3-64 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements Date Code 20001006
SEL-351 Instruction Manual
The capacitor bank is put on-line at the 0.3 per unit VAR level (lagging) on the bus. The per unit
VAR level immediately changes to the -0.7 per unit VAR level (leading) when the capacitor bank is
put on-line (0.3 - 1.0 = -0.7). There is a margin of 0.5 per unit VARs until the capacitor bank is
then be taken off-line (-0.7 - 0.5 = -1.2).
The capacitor bank is taken off-line at the -1.2 per unit VAR level (leading) on the bus. The per
unit VAR level immediately changes to -0.2 per unit VAR level (leading) when the capacitor bank
is taken off-line (-1.2 + 1.0 = -0.2). There is a margin of 0.5 per unit VARs until the capacitor
bank is put on-line again (-0.2 + 0.5 = 0.3).
From preceding calculations and figures:
9600 kVAR ≈ 1.0 per unit VARs ≈ 80.0 VA secondary (single-phase)
Convert the per unit VAR levels 0.3 and -1.2 to single-phase VA (voltamperes) secondary:
0.3 • 80.0 VA secondary (single-phase) = 24.0 VA secondary (single-phase)
-1.2 • 80.0 VA secondary (single-phase) = -96.0 VA secondary (single-phase)
Make the following power element settings for the SEL-351R (B) Relay:
EPWR = 2 (enable two power elements)
PWR1P = 24.0 (power element pickup; VA secondary [single-phase])
PWR1T = +VARS (power element type; lagging VARs)
PWR1D = ____ (power element time delay; cycles)
PWR2P = 96.0 (power element pickup; VA secondary [single-phase])
PWR2T = -VARS (power element type; leading VARs)
PWR2D = ____ (power element time delay; cycles)
To override transient reactive power conditions, set the above power element time delay settings
equivalent to several seconds (or perhaps minutes).
Resulting single-phase power elements PWRA1, PWRB1, and PWRC1 assert when the lagging
VAR level exceeds the 0.3 per unit VAR level (lagging) for each respective phase (see Figure 3.37
and left-hand side of Figure 3.35). These elements are used in close logic in the SEL-351(B) Relay
to automatically put the 9600 kVAR capacitor bank on-line.
Resulting single-phase power elements PWRA2, PWRB2, and PWRC2 assert when the leading
VAR level exceeds the -1.2 per unit VAR level (leading) for each respective phase (see Figure 3.37
and left-hand side of Figure 3.35). These elements are used in trip logic in the SEL-351(B) Relay
to automatically take the 9600 kVAR capacitor bank off-line.
The exact implementation of this capacitor close and trip logic in SELOGIC Control Equations in
the SEL-351(B) Relay requires an application note beyond the scope of this discussion.
Courtesy of NationalSwitchgear.com
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