The step down transformers used for electric power distribution purpose are referred as distribution transformer. There are several types of transformer used in the distribution system. Such as single phase transformer, three phase transformer, pole mounted transformer, pad mounted transformer, and underground transformer. Distribution transformers are generally small in size and filled with insulating oil. These transformers are available in the market in various sizes and efficiencies. Selection of distribution transformer depends upon the purpose and budget of the end users.
Secondary Terminals of Distribution Transformer
Secondary terminals of distribution transformer deliver electrical power at a utilization voltage level to the consumer end via energy metering system. In case of three phase distribution system three phase four wire secondary systems are adopted. Here, three phases, which means red, yellow and blue phase conductors, come out from three low voltage bushing studs of the transformer. The neutral wire is connected to the fourth bushing which is also referred as a neutral bushing of the transformer.
The neutral point of the distribution transformer is projected from the tie point of 3–phase winding inside the transformer. In case of industrial heavy three phase load, four wire system is directly delivered to the consumer end, but in case of single phase light load, one phase and neutral connection of the three phase distribution transformer, are connected to the consumer’s energy meter.
The secondary terminals of distribution transformer at first is connected to the three phase bus of distribution kiosk, located nearby the transformer. This connection is essentially made via a fuse unit to provide protection from external fault to the transformer. From this three phase bus in the distribution kiosk, different, three phase or single phase service mains or sub distributors are connected.
A typical type of-single phase system is available in United State of America for supplying single phase residential tons. Here, single phase pole mounted transformer is used for supplying single phase residential load. This type pole mounted transformer has total three secondary terminals, one of which is ground and other two are phase terminals. Two phase wires provide 240 volts across them, and the voltage across any of the phase wire and the ground wire is 120 volts. So a consumer can be supplied either of 240 volts or 120 volts supply depending upon his requirement. Actually in this system, the secondary winding of the distribution transformer is center tapped. The center wire is grounded and marked as neutral. The two end conductors of the winding is called hot wire.
Efficiency of Distribution Transformer
The efficiency of distribution transformer is defined as the ratio between output power to input power of the transformer at full load condition, but in case of a distribution transformer, the concept is a little bit different as the possibility of running a distribution transformer at its full load condition is nearly nil. The efficiency of the transformer is maximum at 50% of full load.
A transformer has mainly two types of losses, these are, iron losses and copper losses. Iron loss, which is also referred as core loss, consists of hysteresis loss and eddy current loss. These two losses are constant when the transformer is charged. That means the amount of these losses does not depend upon the condition of secondary load of the transformer. In all loading condition, these are fixed. But the copper loss which is also referred as I2R loss entirely depends upon load I. A distribution transformer cannot be run with constant load throughout 24 hours. At day peak time it’s loading is high, whereas in night lean time its loading may be negligible.
So selecting a transformer depending upon its conventional efficiency is not practical and economical, too. As a solution of these problems, the concept of all day efficiency of distribution transformer came into the picture. So this efficiency is same as the efficiency of distribution transformer.
In this concept, we use the ratio of total energy delivered by the transformer to the total energy fed to the transformer, during a 24 hrs span of time instead of ratio of power output and input of the transformer. Hence, all day efficiency is determined as, total KWh at the secondary to the total KWh at the primary of the transformer for a long specific time period preferably 24 hrs. i.e,
This is very much useful to judge the performance of a distribution transformer, whose primary is connected to the system forever, but secondary load varies tremendously throughout the day.