ABB RELION SMU615 Technical Manual page 146

Supervision functions
Differential current monitoring
Differential current monitoring supervises the difference between the summed
phase currents I_A, I_B and I_C and the reference current I_REF.
The current operating characteristics can be selected with the
When the highest phase current is less than 1.0 × In, the differential current limit is
defined with
differential current limit is calculated with the equation.
MAX I A I B I C
The differential current is limited to 1.0 × In.
Figure 96: CCSPVC operating characteristics
When the differential current I_DIFF is in the operating region, the FAIL output is
activated.
The function is internally blocked if any phase current is higher than the set
Max operate current . When the internal blocking activates, the FAIL output is
deactivated immediately. The internal blocking is used for avoiding false operation
during a fault situation when the current transformers are saturated due to high
fault currents.
The value of the differential current is available in the monitored data view on
the WHMI or through other communication tools. The value is calculated with the
equation.
_
I DIFF
146
Start value . When the highest phase current is more than 1.0 × In, the
( _ , _ , _ ) × Start value
_ _ _
I A I B I C
= + +
_
I REF
−
1MRS758407 C
Start value setting.
(Equation 5)
(Equation 6)
SMU615
Technical Manual