ADR233B
Ref ID : ADR233B/IM/PS
Rev No. : 05
Page No. : 134 of 479
The Low Impedance REF or High Impedance REF Protection will be selected either on winding1
side or winding2 side using “Ref. Winding” setting, Which can be set as “W1” or “W2” through
software in the relay.
Low Impedance REF:
Low impedance REF uses a bias characteristic for increasing sensitivity and stabilising for
through faults. The current required to trip the differential element is called the Operate current.
This Operate current is a Function of the differential current and the bias current according to
the bias characteristic and those formulas is given below
The differential current is defined as follows:
Idiff = K(I
A + IB + IC) + IN
The bias current is as follows
IBias
= ½{K(max[|IA| + |IB| + |IC|
]) + |
IN|}
Where:
• Scaling Factor K = Line CT ratio / Neutral CT ratio (0.5 < K < 12).
• IN = current measured by the neutral CT.
The following settings are provided to define this bias characteristic:
• Isr>1 Set: sets the minimum trip threshold
• Isr>2 Set: sets the bias current knee point whereby the required trip current starts increasing.
• Slope1: defines the first slope.
• Slope2: defines the second slope.
Similarly the bias current and differential current are calculated for winding2
Bias Characteristics for Low Impedance:
The flat slope of the characteristic is the minimum differential current required to cause a trip at
low bias currents. From the first knee point onwards, the operating 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 operating current further
increases linearly with bias current, but at a higher rate. The second slope provides stability
under through fault conditions. Bias Characteristics for Low Impedance is shown in the following
figure.
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