Sel 411L - Page 275

P.3.167
Date Code 20151029 Protection Manual SEL-411L Relay
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 3.105 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-distance elements (Z2G–Z5G) set to be blocked during a power
swing.
Enable each distance zone you want included in the power swing blocking
function 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 3.116) and open
pole conditions. Similarly, ground-distance elements included in the power
swing blocking function 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_unfiltMAX is greater than 0.001, the bottom leg of AND 1
asserts.