Specifications

7SR242 Duobias Applications Guide

2.12 Over Fluxing Protection (24)

An underfrequency condition at nominal voltage can cause over-fluxing (or over-excitation) of the transformer.

Excess flux can cause transformer core saturation and some of the flux will radiate as leakage flux through the

transformer tank. This leakage flux causes eddy currents and the I

R losses from these currents heat the

transformer tank and can cause overheating.

Overfluxing protection is applied to generator step-up transformers and other plant which may be subject to this

condition.

This function measures the ratio of voltage to frequency (V/f) applied the transformer to determine operation.

The relay has two types of V/f characteristics:

x User Definable Inverse curve

x Two Independent Definite Time Lag elements(DTL)

User Definable V/f Curve

As the leakage flux will cause overheating, an inverse type curve provides an appropriate match to the over

fluxing withstand characteristics of a transformer.

The relay includes a user definable curve. Where the transformer manufacturer has provided an over-fluxing

withstand curve the user can define up to seven points to provide correlation between the relay characteristic and

transformer withstand curves.

X - (V/f)

X0, Y0 point

defines

curve pick-up

X6, Y6 point

defines

curve cut-off

Straight-line

between points

Two Stage DTL Over fluxing

In addition to the inverse curve, two independent DTL V/f elements are included and are used where the over

excitation withstand curve of the transformer is not known. In this case the inverse V/f curve should be set to

[Disabled] and both DTL elements should be set to [Enabled]. The default DTL settings are adequate to protect

almost all transformer designs, and can be used with confidence.