ABB REL 356

ABB REL 356 Current Differential Protection
3-24
Settings and Application
High Set Ground Overcurrent Unit (IGH) IGH = 62.5 A
The high set ground unit should follow the same guidelines as the high set phase units.
IGH = 1.25 x 50 A = 62.5 A
The high set ground overcurrent unit can be made directional when FDOG, the ground directional unit, is set to IN, and
three phase voltage is applied to the relay. IGH is then supervised by FDOG.
Current Differential System Settings
Trip Desensitizing (TDES) TDES = OUT
This setting temporarily desensitizes the differential protection by increasing the operating threshold, OTH, for 30 cycles
after closing the line breaker (de-energization of the 52b input). This setting can be used to prevent false tripping on large
inrush currents that may be produces by a tapped transformer in the protected zone.
The setting is a multiple of the nominal operating threshold, OTH, and has a range of 1 to 10 times nominal pickup. It can
also be set to OUT and no desensitizing on 52b de-energization will occur.
The trip desensitizing characteristic is a linear ramp which multiplies the tripping threshold immediately on de-energization
of the 52b input to the value to the TDES setting and then slowly over a time period of 30 cycles lowers the trip threshold
back to its nominal value.
Trip desensitizing is not recommended when the transformers are external to the protected zone. In this case the inrush
current will be seen as an external fault and the REL 356 relays will remain stable.
Operating Threshold (OTH) OTH = 0.56
The operating threshold determines the differential current required for operation according to the formula:
where
OP = |IT
L
+ IT
R
|
and
RES = |IT
L
| + |IT
R
|
IT = - C1I1 + C0I0 + C2I2
For internal three phase faults, IT = -C1I1, ÐIT
L
@ ÐIT
R
A 20% margin is recommended, i.e.
OTH < 0.80 (OP
min
– RES
min
) = 0.80 (|IT
L
| + |IT
R
|) x 0.3
OP = |IT
L
+ IT
R
| = (15 + 9)x 0.1 = 2.4
RES = |IT
L
|+ |IT
R
| = (15 + 9) x 0.1 = 2.4
OTH < 0.8 (OP – 0.7RES) = 0.8(2.4 – 0.7 x 2.4) = 0.576
Select OTH = 0.56
For single end infeed conditions, only one current is present, IT
L
or IT
R
. The equations then turn into:
OP = |IT
L
|
RES = |IT
L
|
OTHRESOP ≥⋅− 7.0