ABB Relion 670 Series Applications Manual page 511

1MRK 511 358-UUS A
wires are connected to the common neutral wire. The most common practice is to use four wires
secondary cables so it normally is sufficient to consider just a single secondary wire for the three-
phase case.
The conclusion is that the loop resistance, twice the resistance of the single secondary wire, must
be used in the calculation for phase-to-ground faults and the phase 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.
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
al
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
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
the required rated equivalent limiting secondary e.m.f. E
Application manual
below can be used. The characteristic of the non remanence type CT (TPZ) is not well
according to the IEC 61869-2 standard. Requirements for CTs specified
al
that is larger than or equal to
al
below:
alreq
Section 22
Requirements
M11615-3 v3
SEMOD53723-1 v1
M11616-3 v2
M11621-3 v5
505