ABB RELION 620 Series Technical Manual page 595

1MAC504801-IB E
6.3.3
6.3.4
620 series ANSI
Technical Manual
Functionality
The current circuit supervision function CCM is used for monitoring current transformers.
CCM 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 protection relay.
CCM 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 "Enable" and "Disable".
The operation of current circuit supervision 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 299: 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 Pickup value setting. When
the highest phase current is less than 1.0 × In, the differential current limit is defined with
Pickup 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 Pickup value
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The differential current is limited to 1.0 × In.
Section 6
Supervision functions
(Equation 90)
589