ABB REL 356 Current Differential Protection
3-8
Settings and Application
Current Differential System Settings
Trip Desensitizing (TDES)
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 REL356 relays will remain stable.
Operating Threshold (OTH)
The operating threshold determines the differential current required for operation according to the formula:
OTH
RES
OP
≥
⋅
−
7
.
0
where
OP = |IT
L
+ IT
R
|
and
RES = |IT
L
| + |IT
R
|
IT = - C1I1 + C0I0 + C2I2
For internal three phase faults, IT = -C1I1,
Ð
IT
L
@
Ð
IT
R
A 20% margin is recommended, i.e.
OTH < 0.80 (OP
min
– RES
min
) = 0.80 (|IT
L
| + |IT
R
|) x 0.3
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
|
With the recommended 20% margin, OTH should then be set to:
OTH < 0.80 (OP
min
– RES
min
) = 0.80 x ITL x 0.3 = 0.80 x C1I1 x 0.3
Different CT ratios can be used in the two line ends by adjusting the operating threshold, OTH, so that the same level is
achieved on a primary basis. For example, if one end has a ratio of 240 and OTH is set to 0.5 A, the other end with ratio
120 should have an OTH setting of 1.0 A. Note that the sequence filter settings, C1, C2 and C0, should always be the
same at the two line ends.
Note that for versions 1.15 and earlier, OTH is set in peak value why the above setting recommendations
should be
adjusted with a factor
.
This means that the recommended setting for double end infeed is:
)
(
8
.
0
2
min
min
RES
OP
OTH
−
⋅
⋅
≤
2
Summary of Contents for REL 356
Page 23: ...ABB REL 356 Current Differential Protection 1 10 Product Overview and Specifications ...
Page 83: ...ABB REL 356 Current Differential Protection 3 36 Settings and Application ...
Page 127: ...ABB REL 356 Current Differential Protection 5 28 Testing ...
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