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
Ideal transformer magnetic flux

leakage flux fields
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.

transformer reactance formula
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.
power transformer core

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