Tree-related outages are the major cause of system interruptions at Cemig Distribuição. Since 2004, the utility has been facing an increasing number of tree-related outages. In 2009, total tree-related outages numbered 2,250, and the average cost of tending to an event was $180 per outage. The main reason for the increase in outage numbers — tree related or not — is the decrease in maintenance budget, a situation similar to that of other utilities in Brazil and around the world.
Cemig Distribuição operates a distribution network of 360,000 km (223,694 miles) of rural lines and 89,500 km (55,613 miles) of urban lines in the state of Minas Gerais in southeast Brazil. The utility's major market is the metropolitan area of Belo Horizonte, the state capital, where more than 40% of the energy distribution is concentrated in a 20,000-km (12,427-mile) grid of 13.8 kV.
Implementing an IVM Program
Due to such a high number of interruptions, their high cost, the unacceptable dissatisfaction of customers and the potential penalties by federal regulatory authorities, Cemig launched an integrated vegetation management (IVM) program in 2009 — named Premiar, which stands for special integrated program of vegetation and lines management — to reduce the level of outages to acceptable figures and guarantee operational safety of the system in the Belo Horizonte region of its service area.
The program strives to keep the community involved in its actions; therefore, Cemig conducted opinion surveys with residents of Belo Horizonte and utility employees to determine their perception about the subject that covers urban forest and energy networks. The utility held meetings and workshops to present the IVM program to community leaders, professionals from environmental agencies, representatives from non-governmental organizations, members of professional associations, the public and the utility's employees.
In most of these meetings, it was clear Belo Horizonte lacks true leadership in how arboriculture must be carried out. Above all, there was a strong perception that tree-related problems — not only electrical outages — were caused by misuse of inadequate tree species, poor planting techniques and a lack of steady tree-maintenance programs. It also was reported by almost all present at the public meetings that a tree inventory is a primary issue in the city.
Diagnosis of Urban Forest
With more than 400,000 public trees in the city, most of them on sidewalks since the first half of the 20th century, it is urgent and of great relevance to perform a tree inventory and hazard-risk assessment on a professional basis, using technologies to allow further guidance for the care of these assets. For this purpose, the municipal administration and utility signed an agreement to bid the city's global tree-inventory project, encompassing public trees located on sidewalks and trees on private property that were high enough to reach the utility's line should it fail.
Cemig estimates it will take approximately 18 months to complete the inventory, which is scheduled to begin in mid-2011. In the meantime, the utility hired a five-year tree-management service whose tasks include the risk assessment of trees close to lines. Data from the risk assessment will be fed to a common database being created in partnership with the municipality.
This is surely the greatest gain of the whole program: Having full-time professionals working side by side with contractors and the municipality resulted in huge improvements in the quality of utility pruning, public relations and the quality of Cemig's service. Up to now, about 9,000 trees have been assessed by the arborist's crews, and, among them, more than 700 have been found to pose a serious threat to the system and have been replaced either by trees friendly to the system or by tall tree species planted away from the wires.
Some best practices introduced by the program are the use of high-quality trees and large holes (by Brazilian standards) for new plantings. Going beyond line clearance, it is the utility's intention to show stakeholders that trees are important and need care, and that good arboricultural practices (especially the right tree in the right place) carried out by good professionals will result in an improved quality of life for the whole community, increase tree canopy and improve the utility's electricity supply service.
Another gain in quality of electricity supply is related to clearance distances between trees and wires. What used to be a very short distance — 3.5 ft (1.1 m) with a lot of overhanging — has been increased to a more comfortable distance where trees are pruned back to 5 ft (1.5 m) from the wires and pruned so that no crown grows above bare wires.
This clearance may seem very slight when compared to clearance distances practiced in North America. Due to the lack of pruning standards (like ANSI A300 or Z133.1) or even enforceable ordinances, Cemig referred to available Brazilian official practice, the Ministry of Labor Regulatory Norm No. 10 “Safety Working Practices in Electrical Facilities,” which establishes a “controlled approach zone” where only authorized personnel, with the use of appropriate hot line work practices, may work. With this reference, it was easier for Cemig to discuss increases in clearance distances.
