What is power transformer.

ANSI/IEEE defines a transformer as a static electrical device, involving no continuously moving parts,
used in electric power systems to transfer power between circuits through the use of electromagnetic
induction.

The term power transformer is used to refer to those transformers used between the generator
and the distribution circuits, and these are usually rated at 500 kVA and above.

1. Small power transformers: 500 to 7500 kVA
2. Medium power transformers: 7500 to 100 MVA
3. Large power transformers: 100 MVA and above

Transformers are devices that transfer energy from one circuit to another by means of a common magnetic field. In all cases except autotransformers, there is no direct electrical connection from one circuit to the other.

When an alternating current flows in a conductor, a magnetic field exists around the conductor. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor(as per Faraday’s laws of electromagnetic induction), then a voltage is induced into the second conductor. The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based.

Leakage reactance in Power Transformer

In practical transformer the leakage reactance is an important fact to take in to consideration. This is because leakage reactance causes loss decreasing the transformer efficiency and disturbance to normal operation of transformer.

What is Leakage reactance in transformer:

In ideal transformer the magnetic flux created by excitation circuit induces voltage to the secondary circuit. But some of this flux “leaked” out of the core. This flux exists in the spaces between windings and in the spaces occupied by the windings these flux lines effectively result in an impedance between the windings, which is termed “leakage reactance” in the industry.

The magnitude of this reactance is a function of the number of turns in the windings, the current in the windings, the leakage field, and the geometry of the core and windings. The magnitude of the leakage reactance is usually in the range of 4 to 20% at the base rating of power transformers.

Ideal transformer magnetic flux

Leakage magnetic flux that causes the Leakage reactance.

Calculation of leakage reactance

It is calculated by designers using the number of turns, the magnitudes of the current and the leakage field, and the geometry of the transformer. It is measured by short-circuiting one winding of the transformer and increasing the voltage on the other winding until rated current flows in the windings. This voltage divided by the rated winding voltage

The percent regulation can be determined using the following equation for inductive loads.

where
%Reg = percentage voltage drop across the resistance and the leakage reactance
%R = percentage resistance = (kW of load loss/kVA of transformer)

%x = percentage leakage reactance

Φ=angle corresponding to the power factor of the load.

For capacitance loads, change the sign of the sine terms.