The main purpose of these electrical machines is to provide a step-down mechanism for the incoming voltage from the primary substation, located away from consumers.It should be noted that it steps down voltage and steps up current to feed the consumer directly.It is the last in the line of machines that modify the voltage or current before going to small consumers.
In countries that use the BS 240/415V Standard, their transfomers usually receive and incoming supply of 11kV from the overhead feeders.It is then from this 11kV is is able to step it down to 240/415V 3phase voltage.
One major drawback of these machines is the heat energy generated therefore it does not operate at full efficiency.It is also prone to vandalism, because it houses a rather rare expensive oil that is used as a cooling mechanism. One good as advantage of this machine however is that it does not require regular maintenance and can withstand the weather elements as long as the mounting is mechanically sound.
1.The Transformer UnitUsually rated in kVA these units have the ability to step down large quantities of voltage.Inside this large structure are two coils known as the primary coil and secondary coil. The primary coil receives the 11kV and through magnetic induction smoothly passes it to the secondary coil at 415V.This unit is very expensive to purchase so periodic maintenance to check for oil levels is mandatory.The oil here as said earlier is for passing the heat to the casing for cooling.
2.The Porcelain InsulatorsMade from a material that is a good insulator, these insulators are for holding the cable at it dives down to the transformer.
3.Outgoing Line CablesSoon after the secondary coils of the transformer, curren flows to the fuses and then to the outgoing line for the customers.The current flowing here is very high since large cables can be seen.
4.Fuse BoxSafety for customers
5.Mechanical SwitchesThe mechanical switches are located up at the top of the poles so as to minimise anaouthrised operation of them.They have the ability to open the circuit under load during maintenance procedures.Some are manual while others can be automatic.
6.Incoming SwitchesUsed to open the two incoming power supplies.