Guide For Calculation Of Ct For Generator Differential Protection - ABB RELION 670 SERIES REG670 Applications Manual

Section 24
Requirements
24.1.6.1
798

Guide for calculation of CT for generator differential protection

This section is an informative guide describing the practical procedure when
dimensioning CTs for the generator differential 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.
I
tf
CT1 CT2
~
GDP
IEC11000215 V1 EN-US
In IED the generator differential and the transformer differential functions have the
same CT requirements. According to the manual the CTs must have a rated equivalent
limiting secondary e.m.f. E
required rated equivalent limiting secondary e.m.f. E
I
³ = × ×
NG
E E 30 I
al alreqRat
I
pr
EQUATION2525 V2 EN-US
I
tf
³ = × ×
E E 2 I
al alreqExt sr
I
pr
EQUATION2526 V2 EN-US
where:
I The rated primary current of the generator
NG
I Maximum primary fault current through the CTs for external faults. Generally both three phase
tf
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 CT rated primary current
pr
I CT rated secondary current
sr
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Ext fault
I
tf
that is larger than or equal to the maximum of the
al
( )
+ +
R R R
sr ct w addbu
( )
+ +
R R R
ct w addbu
1MRK 502 071-UUS A
GUID-590F218B-5DE3-48FB-ADBE-59CAE1A96B06 v2
IEC11000215-1-en.vsd
and E below:
alreqRat alreqExt
(Equation 273)
(Equation 274)
Application manual