ABB Relion REC670, Applications Manual

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resistance, the resistance of a single secondary wire, may normally be used in the
calculation for three-phase faults.
As the burden can be considerable different for three-phase faults and phase-to-ground
faults it is important to consider both cases. Even in a case where the phase-to-ground
fault current is smaller than the three-phase fault current the phase-to-ground fault can
be dimensioning for the CT depending on the higher burden.
In isolated or high impedance grounded systems the phase-to-ground fault is not the
dimensioning case. Therefore, the resistance of the single secondary wire can always
be used in the calculation for this kind of power systems.
24.1.5 General current transformer requirements
M11615-3 v3
The current transformer ratio is mainly selected based on power system data for
example, maximum load and/or maximum fault current. It should be verified that the
current to the protection is higher than the minimum operating value for all faults that
are to be detected with the selected CT ratio. It should also be verified that the
maximum possible fault current is within the limits of the IED.
The current error of the current transformer can limit the possibility to use a very
sensitive setting of a sensitive residual overcurrent protection. If a very sensitive
setting of this function will be used it is recommended that the current transformer
should have an accuracy class which have an current error at rated primary current that
is less than ±1% (for example, 5P). If current transformers with less accuracy are used
it is advisable to check the actual unwanted residual current during the commissioning.
24.1.6 Rated equivalent secondary e.m.f. requirements
SEMOD53723-1 v1
M11616-3 v2
With regard to saturation of the current transformer all current transformers of high
remanence and low remanence type that fulfill the requirements on the rated equivalent
limiting secondary e.m.f. E
al
below can be used. The characteristic of the non
remanence type CT (TPZ) is not well defined as far as the phase angle error is
concerned. If no explicit recommendation is given for a specific function we therefore
recommend contacting ABB to confirm that the non remanence type can be used.
The CT requirements for the different functions below are specified as a rated
equivalent limiting secondary e.m.f. E
al
according to the IEC 61869-2 standard.
Requirements for CTs specified according to other classes and standards are given at
the end of this section.
1MRK 511 401-UUS A
Section 24
Requirements
Bay control REC670 2.2 ANSI 631
Application manual