Schweitzer Engineering Laboratories SEL-351-5, Instruction Manual

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F.3
Date Code 20070117
Instruction Manual SEL-351-5, -6, -7 Relay
Setting Negative-Sequence Overcurrent Elements
Coordinating Negative-Sequence Overcurrent Elements
Coordinating Negative-Sequence Overcurrent
Elements
The following coordination guidelines and example assume that the negative-
sequence overcurrent elements operate on 3I
2
magnitude negative-sequence
current and that the power system is radial. The negative-sequence overcurrent
elements in the SEL-351 Relay operate on 3I
2
magnitude negative-sequence
current.
The coordination example is a generic example that can be used with any relay
containing negative-sequence overcurrent elements that operate on 3I
2
magnitude negative-sequence current. The SEL-351 can be inserted as the
feeder relay in this example. Note that the overcurrent element labels in the
example are not the same as the labels of the corresponding SEL-351
overcurrent elements.
Coordination
Guidelines
Step 1. Start with the furthest downstream negative-sequence
overcurrent element (e.g., distribution feeder relay in a
substation).
Step 2. Identify the phase overcurrent device (e.g., line recloser, fuse)
downstream from the negative-sequence overcurrent element
that is of greatest concern for coordination. This is usually the
phase overcurrent device with the longest clearing time.
Step 3. Consider the negative-sequence overcurrent element as an
“equivalent” phase overcurrent element. Derive pickup, time
dial (lever), curve type, or time-delay settings for this
“equivalent” element to coordinate with the downstream phase
overcurrent device, as any phase coordination would be
performed. Load considerations can be disregarded when
deriving the “equivalent” phase overcurrent element settings.
Step 4. Multiply the “equivalent” phase overcurrent element pickup
setting by
√
3 to convert it to the negative-sequence overcurrent
element pickup setting in terms of 3I
2
current.
Any time dial (lever), curve type, or time delay calculated for
the “equivalent” phase overcurrent element is also used for the
negative-sequence overcurrent element with no conversion
factor applied.
Step 5. Set the next upstream negative-sequence overcurrent element to
coordinate with the first downstream negative-sequence
overcurrent element and so on. Again, coordination is not
influenced by load considerations.
Coordination Example
In Figure F.2 , the phase and negative-sequence overcurrent elements of the
feeder relay (51F and 51QF, respectively) must coordinate with the phase
overcurrent element of the line recloser (51R).
Negative-
sequence
overcurrent
element
pickup
=
√
3 • ("equivalent" phase overcurrent element pickup)
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