Compact 500-kV Line Designed for Live Work

Dec. 1, 2010
Safely accommodating expected line maintenance is a critical design challenge for new compact 500-kV lines in Thailand.

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 kVPEAK (3.7 p.u. [1 p.u. = 450 kVPEAK]) 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 kVPEAK (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.

Switching Impulse Tests

Switching impulse withstand voltage tests were undertaken to ensure the safety of EGAT's live-line maintenance teams. The test results also were used to analyze and calculate the minimum-approach distance (MAD), check the feasibility of performing live-line work and select safer bare-hand methods. The switching impulse tests on the actual conductor-tower window configuration, as well as the actual tower crossarm and insulator string assembly, were performed at the NGK High Voltage Laboratory in Japan and at EGAT's high-voltage laboratory in Thailand.

The results indicated that the worst-case condition is the wet switching impulse voltage. Therefore, to ensure the safety of workers and minimize the risk of flashover, a switching impulse withstand voltage of 1028 kV (2.28 p.u.) is used for calculating the MAD. Based on IEC 71-1 and 71-2, CIGRÉ Publication 72 and IEEE 516-1995, the calculated MAD is 3.18 m (10.4 ft), which is less than the 3.25-m minimum phase-to-ground clearance of the 500-kV compact lines.

IEEE Standard 516-1995 recommends the MAD for ac energized work on 500-kV to 550-kV systems should not be less than 3.42 m (11.22 ft), which would accommodate an overvoltage condition of 2.4 p.u. Since EGAT's live-line teams did not accept live-line working with a MAD of 4-m (13.1-ft) using existing methods, special live-line methods had to be considered.

Work-Site Overvoltage Control

To determine how best to deliver a lineman to an energized circuit on the compact line, EGAT explored different methods, including an insulated aerial device, an insulated ladder, a cable cart and a helicopter. Based on the technical feasibility and economic aspects, EGAT decided the insulated ladder worked best for its needs.

To overcome the utility's concern for situations where the physical distances at the tower window of the 500-kV compact line failed to meet the required MAD, EGAT sought the use of additional devices to control overvoltage at the work site.

Work-site control ensures the physical and electrical safety of workers, with the important psychological advantage of visibility to workers. An appropriate device for this purpose was the portable protective air gap (PPAG), which is typically applied for the duration of work and is placed on a structure adjacent to the structure where work is being performed. A properly designed and coordinated PPAG offers several key benefits:

  • PPAG will prevent possible flashover at the work site.
  • The MAD can be reduced by the PPAG at the work site.
  • The insulated tools for 500-kV lines are bulky, heavy and uncontrollable, and PPAG can reduce these problems.
  • PPAG is beneficial at all voltages, especially for the compact line design, where normal operating phase-to-phase and phase-to-ground clearances have been reduced.

The PPAG development program for the live work on EGAT's compact 500-kV line included two important steps. First, EGAT staff visited the United States to meet with several electric utilities and attend a seminar with a PPAG demonstration at the Electric Power Research Institute/EPRI Solutions' High Voltage Transmission Center in Lenox, Massachusetts. Second, staff participated in the 2003 IEEE ESMO International Conference on Transmission and Distribution Construction, Operation and Live-Line Maintenance in Orlando, Florida. This visit allowed EGAT personnel to witness an actual real-time installation of a PPAG on an energized 230-kV structure.

Subsequently, the PPAG development involved tests at EGAT's high-voltage laboratory, including a full-scale mockup of a compact suspension 500-kV structure assembled for testing and for demonstrations to EGAT's crews.

Safety Delivered

By using a compact design, EGAT achieved its objective of upgrading a transmission line within the existing rights-of-way. The reduced safety clearances of the 500-kV compact line demanded the development of innovative live-line maintenance techniques.

EGAT's long-term experience using live-line maintenance teams led to the development of special bare-hand techniques for the 500-kV compact line for use on circuits that could no longer be de-energized for maintenance work. However, in these situations, the insulated-ladder bare-hand method — including the use of PPAG — was recommended to control overvoltages at the work site. Special work-site overvoltage-control strategies had to be developed, and EGAT worked with EPRI Solutions to develop a 500-kV PPAG. The collaboration also included basic training for testing the PPAG at EGAT's laboratory and demonstrations of PPAG operation, including work-site overvoltage-control strategies.

EGAT is now planning the construction of a 500-kV live-line training facility to perform research and development in the field of live-line bare-hand methods.

Design Criteria for 500-kV Compact LinesParametersDesign values Design ruling span length 430 m (1411 ft) Right-of-way width 40 m (131 ft) Maximum operating voltage 550 kV Design switching overvoltage 930 kV (2.1 p.u.) Electrical clearance:
Conductor to tower
Conductor to ground
Phase to phase 3.25 m (10.7 ft)
16.0 m (52.5 ft)
9.5 m (31.17 ft) Phase conductor 4 × 1272-kcmil ACSR Shield wire 9.5 mm (0.375 inches) GSW and OPGW Insulator V-strings with 2 × 27 discs Maximum operating temperature 75°C (167°F) Conductor tension limits 26% of RTS at 27°C (80°F) Shield wire tension limits 85% of phase conductor Wind speed 28.5 m/sec (64 mph) Tower type Lattice-steel tower Foundation type Deep drive piles (majority) Test Lab Results From Two LabsTest condition, laboratory locationFlashover voltage (kV)Withstand voltage (kV)No. flash and position12345 Positive (dry) Thailand 1236 1139 4 1 6 4 1 Negative (dry) Thailand 1699 1699 Withstood Positive (wet) Thailand 1227 1131 3 3 5 — 2 Positive (wet) Japan 1111 1028 3 6 6 4 1 Negative (wet) Thailand 1670 1540 — — 15 — — Negative (wet) Japan 1480 1492 Withstood

Kitti Petchsanthad ([email protected]) earned his BSEE degree from KMITL (Thailand) in 1986 and his master's degree in industrial engineering and management from the School of Advanced Technology (Thailand) in 1993. Petchsanthad joined the Electricity Generating Authority of Thailand in 1987 and has been involved in the design, construction technology, and operations and maintenance of high-voltage/extra-high-voltage transmission line systems up to 500 kV. Currently, he is chief of transmission line technology in the aviation department, Transmission System Maintenance division, Transmission System group.

Companies mentioned:

Electric Power Research Institute

Electricity Generating Authority of Thailand

NGK High Voltage Laboratory

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