3.61
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Overcurrent Element
Time-Dial Settings (51PnTD, 51QnTD, 51NnTD, 51PCmTD, 51NCmTD)
Range: US 0.5–15, IEC 0.05–1, in 0.01 steps
The time-dial setting acts to scale an inverse-time curve vertically, to vary the
output timing for a given multiple of pickup.
Electromechanical Reset Settings
(51PnRS, 51QnRS, 51NnRS, 51PCmRS, 51NCmRS)
Range: Y, N
This setting defines whether an inverse-time curve emulates an
electromechanical disk and resets slowly or instantaneously when current
drops below pickup. A setting of Y causes the relay to emulate an
electromechanical disk. A setting of N causes full reset of the time-
overcurrent element one cycle after current drops below the element pickup
setting.
Torque-Control Settings
(50PnmTC, 50QnmTC, 50NnmTC, 51PnTC, 51QnTC, 51NnTC)
Range: SELOGIC control equation
The torque-control setting is an enable setting for which you have three
options: a setting of logical 0 disables the associated definite-time element, a
logical 1 permits the element to operate, and SEL
OGIC control equations allow
conditional assertion of the element.
Application
Guidelines
Transformer Overcurrent Protection
Instantaneous overcurrent elements typically provide high-speed protection
for high-current, internal transformer faults and coordinated backup protection
for faults on the adjacent bus and/or feeders. You can use inverse-time
overcurrent elements to prevent transformer damage due to excessive through
currents caused by slow clearing external faults. Thermal and mechanical
damage curves should be available from the transformer manufacturer for
specific transformer designs. You can consult several references, including the
IEEE C37.91, Guide for Protective Relay Applications to Power
Transformers, that provide generic through-current limitations for various
classes of transformers.
Set the SEL-387 instantaneous overcurrent elements to detect high-current
faults within the transformer differential protection zone. Use definite-time
and time-overcurrent elements to detect lower current faults inside and outside
the transformer differential protection zone. Use appropriate delays to
coordinate with upstream and downstream protection.
Conventional instantaneous overcurrent elements must be set sufficiently high
to avoid tripping on transformer magnetizing inrush current, where peak
currents can be 30 times the transformer full load current. Transformer
magnetizing inrush current contains substantial second-harmonic current and
often contains a significant dc component. Unlike conventional
electromechanical overcurrent elements, the SEL-387 overcurrent elements
ignore all but the fundamental frequency current, making them insensitive to
NOTE: For combined winding
element settings, m = 1: W1 + W2;
m = 2: W3 + W4.
NOTE: For combined winding
element settings, m = 1: W1 + W2;
m = 2: W3 + W4.
NOTE: For combined winding
element settings, m = 1: W1 + W2;
m = 2: W3 + W4.
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