ABB 615 series Technical Manual page 576

Section 4
Protection functions
570
currents and denote the fundamental frequency components on the phase and
I I
1 2
neutral sides of the current. The amplitude of the differential current I
the equation (assuming that the positive direction of the current is towards the machine):
I I I
1 2
= +
d
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During normal conditions, there is no fault in the area protected by the function block, so
the currents and are equal and the differential current I
I I
1 2
some differential current exists due to inaccuracies in the current transformer on the phase
and neutral sides, but it is very small during normal conditions.
The module calculates the differential current for all three phases.
The low-stage differential protection is stabilized with a bias current. The bias current is
also known as the stabilizing current. Stabilization means that the differential current
required for tripping increases according to the bias current and the operation
characteristics. When an internal fault occurs, the currents on both sides of the protected
object are flowing into it. This causes the biasing current to be considerably smaller, which
makes the operation more sensitive during internal faults.
The traditional way for calculating the stabilized current is:
I I
−
1 2
I
=
b
2
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The module calculates the bias current for all three phases.
Through-fault detection
Through-fault (TF) detection module is for detecting whether the fault is external, that is,
going through, or internal. This information is essential for ensuring the correct operation
of the protection in case of the CT saturation.
• In a through-fault situation, CTs can saturate because of a high fault current
magnitude. Such AC saturation does not happen immediately when the fault begins.
Thus, the TF module sees the fault as external because the bias current is high but the
differential current remains low. If the AC saturation then occurs, a CT saturation-
based blocking is allowed to work to prevent tripping.
• Normally, the phase angle between the machine neutral and line side CTs is 180
degrees. If an internal fault occurs during a through fault, an angle less than 50
degrees clearly indicates an internal fault and the TF module overrules, that is,
deblocks the presence of any blocking due to CT saturation.
1MAC059074-MB A
is obtained using
d
(Equation 105)
= 0. However, in practice
d
(Equation 106)
615 series ANSI
Technical Manual