Date Code 20080110 Testing and Troubleshooting 10-25
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. Because this test
is for the SLP1 threshold, select a point above the O87P intersection point
and below IRS1. If SLP2 = OFF, IRS1 and SLP2 are not functional.
1IRSIRT
1SLP
100
•P87O <<
The value of IOP corresponding to the selected IRT equals the following:
IRT•
100
1SLP
IOP =
Both IRT and IOP are in multiples of tap.
Step 4.
Purpose: Calculate the expected current for Windings 1 and 2 at the restrained
differential element SLP1 threshold for the test point selected above.
Method: Calculate the Winding 1 current for the test using the following formula:
A•1TAP•
200
1SLP
1•IRT1IAW
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
Calculate the Winding 2 current for the test using the following formula:
A•2TAP•
200
1SLP
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 for the test using the following
formula:
1TAP
2TAP
•1IAW2IAW =
This formula determines the current necessary for IOP = 0 given the
IAW1 calculated above.
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