The interconnected Uruguayan — Argentinean transmission system consists of 500-kV lines and substations. Of those, UTE Uruguay has two substations, Montevideo B and Palmar. The Comision Técnica Mixta de Salto Grande (CTMSG) has the San Javier and Salto Grande Uruguay substations, both located in Uruguay. The Uruguayan government owns UTE and 50% of CTMSG, with the Argentinean government owning the other half of CTMSG.

The equipment in these four substations was built in 1979 (by the same constructor) as part of a major hydroelectric dam project, the “Salto Grande.” Since then, the similar equipment has failed at the same time in each of the four substations. Porcelain contamination that adversely affects the insulation characteristics of 500-kV line switches (LSs) was the main reason for the failures. To solve this problem, the engineering teams of both utilities jointly developed an innovative live-line procedure to remove and replace the 500-kV LSs.

The support insulators suffered damage from the ingress of the paint used to coat the cement porcelain sheds of the support insulators. Laboratory tests confirmed a reduction in the insulation level, and although a number of methods to remove the paint were tried — including the use of pressurized water and metal brushes — they were unsuccessful. An effective method for decontaminating the insulators was developed for use in the maintenance depot; therefore, it was necessary to disconnect and remove the insulators from site. This action also provided an opportunity to undertake essential maintenance on the LS's moving parts. As this work was linked to a program to maintain all the LSs in Salto Grande's transmission system substations, a bypass was designed to bridge a single phase of each 500-kV LS, thus allowing the LS phase to open prior to removal.

The Bypass Design

The 500-kV, three-phase section line switches that were subject to the live-line procedure had these basic characteristics:

• Trademark: EGIC
• Type: Semi-pantograph
• Model: OH (horizontal)
• Nominal current: 2000 A
• Space between contacts: 5.47 m (17.87 ft)
• Weight of each phase: 1.350 kg (2970 lb).

The bypass device had to be able to carry the same current as that of the LS being replaced. Therefore, it was agreed that the design should have a maximum nominal current rating of 2000 A and a short-circuit rating of 25 kA for 80 msec.

After several attempts, it was decided that the most appropriate bypass support would comprise a central rigid section with flexible conductors at each end. With this design, the bypass would hang from the rigid section in order to improve the versatility for the connections by means of very flexible conductors. The bypass had a 7-m (22.9-ft)-long central rigid section made of an electric-grade aluminum tube with an 8-mm wall. On the ends, clamps were bolted and two copper multifilament conductors 600 mm² (0.93 in²) were attached to the clamps on each end. The conductors were 1.5 m (4.9 ft) long and flexible enough to connect the bypass to the relevant points.

Specialized Access Equipment

1. Self-Elevator

   Site and personnel safety considerations coupled with the electric clearances and equipment weight were the main reasons why it proved impossible to employ a crane between the energized conductors to remove each LS component. Therefore, it was necessary to adapt the self-elevator, the utilities' other elevating device, because it had reduced dimensions and a suitable operating mode for this procedure. The self-elevator was equipped with an insulated boom that was anchored to the elevator by means of a base and crosspiece to provide a hinge joint. The anchorage and angle positions were achieved using poles and chain blocks. The adaptation was successful, as it proved possible to operate the elevator in a very reduced area with a wide range of distances. Also, two metallic devices were designed to fit the insulated boom, one was used to support the insulator and the other to take the LS moving contact. It was possible to perform these functions by interchanging these two devices.

2. Scaffolding

   To place the operators to circuit potential, several possibilities were examined. The utilities had to use modular-insulated scaffolding because of a limited amount of space available due to the close proximity of other equipment. The scaffolding towers had to be small and stable enough to comfortably accommodate two operators at a height of some 9 m (29.4 ft), and they had to comply with ASTM and IEC standards. The utilities chose Brazilian Ritz scaffolding because it has a 1-m (3.1-ft)-wide design with wheels and rails for movement, a guy system for stability, and dielectric and mechanical characteristics that meet the IEC 855 and ASTM F-711 standards.

3. Equipment to access the operators to the 500-kV busbars

   The 500-kV bus bars some 20 m (65 ft) aboveground level consisted of two aluminum conductors each with a section of 1200 mm² (1.86 in²). To manually position the bypass on these conductors, the utilities had to provide a means of access for an operator at bus bar potential. Two possible solutions were considered, the first being a cable car to go to the area. The second was a Grove RT 518 crane with an insulated arm, which allows for the transportation of an operator in a cradle at the end of the insulated arm. The latter solution was selected because it was simpler and faster, and furthermore, it could be used in places where crane access was impossible.

4. Scaffolding assembly

   Two dielectric modular scaffolding units were assembled on moving platforms and precisely located so as to give access to each of the two support insulators. Once the assembly was complete, the scaffolding was moved so that it made contact with the energized part of the insulator. The leakage current was measured, and when the current was below 30 µA, the operators were placed at bus bar potential on each scaffolding unit.

The Bypass Setup and Connection

The bus bars positioned above the LS were used to support the bypass. To make the connection, an operator was placed at bus bar potential using the Grove RT 518 Crane's insulated arm. The operator fastened two 3.6-m (11.8-ft)-long, 500-kV polymeric insulators to the bus bars, placing a snatch block at this position. Poly-Dacron ropes were fastened to the snatch blocks, which were used to hoist the bypass. These ropes were bound to the bypass by means of another set of polymeric insulators. Lateral ropes also were used to control the position of the bypass during the hoist operation.

The bypass was located approximately 1 m (3.1 ft) above the top of the LS and then moved sideways to allow for the free opening of the LS. Once located as required, the ropes were fastened to ensure the correct positioning of the bypass at all times. All the ropes used were clean, having been subject to dielectric testing prior to use, but the inserted polymeric insulators prevented any danger from discharges.

