7 TRANSIENT BIAS
Phase current differential protection stability for current transformers is assisted by a feature called Transient
Bias. This can be enabled or disabled with the transient Bias setting in the CURRENT DIFF column.
Saturation of current transformers (CTs) under heavy load or external fault conditions can cause the protection to
see differential current and could lead to tripping. Preventing CT saturation can impose high specifications and
high costs for the CTs. The Transient Bias feature allows the CT requirements to be relaxed – typically by 25%.
The Transient Bias feature recognises the changes in bias and differential currents under different conditions and
reacts as follows:
● For an internal fault, bias and differential currents start to increase together. For an external condition, bias
current will start to rise, but differential current will not. If CT saturation starts to occur for an external
condition, the differential current starts to increase after the bias current has increased.
● Detecting the relative changes in the bias and differential currents allows the device to detect whether the
differential current is due to an internal fault or due to an external condition causing CT saturation. For
external conditions, the biasing quantity is raised, transiently, to provide stability. For internal faults,
sensitivity is maintained.
P543i/P545i Chapter 6 - Current Differential Protection
P54x1i-TM-EN-1 111
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