Schweitzer Engineering Laboratories SEL-311L - Demand Current Logic Output Application-Raise Pickup for Unbalance Current

Date Code 20010625 Breaker Monitor and Metering Functions 8-27
SEL-311L Instruction Manual
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 subsection.
The demand current pickup settings in Table 8.3 are applied to demand current meter outputs as
shown in Figure 8.15. 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, GDEM, and QDEM) to alarm for high loading or
unbalance conditions. Use in other schemes such as the following example.
Figure 8.15: 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 51GT. This is accomplished with the