To complete the connection, two operators located on their scaffolding were put to bypass potential. After that, the flexible conductors on the end were separated and connected to the LS branches. It was then possible to open the LS diverting the circulating current through the bypass connection.

Removal of the Section Line Switch

Once the LS was open, the operator could start dismantling the fixed contact using the self-elevator. An insulating mast was mounted on the self-elevator to hoist the insulator and the fixed contact on its end. This compact equipment was convenient to operate, as it required little space operating vertically. Once the fixed contact was taken apart, the support insulator was separated. Afterward, the new support insulator was installed, and finally, the installation of new fixed contact completed the task in this area.

Next, a device to dismount the LS's moving contact was installed on the insulating mast end of the LS. This device consisted of a U-hook that, together with a bolted clamp installed on the LS, helped in dismounting the moving contact. The bolted clamp was positioned at the center of gravity of the moving contact that hung from the self-elevator used in the dismantling operation.

The sequence of work was undertaken in nine stages:

1. The operators at bus bar potential in the moving contact area placed the clamp in its precise position.

2. The self-elevator adjusted the insulated boom to hang the moving contact.

3. The operators at bus bar potential separated the moving contact.

4. The moving contact was taken to the ground.

5. The device on the self-elevator insulated boom was replaced with a device that allows for the removal of the support insulator from the moving contact.

6. The support insulator of the moving contact was changed.

7. Again, the device was located on the self-elevator to move the moving contact.

8. The new moving contact was installed.

9. The new LS was opened and closed repeatedly to ensure proper functioning.

Reconnection of the Section Insulator

The operators on the insulated scaffolding reconnected the phase of the LS that was in the open position. The LS was then closed; the bypass was removed and taken to the ground, an operation that completed the task. A single phase of a 500-kV LS could indeed be replaced.

Field Experience of the Live Bus Bar Procedure

In December 2003, while attempting to operate a 500-kV LS — similar to the one for which the live bus bar procedure was developed, except this time it was installed for vertical operation — leakage current was recorded in one of the phases, preventing the opening of the LS. Hence, CTMSG had to develop a revised live bus bar procedure and the LS was replaced with the substation energized. This action prevented a major part of the substation from being out of service for a period of some 10 hours at a time coincident with maximum generation. In this instance, the live bus bar procedure avoided important economic losses. CTMSG and UTE both have 10 staff trained for live-line procedures in 500-kV substations and on 500-kV lines. Because the procedures are common, the teams work together coordinating their substation maintenance work and sharing equipment. In addition to 20 years experience of 500-kV live-line maintenance, during the past five years, UTE has completed 350 live-line operations at 500-kV and 150-kV substations and successfully replaced a complete substation without the need for a circuit outage.

Summary

This procedure will allow UTE and CTMSG to service and maintain a large quantity of strategically positioned 500-kV LSs without having any limitations at minimum investment cost. This is a significant development and achievement since the replacement time for in-service or out-of-service installations is considerable. Additionally, being able to work on-plant within an energized substation does not involve the additional costs of purchasing energy from an alternative source of generation.

Uruguay and Argentina are energy dependent on the Salto Grande hydro plant (14 135-MW units) and under normal conditions with eight generators in operation; the plant supplies 500 MW to Uruguay. In the event of an LS fault or support insulator failure, the time taken to replace or repair the faulted component is five hours, with an energy cost of US$2000/MWh. The cost of the failure is $5 million. Alternatively, if the utilities arrange a circuit outage to maintain and repair a LS and change the insulator supports, the work takes some 10 hours per phase. Assuming the difference in cost between hydro and thermal generation is $50/MWh, the cost of the 500-MW demand is increased by $250,000, excluding the daily cost of 20 linemen.

The estimated cost of LS live-line maintenance based on the cost of using 20 linemen and on hardware amortization is less than $3000.

It should be noted that the development of this procedure continues to be built on for the replacement of low-level bus bar support insulators in substations with the same voltage. As a consequence, the utilities anticipate that with this new development they will be able to extend these satisfactory results to the majority of similar equipment installed in EHV substations.

Guillermo R. Lockhart is the manager of UTE's transmission lines department of engineering. He has been a senior engineer in the transmission line engineering and project management department in Salto Grande, where he has worked for 18 years. Formerly, he was a professor in the Republic's University of Uruguay, and as a member of the Engineering Association of Uruguay, he has lectured in the National Academy of Engineering and presented many technical articles at International Congresses. Lockhart has specialized in live-line work on EHV and HV transmission lines and substations, and has 25 years experience in the maintenance of transmission lines and substations. glockhart@ute.com.uy

Carlos E. Curbelo is a lines and cables specialist engineer for UTE, where he's worked since 1995 after having graduated with a degree in mechanical engineering from the Republic's University of Uruguay. He also has degrees in protection radiation and nuclear safety from the International Atomic Energy Agency and the University of Buenos Aires. Curbelo has presented several papers at national and international conferences, and for the past 10 years, he has specialized in live-line work in EHV and HV transmission lines and substations, including live-line OPGW stringing projects. ccurbelo@ute.com.uy

Luis L. Neira is a senior engineer and heads the department for transmission lines for the Comision Técnica Mixta de Salto Grande, where he has worked for 27 years. Neira also is a professor at the Technological National University and Regional Concordia, and serves on the Association Electrotécnica Argentina's Commission 21, Live-Line Work. Since 1985, Neira has specialized in work associated with EHV transmission lines and substations, and he has presented numerous papers to national and international conferences, receiving the first prize and special mentions on several occasions. neiral@saltogrande.org