5 m (25 ft)

• 500-kV line with quad 795 kcmil ACSR 54/7 “Condor” conductor, ground clearance of 11 m (36 ft).

According to the comparative analysis, the electric fields for the novel designs are within the specified value, and the magnetic fields are significantly reduced for the more compact single pole and portal towers (Figs. 2b and 2c). For the same phase current, the level of EMFs are comparable to those produced by existing designs.

500-kV Line — Electrical Studies

The electrical performance of the new 500-kV compact lines in the restricted R/W under switching and lightning conditions were thoroughly analyzed using BPA's Electromagnetic Transients Analysis Program (EMTP) and EPRI's lightning-analysis program, “Multiflash.”

As a result of the switching overvoltage studies, surge arresters having a minimum-rated voltage of 420 kV (continuous operating voltage 336 kV) were selected for installation at the line terminals to limit switching overvoltages. Thus, the air gap clearances of the compact tower geometry were designed based on the magnitude of the switching overvoltage.

The EMTP studies confirmed that the highest voltages — and also the worst distribution of switching overvoltages for the 500-kV compact line — occurred during high-speed reclosing operations. Furthermore, the distribution of overvoltages was found to differ greatly from that of a non-compact conventional line. The maximum switching overvoltage during re-energization was used as the basis for the design of the insulation level of insulator string plus the conductor-to-tower clearance.

The conductor-to-tower clearance was calculated using CIGRÉ Guide No. 72, allowing for any differences between the average atmospheric conditions in the Bangkok area and the standard conditions indicated in IEC 60. Under the above conditions, a clearance distance to the tower of 3.25 m (11 ft) was found to be appropriate for slow front re-energization overvoltages.

A minimum ground clearance of 11 m (36 ft) was used in the preliminary analysis to determine the minimum tower height; however, a ground clearance of 16 m (52 ft) was recommended for tower spotting of this line. Based on the limited R/W, the minimum conductor blowout clearance to the edge of R/W was determined using a wind pressure of 50 kg/sq. m (approximately 10-year return period wind).

The Multiflash program attributed shielding failures and back flashover to lightning. An isokeuraunic level of 100 thunderstorm days per year was used to evaluate the lightning performance. Results confirmed that the shielding wires should be positioned at an angle of zero degrees. Using a specified tower-footing resistance of less than 7 ohms and an acceptable lightning performance of less than one circuit outage per 100 km (62 miles) per year is predicted.

Design of the 500-kV Line

For these 500-kV lines, data from six weather stations reasonably close to the route of the line, each with 40 years of wind records, were computed. Worst-case projections were based on the maximum wind speed recorded each year at any one of the six weather stations located in close proximity to the other line route, presuming that this maximum velocity would be experienced anywhere along the line. Finally, the result from calculations of a 50-year hourly mean wind speed of 28.5 m/s at 10 m (33 ft) was used for design of the 500-kV compact lines.

• Conductor Tension Limits. The conductor tension limits are mainly evaluated from the basis of span length, conductor sag and vibration/oscillation performances. The tension limits for the new 500-kV compact line were calculated based on the vibration/oscillation performance of the existing lines. As there is no ice load in the Greater Bangkok area and wind loads are moderate, the vibration/oscillation performance criteria govern the conductor tensions and sags. The tension over mass (T/m) concept was applied for transferring the good vibration/oscillation performance of the existing 230-kV lines to the new 500-kV compact-line design. The analysis resulted in EDT of 26% of RTS being specified for the new 500-kV compact-line design.

• Line Optimization. The optimization of the 500-kV compact line was considerably influenced by the flatness of the terrain in the Bangkok area, the soil conditions requiring expensive foundations and the narrow corridor with restricted conductor blowout. Short spans with more towers per kilometer is expensive because of the cost of foundations and civil works at each tower position. The use of T/m as the vibration-control parameter resulted in an optimum average span of about 430 m (1410 ft).

  Using optimized tower spotting, the final number of towers was reduced by four units. This resulted in a 4% reduction in the cost of materials and construction. The optimization study also showed that the use of I-string insulators resulted in increased conductor blowout thereby increasing the cost because of less efficient use of the tower span capacity. Thus, V-insulator strings were retained in the final design.

• Final Tower Selection. Tower alternatives shown in Figs. 2a, 2b and 2c were evaluated for the design loads and for a nominal span of 430 m (1410 ft). The installed costs of the single steel-pole tower and the portal tower were 28% and 17%, respectively, more expensive than the conventional steel-lattice towers. This depicts results that highlight the premium to be paid for line compaction. However, because of possible requirements of future road and building developments in the Greater Bangkok area, EGAT decided at this stage to increase the ground clearance from 11 to 16 m (36 to 52 ft). This extended the envelope regarding all the electrical criteria, so that the extreme compaction offered by the towers was no longer required.

Consideration of other factors, such as maintainability, lead to the conclusion that the conventional steel-lattice tower was the most cost-effective compact configuration.

500-kV Compact-Line Performance

The design of a cost-effective 500-kV compact line resulted in the following clearances:

• Conductor-to-tower clearance: 3.25 m (11 ft)

• Conductor-to-ground clearance: 16 m (52 ft)

• Phase-to-phase clearance: 9.5 m (31.2 ft).

  The final design resulted in the following materials being specified:

• Phase conductor: Quad 1272 kcmil ACSR/GA, Bundle spacing 457 mm (18 inches)

• Shield wire: ⅜-inch GSW, 70 sq mm OPGW

• Insulator: V-Suspension strings with 2×27 discs.

  The performance of the 500-kV compact lines with towers for a ground clearance of 16 m (52 ft) is as follows:

• Power-frequency electric fields at 1 m (3 ft) above ground level under normal operating conditions of less than 3.6 kV/m within the R/W and 1.7 kV/m at the edge of the R/W.

• Power-frequency magnetic field at 1 m (3 ft) above ground level, with a balanced load of 4 kA per phase, less than 26mT within the R/W, 11mT at the edge of the R/W.

• The predicted L₅₀ in foul weather for the 500-kV compact-line designs is 43-dB (A) at the edge of the R/W.

• With an isokeuraunic level of 100 thunderstorm days per year, the lightning performance of the 500-kV compact line is less than one trip out per 100 km per year based on tower footing resistance 7 ohms.

• The electrical and mechanical performance of the line components, such as corona, radio influence voltage and switching impulse tests for the insulator string and hardware assemblies, successfully passed tests performed at NGK Laboratory Test Station in Japan. The compact towers successfully passed the loading tests at ABB SAE Test Station in Italy.

• The temperature of the conductor is not to exceed 75°C (167°F).

Summarizing Success

EGAT has successfully increased the power-transfer capability by constructing the 500-kV compact lines in the narrow corridor of the existing 230-kV lines. This avoided the problem and cost of acquiring additional land for the 500-kV line that would have cost almost US$1 million per mile in Bangkok. By employing innovative technology and local labor resources, this 500-kV line is both cost effective and environmentally acceptable.

Acknowledgments

The author wishes to thank EGAT and NGC for the permission to transfer the innovative technical experience in the 500-kV compact-line design and wishes to acknowledge H. B. White and NGC's Technology & Science Team for their technical training and technology transfer.

Kitti Petchsanthad received the bachelor's degree in electrical engineering from King Mongkut's Institute of Technology, Thailand, in 1986 and the master's degree in IE&M, School of Advanced Technologies from Asian Institute of Technology in Thailand in 1993. Petchsanthad joined EGAT in 1987 and has been involved in all the design aspects of HV/EHV transmission systems and is currently assistant manager of the 500-kV transmission line project department.