The interconnected lines which facilitate this movement are known as a transmission network. Underground transmission is sometimes used in urban areas or environmentally sensitive locations. A lack of electrical energy storage facilities in transmission systems leads to a key limitation. Electrical energy must be generated at the same rate at which it generation transmission and distribution pdf consumed.
The conductor consists of seven strands of steel surrounded by four layers of aluminium. High-voltage overhead conductors are not covered by insulation. Copper was sometimes used for overhead transmission, but aluminum is lighter, yields only marginally reduced performance and costs much less. Overhead conductors are a commodity supplied by several companies worldwide.
Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Today, transmission-level voltages are usually considered to be 110 kV and above. Since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed to maintain safety. Adverse weather conditions, such as high wind and low temperatures, can lead to power outages. Underground cables take up less right-of-way than overhead lines, have lower visibility, and are less affected by bad weather.
However, costs of insulated cable and excavation are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair. Underground lines are strictly limited by their thermal capacity, which permits less overload or re-rating than overhead lines. DC cables are not limited in length by their capacitance.
New York City streets in 1890. In the early days of commercial electric power, transmission of electric power at the same voltage as used by lighting and mechanical loads restricted the distance between generating plant and consumers. Due to this specialization of lines and because transmission was inefficient for low-voltage high-current circuits, generators needed to be near their loads. 1:1 turn ratio and open magnetic circuit, in 1881. Gaulard secondary generators with their primary windings connected in series, which fed incandescent lamps. The system proved the feasibility of AC electric power transmission on long distances.