Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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5.60
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Out-of-Step Logic (Zero Settings)
In particular, if distance elements pick up without an associated step change in
the system voltage, the swing signature detector declares this as a power swing
condition and asserts Output SSD. However, if distance elements pick up and
there is an associated step change in the system voltage, the swing signature
detector does not assert as this is considered a system fault.
Figure 5.41 shows the logic for the swing signature detector. When both inputs
into AND 1 asserts, SSD (the swing signature detection) asserts. The top input
into AND 1 consists of the phase-distance elements (Z2P–Z5P) and ground-dis-
tance elements (Z2G–Z5G) set to be blocked during a power swing.
Enable each distance zone you want included in the power swing blocking func-
tion on an individual basis with the OOSB2–OOSB5 settings. Note that you
include both phase-distance element (Z2P–Z5P) and ground-distance elements
(Z2G–Z5G) with the OOSB2–OOSB5 settings. Phase-distance elements
included in the power swing blocking function are further supervised by the OSB
unbalance reset conditions (67QUBF, see
Figure 5.52 ) and open-pole conditions.
Similarly, ground-distance elements included in the power swing blocking func-
tion are further supervised by open-pole conditions.
If any of these distance elements asserts (no supervisory conditions), then the top
input into AND 1 asserts.
In a separate calculation, the algorithm calculates and stores three cycles of the
absolute values of the first order derivative, dSCV1_unfilt, in a buffer. From these
values, the algorithm calculates dSCV1_unfiltMAX, the maximum value of
dSCV1_unfilt over the three cycles.
For the summation, the logic uses values from the oldest cycle of the three-cycle
buffer (i.e., two cycle old values). This choice of values effectively delays the
assertion of the bottom leg of AND 1 for at least two and a half cycles. If three
samples in the buffer exceed 5 percent of dSCV1_unfiltMAX, and if dSCV1_un-
filtMAX is greater than 0.001, the bottom leg of AND 1 asserts.
Figure 5.40 Swing Signature Detector Logic
AND 1 SSD
Swing Signature Based
on Values of | dSCV1 |
Stored in a Memory Buffer
Distance-Element-Based
Fault Detection