3-10
Differential, Restricted Earth Fault,
Date Code 20020129
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
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
Energization of a transformer causes a temporary large flow of magnetizing inrush current into
one terminal of a transformer, without other terminals seeing this current. Thus, it appears as a
differential current that could cause improper relay operation. Magnetizing inrush currents
contain greater amounts of second-harmonic current than do fault currents. This second-
harmonic current can be used to identify the inrush phenomenon and to prevent relay
misoperation. The SEL-387E Relay measures the amount of second-harmonic current flowing in
the transformer. You can set the relay to block the percentage restrained differential element if
the ratio of second-harmonic current to fundamental current (IF2/IF1) is greater than the PCT2
setting.
Содержание SEL-387E
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