General Current Transformer Requirements; Rated Equivalent Secondary E.m.f. Requirements; Breaker Failure Protection - ABB Relion REC670 Applications Manual

1MRK 511 358-UEN A
As the burden can be considerable different for three-phase faults and phase-to-earth faults it
is important to consider both cases. Even in a case where the phase-to-earth fault current is
smaller than the three-phase fault current the phase-to-earth fault can be dimensioning for the
CT depending on the higher burden.
In isolated or high impedance earthed systems the phase-to-earth 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.
22.1.5

General current transformer requirements

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.
22.1.6

Rated equivalent secondary e.m.f. requirements

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
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
specified according to other classes and standards are given at the end of this section.
22.1.6.1

Breaker failure protection

The CTs must have a rated equivalent limiting secondary e.m.f. E
to the required rated equivalent limiting secondary e.m.f. E
Application manual
below can be used. The characteristic of the non remanence type
al
according to the IEC 61869-2 standard. Requirements for CTs
al
that is larger than or equal
al
below:
alreq
Section 22
Requirements
M11615-3 v3
SEMOD53723-1 v1
M11616-3 v2
M11621-3 v5
459