Reaching the quality levels of energy supply found in major electrical utilities has been the recent goal of Cia Energética de Minas Gerias (CEMIG, Minas Gerais, Brazil), as it strives to meet customer and environmental needs. CEMIG has invested in system models to afford better coexistence between the electricity supply network and urban tree areas, and to improve the reliability of the network and the quality of supply to customers. CEMIG has fought drastic pruning, which leads to the mutilation of trees close to lines, through a bold initiative that allies environmental policies with investment in new system technologies throughout its supply area.

Conventional overhead lines — developed more than 50 years ago and characterized by bare conductors displaced horizontally over wooden crossarms on medium-voltage (MV) lines and vertically for low-voltage (LV) circuits — are now enveloped in areas of high tree density. Unprotected against environmental interference, the bare conductors on the conventional lines installed on CEMIG's distribution network develop many tree-related faults and require frequent tree pruning. Following the success of several pilot projects, CEMIG has standardized its urban supply systems by adopting MV spacer cable and LV aerial bunched conductors (ABC), which minimize urban tree management.

Table 1. Comparison of the System Average Interrupting Duration Index (SAIDI).
Type of MV/LV Overhead Line Conventional Bare
Conductors
Spacer-Cable System Insulated ABC Line
* SAIDI for all CEMIG Circuits 9.9 4.7 3.0
* SAIDI — Scheduled Interruptions 2.5 1.4 1.0
- Accidental Interruptions 5.2 1.1 0.4
- Total Interruptions 7.7 2.5 1.4
*SAIDI — All values are expressed in terms of the average number of hours without energy, per customer, per annum.

CEMIG's Pilot Projects

The solution CEMIG adopted to improve overhead line performance called for the development and application of the following standard configurations:

  • Medium-voltage compact lines. Use covered conductors, also named the spacer-cable system.

  • Low- and medium-voltage insulated overhead lines. Use aerial bundled cable, also known as the ABC system.

First, CEMIG constructed several pilot projects in the city of Belo Horizonte, the capital of Minas Gerais, as well as in the medium-sized Brazilian cities of Governador Val-adares, Divinopolis, Curvelo and Al-fenas. These projects were designed specifically to provide a comparison to techniques practiced by utilities in North America, Japan and Europe. To improve system-fault performance, CEMIG set out to examine the current practice using cost/benefit factors to establish criteria for project construction, operation and maintenance procedures. The pilot projects yielded excellent results, and in 1998, CEMIG changed its minimum urban-servicing standards to the MV spacer-cable and LV ABC systems.

Characteristics of 15-kV Spacer-Cable System

The 15-kV spacer-cable system uses a steel messenger cable to position the spacers, which are set at 8- to 10-m (26- to 33-ft) intervals between poles. In turn, the spacers support the conductors, holding them in a compact triangular shape in such a way as to apply all the mechanical strength to the messenger cable, leaving the conductors slightly taut. The main components of the system include:

  • Conductors: 50-mm2 and 150-mm2 (0.08-in2 and 0.23-in2) cables with a cross-linked polyethylene (XLPE) sheath that has a radial thickness of 3 mm (0.12 inches). Although the conductors are protected with an XLPE sheath that has a thermal rating of 90°C (194°F), they cannot be classified as “electrically insulated.”

  • Messenger: HS 9.5-mm (0.38-inch) steel cable.

  • Spacers: Diamond-shaped, injection-molded, high-density polyethylene (HDPE). To secure the conductors and the messenger, either ring or preformed ties made of polymer compounds are used.

  • Pin-type insulators: HDPE insulators to provide the same electrical insulation coordination to the system in conjunction with the spacers.

  • Dead-end insulators: Comprising a fiberglass core with skirts made of silicone or a molded rubber (EPDM) compound to provide insulation at the anchoring points of the covered conductors.

  • Brackets: Specially designed to support the line and maintain compact configuration to the line.

