8.26
SEL-351A Relay Instruction Manual Date Code 20080213
Breaker Monitor and Metering
Demand Metering
Residual-Ground Demand Current Below Pickup GDEMP
When unbalance current I
G
is low, unbalance demand current I
G(DEM)
is
below corresponding demand pickup GDEMP = 1.00 A secondary, and Relay
Word bit GDEM is deasserted to logical 0. This results in SEL
OGIC control
equation torque control setting 51GTC being in the state:
51GTC = !GDEM + GDEM * 50G5 = NOT(GDEM) + GDEM * 50G5
= NOT(logical 0) + (logical 0) * 50G5 = logical 1
Thus, the residual-ground time-overcurrent element 51GT operates on its
standard pickup:
51GP = 1.50 A secondary
If a ground fault occurs, the residual-ground time-overcurrent element 51GT
operates with the sensitivity provided by pickup 51GP = 1.50 A secondary.
The thermal demand meter, even with setting DMTC = 5 minutes, does not
respond fast enough to the ground fault to make a change to the effective
residual-ground time-overcurrent element pickup; it remains at 1.50 A
secondary. Demand meters respond to more “slow moving” general trends.
Residual-Ground Demand Current Goes Above Pickup GDEMP
When unbalance current I
G
increases, unbalance demand current I
G(DEM)
follows, going above corresponding demand pickup GDEMP = 1.00 A
secondary, and Relay Word bit GDEM asserts to logical 1. This results in
SEL
OGIC control equation torque control setting 51GTC being in the state:
51GTC = !GDEM + GDEM * 50G5 = NOT(GDEM) + GDEM * 50G5
= NOT(logical 1) + (logical 1) * 50G5 = logical 0 + 50G5 = 50G5
Thus, the residual-ground time-overcurrent element 51GT operates with an
effective, less-sensitive pickup:
50G5P = 2.30 A secondary
The reduced sensitivity keeps the residual-ground time-overcurrent element
51GT from tripping on higher unbalance current I
G
.
Residual-Ground Demand Current Goes Below Pickup GDEMP Again
When unbalance current I
G
decreases again, unbalance demand current
I
G(DEM)
follows, going below corresponding demand pickup GDEMP =
1.00 A secondary, and Relay Word bit GDEM deasserts to logical 0. This
results in SEL
OGIC control equation torque-control setting 51GTC being in
the state:
51GTC = !GDEM + GDEM * 50G5 = NOT(GDEM) + GDEM * 50G5
= NOT(logical 0) + (logical 0) * 50G5 = logical 1
Thus, the residual-ground time-overcurrent element 51GT operates on its
standard pickup again:
51GP = 1.50 A secondary
Courtesy of NationalSwitchgear.com
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