Schweitzer Engineering Laboratories SEL-387E - Restrained Element Operating Current Pickup (O87 P); Restraint Current Slope 1 Limit (IRS1); Restraint Slope Percentage (SLP1, SLP2); Second-Harmonic Blocking Percentage of Fundamental (PCT2)

3-10 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
Note: TAP
MAX
/TAP
MIN
must be less than or equal to 7.5
When a value is entered in the MVA setting (i.e., MVA is not set to “OFF”), the relay uses the
MVA, winding voltage, CT ratio, and CT connection settings you have entered and automatically
calculates the “TAPn” values.
You can also directly enter tap values. Set MVA = OFF, and enter the TAP1 through TAP3
values directly, along with the other pertinent settings.
Restrained Element Operating Current Pickup (O87P)
Range: 0.10–1.00 • TAP
Note: TAP
MIN
• O87P ≥ 0.1 I
n
Set the operating current pickup at a minimum for increased sensitivity but high enough to avoid
operation because of steady-state CT error and transformer excitation current.
Restraint Slope Percentage (SLP1, SLP2)
Range: SLP1: 5–100%, in 1% steps; SLP2: OFF, 25–200%
Use restraint slope percentage settings to discriminate between internal and external faults. Set
SLP1 or SLP2 to accommodate current differences from power transformer tap-changer, CT
saturation, CT errors, and relay error.
Restraint Current Slope 1 Limit (IRS1)
Range: 1.0–20.0, in 0.1 steps • TAP
Note: 1 A: TAP
MAX
• IRS1 ≤ 31.0
5 A: TAP
MAX
• IRS1 ≤ 155.0
A two-slope, or variable-percentage differential application, improves sensitivity in the region
where CT error is less and increases security in the high-current region where CT error is greater.
We must define both slopes, as well as the slope 1 limit or point IRS1, where SLP1 and SLP2
intersect.
Unrestrained Element Current Pickup (U87P)
Range: 1.0–20.0, in 0.1 steps • TAP
The purpose of the instantaneous unrestrained current element is to react quickly to very heavy
current levels that clearly indicate an internal fault. Set the pickup level (U87P) to about 10 times
tap. The unrestrained differential element only responds to the fundamental frequency
component of the differential operating current. It is unaffected by the SLP1, SLP2, IRS1, PCT2,
PCT5, or IHBL settings. Thus, you must set the element pickup level high enough so as not to
react to large inrush currents.
Second-Harmonic Blocking Percentage of Fundamental (PCT2)
Range: OFF, 5–100%, in 1% steps