Protection Schemes
745
Transformer Management Relay
Commission
ing
http://www.GEindustrial.com/multilin
7–15
GE Multilin
h) Slope Kneepoint
1. To measure the approximate kneepoint location, follow the procedure above,
setting
I
1
equal to the kneepoint. Gradually increase
I
2
until the element resets.
Calculate the first slope at this point. This value should be equal to the initial
slope setting. Increase
I
2
until the element operates again. Calculate the slope
at this point – it should be equal to the final slope. If the kneepoint is much dif-
ferent than the selected value of
I
1
, the two values of slope will be the same.
2. For an accurate measurement of the kneepoint, select a value of
I
1
just above
the kneepoint value.
3. Increase
I
2
until the element resets. Calculate the slope – the value should be
equal to the initial slope value.
4. Increase
I
1
by a small amount, say 10%, and adjust
I
2
until a new operating
point is obtained. Calculate the slope. Repeat until the slope value equals the
final slope. The kneepoint value is the value of the restraint current at which the
slope changed in value.
Keep in mind the effects of auto-configuration on the magnitude of the current
signal fed to the differential elements when conducting the slope kneepoint test.
i) 2nd Harmonic Restraint
To measure the percentage of second harmonic required to block the operation of
the harmonic-restraint differential elements, use the connection diagram shown
below. Current is supplied as an operating current to the Phase A element.
FIGURE 7–7: 2nd Harmonic Restraint Testing
1. Close switch S1. Set the AC current,
I
AC
to 2
×
rated CT secondary. Set
I
DC
to
obtain harmonic content above the 2nd harmonic restraint setting under
S4 ELE-
MENTS
!"
DIFFERENTIAL
!"
HARMONIC INHIBIT
!"
HARMONIC INHIBIT LEVEL
.
2. Calculate the percent second harmonic content from the following equations. If
the current is measured with average-responding/reading meters:
(EQ 7.12)
If the current is measured with RMS-responding/reading meters, then:
(EQ 7.13)
3. Open and reclose S1. The relay should not operate.
4. Decrease
I
DC
until the element operates. Calculate the percent of second
harmonic at this point using the equations above. The calculated percent
harmonic value should equal the relay setting.
NOTE
%2nd
100 0.424
I
DC
×
×
I
DC
0.9
I
AC
×
+
---------------------------------------------------
=
%2nd
100 0.424
I
DC
×
×
I
DC
1.414
I
AC
×
+
---------------------------------------------------
=
Summary of Contents for Multilin 745
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