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.
21.1.5 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.
21.1.6 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.
21.1.6.1 Busbar protection
SEMOD129059-4 v6
The CT can be of high remanence or low remanence type and they can be used together within
the same zone of protection. Each of them must have a rated equivalent limiting secondary
e.m.f. E
al
that is larger than or equal to the required rated equivalent limiting secondary e.m.f.
E
alreq
below:
The high remanence type CT must fulfill
sr
R
al alreq f max ct L
2
pr
r
I
S
E E 0.5 I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION1534 V2 EN-US (Equation 92)
The low remanence type CT must fulfill
sr
R
al alreq f max ct L
2
pr
r
I
S
E E 0.2 I R R
I
I
æ ö
³ = × × × + +
ç ÷
è ø
EQUATION1535 V2 EN-US (Equation 93)
where
Section 21 1MRK 505 370-UEN D
Requirements
452 Busbar protection REB670
Application manual
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