No doubt clearance distance was a determinant factor in reducing tree-related outages, despite some complaints from part of the population that was used to seeing trees growing very close to the wires. With a little help from the utility's friends, among them Derek Vannice, executive director of the Utility Arborist Association at that time, Cemig managed to calm down most of the complaints and make the public recognize that having trees encroaching on the wires is not the best solution for the urban environmental problems the community faces.
Another investment Cemig made in its IVM program was replacing conventional bare wires with coated wires (Hendrix Wire & Cable) and insulated wires (triplex wires) where tree density is more relevant for quality of life in the city. When practicing such small clearances, as expressed before, the risk of outages remains high where wires do not tolerate contact with grounded infrastructure. As the arborist crews accomplish their work of tree assessment, information about tree density and health, canopy spread and site suitability for large tree growth is collected and used to determine the use of electrical engineering solutions. After all, Cemig needs to acknowledge that, in some situations, trees are much more important than the utility's facilities and, because of the high cost of underground wires, Cemig chose to apply these alternatives.
In 2010, 61 projects were concluded, in which 52 km (32.3 miles) of lines were replaced in Belo Horizonte. For 2011, the target is to replace 183 km (114 miles) of bare wires with coated wires.
Geoárvores are the activities related to the manipulation, organization and creation of a geographic database in the form of a geographical information system (GIS) for Belo Horizonte, containing information about trees, electric power, road systems and administrative boundaries. The tree database of the GIS contains a large amount of tree information for each individual tree plus the risk assessment. In the near future, it will disclose information on pruning needs and other interferences carried out by any of the database users — municipality or utility.
It is an old vision of Cemig to bid on tree maintenance for the city with the municipality. By this model, contractors would be in charge of pruning trees wherever they are placed; both Cemig and the municipality would have their needs tended to and would share proportionally the costs of the work. Several legal and administrative barriers need to be overcome to make this vision a reality, but the co-work that will take place for the tree inventory is a step in the right direction.
Planting includes the replacement of hazard trees for new trees with higher-quality planting and maintenance techniques. Residents in the neighborhoods of planting sites are consulted in advance about the locations of trees to be planted and to participate in the process of planting. The use of better arboricultural practices and community involvement have contributed to lowering the number of trees lost by vandalism from the usual 80% to about 25%.
Down the Road
Other initiatives are on the way, and some of them are worth reporting. It may seem a little behind the times, but Cemig intends to implement the establishment of pruning cycles, which is tremendous news in Brazil and has great potential for lowering costs and improving quality.
Another one is the use of chippers in routine operations. These machines are rarely used in Brazil by commercial, municipal or utility arborists and contractors. Cemig is now working with Vermeer and a national manufacturer to assess the cost-benefit relationships and develop operational procedures for the use of the equipment.
With these actions and its IVM program, Cemig reinforces its commitment to carry out actions to improve the quality of electricity supply and reiterates its concern and respect for the environment, dialogue with society and sharing with its consumers, and solutions to the efficiency of the utility's service, reliability of its system and improvement of the urban landscape.
Pedro Mendes Castro (firstname.lastname@example.org) is technical officer for vegetation management at Cemig Distribuição S.A. He holds a degree in agronomy from the Universidade Federal de Viçosa and is an International Society of Arboriculture Certified Arborist/Utility Specialist.
Gleiston Bianch Andrade (email@example.com) is an electrical engineer for Cemig Distribuição S.A. He holds a degree in electrical engineering from the University Vale do Rio Doce, Minas Gerais, Brazil.
Wagner Braga Filho (firstname.lastname@example.org) is technician of distribution services and supervisor of vegetation management processes at Cemig Distribuição S.A.