General Current Transformer Requirements - ABB RET650 Applications Manual

1MRK 504 169-UEN A
19.1.5
19.1.6
19.1.6.1
Transformer protection RET650 2.2 IEC
Application manual
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.

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.
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
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
Requirements for CTs specified according to other classes and standards are given
at the end of this section.
Transformer differential protection
The current transformers must have a rated equivalent limiting secondary e.m.f. E
that is larger than the maximum of the required rated equivalent limiting secondary
e.m.f. E below:
alreq
I
³ = × ×
E E 30 I
al alreq rt
I
EQUATION1412 V2 EN-US
below can be used. The characteristic of
al
according to the IEC 61869-2 standard.
al
æ ö
S
× + +
sr R
R R
ç ÷
ct L
2
I
è ø
pr
r
Section 19
Requirements
M11615-3 v3
SEMOD53723-1 v1
M11616-3 v2
SEMOD54689-4 v4
al
(Equation 58)
365