General. The transformer is a static device which, by electromagnetic induction, transforms electric energy from one circuit to another circuit at the same frequency

Chapter 6

 

Transformers

 

General

 

The transformer is a static device which, by electromagnetic induction, transforms electric energy from one circuit to another circuit at the same frequency, but usually at different voltage and current values.

Transformers have found very wide practical applications for the transfer of electric energy over large distances, for the distribution of this energy among users, and also in various rectifier and ampli­fier arrangements, and other devices.

In transmission of electric energy from a generating plant to users, of much importance is the value of current passed through the line wires. The value of current determines the cross section of wires of the power line and hence the cost of wires and the power load in the line.

The transmission of the same amount of energy at a higher volt­age and, accordingly, at a lower current allows us to employ the line wires of a smaller cross-sectional area and thus to effect savings in nonferrous metals and to reduce the power loss. The cross-sectional area A in mm² of a conductor and the power loss dP in the line are given by the expressions: A= I/J and dP =I²R=ρlJP/V since R = ρl/J = ρJl/I. Here I is current, A; J is current density, A/mm²; V is power line voltage, V; P is the power transferred, W; R is wire resistance, Ω; l is the line length, m; and ρ is the resistivity of the wire material, Ωmm²/m. Thus, assuming that the power delivered to the consumer is the same, the cross section of a conductor and the power lost in the line are inversely proportional to voltage.

At power plants, synchronous generators produce electric energy at 11 to 18 kV and sometimes at 30 to 35 kV. These voltages are very high and unsuitable for operating the consumer's loads, but they are not high enough for economical power transmission over long distances. Step-up transformers make possible the transmission of energy at very high voltages.

The loads consuming electric energy, such as incandescent lamps and motors, are designed for low voltages, 110 to 380 V, from the consideration of safety of the personnel handling these loads. Also, high voltages call for enhanced insulation of current-carrying parts which makes the designs of machinery and devices very complex. This explains why the energy transmitted at high voltages cannot be directly used at the receiver end of the line, but must be delivered to loads through step-down transformers.

Transmission lines supplying energy from power plants terminate in step-down transformers of substations, which step down the voltage to a value suitable for secondary transmission and for loads. Not all the step-down transformers operate simultaneously, but only some of them, depending on the power demand, so that the total power of transformers installed on transmission and distribu­tion networks is 7 or 8 times higher than the power of generators at power plants.

A transformer has two insulated windings arranged on a steel core: one is a primary connected to a source of power and the other is a secondary connected to a load. Any winding of a transformer can be used as primary or secondary. In a step-up transformer the primary voltage is lower than the secondary voltage. A step-down transformer uses the reverse connection, in which the primary volt­age is higher than the secondary voltage. Any transformer can operate as a step-up or a step-down transformer.