Rockwell Automation 865 Differential Protection Relay - Manual (Page 150)

8-8 Applications
865-UM001A-EN-P – July 2009
CT Requirement for Protection
When the through current equals and exceeds k
A
xI
N
there may be
enough secondary differential current to trip a relay although there is
no in zone fault. This is because the CTs are unique and they do not
behave equally when approaching saturation.
To avoid false trips caused by heavy through faults the actual
accuracy limit factor k
A
of the CTs should exceed the relative setting
I
SET
of the non-stabilized differential stage.
Equation 8.8
NCT
NTra
SET
A I
I
I c k ⋅ ⋅ >
c = Safety factor
I
SET
= Relative setting of the non-stabilized differential current
stage
I
NTra
= Rated current of the transformer (primary side or
secondary side)
I
NCT
= Rated primary current of the CT (primary side or
secondary side)
Using slightly smaller safety factor than indicated in Table 8.2 will
increase the setting inaccuracy.
Table 8.2 – Safety Factors
Protection Application Safety Factor c
Overcurrent 2
Earth-fault, cable transformer 3
Earth-fault overcurrent, sum of three phase currents 6
Transformer differential, -winding or unearthed Y-winding 3
Transformer differential, earthed Y-winding 4
Generator differential 3
Sensitive earth-fault current settings, < 5% x In, should be avoided in this configuration
because a set of three CTs are not exactly similar and will produce some secondary
residual current, even though there is no residual current in the primary side.