6
set lower if the relay filters harmonic currents
and responds only to fundamental currents.)
Assuming a maximum fault current of 8A primary
in the neutral and a relay set to pick up at 1A
primary, 88% of the stator winding is covered.
As with the 59N relay, the 51GN delay will allow
it to override clearing of a high-side ground fault.
An instantaneous overcurrent element can also
be employed, set at about three times the time-
overcurrent element pickup, although it may not
coordinate with primary vt fuses that are con-
nected to the generator terminals.
Multiple generators, per Fig. 7, can be high-
resistance grounded, but the 59N relays will not
be selective. A ground fault anywhere on the
generation bus or on the individual generators will
be seen by all 59N relays, and the tendency will
be for all generators to trip. The 51N relay, when
connected to a flux summation CT, will provide
selective tripping if at least three generators are
in service. In this case, the faulted generator
51N relay will then see more current than the
other 51N relays. The proper 51N will operate
before the others because of the inverse charac-
teristic of the relays. Use of the flux summation
CT is limited to those cases where the CT
window can accommodate the three cables.
Fault currents are relatively low, so care must be
exercised in selecting appropriate nominal relay
current level (e.g., 5A vs. 1A) and CT ratio. For
example, with a 30A fault level and a 50 to 5A
CT, a 1A nominal 51N with a pickup of 0.1A
might be used. With two generators, each
contributing 10A to a terminal fault in a third
generator, the faulted-generator 51N relay sees
2*10/(50/5) = 2A. Then the relay protects down
to (0.1/2)*100 = 5% from the neutral.
When feeder cables are connected to the gen-
erator bus, the additional capacitance dictates a
much lower level of grounding resistance than
achieved with a unit-connected case. A lower re-
sistance is required to minimize transient over-
voltages during an arcing fault.
FIGURE 7. 59N RELAY OPERATION WITH MULTIPLE
UNITS WILL NOT BE SELECTIVE; 51N RELAYS PRO-
VIDE SELECTIVE PROTECTION IF AT LEAST THREE
GENERATORS ARE IN SERVICE.
Ground differential (Fig. 8) is a good method to
sense ground faults on low and medium imped-
ance grounded units. It would more commonly be
seen on generators that have the CTs required
for phase differential relaying. In Fig. 8, the
protective function is labeled 87N, but the Basler
BE1-CDS220 or the BE1-67N is applied. The
BE1-CDS220 approach is more applicable to low
and medium impedance grounded generators
with ground faults as low as 50% of phase fault
current. The BE1-67N approach is more appli-
cable to medium impedance generators with low
ground fault current levels. The BE1-CDS220 is
limited in sensitivity to ground faults in excess of
10% of the phase CT tap setting, but the use of
the auxiliary CT in the BE1-67N approach allows
for amplification of the ground current in the
phase CTs, yielding increased sensitivity.
Whichever approach is used, an effort should be
made to select relay settings to trip for faults as
low as 10% of maximum ground fault current
levels. During external phase faults, considerable
87N operating current can occur when there is
dissimilar saturation of the phase CTs due to
high AC current or due to transient DC offset
effects, while the generator neutral current still
will be zero, assuming balanced conductor
impedances to the fault. One method to compen-
sate for transient CT saturation is to have
sufficient delay in the relay to ride through
external high-current two-phase-ground faults.
Содержание BE1-1051
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