CEMIG (Companhia Energetica de Minas Gerais) is a large power company in Minas Gerais in southeast Brazil. While its electrical system was reliable, the utility's management realized that it could be better. In the past, the average interruption duration in the supply of electricity had been 15 hours per customer/year, which was a rather high rate. One of the main factors contributing to this high rate was the construction standard, which used overhead lines and bare conductors. Another serious problem was the high number of accidents related to electric shock. To resolve these problems, CEMIG has adopted a spacer cable and aerial bundled cable (ABC) technology for its overhead lines, which has improved its reliability to near that of underground lines.
CEMIG serves 4,430,000 electric customers in a 560,000 km2 (218,000 sq miles) area and has more than 265,000 km (164,663 miles) of overhead distribution lines of which 48,000 km (29,826 miles) are in urban areas. It has 395,000 distribution transformers installed--130,000 of those are in urban lines.
Reliability Table 1 presents the average duration and frequency of the interruptions in CEMIG's generation, transmission and distribution system. Sixty percent of the interruptions are in the primary urban lines (13.8 kV).
The causes of the distribution accidental interruptions are below: Causes - % Accidental Duration Lightning and Rain 35 % Trees or Animals/Objects on Lines 25 % Material and Equipment Failures 20 % Others (10 % due to vandalism, collisions and overloads) 20 %
According to these causes, the major problems are lightning strikes and thunderstorms. Minas Gerais State is one of the most critical lightning regions in the world.
Another cause of interruptions is trees. Belo Horizonte, the capital of Minas Gerais, with 2,500,000 inhabitants, has 250,000 trees interfering with the overhead lines. Most of the line spans have two to three trees, and trimming is troublesome for various reasons, including environmental concerns. Brazilian trees have a high growth rate, and annual trimmings are necessary to unblock the circuits. Depending on the species of the tree, two trimmings are necessary per year.
As mentioned, one of the main reasons for the high failure rate is the line standard: overhead construction with bare aluminum conductors laid horizontally on wood crossarms in medium voltage (13.8 kV) and vertically in low voltage, (220/127 V). The average failure rate in CEMIG's standard urban line is 0.9 failure/km/year in medium voltage (MV) and 1.2 failure/km/year in low voltage (LV). Another negative feature of lines with bare conductors is the high rate of accidents involving electric shock of linemen and the general public.
The bare conductor line standard and associated materials are now technologically obsolete; therefore, aiming at improving the reliability of its distribution system, reducing operating costs, and increasing personnel security, CEMIG has researched new line technology, which would not demand great investments to be implemented and which would reach near underground line performance.
Pilot Projects CEMIG started the research to develop new distribution line technologies more than 15 years ago. The company has chosen the use of insulated and covered cables, in compact overhead lines, which has a high basic insulation level (BIL). The selected systems were: ABC (aerial bundled cable) and spacer cable distribution lines.
The first pilot project was an ABC line built in 1982 with 2.5 km (1.55 miles) of MV and 4.6 km (2.86 miles) of LV in Governador Valadares City (east of Minas Gerais). Five years later CEMIG built a second ABC line with 4.5 km (2.80 miles) of MV and 7 km (4.35 miles) of LV in Belo Horizonte City.
The first pilot project of spacer cable lines with covered conductors was built in 1990 with a length of 0.5 km (0.31 miles) in a residential condominium neighborhood of Belo Horizonte. Another five projects were built up to 1992 with a total length of 10.9 km (6.77 miles) of primary circuit.
These pilot projects were extremely useful to assess the operational performance, as well as to define the material and equipment specifications, standard assemblies, and methods and tools for construction.
All ABC and spacer cable lines were built in heavily treed streets, which presented trimming problems. The old lines presented frequent interruptions due to the tree contact with their bare conductors, as well as problems with lightning.
In the first two ABC lines, which were built 15 years ago, only four accidental outages occurred. Their causes were: incorrect installation of an elbow, vandalism, infiltration of water into a transformer and a falling tree on the line, which ruptured the pre-assembled cable. This represents an average rate of 0.05 failure/km/year in MV of the ABC line.
In the spacer cable lines, operating from five to six years, two accidental interruptions occurred: The rupture of two conductors, due to a falling object from a building, and the extended rubbing of a covered cable against a tree branch. These interruptions represent an average rate of 0.03 failure/km/year in MV of the six pilot projects of the spacer cable lines.
