GE 745 Instruction Manual page 258

PROTECTION SCHEMES
7.5.2.8 Slope Kneepoint
Note
NOTE
7.5.2.9 Second Harmonic Restraint
7–20
 Slowly increase I
differential current drops below the minimum pickup.
As I continues to increase, the element operates again when both the initial slope and the
2
minimum pickup conditions are satisfied.
 Calculate the initial slope 1 value at this point.
As I increases further, the element may reset again, depending on the setting of the slope
2
kneepoint. This is caused by the current values moving into the slope 2 region.
 Continue increasing I
 Compute the slope 2 value at this point.
 To measure the approximate kneepoint location, follow the procedure
above, setting I
 Gradually increase I
 Calculate the first slope at this point.
This value should be equal to the initial slope setting.
 Increase I
 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
values of slope will be the same.
 For an accurate measurement of the kneepoint, select a value of I
above the kneepoint value.
 Increase I
 Calculate the slope.
The value should be equal to the initial slope value.
 Increase I
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.
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.
. As I is increased, the element will reset when the
2 2
until the element operates again.
2
equal to the kneepoint.
1
until the element resets.
2
until the element operates again.
2
until the element resets.
2
by a small amount, say 10%, and adjust I
1
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: COMMISSIONING
, the two
1
just
1
until a new
2