ABSTRACT
Transformers efficiently convert electricity to higher voltage for long-distance transmission and back down to low voltages suitable for customer usage. The distribution transformer normally serves as the final transition to the customer and often provides a local grounding reference. Distribution feeders may have other transformers: voltage regulators, feeder step banks to interface circuits of different voltages, and grounding banks. Mineral oil fills most distribution and substation transformers. The oil provides two critical functions: conducting heat and insulation. Because the oil is a good heat conductor, an oil-filled transformer has more load-carrying capability than a dry-type transformer. Transformers in underground vaults are often used in cities, especially for network transformers. Increasing sizes of distribution transformers caused overlap between “distribution” and “power” transformers, so larger distribution transformers were made with subtractive polarity for consistency. Completely self-protected transformers are a widely used single-phase distribution transformer with several built-in features: tank-mounted arrester; internal “weak-link” fuse; and secondary breaker.
