General Current Transformer Requirements - ABB REL650 Applications Manual

Section 18
Requirements
18.1.5
18.1.6
18.1.6.1
492

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
limiting secondary e.m.f. E
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
CTs specified according to other classes and standards are given at the end of this section.
Distance protection
The current transformers must have a rated equivalent secondary e.m.f. E
than the maximum of the required secondary e.m.f. E
×
I I
³ = k ma x ×
s n
E E
a l a lre q
I
pn
EQUATION1675 V1 EN
×
I I
³ = ×
kzone 1
s n
E E
a l a lre q
I
pn
EQUATION1676 V1 EN
below can be used. The characteristic of the non remanence
al
according to the IEC 61869-2 standard. Requirements for
al
æ ö
S
+ +
R R R
ç ÷
CT L
2
I
è ø
n
æ ö
S
+ +
R
R R
ç ÷
CT L
2
I
è ø
n
1MRK 506 334-UUS A
that is larger
al
below:
alreq
(Equation 178)
(Equation 179)
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