8.24
SEL-351A Relay Instruction Manual Date Code 20080213
Breaker Monitor and Metering
Demand Metering
Rolling demand meter response at “Time = 15 minutes” = 3.0/3 = 1.0 per unit.
Demand Meter
Settings
The demand current pickup settings in Table 8.3 are applied to demand current
meter outputs as shown in Figure 8.13. For example, when residual-ground
demand current I
G(DEM)
goes above corresponding demand pickup GDEMP,
Relay Word bit GDEM asserts to logical 1. Use these demand current logic
outputs (PDEM, NDEM, GDEM, and QDEM) to alarm for high loading or
unbalance conditions. Use in other schemes such as the following example.
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 51GT. This is accomplished
with the following settings from Tab le 8.3, pertinent residual-ground
overcurrent element settings, and SEL
OGIC control equation torque control
setting 51GTC:
EDEM = THM
DMTC = 5
GDEMP = 1.0
51GP = 1.50
50G5P = 2.30
51GTC = !GDEM + GDEM * 50G5
Refer to Figure 8.13, Figure 8.14, and Figure 3.19.
Table 8.3 Demand Meter Settings and Settings Range
Setting Definition Range
EDEM Demand meter type THM = thermal ROL = rolling
DMTC Demand meter
time constant
5, 10, 15, 30, or 60 minutes
PDEMP Phase demand
current pickup
OFF
0.50–16.00 A {5 A nominal}
0.10–3.20 A {1 A nominal}
Additional ranges for NDEMP:
0.005–0.640 A
{0.2 A nominal channel IN current input}
0.005–0.160 A
{0.05 A nominal channel IN current input}
NDEMP Neutral-ground
demand current pickup
GDEMP Residual-ground
demand current pickup
QDEMP Negative-sequence
demand current pickup
NOTE: Changing setting EDEM or
DMTC resets the demand meter
values to zero. This also applies to
changing the active setting group, and
setting EDEM or DMTC is different in
the new active setting group. Demand
current pickup settings PDEMP,
NDEMP, GDEMP, and QDEMP can be
changed without affecting the
demand meters.
The examples in this section discuss
demand current, but MW and MVAR
demand values are also available, as
stated at the beginning of this
Courtesy of NationalSwitchgear.com
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