Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 38 of 64
2.12 Over Fluxing Protection (24)
An under-frequency 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
2
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:
User Definable Inverse curve
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.
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.