ABB RELION 620 Series Technical Manual page 412

Section 4
Protection functions
406
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Figure 185: Functional module diagram
Differential and bias calculation
Differential calculation module calculates the differential current. The differential current
is the difference in current between the phase and neutral sides of the machine. The phase
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
1MAC504801-IB E
is obtained using
d
(Equation 32)
= 0. However, in practice
d
620 series ANSI
Technical Manual