ABB REG670 - Calculation of Differential Current and Bias Current

6.4.7.3 Calculation of differential current and bias current
The differential current (operate current), as a fundamental frequency phasor, is
calculated as (with designations as in Figure 61 and Figure 62):
3Idiff IN Io= +
EQUATION1533 V1 EN (Equation 29)
where:
I
N
is current in the power transformer neutral as a fundamental frequency phasor.
3I
o
is residual current of the power transformer terminal currents as a phasor.
If there are two three-phase CT inputs, as in breaker-and-a-half configurations, see
figure 10, then their respective residual currents are added within the REFPDIF
function so that:
I3PW1 = I3PW1CT1 + I3PW1CT2
where the signals are defined in the input and output signal tables for REFPDIF.
The bias current is a measure (expressed internally as a true fundamental frequency
current in Amperes) of how difficult the conditions are under which the instrument
current transformers operate. Dependent on the magnitude of the bias current, the
corresponding zone (section) of the operate-bias characteristic is applied, when
deciding whether to trip, or not to trip. In general, the higher the bias current, the higher
the differential current required to produce a trip.
The bias current is the highest current of all separate input currents to REFPDIF, that
is, of current in phase L1, phase L2, phase L3, and the current in the neutral point
(designated as IN in Figure
61 and in Figure 62).
If there are two feeders included in the zone of protection of REFPDIF, as in case of
an auto-transformer with two feeders included on both sides, then the respective bias
current is found as the relatively highest of the following currents:
1
current[1] = max (I3PW1CT1)
CTFactorPri1
×
EQUATION1526 V1 EN (Equation 30)
1
current[2] = max (I3PW1CT2)
CTFactorPri2
×
EQUATION1527 V1 EN (Equation 31)
1
current[3] = max (I3PW2CT1)
CTFactorSec1
×
EQUATION1528 V1 EN (Equation 32)
Section 6 1MRK502052-UEN B
Differential protection
172
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