5.14
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Polarizing Quantity for Distance Element Calculations
Using ALTI and ALTV
SELOGIC control equations ALTI and ALTV give great flexibility in choosing
alternate CT and PT inputs to the SEL-421. The relay switches to the alternate
source when these SEL
OGIC control equations become true. The relay delays a
subsequent ALTI or ALTV switch for eight cycles after the initial switch to give
time for the system to settle. The status of ALTI and ALTV will be displayed in
the SER report. This confirms if the relay has switched the source it was using.
Test the SEL
OGIC control equation programming that you use to switch ALTI and
ALTV alternate sources. It is possible to create a toggling condition where the
relay repeatedly switches between sources. Examine each line of SEL
OGIC con-
trol equation programming to verify that this toggling condition does not occur in
your protection/control scheme.
One method for exercising caution when implementing alternate current source
and alternate voltage source switching is to use SEL
OGIC control equation protec-
tion latches (PLT01–PLT32) to switch alternate sources. For example, to switch
to an alternate voltage, set ALINEV to VZ (enables setting ALTV) and then set
ALTV to PLT31. To perform the switch use the protection latch control inputs
PLT31S and PLT31R (Set and Reset, respectively).
Polarizing Quantity for Distance Element Calculations
The relay uses positive-sequence memory voltage as the polarizing quantity for
distance element calculations. Memory polarization ensures proper operation
during zero-voltage three-phase faults and provides expansion of the mho charac-
teristic back to the source impedance, improving fault-resistance coverage. How-
ever, longer memory may impair distance element security when a power system
disturbance causes a fast frequency excursion.
The polarization memory is adaptive. The relay normally uses positive-sequence
voltage with short or medium length memory. This short or medium length mem-
ory works satisfactorily for all faults other than zero-voltage three-phase faults.
When the relay measures positive-sequence voltage magnitude lower than a
threshold, it automatically switches to a long memory polarizing quantity.
The VMEMC setting allows you to choose between short or medium length
memory voltage for the normal polarizing quantity. To closely follow the power
system frequency, set VMEMC = 0. When VMEMC is deasserted (logical 0), the
relay normally uses a short memory time constant that closely follows the posi-
tive-sequence voltage, yet automatically switches to the long memory when nec-
ALTI Alternate Current Source (SELOGIC Equation) NA Hidden
BK1I Breaker 1 Current Source (IW, IX, NA) IW
BK2I Breaker 2 Current Source (NA) NA Hidden
IPOL Polarizing Current (IAX, IBX, ICX, NA) IAX
ALINEV Alternate Line Voltage Source (VZ, NA) NA Default
ALTV Alternate Voltage Source (SEL
OGIC Equation) NA Hidden
Table 5.11 ESS := Y, Current Polarizing Source (Sheet 2 of 2)
Setting Prompt Entry Comments
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