ABB REL 356

ABB REL 356 Current Differential Protection
1-8
Product Overview and Specifications
Operation is obtained when OP - 0.7 RES > OTH
where
OTH = set operating threshold
The worst case to consider is an external fault with one CT non-saturated while the other saturates. (This is an unlikely
case since the CT’s will have the same current and with similar characteristic and burden, both will have similar satura-
tion.)
One end will then deliver the undistorted ITL. The other end will deliver some lower value of current, corresponding to x%
of a “true” ITR. As this is an external fault ITR and ITL are 180 degrees out of phase and the magnitude of the “true” ITR
is equal to the magnitude of ITL. Thus:
TRTL
TRTL
xIIRES
xIIOP
+=
−+= )(
For operation OP > 0.7 RES + OTH
TL
TL
TL
TLTLTLTL
TRTLTRTL
I
OTHI
x
OTHxIIxII
OTHxIIxII
I OTH as 176.0
7.1
3.0
)(7.0)(
)(7.0)(
<<≈
⋅
−⋅
<
++>−+
++>−+
The example shows that for an external fault, operation will only take place if the saturated CT delivers less than 17.6%
current compared to true current. Even for the most severe saturation, a considerably higher amount current of funda-
mental frequency will be available.
The above example illustrates the high security of the REL 356 relay. It is difficult to derive an exact mathematical
formula due to the complexity in mathematically describing CT saturation as a factor of fault current and system dc
component. One important factor for the amount of saturation is the remanence present in the CT and this can not easily
be taken into account in a theoretical study.
The formula we are giving for CT verification is based on experience. As it is impossible to test all types of CTs for all
cases of remanence a factor 2 has been introduced as a safety margin. This means that the relay will operate correctly
if the CT satisfies the requirement:
()
356maxmin
2 RRRIV
LCTkk
++⋅=
where
V
kmin
= minimum knee-point voltage required
I
kmax
= secondary maximum through fault current, i.e. maximum current for an external fault
R
CT
= CT secondary resistance
R
L
= CT secondary burden, i.e. the sum of the cable resistance and the resistance of any other devices connected to the
same CT core
R
356
= burden of REL 356; 0.06 ohms for 1A and 0.02 ohms for 5A rated current