Three winding transformer basics
Electrical power is delivered to a substation via 3-phase transmission lines (electricity grid), usually at 115 kV or higher, to the primary, or high voltage coils of power transformers, of which there is generally at least two for redundancy. High voltage switches between the power transmission lines and transformers allow the substation to be isolated from the transmission line. The secondary side of the transformer is usually connected to an overhead three-phase bus system, which is generally made of 4 aluminum tubes, to carry each of three phase conductors and a grounded neutral. From the substation, electrical power is distributed to one or more, three-phase, electrical distribution feeders.
These feeders are generally a minimum of 12,470 volts, line-to-line, and as high as 69,000 volts, and each have switches to allow isolation. The substation transformer and distribution feeders have an electrical protection system, electrical and thermal monitoring system, and security systems. The electrical protection system isolates distribution feeders or the substation transformers if there are any adverse conditions that develop, such as short circuits or other failures.
Basics of transformers
The distribution feeder lines may be above ground, on poles, or buried, or a combination of both, and they are typically arranged to run in road right-of-ways to distribute power to distribution transformers. The distribution transformers can be either single-phase or three-phase, and each is usually within several hundred feet of the electrical service entrances of the facilities that they serve. Transformers usually are filled with dielectric oil because the air does not have necessary dielectric strength.
Used oil reclamation
That oil requires maintenance each 10 years — oil purification and also reclamation of oil. It’s done with special oil reclamation machines.
Household Electricity Grid
Each electrical service has a service meter a main service disconnect, and overcurrent protection to isolate the service in the event of an electrical short circuit. The main service disconnect and protection is usually either a circuit breaker, or a manual switch and fuse protection. The service is also grounded near the electrical service disconnect, which connects the neutral and grounding conductors to a ground rod (grounding electrode) driven into soil at the electrical service entrance. The grounding system at the service entrance also connects to buried metallic lines and to any structural steel used in the construction of the house or building, if there is such.