Basler BE1-CDS220, Instruction Manual

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BE1-CDS220 Settings Calculations D-9
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%
Equation D-17
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
upward 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).
unmon offset
I IE I
+ =
24 . 0 0 24 . 0
= + =
offset
I
Where:
IE is assumed to be less than 4% of the self cooled rating.
d forcecoole
selfcooled
X selfcooled
MVA
MVA
IE IE
∗ ∗
=
Equation D-18
024 . 0
20
12
1 04 . 0
= ∗ ∗ =
IE
Equation D-19
Where:
X is the tap conversion factor defined in Calculate Minpu, Step 1, Equation D-6.
I
unmon
is the unmonitored load calculated in Calculate Minpu, Step 2, Equation D-7.
Step 4: Calculate Operating Margin M
o
. The Operating Margin M
o
is given by Equation D-20.
offset
i
o
I
S
S
M
−






=
- 1 Minpu
Equation D-20
The conservative calculation of margin at the knee point uses S
i
as calculated in Calculate Slope, Step 1.
For this example, M
o
in times tap is Equation D-21.
tap times 040 . 0 024 . 0
39
29
1 25 . 0
= − 





− ∗ =
o
M
Equation D-21
The above calculation of margin is conservative. For the low current level where the margin at the tripping
characteristic knee point is of interest, the CT error will typically be less than 1%. Thus, calculating the
margin using S
i
= 20 (replace 10% CT error with 1% CT error) yields the following result in Equation D-22.