ABB REB670 Series Applications Manual page 462

Section 21
Requirements
the E
knee
TPS must have a rated knee point e.m.f. E
»
E
knee
EQUATION2100 V2 EN-US
21.1.7.3 Current transformers according to ANSI/IEEE
Current transformers according to ANSI/IEEE are partly specified in different ways. A rated
secondary terminal voltage U
terminal voltage the CT will deliver to a standard burden at 20 times rated secondary current
without exceeding 10 % ratio correction. There are a number of standardized U
example, U
e.m.f. E
E
alANSI
EQUATION971 V2 EN-US
where:
Z
bANSI
U
ANSI
The CTs according to class C must have a calculated rated equivalent limiting secondary e.m.f.
E that fulfils the following:
alANSI
E
alANSI
EQUATION1384 V2 EN-US
A CT according to ANSI/IEEE is also specified by the knee point voltage U
graphically defined from an excitation curve. The knee point voltage U
lower value than the knee-point e.m.f. according to IEC and BS. U
estimated to 75 % of the corresponding E
according to ANSI/IEEE must have a knee point voltage U
V
kneeANSI
EQUATION2101 V2 EN-US
21.2 Voltage transformer requirements
The performance of a protection function will depend on the quality of the measured input
signal. Transients caused by capacitive voltage transformers (CVTs) can affect some
protection functions.
Magnetic or capacitive voltage transformers can be used.
456
is approximately 80 % of the E
(
» » > ×
E E U 0.8 maximum of E
k kneeBS al
ANSI
is 400 V for a C400 CT. A corresponding rated equivalent limiting secondary
ANSI
can be estimated as follows:
alANSI
= × × +
20 I R U
sr ct ANSI
The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific C class
(W)
The secondary terminal voltage for the specific C class (V)
>
maximum of E
alreq
> ×
0.75 (max imum of E
. Therefore, the CTs according to class PX, PXR, X and
al
that fulfills the following:
knee
)
alreq
is specified for a CT of class C. U
= × × + ×
20 I R 20 I
sr ct sr
according to IEC 61869-2. Therefore, the CTs
al
)
alreq
1MRK 505 370-UEN D
is the secondary
ANSI
ANSI
×
Z
bANSI
kneeANSI
normally has a
kneeANSI
can approximately be
kneeANSI
that fulfills the following:
kneeANSI
Busbar protection REB670
Application manual
(Equation 100)
M11623-22 v6
values for
(Equation 101)
(Equation 102)
that is
(Equation 103)
M11608-3 v5