Transmission is the process of delivering electrical energy from one point to another, and it is one of the most important phases of energy transport from generation to consumers. These are robust projects, usually covering long distances and operating at high voltages.
What are transmission lines?
In Brazil, transmission lines cover long distances due to the location of hydroelectric plants, which are usually far from consumption centers. This extensive transmission network results in nearly total interconnection throughout the country's territory. The National Interconnected System (SIN), coordinated and controlled by the National Electric System Operator (ONS), is responsible for about 95% of energy supply.
They operate at voltage levels ranging from 138 kV to 750 kV, including values of 230 kV, 345 kV, 440 kV, and 500 kV, predominantly using alternating current. Additionally, there are levels of 600 kV to 800 kV transmitted in direct current, known as HVDC (high-voltage direct current).
To avoid the risk of electrical contact with vegetation, animals, or any other elements present in the region, components of these lines, such as towers and overhead cables, must be maintained at a safe distance from the ground. This measure depends on the characteristics of the installation site.
Furthermore, these structures must be designed to withstand adverse weather conditions such as heavy rains, strong winds, and lightning strikes. Failure of these systems can have significant economic and humanitarian impacts, as energy transmission is responsible for supplying electricity to cities and even entire states.
The main components of a transmission line are:
Transmission Towers: They support the conductor cables of the transmission line at an appropriate height.
Conductor Cables: These are the conductors responsible for carrying electric current along the transmission line.
Insulators: They electrically isolate the conductor cables from the transmission towers.
Connectors and Terminations: They connect the conductor cables to the towers and electrical equipment.
Protection Devices: They protect the transmission line against voltage surges, such as lightning arresters.
Accessories and Auxiliary Equipment: These include spacers, suspension insulators, control and monitoring devices, grounding cables, among others.
These components work together to ensure efficient and reliable transmission of electrical energy along the transmission line, from generation to consumption point.
The importance of insulators in a transmission line
Insulators have the duty of supporting the cables on the tower, preventing the dissipation of energy through the structure. They represent a critical portion of energy transmission systems; failure of this system can result in losses in reliability, stability, and quality of energy, causing problems for consumers and utilities.
Due to environmental conditions, which can vary depending on climate, line location, pollution, and degree of material aging, dry bands may appear in the presence of humidity, causing surface material degradation and triggering partial discharges (PDs).
During operation, high-voltage insulators are subject to all types of pollution deposition on their surface. This pollution, when in the presence of moisture, can form a conductive layer on the insulator surface, causing an increase in leakage current. In coastal cities, for example, insulators are subjected to heavy salt deposition due to salt mist, and during months of little rain, this saline pollution is intensified as rain naturally washes the insulator surface.
Partial discharges and their effects
Partial discharges are small local electrical breakdowns in dielectric materials, whether solid, liquid, or gaseous, i.e., small breakdowns of temporary insulation. They can be external, internal, or superficial.
PDs occur in the presence of high voltage partially connecting two electrodes and can occur, for example, in oil insulator bubbles when internal, or even on external surfaces in angular projections or sharp points along the surface of the insulating material. Over time, partial discharges can lead to failures, short circuits, and even fires due to lack of tracking, faults, asset monitoring, and preventive maintenance.