GE STD15C Instructions Manual page 8

GE:K-45307
PERCENT SLOPE SETTING
The proper percent slope required is determined by the sum of:
a.
The maximum range of manual taps and the load—ratio-control, or automatic
tap changing means, in percent.
b. The maximum percent of mismatch of the relay taps.
Set the desired percent slope by means of R3 (See Figure 6A).
The percentage slope setting selected should be greater than the ratio of maximum
total error current to the smaller of the through currents.
In general, if the
total error current does not exceed 20%, the 25% setting is used.
If it exceeds
20%, but not 35%, the 40% setting is used.
If the movable lead
is
used (as in Figure 1, for example)
the percent slope setting
should be chosen about twice as high, since the movable lead provides no restraint.
EXAMPLE (REFER TO FIGURE 4)
I. Determination of CT Turns and STD Relay Tap Settings
1. Transformer and Line
A B C
2. Maximum I
=
3750/ J3 (Line kV)
19.7 49.5 157
3. 100% Ip
=
3000/ J (Line kV)
15.7 39.6 125
4. Assume CT turns (N)
20 20
60
5.
Maximum I secondary (less than 5a)
0.98 2.47 2.62
6. 100% I secondary
0.79 1.98 2.08
7.
CT connections
Delta Wye Delta
8. Relay Current for 100% I Sec.
1.37 1.98 3.60
Select a relay tap for one of the line currents and calculate what the
currents in other lines would be
if
they were increased in the same ratio.
If any current is greater than V3 times any other, the 8.7 tap should be
chosen for it, and new ideal relay taps calculated for the other lines.
9. Ideal Relay Taps (Set C
8.7) 3.31
4.78 8.7
10. Try Relay Taps
3.2 4.6 8.7
11. Check Mismatch Error
Ratio of Taps on Lines B-A
=
1.43 Ratio of Sec. Lines Currents
1.98
-
1.37
—
1.44
Mismatch 1.44
-
43
=
0.7%
Ratio of Taps on LinesC-B
-!.j-
=
1.89 Ratio of Sec. Line Currents
=
1.82
1.89
-
1.82
Mismatch
1 8
=
3.8%
8