General Current Transformer Requirements; Rated Equivalent Secondary E.m.f. Requirements; Transformer Differential Protection - ABB RET670 Applications Manual

Section 2
Requirements
2.1.5
2.1.6
2.1.6.1
22
In isolated or high impedance grounded systems the phase-to-ground fault is not the
dimensioning case and therefore the resistance of the single secondary wire always can
be used in the calculation, for this case.

General current transformer requirements

The current transformer ratio is mainly selected based on power system data for
example, maximum load. However, 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. The minimum operating current is different for
different functions and normally settable so each function should be checked.
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
secondary e.m.f. E below can be used. The characteristic of the non remanence type
al
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 in different ways are given at the end of this section.

Transformer differential protection

The current transformers must have a rated equivalent secondary e.m.f. E
larger than the maximum of the required secondary e.m.f. E
I
×
³ = ×
E E 30 I s n
nt
a l a lre q
I
pn
EQUATION1672 V1 EN
according to the IEC 60044-6 standard.
al
æ ö
S
× + +
R R R
ç ÷
CT L
2
I
è ø
n
1MRK504116-UUS C
that is
al
below:
alreq
(Equation 1)
Application manual