The differential protection REFPDIF (87N) calculates a differential current and a bias current. In
case of internal ground faults, the differential current is theoretically equal to the total ground
fault current. The bias current is supposed to give stability to REFPDIF(87N). The bias current is a
measure of how high the currents are and how difficult the conditions are under which the CTs
operate. With a high bias, difficult conditions can be suspected, and it will be more likely that the
calculated differential current has a component of a false current, primarily due to CT saturation.
This “law” is formulated by the operate-bias characteristic. This characteristic divides the Idiff -
Ibias plane in two areas. The area above the operate-bias characteristic is the operate area, while
the one below is the block area, see Figure
66.
Figure 66: Operate - bias characteristic of the Restricted earth-fault protection, low
impedance REFPDIF (87N)
6.4.7.3 Calculation of differential current and bias current
M5447-47 v11
The differential current (operate current), as a fundamental frequency phasor, is calculated as
(with designations as in Figure 64 and Figure 65):
EQUATION1533 V1 EN-US (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 (87N) function so that:
I3PW1 = I3PW1CT1 + I3PW1CT2
where the signals are defined in the input and output signal tables for REFPDIF (87N).
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 (87N), that is, of
current in phase A, phase B, phase C, and the current in the neutral point (designated as IN in
Figure
64 and in Figure 65).
If there are two feeders included in the zone of protection of REFPDIF (87N), 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-US (Equation 30)
Section 6 1MRK 502 066-UUS B
Differential protection
172
Technical manual
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