Date Code 20000421 Specifications 2-29
SEL-251, -2, -3 Instruction Manual
The setting option, 27C = 4, enables the following logic for the 27 Relay Word bit:
27 = (Any phase-to-phase voltage less than 0.4 pu) * (Any phase-to-phase voltage
greater than 0.72 pu)
27 = (27LAB + 27LBC + 27LCA) * (!27HAB + !27HBC + !27HCA)
If a transformer fuse operates, one phase-to-phase voltage goes to zero (satisfying the left portion
of the equation above) and the remaining phase-to-phase voltages stay above 0.72 per unit
(satisfying the right portion of the equation). If a VT fuse operates, the phase-to-phase voltages
drop below the 0.72 per unit threshold and the 27 equation is not satisfied.
Use the Detection Logic to Trip or Indicate
Since certain faults may also present these voltages to the SEL-251 Relay, you may wish to use a
nondedicated SELOGIC Control Equation timer, such as the ST timer to provide some coordi-
nated time delay on pickup of the condition.
Set the SELOGIC Control Equation, S(123) = 27. Use the time-delayed pickup timer, TSPU =
300 to 600 cycles to provide a 5- to 10-second time delay. You can use the ST bit, which in-
cludes the time-delayed pickup, in any of the SELOGIC Control Equations for tripping or pro-
grammable output contact operation.
DEMAND AMMETERS
The SEL-251 Relay provides demand ammeters for phase, negative-sequence, and zero-sequence
(ground/residual) currents. Peak demands are saved. The demand ammeters behave much like
low-pass filters, responding to gradual trends. The demand ammeter time constant is used for all
three demand ammeters. The time constant is settable from 5 to 60 minutes.
Figure 2.23 shows the phase, negative-sequence, and ground/residual demand ammeters from top
to bottom. Let's concentrate on the bottom diagram (ground/residual demand ammeter) to under-
stand demand ammeter functions in general.
Present ground/residual current (IR) is the input into the ground/residual demand ammeter and
ground/residual demand current (ND(t)) is the output. If the ground/residual demand current is
ND(0) at t = 0 and the ground/residual current (IR) is constant, at t = DATC the ground/residual
demand current will be:
ND(DATC) = 0.9(IR - ND(0)) + ND(0) = 0.9IR + 0.1ND(0)
If the ground/residual demand current was reset at t = 0 (ND(0) = 0), at t = DATC the
ground/residual demand current would be:
ND(DATC) = 0.9(IR)
For all demand ammeters in general, if demand current is reset at t = 0 and a constant input cur-
rent is applied, the demand current output will be 90% of the constant input current value at t =
DATC.
To Next Page
Loading...
