ABB Relion 670 series - General Current Transformer Requirements

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.
25.1.5 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.
25.1.6 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
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.
25.1.6.1 Distance protection
The current transformers must have a rated equivalent limiting secondary e.m.f. E
al
that
is larger than the maximum of the required rated equivalent limiting secondary e.m.f.
E
alreq
below:
1MRK 506 369-UUS - Section 25
Requirements
Line distance protection REL670 2.2 ANSI 919
Application manual