Note that the specifications of the conductors and accessories consider requirements that define the resistance of their polymer materials on two basic phenomena, which can compromise the performance of the whole system:

  1. The premature aging produced by ultraviolet (UV) radiation (simulation in laboratory — 2000 hr. minimum).

  2. The composite degradation due to electrical tracking and erosion (evaluation in laboratory — 2.75 kV minimum).

It is important to identify the significant differences and advantages to conventional lines, in which the aluminum bare conductors have to operate at less than 70°C (158°F) due to the tension/sag applied to them (the creep phenomena). However, the spacer-cable system can use conductors with a smaller cross-sectional area operating up to 90°C (194°F), because all the mechanical strength is applied over the messenger. Consequently, the spacer-cable circuit line is lighter, yet it has the same power capability.

Compared to the overhead-line bare conductor system, the spacer cable is simple to install (Fig. 1). As a result of this system's compact conductor configuration, the construction of four circuits on the same pole is perfectly viable (Fig. 2).

The spacer cable should be treated as bare for personnel safety. However, with materials and its configuration, the XLPE cover provides sufficient dielectric strength to limit the likelihood of a short circuit in case of momentary contact between covered conductors, spacers and grounded objects, such as leaves and branches on trees, without interrupting the energy supply.

The use of spacer-cable 15-kV circuits has had a dramatic effect on the tree management program. Simpler servicing has replaced the drastic and often unnecessary pruning of trees, corresponding to the cleaning and removal of small branches in permanent contact with the lines. This pruning-control philosophy can reduce the volume of wood cut from trees on the circuit route by two-thirds (Fig. 2 inset).

In addition to spacer cable, CEMIG's design standards to further improve system reliability for 15-kV overhead circuits also included:

  • Zinc-oxide surge arresters to protect the line against lightning.

  • Gas-insulated switches positioned for line sectioning and circuit load transfers.

  • Self-protected transformers, equipped with internal MV fuses and LV three-phase circuit breakers.

Table 2. Relationship between line costs — extension of new circuits.
Type of Circuit Conventional
Cost per Pole (US$)
Spacer-Cable
Cost per Pole (US$)
ABC
Cost per Pole (US$)
Type of Urban Area With tree
close to
the line
No trees With tree
close to
the line
No trees With tree
close to
the line
No trees
Initial Investment $822 $943 $1428
Operational Costs $624 $340 $102 $44 $54 -
Current Value $1446 $1162 $1045 $987 $1482 $1428
Ratio 1.0 1.0 0.72 0.85 1.03 1.23

Characteristics of 15-kV Insulated Lines

The primary insulated overhead lines, or MV ABC lines, constitute an excellent alternative when used with spacer-cable systems. To satisfy environmental and safety requirements, together with the complexity of these installations, insulated overhead conductors offer an economic alternative for specific applications. This system format uses three-phase insulated shielded conductors twisted around the messenger cable. In addition, the connections are designed with “dead-break accessories,” which ensure the system is totally insulated.

The main components used for 15-kV ABC circuits include:

  • Conductor: Aluminum cables in sizes of 50 mm2, 120 mm2 and 185 m2 (0.08 inches2, 0.19 inches2 and 0.29 inches2), insulated with 4.5-mm-thick (0.18-inch-thick) XLPE and shielded with two 0.4-mm-thick (0.02-inch-thick) semiconductive layers to control the electrical field inside the cable, followed by metallic screening of 6 mm2 (0.01 inches2) of copper wires, which is jacketed with an outer layer of low-density polyethylene (LDPE).

  • Dead-break disconnectors: Molded rubber EPDM, used in all connections among conductors and equipment, with specific geometric shapes (elbows, straight and bushing wells) for each application (Fig. 3).

  • Terminations: Molded polymer compounds, providing the transition between insulated cables and incoming branch circuits built with bare or covered conductors. Figure 4 shows an example of MV and LV ABC lines constructed within a residential area. Many trees come in permanent contact with the conductors.

Table 3. A comparison showing the main advantages of the two systems.
MV Spacer Cable MV ABC
Lower initial investment Lower incidence of energy interruption
Greater versatility for pole assembly Less clearance and spacing required
Faster jobs on construction and maintenance
service with linemen
Greater safety against accidental contact
involving personnel or animals
Light servicing to remove branches and
leaves in permanent contact with the conductors
Pruning and clearing services practically
unnecessary

System Reliability Benefits

Since introducing spacer-cable and ABC systems, the overall performance of the MV network has improved, as confirmed by the “material failure index,” which is expressed as the number of failures per kilometer, per annum. The index for the alternative systems is as follows:

  • Conventional bare conductors to spacer cable = 10 to 1.

