ABB REG670 Applications Manual page 754

Section 24
Requirements
748
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
al alreqRat
I
pr
EQUATION2525 V2 EN
I
tf
³ = × ×
E E 2 I
al alreqExt sr
I
pr
EQUATION2526 V2 EN
where:
I The rated primary current of the generator
NG
I Maximum primary fault current through the CTs for external faults. Generally both three
tf
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 CT rated primary current
pr
I CT rated secondary current
sr
R CT secondary winding resistance
ct
R The resistance of the secondary wire. For phase to earth faults the loop resistance containing
w
the phase and neutral wires (double length) shall be used and for three phase faults the
phase wire (single length) can be used.
R The total additional burden from the differential relay and possible other relays
addbu
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:
I
r
= × W
R
w
A
EQUATION2527 V1 EN
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
m.
With this value
I 300
r
= × = ×
0.021
R
w
A 2.5
EQUATION2528 V2 EN
The total additional burden in our example is 0.3 Ω for both CTs.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
that is larger than or equal to the maximum of the
al
( )
+ +
I R R R
sr ct w addbu
( )
+ +
R R R
ct w addbu
2
. The resistivity for cupper at 75 C° is 0.021 Ω m
= W
2.5
1MRK 502 071-UEN -
and E below:
alreqRat alreqExt
(Equation 280)
(Equation 281)
(Equation 282)
(Equation 283)
Application manual
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