In order to provide low transmission loss of high-voltage electricity, TEPCO Power Grid transmits electricity from our power stations at 500,000 V and 275,000 V, and the voltage is sequentially reduced to 154,000 V and 66,000 V as it nears areas of demand until it is finally reduced to 200 V and 100 V for delivery to the homes and offices of our customers. TEPCO Power Grid has installed some 1,600 substations at key points along these transmission routes to serve as voltage reducing equipment.
Tokyo, known as a "world-leading commercial city" and for its "densely packed housing", also requires substations in order to stably provide electricity. Due to the limited amount of land in urban areas that is available for use, we have installed underground substations in some 200 locations such as under office buildings, schools, and temples.
An outdoor substation is where equipment such as transformers (control voltage), circuit breakers (stop power generation when there is an accident), and lightning arresters (protect equipment from lightning) are installed outside. Although in general, these can be installed at a lower cost than underground substations, they are mostly located in suburban areas due to the need for a large area of land.
More than 90% of the transmission lines within the 23 wards of Tokyo are underground (as of 2016). In order to provide a large amount of electricity to city centers in an efficient manner, we use voltages of 500,000 V and 275,000 V due to the necessity of reduced transmission loss. TEPCO was the first electric company in the world to adopt use of 500,000-V long-distance underground transmission cables, which has helped reduce costs.