A distribution transformer is
a transformer that provides the final voltage transformation in
the electric power distribution system, stepping down the voltage used
in the distribution lines to the level used by the customer. The
invention of a practical efficient transformer made AC power
distribution feasible; a system using distribution transformers was
demonstrated as early as 1882.
If mounted on a utility pole, they are called pole-mount transformers.
If the distribution lines are located at ground level or underground,
distribution transformers are mounted on concrete pads and locked in
steel cases, thus known as pad-mount transformers.
Distribution transformers normally have ratings up to 200
kVA, although some national standards can describe units up to 5000 kVA
as distribution transformers. Since distribution transformers are
energized for 24 hours a day (even when they don't carry any load),
reducing iron losses has an important role in their design. As they
usually don't operate at full load, they are designed to have maximum
efficiency at lower loads. To have a better efficiency, voltage
regulation in these transformers should be kept to a minimum. Hence they
are designed to have small leakage reactance.
Distribution transformers are classified into different
categories based on certain factors such as:
- Mounting location - pole, pad, underground vault
- Type of insulation - liquid-immersed or dry-type
- Number of Phases - single-phase or three-phase
- Voltage class
- Basic impulse insulation level (BIL).
Distribution transformers are normally located at a service drop,
where wires run from a utility pole or underground power lines to a
customer's premises. They are often used for the power supply of
facilities outside settlements, such as isolated houses, farmyards
or pumping stations at voltages below 30 kV. Another application is
the power supply of the overhead wire of railways electrified with
AC. In this case single phase distribution transformers are used.
The number of customers fed by a single distribution transformer
varies depending on the number of customers in an area. Several
homes may be fed off a single transformer in urban areas; rural
distribution may require one transformer per customer. A large
commercial or industrial complex will have multiple distribution
transformers. Pad mount transformers are used in urban areas and
neighborhoods where the primary distribution lines run underground.
Many large buildings have electric service provided at primary
distribution voltage. These buildings have customer-owned
transformers in the basement for step-down purposes. In a secondary
network system as used in urban areas, many distribution
transformers may be connected in parallel, each equipped with its
own network protector circuit breaker to isolate it from the
secondary network in case of a fault.
Distribution transformers are also found in the power collector
networks of wind farms, where they step up power from each wind
turbine to connect to a substation that may be several miles (kilo