High Impedance Ref Principle; Figure 74: Low Impedance Ref Connection; Figure 75: Three-Slope Ref Bias Characteristic - GE P642 Technical Manual

P64x
Connecting IED to star winding for Low
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Figure 74: Low Impedance REF Connection

2.4.1.1 LOW IMPEDANCE BIAS CHARACTERISTIC
Usually, a triple slope biased characteristic is used as follows:
Differential current
Minimum operating current
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Figure 75: Three-slope REF bias characteristic

The flat area of the characteristic is the minimum differential current required to cause a trip (operate current) at
low bias currents. From the first kneepoint onwards, the operate current increases linearly with bias current, as
shown by the lower slope on the characteristic. This lower slope provides sensitivity for internal faults. From the
second knee point onwards, the operate current further increases linearly with bias current, but at a higher rate.
The second slope provides stability under through fault conditions.
Note:
In Restricted Earth Fault applications, Bias Current Compensation is also known as Low Impedance REF.
2.4.2

HIGH IMPEDANCE REF PRINCIPLE

This scheme is very sensitive and can protect against low levels of fault current, typical of winding faults.
High Impedance REF protection is based on the differential principle. It works on the circulating current principle as
shown in the following diagram.
P64x-TM-EN-1.3
Phase A
Phase B
Phase C
I
Phase A
I
Phase B
I
Phase C
I
Neutral
IED
Impedance REF
Operate region
Lower slope
First knee point Second knee point
Chapter 8 - Restricted Earth Fault Protection
Connecting IED to delta winding for Low
Impedance REF
Higher
slope
Restraint region
Bias current
Phase A
Phase B
Phase C
I
Phase A
I
Phase B
I
Phase C
IED
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