Other line constructions were developed, and in 1993, CEMIG analyzed the cost-benefit of these lines based on: (a) acquired experience in the line constructions, (b) performance, and (c) the exchange of information and visits to utilities and manufacturers in the United States.
Nowadays, one km (0.62 miles) of standard line constructed with concrete poles, primary and secondary circuits with bare conductors, three-phase transformers, protection and switching equipment, street lighting and meter costs about US$40,000 including contractors. Based on long-term analysis, CEMIG standardized on MV spacer cable and LV multiplex (pre-assembled) cable. The ABC line, in MV, would have a special applicability.
Materials The basic characteristics of the main materials for spacer cable and ABC lines are shown below.
Spacer Cable Materials, class 15 kV CEMIG has been using the specifications of the materials from Hendrix, Milford, New Hampshire, U.S., as reference. The exchange of information with Hendrix, visits to the company's factory, and the experience with its products were useful for CEMIG to elaborate the technical specifications of the cable, spacer, pin insulators and other accessories.
- Covered Cable - Compacted all aluminum conductor; XLPE or HDPE cover with 3.0 mm (0.12 inch) of thickness; tree proof; rated conductor temperature of 90 degrees C; tracking (min. 2.75 kV), abrasion and UV resistant; sizes 50 mm2-150 mm2 (0.08 sq inch-0.23 sq inch). - Spacer and Ties - HDPE; tracking and UV resistant; BIL 110 kV. The tie could be an EPDM/EPR rubber ring or polymeric preformed. Aiming at making the product available in Brazil, CEMIG participated in the development of spacers by PLP do Brasil and Pirelli Cabos S.A, Brazil. - Insulators - Pin type: HDPE; tracking and UV resistant: BIL 125 kV. Dead-end type: EPDM or silicone; UV resistant; BIL 140 kV. - Electrical Connections - Tap wedge connector is used. Due to the use of covered cables, all electrical connectors are covered (except in the stirrup bail where hot line clamps are installed for transformer or grounding connections).
In search of a cover which could allow contacts with trees, CEMIG established a partnership with AMP do Brasil to develop a polymeric cover for 15 kV, tracking, UV and voltage surge resistant, applicable to the AMPACT wedge connector.
The cable joint is made with an aluminum compression splice covered with a tube/sleeve of polymeric material for 15 kV, heat or cold shrinkable, tracking and UV resistant.
ABC Materials The materials used have characteristics similar to those of underground lines, but they are lighter and have a specific focus for installation in overhead lines.
- MV Pre-assembled Cable - Compacted all aluminum conductor; XLPE insulation with 4.5 mm (0.18 inch) of thickness; semiconducting shielding; metallic screening, with 6 mm2 (11.9 kcmil) of copper wire; PE jacket; class 8.7/15 kV; rated conductor temperature of 90 degrees C; messenger in HS steel or aluminum alloy cable; pre-assembly method: lashing type, with PVC/wire tape; sizes: 3x50mm2 +3/8 inch, 3x120mm2+3/8 inch, 3x185mm2+3/8 inch. (Nomenclature refers to three phases with conductor size 50 mm2 plus one messenger cable, size 3/8 inch dia). A partnership between CEMIG and Alcoa Aluminio S.A., Brazil, brought about the development of the latched cable in the Brazilian market. - MV Separable Connector - The same insulated disconnectors used for underground lines. Deadbreak type; 200 A and 600 A class; elbow, T connectors, three point joint. - MV Joint and Terminator -The phase splicing of the cable is made with a kit composed of an aluminum compression splice and polymeric heat or cold shrinkable sleeves, which re-establishes the shielding and insulation of the conductor. The termination kit for insulated cable is composed of an aluminum compression terminal and polymeric heat or cold shrinkable sleeves, or polymeric premolded skirts. - LV Pre-assembled or Multiplex Cable - Compacted all aluminum conductor; XLPE insulation; class 0.6/1 kV; rated conductor temperature of 90 degrees C; messenger/neutral in aluminum alloy cable; pre-assembly method: twisting type around the messenger; sizes: 3x35+35mm2, 3x70+70mm2, 3x120+70mm2. - LV Electrical Connections - The selection of connectors for LV multiplex cable was based on a detailed study that considered the performance and the simplicity of application as fundamental. - Piercing Connector for Secondary Line - It has toothed blades to pierce the cable insulation, torque limiter in the bolthead and an insulating covering for 1 kV. It is quickly installed and it is not necessary to remove the cable insulation to be used. CEMIG has obtained experience and developed specifications for this connector with technical support from SIMEL/AMP, Inc., France. - Little Wedge Connector for Service Entrance - A connector developed together with AMP do Brasil (UDC type) comprises an elastic compliant wedge and a C-member. It is easily installed with flat-nose pliers. It also has an insulating cover. In the last 10 years, CEMIG installed more than 8 million of this connector type which have shown excellent performance.
