Schweitzer Engineering Laboratories SEL-587Z - Residual Overcurrent Protection; Time-Overcurrent Curve Equations

3.16
SEL-587Z Instruction Manual Date Code 20020903
Relay Elements
Overcurrent Protection
While negative-sequence overcurrent elements do not respond to balanced
load, these elements do detect the negative-sequence current present in
unbalanced load. For this reason, select an element pickup setting above the
maximum 3I
2
current expected due to load unbalance.
When applied on the delta side of a delta-wye transformer, negative-sequence
relay elements also provide sensitive fault protection for ground faults on the
wye side of the transformer. This is not possible using only phase and residual
overcurrent elements.
Residual Overcurrent Protection
The residual element responds to 3I
0
current, where 3I
0
= Ia + Ib + Ic.
Residual overcurrent elements detect ground faults and do not respond to
balanced load. The residual element is sensitive to unbalanced load, however,
and should be set above the maximum 3I
0
current expected due to load
unbalance.
When applied on the delta side of a delta-wye transformer, residual
overcurrent elements are insensitive to any type of fault on the wye side of the
transformer and can only detect ground faults on the delta side. This
eliminates any coordination constraints with protection devices on the wye
side of the transformer, permitting very sensitive residual overcurrent element
pickup settings.
Time–Overcurrent
Curve Equations
Use Equation 3.8 through Equation 3.27 on page 3.18 to calculate the TOC
operating and reset times for the 10 most common curves.
tp = operating time in seconds
tr = electromechanical induction-disk emulation reset time
in seconds
(if electromechanical reset setting is made)
TD = time-dial setting
M = applied multiples of pickup current [for operating time
(tp), M>1; for reset time (tr), M
≤ 1]
U.S. Moderately Inverse Curve: U1
Operating Time
Equation 3.8
Reset Time
Equation 3.9
U.S. Inverse Curve: U2
Operating Time
Equation 3.10
tp TD 0.0226
0.0104
M
0.02
1–
----------------------+• =
tr TD
1.08
1M
2
–
----------------
• =
tp TD 0.180
5.95
M
2
1–
----------------+• =