Date Code 20080110 Testing and Troubleshooting 10-27
SEL-387E Instruction Manual
Step 3.
Purpose: Select a test point on the percentage differential curve in Figure 10.7.
Method: Decide where you want to cross the differential characteristic by picking a
restraint value IRT, which is a vertical line on the graph. Since this test is
for the SLP2 threshold, select a point above the IRS1 setting.
IRT
> IRS1
The value of IOP that corresponds to the selected IRT is as follows:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
+=
100
2SLP1SLP
•1IRSIRT•
100
2SLP
IOP
Both IRT and IOP are in multiples of tap.
Step 4.
Purpose: Calculate the expected current for Winding 1 and 2 at the restrained
differential element SLP2 threshold for the test point selected above.
Method: Calculate the Winding 1 current for the test using the following formula:
A•1TAP•
200
2SLP1SLP
•1IRS
200
2SLP
1•IRT1IAW
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
Calculate the Winding 2 current for the test using the following formula:
A•2TAP•
200
2SLP1SLP
•1IRS
200
2SLP
1•IRT2IAW
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−=
The A connection compensation constant is based on
Table 10.3. Because
the windings have the same W
n
CTC setting, the A constant will be the
same for both windings. The A constant must be used to achieve the
exact curve point on which we have based the calculations. The TAP
n
settings can be different for the two windings.
CAU TION
!
The continuous rating of the current inputs is 3
•
I
nom
. If any currents in
this test will exceed this rating, reduce the TAP
n
values as needed, to
prevent possible damage to the input circuits.
Step 5.
Purpose: Calculate the initial current for Winding 2 for this test.
Method: Calculate the Winding 2 initial current by multiplying the Winding 2
expected current calculated above by 110 percent.
IAW2 (initial) = 1.1 • IAW2 (from Step 4)
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