Functionality; Operation Principle - ABB Relion SMU615 Technical Manual

Section 5
Supervision functions
5.2.3
5.2.4
134

Functionality

The current circuit supervision function CCSPVC is used for monitoring current
transformer secondary circuits.
CCSPVC calculates internally the sum of phase currents (I_A, I_B and I_C) and
compares the sum against the measured single reference current (I_REF).The
reference current must originate from other three-phase CT cores than the phase
currents (I_A, I_B and I_C) and it is to be externally summated, that is, outside the
merging unit.
CCSPVC detects a fault in the measurement circuit and issues an alarm or blocks the
protection functions to avoid unwanted tripping.
It must be remembered that the blocking of protection functions at an occurring open
CT circuit means that the situation remains unchanged and extremely high voltages
stress the secondary circuit.

Operation principle

The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of CCSPVC can be described with a module diagram. All the modules
in the diagram are explained in the next sections.
GUID-A4F2DBEE-938F-4961-9DAD-977151DDBA13 V1 EN
Figure 93: Functional module diagram
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 Start value setting.
When the highest phase current is less than 1.0 × In, the differential current limit is
defined with Start value. When the highest phase current is more than 1.0 × In, the
differential current limit is calculated with the equation.
MAX I A I B I C ( _ , _ , _ ) ×
GUID-A0C500CA-4A1A-44AA-AC14-4540676DD8CB V3 EN
The differential current is limited to 1.0 × In.
Start value
1MRS758407 B
(Equation 4)
SMU615
Technical Manual