Schweitzer Engineering Laboratories SEL-751 - Figure 4.22 Logic for Relay Word Bit GNDSW

4.32
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
Figure 4.22 Logic for Relay Word bit GNDSW
Relay Word bit GNDSW indicates whether the directional element for low-
impedance grounded or ungrounded/high-impedance grounded systems is
operating on neutral channel (IN) current I
N
(GNDSW = logical 1) or on resid-
ual ground current I
G
instead (GNDSW = logical 0).
This switching of currents (from I
N
to I
G
) requires the 50NFP/50NRP settings
(based on current I
N
) in the Figure 4.24 logic to be effectively changed to the
new I
G
base. This is done internally with CT ratio settings:
50NFP • CTRN/CTR (I
G
base)
50NRP • CTRN/CTR (I
G
base)
If the logic in Figure 4.25 (and Figure 4.29 and Figure 4.31) operates on neu-
tral current I
N
, then settings 50NFP and 50NRP are not adjusted, and just
operate as:
50NFP (I
N
base)
50NRP (I
N
base)
This transition is “seamless” if the lower detection threshold of the residual
ground current I
G
(0.05 A secondary for 5 A nominal phase; 0.01 A secondary
for 1 A nominal) effectively overlaps with the upper detection threshold of
neutral channel current I
N
(5 A secondary):
CTR/CTRN  (5 A/0.05 A) = 100 (5 A nominal phase inputs)
CTR/CTRN  (5 A/0.01 A) = 500 (1 A nominal phase inputs)
There is no effective overlap if:
CTR/CTRN > 100 (5 A nominal phase inputs)
CTR/CTRN > 500 (1 A nominal phase inputs)
With no effective overlap, when the neutral channel current I
N
exceeds the
upper detection threshold of neutral channel IN (5 A secondary), the unit still
operates on the neutral channel current I
N
until the lower detection threshold
of the residual ground current I
G
(0.05 A secondary for 5 A nominal phase;
0.01 A secondary for 1 A nominal) is reached. It is better to have effective
overlap:
CTR/CTRN  100 (5 A nominal phase inputs)
CTR/CTRN  500 (1 A nominal phase inputs)
2 CYC
0
4.7 A sec.
INOM/100
CTRN
CTR
5.0 A sec •
GNDSW
Reset has priority
INOM = Rating of phase current inputs
Latch
Set
Reset
Output
Relay
Word
Bits
|IN|
|IG|
Settings
CTR
CTRN
4.7 A-sec * 100
INOM
>