Sel 551 - Figure 3.26 Demand Current Logic Outputs

3.44
SEL-551 Relay Instruction Manual Date Code 20110408
Relay Elements and Logic
Demand Ammetering
Figure 3.26 Demand Current Logic Outputs
Demand Current
Logic Output
Application—Raise
Pickup for Unbalance
Current
During times of high loading, the residual ground overcurrent elements can
see relatively high unbalance current I
G
(I
G
= 3I
0
). To avoid tripping on
unbalance current I
G
, use Relay Word bit GDEM to detect the residual
ground (unbalance) demand current I
G(DEM)
and effectively raise the pickup
of the residual ground time-overcurrent element 51G1T. This is accomplished
with the following settings from Table 3.7, pertinent residual ground
overcurrent element settings, and SEL
OGIC control equation torque control
setting 51G1TC (see Figure 3.27).
DMTC = 5
GDEMP = 1.0
51G1P = 1.50
50G1P = 2.30
51G1TC = !GDEM + GDEM * 50G1
Refer to Figure 3.26, Figure 3.27, and Figure 3.11.
PDEM
Relay
Word
Bits
Demand Current
Pickup Settings
(A Secondary)
I
B(DEM)
I
A(DEM)
I
A
I
C(DEM)
I
N(DEM)
Demand
Currents
Instantaneous
Currents
Serial Port
Command
Maximum
Phase
Demand
Current
Reset
Demand
Reset
Demand
Demand Function
Thermal
NDEM
NDEMP
PDEMP
I
N
Thermal
Demand Function
I
G(DEM)
Reset
Demand
GDEM
GDEMP
I
G
(residual)
Thermal
Demand Function
3I
2(DEM)
Reset
Demand
QDEM
QDEMP
3I
2
MET RD
Thermal
Demand Function