Date Code 20000421 Specifications 2-27
SEL-251, -2, -3 Instruction Manual
If input IN4 = TCG, the ground/residual overcurrent elements selected in the ETC(1) setting
(50NL and consequently 50NLT in this example) are enabled for operation when input IN4 is
energized. If input IN4 = !TCG, the ground/residual overcurrent elements selected in the ETC(1)
setting are enabled for operation when input IN4 is deenergized.
If neither TCG nor !TCG is assigned to an input, the ground/residual overcurrent elements
selected in the ETC(1) setting are not externally torque controlled. The selected ground/residual
overcurrent elements are always enabled with respect to external torque control.
Internal Torque Control
The ITC(1) setting selects phase overcurrent elements to be internally torque controlled. Only
phase overcurrent elements in Relay Word row R1 can be selected. As an example:
ITC = 51P, 50H 51P, 50H, and consequently 51T are selected for
internal torque control
The TCI setting selects the elements which perform internal torque control:
TCI = 0, V, I, or 3 0 = none, V = 27, I = 50C, 3 = both
If you set TCI equal to V, the 27 element torque controls phase overcurrent elements selected in
the ITC(1) setting (51P and 50H and consequently 51T in this example). If 27 asserts, the
selected phase overcurrent elements are enabled with respect to internal torque control.
If you set TCI equal to I, the 50C element torque controls phase overcurrent elements selected in
the ITC(1) setting (51P and 50H and consequently 51T in this example). If 50C asserts, the
selected phase overcurrent elements are enabled with respect to internal torque control.
If you set TCI equal to 3, the phase overcurrent elements selected in the ITC(1) setting (51P and
50H and consequently 51T in this example) are torque controlled by "27 + 50C". If either 27 or
50C asserts, the selected phase overcurrent elements are enabled with respect to internal torque
control.
If you set TCI equal to 0, the phase overcurrent elements selected in the ITC(1) setting (51P and
50H and consequently 51T in this example) are not internally torque controlled. The phase
overcurrent elements are always enabled with respect to internal torque control.
TRANSFORMER BLOWN-FUSE DETECTION
Delta-wye connected distribution transformer banks are frequently protected by fuses connected
in the bank high side, as shown in Figure 2.11. When one high-side fuse blows, unbalanced
voltages are applied to the transformer bank and its connected load.
The SEL-251 Relay includes logic that detects high-fuse operations by measuring the low-side
voltages. The logic also rejects operations of low-side Voltage Transformer (VT) fuses.
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