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

1MRK 502 048-UUS A
6.2.7.3
Technical manual
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 46: Operate - bias characteristic of the Restricted earth-fault protection, low
impedance REFPDIF (87N)

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.
5
4
Operate
conditionally
3
2
Section 1
1
IdMin
0
0
1
End Section 1
End Section 2
44 and figure 45):
Differential protection
Characteristic if
IdMin = 1.0 pu
Slope 100%
Section 3
Sec. 2
Restrain
Slope 70%
5
2 3 4
bias current
in per unit
IEC98000017-5-en.vsd
Section 6
6
(Equation 26)
125