24.1.6.1 Guide for calculation of CT for generator differential protection
GUID-590F218B-5DE3-48FB-ADBE-59CAE1A96B06 v2
This section is an informative guide describing the practical procedure when
dimensioning CTs for the generator dif
ferential protection IED. Two different cases are
of interest. The first case describes how to verify that existing CTs fulfill the
requirements in a specific application. The other case describes a method to provide
CT manufacturers with necessary CT data for the application. Below is one example
for each case.
~
GDP
Ext fault
CT1 CT2
I
tf
I
tf
IEC11000215-1-en.vsd
IEC11000215 V1 EN-US
In IED the generator dif
ferential and the transformer dif
ferential functions have the
same CT requirements. According to the manual the CTs must have a rated equivalent
limiting secondary e.m.f. E
al
that is larger than or equal to the maximum of the
required rated equivalent limiting secondary e.m.f. E
alreqRat
and E
alreqExt
below:
( )
30
NG
al alreqRat sr ct w addbu
pr
I
E E I R R R
I
³ = × × + +
EQUATION2525 V2 EN-US (Equation 273)
( )
2
tf
al alreqExt sr ct w addbu
pr
I
E E I R R R
I
³ = × × + +
EQUATION2526 V2 EN-US (Equation 274)
where:
I
NG
The rated primary current of the generator
I
tf
Maximum primary fault current through the CTs for external faults. Generally both three phase
faults and phase to earth faults shall be considered. However, in most generator applications
the system is high impedance earthed and the phase to earth fault current is small which
means that the three phase fault will be the dimensioning case.
I
pr
CT rated primary current
I
sr
CT rated secondary current
Section 24 1MRK 502 071-UUS A
Requirements
798 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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