Calculation Of Differential Current And Bias Current - ABB RET650 Technical Manual

Section 6
Differential protection
6.2.7.3
116
Zero-
sequence
diff. current
in per unit
D
I
operate
slope = ----------------* 100 %
D
I
restrain
IdMin range: 0.04 –1.00 IBase
Reset Ratio in all sections:
0.95 (a constant)
The characteristic
can be moved
up and down
(vertically)
by changing
the setting IdMin
IEC98000017-5 V1 EN
Figure 47: Operate - bias characteristic of the Restricted earth-fault protection,
low impedance REFPDIF

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
Idiff = IN + 3I
0
IECEQUATION2417 V1 EN
where:
I current in the power transformer neutral as a fundamental frequency phasor,
N
3I residual current of the power transformer line (terminal) currents as a phasor.
0
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
5
4
Operate
conditionally
3
2
Section 1
1
IdMin
0
0
1
End Section 1
45 and figure 46):
45 and in figure 46).
1MRK 504 135-UEN A
Characteristic if
IdMin = 1.0 pu
Section 3
Sec. 2
Restrain
Slope 70%
2 3 4
bias current
in per unit
End Section 2
IEC98000017-5-en.vsd
Technical manual
Slope 100%
6
5
(Equation 26)