Fault Current; Rec 561 Current Transformer Requirements - ABB REC 561 User Manual

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3.3 Fault current

3.4 REC 561 current
transformer
requirements
Requirements and basic technical
data - REC 561
All testing was made without any remanence flux in the current trans-
former core. So the requirements below are fully valid for a core with no
remanence flux. General recommendations for additional margins for
remanence flux cannot be provided. They depend on the reliability and
economy requirements.
When TPY current transformers are used, practically no additional margin
is needed due to the anti-remanence air gap.
For TPX current transformers, remember that the small probability of a
fully asymmetrical fault, together with maximum remanence flux in the
same direction as the flux generated by the fault can influence decisions
when about additional margin. Fully asymmetrical fault current is
achieved when the fault occurs at zero voltage (0°). Tests showed that
95% of the faults in the network occurs when the voltage is between 40°
and 90°.
The current transformer requirements are based on the maximum fault
current for faults in different positions. Maximum fault current occurs for
three-phase faults or single-phase-to-earth faults. The current for a single
phase-to-earth fault exceeds the current for a three-phase fault when the
zero sequence impedance in the total fault loop is less than the positive
sequence impedance.
When calculating the current transformer requirements, use maximum
fault current and consider both fault types.
The current transformer secondary limiting emf (E
two requirements below:
⋅
I I
kmax sn
> ⋅ ⋅
E a R
-----------------------------
2max
I
pn
I Maximum primary fundamental frequency current for forward
kmax
and reverse fault
I Primary nominal CT current
pn
I Secondary nominal CT current
sn
I Protection terminal nominal current
R
R CT secondary winding resistance
CT
R CT secondary cable resistance and additional load
L
a This factor is a function of the network frequency and the time con-
stant for the dc component in the fault current
ABB Network Partner AB
 
,
0 5
 
R
+ + --------- -
 
CT L
2
 
I
R
Version 1.0-00
) should meet the
2max