What Is a Distribution Transformer?

A distribution transformer is a device that steps down voltage to convert it from the very high voltage used for distribution to the lower voltages needed by utility customers. A single transformer can serve several customers, and it is also possible for a utility to install a distribution substation for areas with high energy use, like industrial parks. The utility is responsible for installing and maintaining the devices to meet customer needs. Transformers pose a safety risk, and they are typically labeled and secured to keep members of the general public away.

Two basic mounting designs are available. Pole-mounted distribution transformer devices are used for lower voltages, like those needed in industrial neighborhoods, and may be familiar to some utility customers. They look like large canisters mounted to the sides of power poles. Pad-mounted designs are necessary for processing higher voltages and are found in large metal boxes at ground level. They are usually marked with high voltage signs and are locked for safety.

As electricity enters the distribution transformer, it travels through a series of coils that step down the voltage while retaining the original frequency. The transformer feeds the reduced voltage out to distribution lines, which deliver it to individual facilities with service drops. The size of a service drop depends on the facility; a single residence has relatively low requirements, while a factory or similar facility may need a very large voltage drop to meet its power needs.

Insulation used inside a distribution transformer can vary. Some are dry, and rely on air as an insulator, while others have liquids. The choice of insulator, casing, and other materials can depend on the type of voltage the device needs to be able to handle, regional electric codes, and the preference of the utility. Whether wet or dry, the device needs periodic inspection to confirm that the insulation is in good condition and the device is working properly.

Power companies can move or add distribution transformers to the grid as necessary to meet evolving power needs. Brief service outages may occur while the utility performs work, as personnel cannot handle these devices while they are live. Typically the utility serves notice when it knows about work in advance, and provides an outage window for customers to plan around. When a distribution transformer fails, causing a brownout or a full power outage in the region it serves, the utility acts as quickly as possible to restore power.