Chapter 7 - Restricted Earth Fault Protection
Figure 94: REF bias characteristic
The following settings are provided to define this bias characteristic:
IREF> Is1: sets the minimum trip threshold
IREF> Is2: sets the bias current kneepoint whereby the required trip current starts increasing
IREF> k1: defines the first slope (often set to 0%)
IREF> k2: defines the second slope
Is1 and Is2 are relative to the line CT, which is always the reference CT.
The bias quantity used is actually delayed by one cycle. It is the maximum value of the mean bias quantities
calculated within the previous cycle, where the mean bias is the fundamental bias current. This means the bias
level, and thus through-fault stability is maintained after an external fault has been cleared.
The algorithm, shown below, is executed eight times per cycle.
= Maximum [I
It is this delayed bias that is used to calculate the operating current.
If there is a sudden increase in the mean-bias measurement, an additional bias quantity is introduced in the bias
calculation. Transient Bias provides stability for external faults where CT saturation might occur.
The transient bias function enhances the stability of the differential element during external faults and allows for
the time delay in CT saturation caused by small external fault currents and high X/R ratios.
No transient bias is produced under load switching conditions, or when the CT comes out of saturation.
HIGH IMPEDANCE REF
The device provides a high impedance restricted earth fault protection function. An external resistor is required to
provide stability in the presence of saturated line current transformers. Current transformer supervision signals do
, (n – (K-1))]