● Common phase current inputs can be used.
● It pr
ovides internal CT ratio mismatch compensation. It can match CT ratios up to 1:40 resulting flexibility in
substation design and reduced cost.
● Advanced algorithms make the protection secure.
With High Impedance REF, there is no bias characteristic, and the trip threshold is set to a constant level. However,
the High Impedance differential technique ensures that the impedance of the circuit is sufficiently high such that
the differential voltage under external fault conditions is lower than the voltage needed to drive differential current
through the device. This ensures stability against external fault conditions so the device will operate only for faults
occurring inside the protected zone.
High Impedance REF protection responds to a voltage across the differential junction points. During external faults,
even with severe saturation of some of the CTs, the voltage does not rise above certain level, because the other
CTs will provide a lower-impedance path compared with the device input impedance. The principle has been used
for more than half a century. Some advantages of using High Impedance REF are listed below:
● It provides a simple proven algorithm, which is fast, robust and secure.
● It is less sensitive to CT saturation.
2.4.1 LOW IMPEDANCE REF PRINCIPLE
Low Impedance REF can be used for either delta windings or star windings in both solidly grounded and resistance
gr
ounded systems. The connection to a modern IED is as follows:
IED
V00679
I
Phase A
I
Phase B
I
Phase C
I
Neutral
Phase A
Phase B
Phase C
IED
I
Phase A
I
Phase B
I
Phase C
Phase A
Phase B
Phase C
Connecting IED to star winding for Low
Impedance REF
Connecting IED to delta winding for Low
Impedance REF
Figure 90: Low Impedance REF Connection
2.4.1.1 LOW IMPEDANCE BIAS CHARACTERISTIC
Usually, a triple slope biased characteristic is used as follows:
P14x Chapter 7 - Restricted Earth Fault Protection
P14xEd1-TM-EN-1 167
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