Equipment Special, more reliable, compact and easy to operate equipment for protecting and switching class 15 kV are applied to spacer cable and ABC lines. These include: - Complete Self Protected (CSP) Transformer - Three phase; MV fuse and LV circuit breaker inside; MV porcelain bushing for spacer cable, and epoxy bushing for disconnectors for ABC. CEMIG brought the CSP into use seven years ago through a partnership with Toshiba do Brasil, aimed at making the equipment available in the Brazilian market. - Poletop SF6 Switch - Three pole; SF6 gas insulation; types: 600 A, with porcelain or polymeric bushing for spacer cable and 200 A or 600 A, with epoxy bushing for ABC. These switches allow for future automation (SCADA system).
Surge Arrester - Medium Voltage -12 kV; 10 kA; polymeric housing; mounted in all transformers, switches and dead-end lines. Five years ago CEMIG began a partnership with Raychem do Brasil aimed at developing a specification for lightning arresters and performing all required tests from IEC and ANSI standards. Presently, CEMIG acquires high quality arresters. - Low Voltage - 280 V; 10 kA; polymeric housing (a new type); used in LV multiplex, in dead-end line and secondary side of transformer.
Present and Future Situation At the end of 1996 the construction costs of the spacer cable and ABC lines were reassessed, taking into account considerable decrease in prices for materials and equipment, which have been purchased in big lots through international bids. When CEMIG standardized the LV multiplex lines in 1994, its initial investment was 14% superior to the standard construction. Nowadays, this difference is less than 4%. Therefore, the company decided to use 100% of LV multiplex cable in the secondary circuits in all new and rebuilt lines. The initial investment in the spacer cable line, related to the standard construction, has also decreased to about 115 to 120%.
CEMIG plans to construct approximately 1200 km/year (745.8 miles). of the medium voltage lines over the next few years. To reduce initial investment, CEMIG tends to use only spacer cable in the primary circuits. For the secondary circuits, all lines are already LV multiplex. The yearly program is around 2500 km (1554 miles) of cables.
Many utilities in Brazil have been experimenting with these new line technologies and, considering the high level of cost/benefit, they tend to adopt these new line standards.
Conclusions The use of spacer cable and ABC systems, with covered and insulated conductors, brings several benefits to the utility and its customers. These systems have a high cost-effectiveness and reliability. The initial investment for these lines is a little higher than for standard construction with bare conductors, and the performance is close to that of underground lines.
The main benefits of spacer cable and ABC systems are: - High reduction of operating costs; the number of line interventions is lower and, consequently, the number of linemen can be reduced. - Increase in personnel safety; considerable reduction in the number of accidents with linemen and the public in general. - Improved reliability; considerable reduction in the interruption frequency and duration, per customer; - Preservation of trees. - Better relationship with city hall, environmental non-government organizations and with the population in general.
CEMIG was a pioneer in the development of these systems in Brazil and has gained much experience in the past 15 years. Many requests from Brazilian utilities have been received by CEMIG to transfer its accumulated knowledge of spacer cable and ABC system technology. The partnership between CEMIG and the materials and equipment manufacturers, along with the international bids of materials, have been important in improving materials and reducing cost.
Mauricio R. Soares is manager of the Distribution Lines Engineering Dept. at CEMIG, Brazil. He has the BSEE degree, from Minas Gerais Federal University. He did postgraduate studies in Power Systems and Distribution Engineering. Soares joined CEMIG in 1974 and has worked with distribution material, equipment and tool specifications, and with development of new line designs. He is a member of IEEE and has presented papers about distribution technologies at IEEE and CIRED conferences.