Basler BE1-CDS240, Instruction Manual, Page: 560 / 580

OFFSET
i
o
OFFSET o
o
I
S
S
minpu M
I
S
minpu S
minpu M
AC AB M






 







  
 
1
i
Figure D-2. Slope and Operating Margin
Step 2. Choose slope setting S. The tripping slope S must be greater than S
i
to provide a safety margin
at the differential tripping characteristic knee point (intersection of the minimum pickup tripping
characteristic and the slope tripping characteristic). This safety margin is required to
accommodate the additional mismatch caused by the excitation current and the unmonitored load
current. Refer to Figure D-2. A slope margin of 10% is recommended.
S = S
i
+ Slope Margin S = 29% + 10% = 39%
Step 3. Calculate Offset Current I
OFFSET
. The margin at the knee point of the tripping characteristic is
illustrated in Figure D-2. The additional mismatch caused by excitation current and unmonitored
loads does not vary with through current loading so it tends to offset the operating slope S
i
up-
ward from the origin. Thus, it decreases the safety margin at the differential tripping characteristic
knee point (intersection of the minimum pickup tripping characteristic and the slope tripping
characteristic). The offset current is calculated as:
unmon Exc Offset
I I I
 
Assume for the example that excitation current is 4% of full load current on the self-cooled basis.
Converting over to percent on the same MVA basis that the tap setting was chosen and including
the previously discussed
X
factor:
cooled forced MVA
cooled self MVA
* X * cooled self IE IE

024 . 0
20
12
* 1 * 04 . 0
 
IE
where:
X
is the tap conversion factor defined in Calculate Minpu, Step 1 (equation, page D-7).
I
unmon
is the unmonitored load calculated in Calculate Minpu, Step 2 (equation, page D-7.
D-10 BE1-CDS240 Settings Calculations 9365200990 Rev F