2-26 Specifications Date Code 20000421
SEL-251, -2, -3 Instruction Manual
The digital filter has the properties of a double differentiator smoother and requires only addition
and subtraction of data samples. Let the latest four samples of one channel be X1, X2, X3, and
X4. Then the digital filter is defined:
P = X1 - X2 - X3 + X4.
This filter eliminates dc offsets. When all samples are set to the same value, the filter output is
zero. It also eliminates ramps, which you may verify by setting the samples equal to 1, 2, 3, and
4. Again, the output is zero.
Every quarter-cycle, the relay computes a new value of P for each input. The current value of P
combines with the previous value (renamed Q) to form a Cartesian coordinate pair. This pair
represents the input signal as a phasor (P, Q). The relay processes these phasor representations
of the input signals.
TORQUE CONTROL
Elements in Relay Word row R1 may be torque controlled. Elements derived from row R1
elements are torque controlled if the row R1 element is torque controlled. For example, if row
R1 elements 51P and 50NL are torque controlled, row R2 elements 51T and 50NLT are also
torque controlled. 51P and 50NL are the pickups for 51T and 50NLT, respectively. See
Figure 2.18, Figure 2.19, and Figure 2.20 for more information.
Phase overcurrent elements can be externally and internally torque controlled. Negative-
sequence and ground/residual overcurrent elements can only be externally torque controlled.
External Torque Control
The ETC(1) setting selects overcurrent elements to be externally torque controlled. Only over-
current elements in Relay Word row R1 can be selected. As an example:
ETC = 51P, 50Q, 50NL 51P, 50Q, 50NL, and consequently 51T, 50QT, and
50NLT are selected for external torque control
TCP External Torque Control (Phase and Negative-Sequence Overcurrent Elements)
TCG External Torque Control (Ground/Residual Overcurrent Elements)
TCP and TCG are assigned to programmable inputs. The inverted sense of TCP or TCG is
available, too (!TCP or !TCG, respectively).
If input IN3 = TCP, the phase and negative-sequence overcurrent elements selected in the
ETC(1) setting (51P and 50Q and consequently 51T and 50QT in this example) are enabled for
operation when input IN3 is energized. If input IN3 = !TCP, the phase and negative-sequence
overcurrent elements selected in the ETC(1) setting are enabled for operation when input IN3 is
deenergized.
If neither TCP or !TCP is assigned to an input, the phase and negative-sequence overcurrent
elements selected in the ETC(1) setting are not externally torque controlled. The selected phase
and negative-sequence overcurrent elements are always enabled with respect to external torque
control.
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