  • Conventional bare conductors to ABC = 20 to 1.

Comparing the accidental System Average Interrupting Duration Index (SAIDI) for each system also can improve performance. SAIDI is a system fault performance indicator expressed as the average number of hours without energy, per customer, per year.

Table 1 shows the comparable values for the different systems. It is evident that spacer cable and ABC have reduced the number of environmental faults (for example, lightning strikes, trees and pollution), which has reduced material failures. Moreover, scheduled maintenance outage times are much shorter.

Cost/Benefit Comparisons

To verify the cost-benefit ratio of the spacer-cable and ABC systems, CEMIG conducted several economic analyses. The utility considered the following variables for each analysis over a 25-year period, with a minimum interest rate of 10% per year (corresponding to a distribution system depreciation of 4%, plus a capital payment interest rate of 6% per year):

  • Initial investment: Material and project construction costs.

  • Corrective and preventative maintenance: Costs that correspond to tree pruning, line inspection and repair services.

  • Cost of suspended profit: Cost based on the amount of energy not supplied.

Table 2 shows the results of the economic comparison for the two systems (spacer and ABC). Although spacer-cable systems incur a 15% higher initial investment cost than conventional bare systems, they are economically viable because of the reduction in maintenance costs. The ABC system is less viable because of the higher initial investment of approximately 70%.

Even in situations where tree growth control is not required, spacer cable is the most feasible economic solution. Based on these values, payback for this system is realized in the third year.

Summary

The evolution of the consumer market ran parallel to social awareness of environmental factors, making it imperative for service industries to strive for excellence and aim for the standards found in developed countries. The adoption of more efficient overhead systems is fundamentally important in meeting these objectives, mainly in regards to the preservation of urban trees to comply with new environmental laws.

CEMIG believes spacer cable is the most economical and technical solution for all the urban primary circuits, a standard that applies to new circuits or the refurbishment of existing lines. Rebuilds can be performed irrespective of the proximity of trees along the overhead line route. CEMIG has been implementing this policy intensively over the past few years, using spacer-cable systems as the new minimum standard to supply MV energy throughout its distribution market. Already, the utility has progressively constructed some 1200 km (746 miles) of primary circuit to replace the existing bare conductor overhead line systems. Minimizing tree-pruning services can reduce faults and supply interruptions and increase personnel safety. These are just some of the benefits these lines have supplied.

Similarly, MV ABC systems, which account for more than 120 km (75 miles), will continue to be installed where spacer-cable systems are not a good application. Such situations might include:

  • Areas with dense tree growth that offer no possibility of small pruning or minimal removal of branches.

  • On congested poles that could create operational hazards.

  • On systems installed near buildings, bridges and viaducts.

Naturally, this transformation is not immediate because there are more than 28 km (17 miles) of primary circuits to rebuild. However, in accordance with its responsibilities for the distribution system, CEMIG has adopted new technical standards for its 15-kV and LV circuits that satisfy the company's environmental policies.

As CEMIG's distribution CEO Aloísio V. Novais says, “The process of changing philosophies about mutilating urban trees near the lines has taken off. Despite needing time to consolidate itself completely, its reflexes can be perceived with each new project, planned according to our construction program with the municipal councils and city halls. This is already a source of pride for all the people served by CEMIG.”

Renato A.O. Bernis received the BSEE degree from Pontiff Catholic University in 1978 and a postgraduate degree in economic engineering from the Dom Cabral Foundation in 1983, both in Belo Horizonte, Brazil. During 1978, he started his professional career at the consulting firm IESA Co., working with projects for Brazilian steel and copper mill plants. Bernis joined CEMIG in 1983 and has been a senior distribution research and design engineer since 1990. He is the coordinator for the development of spacer-cable systems, insulated overhead lines and underground cable networks. His career has included trips for congressional purposes to the United States and Europe.