Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.132
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
selected to allow a ground fault current through the resistor equal to or
somewhat more than the capacitive charging current of the system. To detect a
ground fault internal to the generator, which is forward looking from the core-
balance CT into the generator, a Z0MTAX value of -45 degrees should suffice.
To determine a more accurate value of Z0MTAX, a system study is
recommended.
Figure 4.93 Hybrid Power System With Neutral-Ground Resistor
50NFP/50NRP—Forward/Reverse Directional Neutral-Ground
Current Pickup.
Note that 50NFP and 50NRP settings are tied into X-side
settings of the relay. If setting ORDERX does not contain U, then settings
50NFP and 50NRP are not made or displayed.
The 50NFP setting (IN current value) is the pickup for the forward fault
detector 50NF of the zero-sequence voltage-polarized directional element
with IN as operate quantity (see Figure 4.76). Ideally, this setting is above
normal load unbalance and below the lowest expected zero-sequence current
magnitude for unbalanced forward faults.
The 50NRP setting (IN current value) is the pickup for the reverse fault
detector 50NR of the zero-sequence voltage-polarized directional element
with IN as operate quantity (see Figure 4.76). Ideally, this setting is above
normal load unbalance and below the lowest expected zero-sequence current
magnitude for unbalanced reverse faults.
a0N—Positive-Sequence Current Restraint Factor, IN/I1.
If setting
ORDER does not contain U, then setting a0N is not made or displayed (refer to
Figure 4.76). The following comparison is made as part of internal enable DIRNE:
|IN| > a0N • |I1|
IN is the secondary current measured by neutral channel IN. I1 is the positive-
sequence secondary current derived from the phase current channels IA, IB,
and IC. Presumably, channel IN is connected in such a manner that it sees the
system zero-sequence current (e.g., channel IN is connected to a core-balance
CT through which the three-phase conductors pass; in such a connection,
channel IN sees 3I0 zero- sequence current, IN = 3I0; see Figure 2.25). If a
core-balance current transformer is connected to neutral channel IN, it most
likely has a different ratio, compared to the current transformers connected to
the phase current channels IA, IB, and IC (CT ratio settings CTRN and
CTRX, respectively).
From a primary system study, load profile values, or metering values, derive
a0N as follows:
a0N = (3I0 pri./I1 pri.) • (CTRX/CTRN)
3I0 pri. = standing system unbalance current (zero-sequence; A primary)
I1 pri. = maximum load current (positive-sequence; A primary)
Neutral ground
resistor