GE 745 - 7.5.2.8 Slope Kneepoint

7–20 745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION SCHEMES CHAPTER 7: COMMISSIONING
 Slowly increase I
2
. As I
2
is increased, the element will reset when the
differential current drops below the minimum pickup.
As I
2
continues to increase, the element operates again when both the initial slope and the
minimum pickup conditions are satisfied.
 Calculate the initial slope 1 value at this point.
As I
2
increases further, the element may reset again, depending on the setting of the slope
kneepoint. This is caused by the current values moving into the slope 2 region.
 Continue increasing I
2
until the element operates again.
 Compute the slope 2 value at this point.
7.5.2.8 Slope Kneepoint
 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 different than the selected value of I
1
, the two
values of slope will be the same.
 For an accurate measurement of the kneepoint, select a value of I
1
just
above the kneepoint value.
 Increase I
2
until the element resets.
 Calculate the slope.
The value should be equal to the initial slope value.
 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.
NOTE
Note
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.
7.5.2.9 Second 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.