ABB REG670 - Page 754

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
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 (Equation 280)
( )
2
tf
al alreqExt sr ct w addbu
pr
I
E E I R R R
I
³ = × × + +
EQUATION2526 V2 EN (Equation 281)
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
R
ct
CT secondary winding resistance
R
w
The resistance of the secondary wire. For phase to earth faults the loop resistance containing
the phase and neutral wires (double length) shall be used and for three phase faults the
phase wire (single length) can be used.
R
addbu
The total additional burden from the differential relay and possible other relays
We assume that the secondary wire and additional burden are the same for the two
examples. The resistance of the secondary wires can be calculated with the following
expression:
w
R
I
A
r = × W
EQUATION2527 V1 EN (Equation 282)
In our example the single length of the secondary wire is 300 m both to CT1 and CT2.
The cross-section area is 2.5 mm
2
. The resistivity for cupper at 75 C° is 0.021 Ω m
2
/
m.
With this value
300
0.021 2.5
2.5
r = × = × = W
w
R
I
A
EQUATION2528 V2 EN (Equation 283)
The total additional burden in our example is 0.3 Ω for both CTs.
Section 24 1MRK 502 071-UEN -
Requirements
748 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual