Distribution Transformer | All Day Efficiency of Distribution Transformer

Distribution Transformer

The step down transformers used near the load end of an electric power network are called a distribution transformer. They reduce distribution voltage to the utilisation level required by customers. Designs include single-phase and three-phase units installed on poles, pads, underground networks or inside buildings. Distribution transformers may be liquid-immersed or dry-type and span many ratings. Selection depends on the voltage ratio, kVA rating, phase arrangement, insulation, cooling, installation, losses, protection, applicable standards and lifecycle cost.

Secondary Terminals of Distribution Transformer

Secondary terminals deliver electrical power at a utilisation voltage level through the service and metering equipment. The secondary winding and terminal arrangement depend on the network. A three-phase four-wire service normally uses a wye or star secondary with three line terminals and a neutral point. Delta and three-wire secondaries also exist. Phase-conductor colours are set by local wiring rules and should not be assumed from the transformer layout.

A neutral point exists only when the winding connection provides one, such as the common point of a wye secondary or the centre tap of a single-phase secondary. A three-phase four-wire service can supply line-to-line and line-to-neutral loads. A single-phase load may use one line and the neutral where the network design permits it, with the supply connected through the consumer’s energy meter.

The arrangement between the transformer terminals and customer services varies by network and installation. It may include a bus, kiosk, fuse, circuit breaker, service main or sub-distributor selected for the required fault protection and isolation.
A common single-phase residential service in the United States uses a centre-tapped 120/240 V transformer secondary. It has two hot terminals and a grounded centre-tap neutral, not two phases. The voltage is 240 V between the hot terminals and 120 V from either hot terminal to neutral. The equipment grounding conductor is separate from the neutral on the customer side where required by the installation rules. The two end conductors of the winding are the hot conductors.

Efficiency of Distribution Transformer

The efficiency of a distribution transformer at a stated operating point is the ratio of output power to input power. The load fraction, power factor, voltage, frequency and temperature affect the result. Maximum efficiency does not always occur at 50% load. Under the usual simplified loss model, it occurs when load-dependent loss equals no-load loss. Some regulations use 35% or 50% load as a test point for particular transformer classes, but that test point is not a universal maximum.
Transformer losses are often grouped as no-load and load losses. No-load or core loss includes hysteresis loss and eddy current loss. At fixed voltage and frequency, no-load loss is approximately constant over the load range. Load loss includes I²R winding loss and stray components and generally rises with current. A distribution transformer usually carries a changing load during the day, so its losses and point efficiency also change.

Point efficiency alone does not describe performance across a varying daily load. The all day efficiency of distribution transformer is an energy-efficiency measure for a stated load cycle. It differs from the efficiency of distribution transformer at one operating point.
Over the selected interval, it is the ratio of energy delivered to energy supplied, including the losses incurred throughout that interval. A 24-hour period is common, but the interval and load profile should be stated. For the transformer, all day efficiency equals secondary energy in kWh divided by primary energy in kWh over the same interval. That is,

This measure is useful for assessing a distribution transformer that remains energised for long periods while its secondary load varies through the day.